2.2.1. Satellite Data

TRMM Multi-Satellite Precipitation Analysis (TMPA) (50◦ N–50◦ S) can retrieve microwave–infrared satellite precipitation estimates with gauge adjustments and can generate rational precipitation estimates at fine spatiotemporal scales (0.25◦ × 0.25◦ and 3-hourly temporal intervals) within the global scope (60◦ N–60◦ S) [21,22]. As the successor of TRMM, the GPM constellation consists of a core observation platform and 10 cooperative satellites to observe global precipitation through inter-satellite cooperation. The GPM program offers three different levels of data products, all of which are available on the NASA website (http://www.nasa.gov/mission\_pages/GPM). This study focuses on the GPM Level 3 products based on the IMERG algorithm. IMERG not only has a high spatial resolution (0.1◦ × 0.1◦, 0.5 h) and fully global coverage (60◦ S~60◦ N), but also predicts flood disasters by reducing the uncertainty associated with short-term precipitation accumulation. At present, IMERG data has been developed from the first version to the sixth version, and IMERG-V06B is the latest version of the satellite rainfall data. However, Mohammad et al. (2019) verified that IMERG V05 performs better than V06, and therefore this study has employed IMERG V05 to estimate the precipitation data [23].

IMERG has three different products: Early, Late, and Final. In the real-time phase, the IMERG data generation system generates Early products after running once and then generates Late products after running again. The main difference between them is that the Early product is generated only by the forward propagation algorithm in the cloud mobile vector propagation algorithm, while the Late product is added with the backpropagation algorithm. The time-lag product Final introduces more

sensor data sources based on the Late product. Meanwhile, the time of the extracted satellite data is matched with the station's data (08:00 a.m.) [24]. Table 1 shows the detailed information of the three IMERG products, with a study period of 2015–2018. Since IMERG-E and IMERG-L products are NRT products, they are released after 4 h and 12 h after observation, respectively; in turn, the IMERG-F is a PRT product, which has been calibrated for the deviation of the ground rainfall station, so it has a high accuracy and is usually released after two months of observation. Therefore, this study has adopted CMA data as calibration data, pays attention to the accuracy of the IMERG rainfall data to capture flash flood disasters, and combines the precipitation data estimated by IMERG-E and IMERG-F to discuss the accuracy of capturing flash flood, and thereby contributes to obtaining the static early warning thresholds.

**Table 1.** List of Integrated Multi-Satellite Retrievals for Global Precipitation Measurement (IMERG) products.

#### 2.2.2. Ground Observation Precipitation Data

The regional hourly precipitation integration products of the CMA (http://cdc.cma.gov.cn) is a reference product, which is based on the hourly precipitation observed by more than 30,000 automatic weather stations nationwide and the Climate Prediction Center morphing (CMORPH) satellite. Using the probability density function and optimal interpolation method, the precipitation fusion products with a high spatiotemporal resolution (0.1◦ × 0.1◦ and 1-h interval) have been generated in China. Among them, the optimal interpolation method is to combine the semi-hourly infrared data of the Geostationary Earth Orbit (GEO) satellite to interpolate the Passive microwave (PMW) inversion data, and then obtain the relatively fine precipitation. The gauge data involved in the CMA are subject to extreme quality control, including consistency checks, both internal and spatiotemporal. Meanwhile, the complex terrain, the low economic level in remote mountainous areas, and the sparse distribution of the ground stations have led to large errors in the measured data. The CMA data has a wide coverage and high spatiotemporal resolution; the overall error of this product is within 10%, and the error for heavy precipitation and site sparse areas is within 20%, which is more accurate than other similar products in China. Therefore, CMA products are suitable as the calibration data for product evaluation, but their scale should be consistent with the IMERG product in the calibration process [25]. Besides, when using CMA data to estimate the rainfall distribution, we should consider that the dataset may have a delayed phenomenon since it is actually a fusion of measured data and CMORPH satellite rainfall data.

The rainfall level is defined by the CMA, for example, the intraday rainfall is categorized into the rain and light-moderate rain (0–25 mm), heavy rain (25–50 mm), and rainstorm (>50 mm). Meanwhile, the minimum amount of precipitation that the gauges can measure is 0.1 mm. The critical rainfall in Yunnan Province mainly comes from the China rainstorm parameters atlas, which describes the statistical characteristics and laws of China's rainstorms. Based on the study period 2015–2018, the flash flood data were mainly from officially published data. In this study, a total of 120 flash flood events were analyzed, all of which resulted in deaths or missing of people. Additionally, some other data were also employed; for example, the topographic data came from a 1:250,000 digital elevation model (DEM) of the Yunnan Province.


Pathology Diagnosis and Treatment

*Vector-Borne Diseases - Recent Developments in Epidemiology and Control*

Vectors. 2019;**12**(1):151. DOI: 10.1186/

[62] Kocher A, Gantier JC, Gaborit P, Zinger L, Holota H, Valiere S, et al. Vector soup: High-throughput identification of Neotropical phlebotomine sand flies using metabarcoding. Molecular Ecology Resources. 2017;**17**(2):172-182. DOI:

s13071-019-3402-2

10.1111/1755-0998.12556

**100**

**103**

**Chapter 8**

**Abstract**

resistance.

**1. Introduction**

Wnt5A Signaling Antagonizes

*Leishmania donovani* Infection

Infection by the parasite *Leishmania donovani* causes Visceral Leishmaniasis, a deadly disease if not properly treated. *L. donovani* captures the immune defense potential of host macrophages. It disrupts immune homeostasis at least partly by inducing expression of anti-inflammatory cytokines and altering the cellular cytoskeletal framework, thereby creating a niche for its own survival. While inhibition of macrophage Wnt5A signaling promotes infection by the parasite, activation of Wnt5A signaling significantly dampens infection. Experimental evidence suggests that the antagonistic effect of Wnt5A signaling on parasite infection in macrophages may be on account of its influence on the actin cytoskeleton. Importantly, the inhibitory effect of Wnt5A on infection is sustained independent of drug sensitivity and resistance. Keeping in mind that drugs used to treat Visceral Leishmaniasis are quite toxic and the parasite develops drug resistance, revamping host Wnt5A signaling may be useful for eliminating infection independent of drug sensitivity or

*Arijit Chakraborty, Shreyasi Maity and Malini Sen*

**Keywords:** Wnt5A, *L. donovani*, macrophage, cytoskeleton, Rac1, actin

wall of the central vein, and pericellular fibrosis [4].

Leishmaniasis is a group of neglected tropical zoonotic diseases caused by intracellular obligate parasites belonging to the genus *Leishmania*. The disease is endemic to about 60 countries and may turn out life threatening if not properly treated [1]. Clinical manifestations of the disease range from cutaneous lesions (as in Cutaneous Leishmaniasis) to visceral pathologies (as in Visceral Leishmaniasis) [2]. In this review we will focus on Visceral Leishmaniasis (VL), its causative organism *Leishmania donovani* and host immune response to *L. donovani* infection.

Like many other intracellular parasites, *L. donovani* has evolved a unique ability to sustain a parasitic mode of life. In VL, *L. donovani* resides and then proliferates mainly in the spleen, the liver and the bone marrow. This can cause splenomegaly, hepatomegaly, fever and gray discoloration of the skin—hence the name "kala azar" or "black disease". Associated hematological changes include a hemoglobin level of 7–10 g/dl, leukopenia, thrombocytopenia, pancytopenia, increased ESR and erythroid hyperplasia in the bone marrow and varying degrees of erythrophagocytosis, leukophagocytosis and granulomatous reactions. In some cases clotting abnormalities and deranged platelet function are also observed [3]. Few studies, furthermore, indicate inflammatory changes in the liver, which include hypertrophy and hyperplasia of Kupffer cells, diffused intralobular fibrosis, fibrosis of the


**4. The future**

The future of family planning is highlighted by the fact that it is an important component of national health promotion and disease prevention programs. Research on improving family planning service delivery is closely related to the broader research effort that relates to the betterment of the general quality of health care that would inform practitioners about best practices. It is, therefore, necessary to foster research that results in improving family planning's effective and timely dissemination of information to service providers.

The future of family planning revolves around the prediction that methods would become 100% effective, especially those used a day after coitus, and producing vaginal spermicides that are bactericidal and virucidal against sexually transmitted infections. Albeit, the advancement of contraceptive pills or injections for men would be influenced by the reluctance of men from certain cultures and societies to adopt such methods.

### *Studies in Family Planning*

As with the health care system as a whole, the family planning future agenda should include some key aspects that relate to safety, effectiveness, patient-centered care, efficiency, and equity of health care.


**1. Introduction**

The brain is the most complex organ in the human body. It has over 100 billion nerve cells with trillions of synapses [1]. In other words, the human brain is the primary command and control center of the neurological system. Therefore, an injury in the brain has a catastrophic influence on human health. For example, in a brain tumor, the development of

**Citation:** Almalki, Y.E.; Ali, M.U.; Ahmed, W.; Kallu, K.D.; Zafar, A.; Alduraibi, S.K.; Irfan, M.; Basha, M.A.A.; Alshamrani, H.A.; Alduraibi, A.K. Robust Gaussian and Nonlinear Hybrid Invariant Clustered Features Aided Approach for Speeded Brain Tumor Diagnosis. *Life* **2022**, *12*, 1084. https://doi.org/10.3390/life12071084

Academic Editor: Yudong Cai

Received: 20 June 2022 Accepted: 17 July 2022 Published: 20 July 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

abnormal brain cells may damage the brain and may even threaten a patient's life. Because brain tumors have long-term and life-altering physical and psychological implications, they can significantly influence a patient's living quality and affect their entire life [2]. According to a World Health Organization (WHO) report [3], cancer is the second greatest cause of mortality globally. It is responsible for around 10 million fatalities. Therefore, early cancer identification improves the patient's survival chances.

The location, progression stage, type, and rate of growth of brain tumors determine whether they are benign or malignant [5,6]. The affected cells rarely attack nearly healthy cells in benign brain tumors. They also progress slowly and have clear limits, such as in meningioma and pituitary tumors. In contrast, neighboring healthy cells are influenced by affected cells in malignant brain tumors. These tumors also have a fast advancement rate with broad limitations, such as gliomas. Furthermore, brain tumors may be divided into two types based on their origin: primary and secondary brain tumors [7]. The brain tumors that start in the brain tissues are known as primary tumors. In contrast, secondary brain tumors develop in many areas of the central nervous system (CNS) and move to the brain via the blood vessels. Therefore, early cancer type detection (meningioma, pituitary, and glioma) is crucial for cancer treatment to save the patient's life.

For brain tumor detection, several diagnostic methods, both invasive and non-invasive, are utilized [8]. A biopsy is an invasive approach: a sample is retrieved by an incision and is inspected under a microscope to assess malignancy. Unlike other tumors in other areas of the body, the biopsy is usually delayed until the final brain surgery. Due to this, computer-aided diagnostics (CAD) (non-invasive) such as computed tomography (CT), positron emission tomography (PET), and magnetic resonance imaging (MRI) are thought to be faster and safer than a biopsy for diagnosing brain tumors. Brain MRI is considered to be the most recommended method owing to its ability to provide extensive information regarding the position, extension, nature, and size of the brain tumor [9]. Meanwhile, manual MRI scan interpretation takes a long time and has a significant risk of mistakes. Therefore, an automatic computer-aided diagnostic approach is required for injury detection in the brain.

The evolution of machine learning methods has increased CAD systems' efficiency in assisting doctors in identifying brain tumors [7,10,11]. Numerous learning methods have been presented in the literature to diagnose brain tumors; they can be further categorized as deep learning and classical learning methods based on the literature [12]. In deep learning approaches, convolution neural networks (CNNs) are generally utilized to identify brain tumors using MRI [13]. Various researchers have used pre-trained and developed learning models to classify MRI images. In one work [14], the authors developed a CNN model to classify brain MRI images into two classes (tumor and no tumor). The main shortcoming of their model was the detection of the subclasses of the tumor. Abiwinanda et al. [15] designed a CNN model to detect brain tumor subclasses (glioma, meningioma, and pituitary). However, their model had a low accuracy of only 84.19%. Recently, a new CNN model was developed to classify brain MRI images into three subclasses [8]. The authors also performed data augmentation to enhance the classification accuracy of brain MRI images. A classification accuracy of 96.56% was achieved using a 10-fold cross-validation approach. Irmak [16] developed a 25-layer CNN model to classify brain images into five classes, with an accuracy of 92.66%. Pre-trained networks such as GoogLeNet and ResNet-50 are also used to classify brain images [17–19]. However, the deep networks require long training times, have a complex architecture, high memory requirements, a strong processing unit (GPU), etc.

In contrast to deep learning models, classical models require the most basic features of brain MRI images to diagnose a brain tumor. Therefore, they require less time to train the models; methods include support vector machine (SVM), tree, Naïve Bayes, etc. Kumari et al. [20] computed the gray-level co-occurrence matrix of brain MRI images to

classify them into two classes. The model's accuracy was high; however, the authors only detected the tumors on the brain MRI images. The accuracy of these global-level features is not high due to the high similarity in the brain MRI images. Therefore, locallevel features such as the bag of words [21], Fisher vector [22], and scale-invariant feature transformation [23] are also used to classify brain MRI images.  They achieved a classification accuracy of 91.28% for the three-class classification MRI dataset. In a recent study [25], the authors calculated the deep features of brain MRI image datasets using pre-trained CNN models. The results showed that the hybrid features of the pre-trained model had the best accuracy of 93.72% when using an SVM classifier. However, the size of their dataset was large, and it required a long training time. Moreover, in machine learning images/MRI feature extraction approaches, features such as texture (extracted through gray-level co-occurrence matrix), local binary pattern, histogram of oriented gradient, etc., are quite sensitive to noise, scaling, rotation, visibility, etc., which affect the performance, memory requirement, execution time, etc.

Considering the shortcomings of deep and machine learning approaches, the following are the main contributions of this work:

The paper's organization is as follows: Section 2 presents the feature extraction and the workings of the proposed approach. Then, the dataset and results are presented in the third section. Finally, the results are discussed and concluded in Sections 4 and 5.


**2.1. Subcritical and supercritical water extraction**

Subcritical and supercritical water extractions have been employed extensively in biomass utilization due to the tunable physical and chemical properties of water, potentially valuable products, and environmental friendliness. Furthermore, these two fractionation methods are known as the promising methods to make the biorefinery concept more practical with sufficient and sustainable profit.

Typically, subcritical water is defined as the use of water at a temperature between the boiling point and critical temperature (373–647 K) under pressure, which is high enough to maintain its liquid state. Supercritical water occurs at a temperature and pressure higher than its critical point (22.1 MPa and 647 K). In the supercritical region, the properties of liquid and vapor fuse [6, 7]. The behavior of subcritical and supercritical water near critical point mainly depends on pressure and temperature; therefore, some important properties of water could be tuned

**Figure 4.** Static dielectric constant of water at various temperatures and pressures [14].

certainly takes great effects. The essential function of water in common fractionation includes the following: (a) it acts as a mass transfer medium, (b) it plays as a reactant constructing a mild acidic state due to the mitigation of pKw at an increased temperature, (c) it performs as a heat transfer medium, and (d) it represents as an explosion medium for explosion pretreatment to tear biomass into small pieces. Due to the advantages of water-lignocellulose interaction and efficacy, many attempts have practically focused on applying water into the fractionation process to separate the mixture of lignocellulosic biomass into an individual composition called aqueous fractionation, hydro-based fractionation, or "hydro-fractionation." The overall process of bio-based product production from lignocellulosic biomass is shown in **Figure 3**. Hydro-fractionations or the processes utilizing water as a medium, reactant, or catalyst for separating mixture compositions including subcritical extraction, supercritical extraction, and steam explosion are mainly discussed in terms of their fractionation principle, current

Subcritical and supercritical water extractions have been employed extensively in biomass utilization due to the tunable physical and chemical properties of water, potentially valuable products, and environmental friendliness. Furthermore, these two fractionation methods are known as the promising methods to make the biorefinery concept more practical with suf-

Typically, subcritical water is defined as the use of water at a temperature between the boiling point and critical temperature (373–647 K) under pressure, which is high enough to maintain its liquid state. Supercritical water occurs at a temperature and pressure higher than its critical point (22.1 MPa and 647 K). In the supercritical region, the properties of liquid and vapor fuse [6, 7]. The behavior of subcritical and supercritical water near critical point mainly depends on pressure and temperature; therefore, some important properties of water could be tuned

status and potential uses, life cycle and bioeconomy.

**Figure 3.** Process for sugar-based chemical platform production from biomass.

**2.1. Subcritical and supercritical water extraction**

**2. Principle of hydro-fractionation**

ficient and sustainable profit.

72 Renewable Resources and Biorefineries

by varying the temperature and pressure for particular conditions of biomass fractionation. In this section, the important properties of water at the subcritical and supercritical state related to biomass fractionation including dielectric constant, ionization constant, density, and viscosity are demonstrated and discussed. A better understanding of water properties under various temperatures and pressures can allow an appropriate experimental design and suitable operating conditions for some specific proposes.

Dielectric constant is a dimensionless value showing the relative permittivity of a material compared with the permittivity of free space. Typically, the high dielectric constant of a solvent means that it has high polarity and vice versa. **Figure 4** shows the influence of temperature and pressure on the dielectric constant. The value of the dielectric constant tends to decrease with the increasing temperatures while it is slightly affected by pressure around the critical point. This phenomenon hints that the polarity of water can be reduced by increasing the temperature which indicated that the solubility of hydrophobic organic compounds and low molecular biopolymers in biomass could be enhanced by using low polarity of water generated at elevated temperatures [8–10]. It is worth mentioning that low polarity of water also reduces the solubility of salt in the process, especially type 2 salts (classified by solubility behavior) such as Na2 SO4 , Na2 CO3 , and K2 SO4 [11], and the participation of salt might cause fouling that diminishes the efficiency of the process or even terminates the process. Therefore, the water supply should be treated to eliminate type 2 salts before its use in the process; also, a special design of a reactor might be required in case of raw material containing high contents of type 2 salts [12, 13].

The ionization constant of water is the ratio between the concentration of ionic ([H3 O]+ and [OH]− ) products and the reactant at the equilibrium condition. The influence of temperature and pressure on the ionization constant is shown in **Figure 5**. In the subcritical region, the ionization constant increases with the raising of temperature and is slightly affected by the increase of pressure. On the other hand, beyond critical temperature, the ionic constant

**Figure 5.** Negative log (base 10) of ionization constant of water at various temperatures and pressures [19].

declines with the increase of reaction temperature and dramatically drops with the reduction of pressure. Therefore, the reaction of biomass degradation takes place in ionic media for subcritical water extraction. On the other hand, the supercritical water extraction provided a radical-oriented environment for biomass fractionation [15–18].

(433–533 K) for a period ranging from seconds to several minutes and then suddenly depressurizing it to atmospheric pressure, making the biomass undergo an explosive decompression. This pretreatment is the combination of mechanical forces and chemical effects due to autohydrolysis of the acetyl group in hemicellulose. Autohydrolysis takes place from the formation of acetic acid from the acetyl group in the hemicellulose structure at high temperature where water acts as an acid at high temperature. The hemicellulose and lignin bonds are cleaved during the explosion, allowing the hemicellulose become water soluble; water-soluble lignin from plant cell wall is also released from the cleavage action into water phase. The mechanical effect is caused by explosive decompression that occurred from suddenly dropped pressure at the termination of the pretreatment, which induced the cell walls in biomass to undergo structural disruption and expansion. Because of these effects, a part of hemicellulose hydrolyzed and solubilized; lignin was redistributed, lignocellulosic matrix polymer was broken down, particle size was decreased, the degree of polymerization was reduced, and porosity was increased; moreover, cellulose was slightly depolymerized, which led to the improvement of lignocellulose digestibility [22–24].

**Figure 6.** Viscosity of water at various temperatures and pressures [21].

Supercritical extraction in terms of operating conditions, reaction mechanism, and preferred

>647 >22.1 Radical reaction Extract desired product

expansion of water

**Fractionation route Application**

Liquid ionic reaction Extract desired product

Reduce crystallinity of

biopolymer

Hydro-Fractionation for Biomass Upgrading http://dx.doi.org/10.5772/intechopen.79396 75

**Pressure range (MPa)**

liquid phase)

373–647 >0.001 (maintain

Steam explosion 433–533 0.69–4.83 Rapid volume

biomass is shown in **Table 1**.

**Temperature range (K)**

**Table 1.** Comparison of different hydro-fractionation methods.

**Hydro-fractionation**

Subcritical water extraction

Supercritical water extraction

**method**

Density of water is defined as the ratio between the mass and volume of water at a specific temperature and pressure. The density of water is decreased with the increase of temperature due to the expansion of the volume. With the increase of pressure, the density of water increases. The higher density of water at specific conditions provides a better chance to penetrate the biomass structure [20].

The viscosity of water is the resistance of water from the external stress such as tensile strength and shear strength. It refers to the resistivity of the water over movement or deformity. The viscosity of water decreased with the increase of temperature but only a slight change was observed when the pressure increased in the subcritical region. However, a more effect of higher pressure was found in the supercritical region on the higher value of viscosity. The viscosity has a direct effect on biomass fractionation. Since the small value of water viscosity provides better wettability of the biomass, the penetration of water to destroy the biomass structure increases (**Figure 6**).

#### **2.2. Steam explosion**

Steam explosion, one of the most widely employed hydrothermal technologies for pretreating lignocellulose in industrial applications to convert biomass into useful chemicals, has been recognized as an environmental friendly pretreatment method that can effectively enhance subsequent enzymatic hydrolysis without the necessity of using chemicals, except water, which can lower environmental impact, lower capital investment, bring more potential for energy efficiency, and give rise to less hazardous process chemicals and conditions; this offers several attractive features when compared to hydrolytic acid and oxidative processes. Steam explosion involves exposing wet lignocellulosic biomass to high-pressure saturated steam (0.69–4.83 MPa) and temperature

**Figure 6.** Viscosity of water at various temperatures and pressures [21].

declines with the increase of reaction temperature and dramatically drops with the reduction of pressure. Therefore, the reaction of biomass degradation takes place in ionic media for subcritical water extraction. On the other hand, the supercritical water extraction provided a

**Figure 5.** Negative log (base 10) of ionization constant of water at various temperatures and pressures [19].

Density of water is defined as the ratio between the mass and volume of water at a specific temperature and pressure. The density of water is decreased with the increase of temperature due to the expansion of the volume. With the increase of pressure, the density of water increases. The higher density of water at specific conditions provides a better chance to pen-

The viscosity of water is the resistance of water from the external stress such as tensile strength and shear strength. It refers to the resistivity of the water over movement or deformity. The viscosity of water decreased with the increase of temperature but only a slight change was observed when the pressure increased in the subcritical region. However, a more effect of higher pressure was found in the supercritical region on the higher value of viscosity. The viscosity has a direct effect on biomass fractionation. Since the small value of water viscosity provides better wettability of the biomass, the penetration of water to destroy the biomass structure increases (**Figure 6**).

Steam explosion, one of the most widely employed hydrothermal technologies for pretreating lignocellulose in industrial applications to convert biomass into useful chemicals, has been recognized as an environmental friendly pretreatment method that can effectively enhance subsequent enzymatic hydrolysis without the necessity of using chemicals, except water, which can lower environmental impact, lower capital investment, bring more potential for energy efficiency, and give rise to less hazardous process chemicals and conditions; this offers several attractive features when compared to hydrolytic acid and oxidative processes. Steam explosion involves exposing wet lignocellulosic biomass to high-pressure saturated steam (0.69–4.83 MPa) and temperature

radical-oriented environment for biomass fractionation [15–18].

etrate the biomass structure [20].

74 Renewable Resources and Biorefineries

**2.2. Steam explosion**

(433–533 K) for a period ranging from seconds to several minutes and then suddenly depressurizing it to atmospheric pressure, making the biomass undergo an explosive decompression. This pretreatment is the combination of mechanical forces and chemical effects due to autohydrolysis of the acetyl group in hemicellulose. Autohydrolysis takes place from the formation of acetic acid from the acetyl group in the hemicellulose structure at high temperature where water acts as an acid at high temperature. The hemicellulose and lignin bonds are cleaved during the explosion, allowing the hemicellulose become water soluble; water-soluble lignin from plant cell wall is also released from the cleavage action into water phase. The mechanical effect is caused by explosive decompression that occurred from suddenly dropped pressure at the termination of the pretreatment, which induced the cell walls in biomass to undergo structural disruption and expansion. Because of these effects, a part of hemicellulose hydrolyzed and solubilized; lignin was redistributed, lignocellulosic matrix polymer was broken down, particle size was decreased, the degree of polymerization was reduced, and porosity was increased; moreover, cellulose was slightly depolymerized, which led to the improvement of lignocellulose digestibility [22–24].

Supercritical extraction in terms of operating conditions, reaction mechanism, and preferred biomass is shown in **Table 1**.

**Table 1.** Comparison of different hydro-fractionation methods.


**2. Lipid hydration and bilayer stabilization**

When dry phosphatidylcholines (PCs) of chain length above 12 hydrocarbon atoms are dispersed in water above their transition temperature as described in **Figure 2**, they form lamellar onion-like structures in which bilayers are separated by aqueous spaces that are available to trap the compounds of interest to vehiculize and deliver (**Figure 4**).

**Figure 4.** *(A) Electronic microscopic traditional image of multilamellar liposomes; (B) the diffraction pattern illustrates the separation between bilayers; (C) water solution trapped in between bilayers is schematically represented.*

After Bangham, Luzzati and others [13–15] determined the thickness of lipid bilayers by SAXS. Different thermal profiles were obtained according to the lipid features and head group structure was found [16–18]. Lipids stabilize differently according to its geometry. Phosphatidylcholines, that have similar areas in the head group region and the acyl chain, form bilayers by stacking molecules in cylindrical shape. In contrast, phosphatidylethanolamine (PE) forms hexagonal phases due to the conical shape of molecules [19, 20].

The process of lipid hydration that derives the formation of liposomes consists of different stages as described in **Figure 5**.

The hydration step consists of the increase in area and decrease in thickness up to around 20% c.a. 22–24 water molecules per lipid. After this stage, area and thickness remain constant and the swelling of liposomes starts by the increase of water in the interlamellar space. This description of lipid swelling illustrates about several structural and physicochemical properties of the bilayers. The first observation is that there is a defined number of water molecules per lipid that determines the area per lipid and the bilayer thickness. This number is around 7–8 below the phase transition temperature and 22–24 above as derived by DSC [17, 18, 21, 22]. Thus, water is a component of the structure of the lipid bilayer, determining its thermodynamic stability.

**13**

must be done.

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems*

At least four features deserve discussion. At equilibrium in fully hydrated state, the membrane thickness is composed by the excluded volume of the lipid molecules and the excluded volume of the water organized by them (the hydration number denoted above). Thus, the barrier properties do not only merge with the head group and the acyl chain region per se but also of the packing and arrangement of water molecules in the hydration shell of the phospholipids. This means that for any solute, to overcome the bilayers, that is, releasing the trapped solute or incorporating some of them must permeate or alter the hydration shell. This can, in principle, be done by some of these mechanisms: insertion in the water network removing or replacing water molecules in the hydration shell or changing the area per lipid by

The second feature of lipid bilayers is derived from the first one. The interbilayer space consists of water not bound to the membrane, that is, it can be displaced by changing the osmotic gradient between the inner spaces and the outer media of the liposome. Water can permeate the lipid bilayer with certain facility depending on the phase state of the lipids, the presence of double bonds or ramifications in the acyl chains [23, 24]. In contrast, membrane is completely or partially impermeable to some solutes, such as sugars, ions, depending on its size and molecular structure. The differences in permeation rates between water and any of these solutes means that at least in the beginning of the process, a gradient of water chemical potential can be built with a difference in solute concentrations between the two sides of the bilayer. Let us consider the interbilayer space described in **Figure 5**. If solute is more concentrated between the bilayers, water will be driven to enter due to a difference in osmotic pressure and them the spacing (and hence the trapped volume) will be larger. An opposite effect can be caused, if the solute is concentrated in the outer media of the liposomes. In this case, liposomes shrink and the interbilayer space decreases.

The third feature is represented by an area per lipid for a membrane thickness of 60 Å, when lipids hydrate with 22–24 water molecules per lipid. Thus, any change in the number of these water molecules will affect thickness and area with concomi-

Finally, the fourth feature is defined by the limit of the volume decrease. This is given by the steric repulsion of the groups in the surface of the bilayer, in which water plays a significant role. Water associated with the lipids is oriented at the bilayer surface constituting an electrical potential that hinder bilayers approach. This repulsive force is named as dipole potential or hydration forces

It is immediate to derive that the presence of these forces hinders the adhesion or fusion of membranes with different kinds of surfaces (inorganic and organic materials, other membranes, proteins, tissues, etc.). On the other way round, those processes will be feasible if water of hydration is totally or partially removed. This point is essential to understand the role of water in terms of membrane response to

Entering details of the dynamic properties of membranes in relation to water, we may again inspect **Figure 5**. *The equilibrium point* corresponding to an area per molecule of 75 Å and 20 water molecules per lipid could be modified by the inclusion or exclusion of water molecules. So, the question is which perturbations can trigger *changes in hydration* that can be dominated in order to promote controlled changes in permeation. In this direction, let us focus on the mechanical and chemical forces at constant temperature. By mechanical forces, we refer to processes that led the membrane to expand or contract and by chemical forces, the competition with water by membrane sites of compounds may form hydrogen bonds. To analyze these points, previous considerations about water as a component of membranes

*DOI: http://dx.doi.org/10.5772/intechopen.85811*

expansion or compression.

tant effect on permeability.

biologically relevant effectors.

[24–27] (**Figure 6**).

**Figure 5.** *Hydration of lipids and swelling of liposomes.*

#### *The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems DOI: http://dx.doi.org/10.5772/intechopen.85811*

At least four features deserve discussion. At equilibrium in fully hydrated state, the membrane thickness is composed by the excluded volume of the lipid molecules and the excluded volume of the water organized by them (the hydration number denoted above). Thus, the barrier properties do not only merge with the head group and the acyl chain region per se but also of the packing and arrangement of water molecules in the hydration shell of the phospholipids. This means that for any solute, to overcome the bilayers, that is, releasing the trapped solute or incorporating some of them must permeate or alter the hydration shell. This can, in principle, be done by some of these mechanisms: insertion in the water network removing or replacing water molecules in the hydration shell or changing the area per lipid by expansion or compression.

The second feature of lipid bilayers is derived from the first one. The interbilayer space consists of water not bound to the membrane, that is, it can be displaced by changing the osmotic gradient between the inner spaces and the outer media of the liposome. Water can permeate the lipid bilayer with certain facility depending on the phase state of the lipids, the presence of double bonds or ramifications in the acyl chains [23, 24]. In contrast, membrane is completely or partially impermeable to some solutes, such as sugars, ions, depending on its size and molecular structure. The differences in permeation rates between water and any of these solutes means that at least in the beginning of the process, a gradient of water chemical potential can be built with a difference in solute concentrations between the two sides of the bilayer. Let us consider the interbilayer space described in **Figure 5**. If solute is more concentrated between the bilayers, water will be driven to enter due to a difference in osmotic pressure and them the spacing (and hence the trapped volume) will be larger. An opposite effect can be caused, if the solute is concentrated in the outer media of the liposomes. In this case, liposomes shrink and the interbilayer space decreases.

The third feature is represented by an area per lipid for a membrane thickness of 60 Å, when lipids hydrate with 22–24 water molecules per lipid. Thus, any change in the number of these water molecules will affect thickness and area with concomitant effect on permeability.

Finally, the fourth feature is defined by the limit of the volume decrease. This is given by the steric repulsion of the groups in the surface of the bilayer, in which water plays a significant role. Water associated with the lipids is oriented at the bilayer surface constituting an electrical potential that hinder bilayers approach. This repulsive force is named as dipole potential or hydration forces [24–27] (**Figure 6**).

It is immediate to derive that the presence of these forces hinders the adhesion or fusion of membranes with different kinds of surfaces (inorganic and organic materials, other membranes, proteins, tissues, etc.). On the other way round, those processes will be feasible if water of hydration is totally or partially removed. This point is essential to understand the role of water in terms of membrane response to biologically relevant effectors.

Entering details of the dynamic properties of membranes in relation to water, we may again inspect **Figure 5**. *The equilibrium point* corresponding to an area per molecule of 75 Å and 20 water molecules per lipid could be modified by the inclusion or exclusion of water molecules. So, the question is which perturbations can trigger *changes in hydration* that can be dominated in order to promote controlled changes in permeation. In this direction, let us focus on the mechanical and chemical forces at constant temperature. By mechanical forces, we refer to processes that led the membrane to expand or contract and by chemical forces, the competition with water by membrane sites of compounds may form hydrogen bonds. To analyze these points, previous considerations about water as a component of membranes must be done.

*Liposomes - Advances and Perspectives*

the conical shape of molecules [19, 20].

**Figure 4.**

of different stages as described in **Figure 5**.

After Bangham, Luzzati and others [13–15] determined the thickness of lipid bilayers by SAXS. Different thermal profiles were obtained according to the lipid features and head group structure was found [16–18]. Lipids stabilize differently according to its geometry. Phosphatidylcholines, that have similar areas in the head group region and the acyl chain, form bilayers by stacking molecules in cylindrical shape. In contrast, phosphatidylethanolamine (PE) forms hexagonal phases due to

*(A) Electronic microscopic traditional image of multilamellar liposomes; (B) the diffraction pattern illustrates the separation between bilayers; (C) water solution trapped in between bilayers is schematically represented.*

The process of lipid hydration that derives the formation of liposomes consists

The hydration step consists of the increase in area and decrease in thickness up to around 20% c.a. 22–24 water molecules per lipid. After this stage, area and thickness remain constant and the swelling of liposomes starts by the increase of water in the interlamellar space. This description of lipid swelling illustrates about several structural and physicochemical properties of the bilayers. The first observation is that there is a defined number of water molecules per lipid that determines the area per lipid and the bilayer thickness. This number is around 7–8 below the phase transition temperature and 22–24 above as derived by DSC [17, 18, 21, 22]. Thus, water is a component of

the structure of the lipid bilayer, determining its thermodynamic stability.

**12**

**Figure 5.**

*Hydration of lipids and swelling of liposomes.*

#### **Figure 6.**

*(A) The limit of approach of lipid bilayers, (B) water organized at the interphase determining the repulsion forces, (C) dipole potential at the lipid interphase. This potential makes the bilayer interior positive and has important consequences in the binding and penetration of charged peptides and proteins.*

#### **3. The bilayer structure and the water ratio**

The understanding of the structural role of water in lipid membranes received considerable attention after Luzzatti et al. and efforts were addressed to give a defined location to water and to determine the appropriated values for area per lipid molecule [22, 27, 28]. Today, it is generally accepted that PCs admit up to a limit of around 22–24 water molecules per lipid above the phase transition temperature to stabilize in a bilayer with an area per lipid of 64 Å2 and a thickness of around 40 A [29, 30]. In the first stages of the hydration process, the water molecules interact with the phosphate group and, at a second step, with the carbonyl groups [31–33]. The formation of hydrogen bonds between these groups and water molecules can be monitored by observing the frequency shift of their stretching frequencies [34, 35]. It is expected that upon hydrogen-bonding, these frequencies decrease as a consequence of the elongation of the chemical bonds in the relative functional groups. Furthermore, the conformation of the acyl chains changes with the hydration [36].

At least three regions of differential hydration can be identified according to the rate of exchange between membrane phase and the aqueous environment: the phosphate group, the carbonyl groups and the hydrocarbon chains. Each of them has different level of water organization in regard to coordination water by H bonding [30]. From them, CO and acyl chains seem to be important to evaluate the role of water in regulatory and functional response [37, 38].

**Figure 7** shows the lipid bilayer denoting the water region (upper part figure) and the profile of water distribution along the lipid bilayer (lower part).

It is observed that water covers all the phosphate and choline regions and partially the carbonyl groups. Moreover, it is also observed that water may penetrate beyond the carbonyl region, that is, the first methylene groups of the hydrocarbon chains. This denotes that different arrangements of water can be found along the membrane thickness each of one with a different energetic profile, that is, the ability

**15**

with which its interacts.

**Figure 7.**

nated waters [35, 39].

described (**Figure 9**) [30].

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems*

to react with polar or ionic solutes. This reactivity can be ascribed to the residual capacity the water molecules have to form H bonds depending on the surface groups

*(orange line) and cholines (green line) groups completely covered by water.*

*The lipid bilayer considering the water interphases. In the lower diagram, it is observed that the water profile (blue line) penetrates up to the region of the carbonyl groups (red solid and dashed lines), being phosphates*

can be ascribed to subpopulations of water molecule in four of the five groups

bilayer. In this regard, the presence of proteins can alter this picture.

In **Figure 8**, water populations are visualized using infrared spectroscopy. Water bands at different frequencies denote the presence of non bonded to tetracoordi-

A closer analysis by molecular simulation allows to explain that the broad bands

Hence, in a population of molecules with three hydrogen bonds, the following combinations can be possible: www, ppp, ccc, wwp, wwc, wpp, wcc, wcp, ppw, ppc, etc. This means that the water distribution and the energy of the surface are extremely heterogeneous and hence a great versatility in reaction can be expected, even more if we consider that these combinations have a mean life time. Hence, the sites can be modified by fluctuations and these can in turn affect the presence of solute from the media and the lateral interaction at different lateral pressure of the

For a better understanding of the influence of the lateral pressure on the membrane properties, we first deal with thermodynamic aspects of lipid monolayer. We will connect these properties with bilayer in the last part of the next section.

*DOI: http://dx.doi.org/10.5772/intechopen.85811*

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems DOI: http://dx.doi.org/10.5772/intechopen.85811*

#### **Figure 7.**

*Liposomes - Advances and Perspectives*

**3. The bilayer structure and the water ratio**

stabilize in a bilayer with an area per lipid of 64 Å2

of water in regulatory and functional response [37, 38].

The understanding of the structural role of water in lipid membranes received considerable attention after Luzzatti et al. and efforts were addressed to give a defined location to water and to determine the appropriated values for area per lipid molecule [22, 27, 28]. Today, it is generally accepted that PCs admit up to a limit of around 22–24 water molecules per lipid above the phase transition temperature to

*(A) The limit of approach of lipid bilayers, (B) water organized at the interphase determining the repulsion forces, (C) dipole potential at the lipid interphase. This potential makes the bilayer interior positive and has*

*important consequences in the binding and penetration of charged peptides and proteins.*

[29, 30]. In the first stages of the hydration process, the water molecules interact with the phosphate group and, at a second step, with the carbonyl groups [31–33]. The formation of hydrogen bonds between these groups and water molecules can be monitored by observing the frequency shift of their stretching frequencies [34, 35]. It is expected that upon hydrogen-bonding, these frequencies decrease as a consequence of the elongation of the chemical bonds in the relative functional groups. Furthermore, the conformation of the acyl chains changes with the hydration [36]. At least three regions of differential hydration can be identified according to the rate of exchange between membrane phase and the aqueous environment: the phosphate group, the carbonyl groups and the hydrocarbon chains. Each of them has different level of water organization in regard to coordination water by H bonding [30]. From them, CO and acyl chains seem to be important to evaluate the role

**Figure 7** shows the lipid bilayer denoting the water region (upper part figure)

It is observed that water covers all the phosphate and choline regions and partially the carbonyl groups. Moreover, it is also observed that water may penetrate beyond the carbonyl region, that is, the first methylene groups of the hydrocarbon chains. This denotes that different arrangements of water can be found along the membrane thickness each of one with a different energetic profile, that is, the ability

and the profile of water distribution along the lipid bilayer (lower part).

and a thickness of around 40 A

**14**

**Figure 6.**

*The lipid bilayer considering the water interphases. In the lower diagram, it is observed that the water profile (blue line) penetrates up to the region of the carbonyl groups (red solid and dashed lines), being phosphates (orange line) and cholines (green line) groups completely covered by water.*

to react with polar or ionic solutes. This reactivity can be ascribed to the residual capacity the water molecules have to form H bonds depending on the surface groups with which its interacts.

In **Figure 8**, water populations are visualized using infrared spectroscopy. Water bands at different frequencies denote the presence of non bonded to tetracoordinated waters [35, 39].

A closer analysis by molecular simulation allows to explain that the broad bands can be ascribed to subpopulations of water molecule in four of the five groups described (**Figure 9**) [30].

Hence, in a population of molecules with three hydrogen bonds, the following combinations can be possible: www, ppp, ccc, wwp, wwc, wpp, wcc, wcp, ppw, ppc, etc. This means that the water distribution and the energy of the surface are extremely heterogeneous and hence a great versatility in reaction can be expected, even more if we consider that these combinations have a mean life time. Hence, the sites can be modified by fluctuations and these can in turn affect the presence of solute from the media and the lateral interaction at different lateral pressure of the bilayer. In this regard, the presence of proteins can alter this picture.

For a better understanding of the influence of the lateral pressure on the membrane properties, we first deal with thermodynamic aspects of lipid monolayer. We will connect these properties with bilayer in the last part of the next section.

#### **Figure 8.**

*Water bands in the adjacencies of lipid membranes in the gel (red line) and in the liquid crystalline state (blue line). Dotted lines denote the populations of water with none, one, two, three and four H-bonds. Central band at approximately 3500 cm<sup>−</sup><sup>1</sup> (red solid line) corresponds to bulk water without lipids.*

#### **Figure 9.**

*Distributions of subpopulations of water forming none, one, two, three and four H bonds. Numbers correspond to water (w), carbonyl (C) and phosphate (P). Thus, 2 1 0 means two H-bonds with water, one with carbonyl, none with phosphates.*

### **4. Stability and membrane response: a dynamic picture**

The thermodynamic stabilization of the lipids in water to form bilayers as a consequence of the hydrophobic interaction between acyl chains has been discussed in several previous paper and will not be analyzed here [40–42].

Instead, the thermodynamic aspects of the bilayer as a reactive surface considering its *mechanical properties* will specifically be discussed. This approach is based

**17**

**Figure 10.**

*pressure (Π) by decreasing the water surface tension (γ*

*area per lipid molecule can be calculated.*

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems*

on the proposals presented earlier by Damoradan et al., Cevc, Disalvo et al. [37, 38, 43–45]. In terms of interfacial properties, it is important to consider lipid interphases results that may give complementary information to present a more rigorous picture in terms of thermodynamic response. This implies to focus on the interfacial properties of the lipid arrangements both in bilayers and monolayers in which water is a component of the structure. For this reason, some general properties of lipid monolayers will be discussed to relate them in response with lipid bilayers of liposomes and vesicles to have a general formalism that can explain the behavior of both systems. The main property inherent to membrane stabilization as a bilayer or as a monolayer is the *surface tension of water*. In the formation of bilayers, the surface tension between the acyl chain and water as described in **Figure 2** is the main force driven the *hydrophobic interaction*. The increase in entropy is expressed by the decrease in the surface tension, that is, of the surface free energy. Hence the process is spontaneous. When lipids spread on the air-water surface (**Figure 10**), monolayers are formed spontaneously by the same reason: decrease of the surface tension with respect to

) [46]. This is generally expressed as the *surface pressure* (Π)

Π = γ<sup>0</sup> − γ (1)

*The addition of lipids to the air water surface in a Langmuir though at constant area increases the surface*

*ο*

*) until a limit value is reached. From this point, the*

*DOI: http://dx.doi.org/10.5772/intechopen.85811*

0

that of pure water (γ

which is given by:

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems DOI: http://dx.doi.org/10.5772/intechopen.85811*

on the proposals presented earlier by Damoradan et al., Cevc, Disalvo et al. [37, 38, 43–45]. In terms of interfacial properties, it is important to consider lipid interphases results that may give complementary information to present a more rigorous picture in terms of thermodynamic response. This implies to focus on the interfacial properties of the lipid arrangements both in bilayers and monolayers in which water is a component of the structure. For this reason, some general properties of lipid monolayers will be discussed to relate them in response with lipid bilayers of liposomes and vesicles to have a general formalism that can explain the behavior of both systems.

The main property inherent to membrane stabilization as a bilayer or as a monolayer is the *surface tension of water*. In the formation of bilayers, the surface tension between the acyl chain and water as described in **Figure 2** is the main force driven the *hydrophobic interaction*. The increase in entropy is expressed by the decrease in the surface tension, that is, of the surface free energy. Hence the process is spontaneous.

When lipids spread on the air-water surface (**Figure 10**), monolayers are formed spontaneously by the same reason: decrease of the surface tension with respect to that of pure water (γ 0 ) [46]. This is generally expressed as the *surface pressure* (Π) which is given by:

$$
\Pi \quad = \begin{array}{c} \lambda\_0 - \lambda \end{array} \tag{1}
$$

#### **Figure 10.**

*The addition of lipids to the air water surface in a Langmuir though at constant area increases the surface pressure (Π) by decreasing the water surface tension (γ ο ) until a limit value is reached. From this point, the area per lipid molecule can be calculated.*

*Liposomes - Advances and Perspectives*

**16**

**Figure 9.**

**Figure 8.**

*at approximately 3500 cm<sup>−</sup><sup>1</sup>*

*none with phosphates.*

**4. Stability and membrane response: a dynamic picture**

in several previous paper and will not be analyzed here [40–42].

The thermodynamic stabilization of the lipids in water to form bilayers as a consequence of the hydrophobic interaction between acyl chains has been discussed

*Distributions of subpopulations of water forming none, one, two, three and four H bonds. Numbers correspond to water (w), carbonyl (C) and phosphate (P). Thus, 2 1 0 means two H-bonds with water, one with carbonyl,*

*Water bands in the adjacencies of lipid membranes in the gel (red line) and in the liquid crystalline state (blue line). Dotted lines denote the populations of water with none, one, two, three and four H-bonds. Central band*

 *(red solid line) corresponds to bulk water without lipids.*

Instead, the thermodynamic aspects of the bilayer as a reactive surface considering its *mechanical properties* will specifically be discussed. This approach is based

**Figure 11.**

*(A) Kinetic of surface pressure changes after the injection of the effector to the subphase. The extent of the change reflects the affinity of the effector by the monolayer, (B) surface pressure increase versus initial pressure plot used for determining the exclusion pressure of the lipid monolayer.*

Once the monolayer is stabilized at different lateral pressures (i. e. different surface excess of lipids), the response to perturbations promoted by solutes in the aqueous subphase can be tested (**Figure 11**).

As observed in **Figure 12**, *the exclusion pressure or cut off*, that is, the surface pressure at which no further effect on the monolayer is observed depends on the head group region for a given acyl chain length. DMPE (mean hydration ratio, 7 water molecules per lipid) shows a lower exclusion pressure (cut off), than DMPC (water ratio 22–24 water per lipid), although the slope of the curves remains constant. Thus, in the whole range of pressure, PE monolayers are less reactive than those of PC in the same conditions [47].

In addition, the magnitude of the surface pressure increase is also dependent on the hydrocarbon composition for similar head group regions. In **Figure 13**, it is

**Figure 12.**

*The effect of a soluble protease on the surface pressure of pure DMPC (black full triangles) and DMPE (red empty triangles).*

**19**

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems*

*Effect on the magnitude of the surface pressure response to a soluble protease of (A) unsaturation of the acyl*

*chains in phosphatidylcholines, (B) inclusion of cholesterol in phosphatidylcholine monolayers.*

observed that the response at a given pressure increases with the unsaturation or ramification of the acyl chains and decreases with the presence of cholesterol [47, 48]. The surface pressures below the critical cut off (Πc) are those states of the membrane interphase with propensity to react when chased by an effector in the subphase. These states correspond to different water/lipid ratio and hence different surface tension of water in the lipid matrix. The decrease in surface pressure Π<sup>c</sup> − Π is related to the increase of water beyond the hydration water (confined water) which part of it is distributed along the hydrocarbon chains. This can be followed by frequency changes in the methylene region by FTIR spectroscopy (**Figure 14**) [49, 50].

*Dehydration produces a decrease in confined water between acyl chains.*

**5. Bilayer barrier properties: permeation and controlled release**

The studies in lipid monolayers allow to conclude that the response of the membrane to stimuli is due to the excess of surface tension due to the increase

*DOI: http://dx.doi.org/10.5772/intechopen.85811*

**Figure 13.**

**Figure 14.**

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems DOI: http://dx.doi.org/10.5772/intechopen.85811*

#### **Figure 13.**

*Liposomes - Advances and Perspectives*

aqueous subphase can be tested (**Figure 11**).

*plot used for determining the exclusion pressure of the lipid monolayer.*

PC in the same conditions [47].

**Figure 11.**

**18**

**Figure 12.**

*empty triangles).*

*The effect of a soluble protease on the surface pressure of pure DMPC (black full triangles) and DMPE (red*

Once the monolayer is stabilized at different lateral pressures (i. e. different surface excess of lipids), the response to perturbations promoted by solutes in the

*(A) Kinetic of surface pressure changes after the injection of the effector to the subphase. The extent of the change reflects the affinity of the effector by the monolayer, (B) surface pressure increase versus initial pressure*

In addition, the magnitude of the surface pressure increase is also dependent on the hydrocarbon composition for similar head group regions. In **Figure 13**, it is

As observed in **Figure 12**, *the exclusion pressure or cut off*, that is, the surface pressure at which no further effect on the monolayer is observed depends on the head group region for a given acyl chain length. DMPE (mean hydration ratio, 7 water molecules per lipid) shows a lower exclusion pressure (cut off), than DMPC (water ratio 22–24 water per lipid), although the slope of the curves remains constant. Thus, in the whole range of pressure, PE monolayers are less reactive than those of

*Effect on the magnitude of the surface pressure response to a soluble protease of (A) unsaturation of the acyl chains in phosphatidylcholines, (B) inclusion of cholesterol in phosphatidylcholine monolayers.*

**Figure 14.** *Dehydration produces a decrease in confined water between acyl chains.*

observed that the response at a given pressure increases with the unsaturation or ramification of the acyl chains and decreases with the presence of cholesterol [47, 48].

The surface pressures below the critical cut off (Πc) are those states of the membrane interphase with propensity to react when chased by an effector in the subphase. These states correspond to different water/lipid ratio and hence different surface tension of water in the lipid matrix. The decrease in surface pressure Π<sup>c</sup> − Π is related to the increase of water beyond the hydration water (confined water) which part of it is distributed along the hydrocarbon chains. This can be followed by frequency changes in the methylene region by FTIR spectroscopy (**Figure 14**) [49, 50].

#### **5. Bilayer barrier properties: permeation and controlled release**

The studies in lipid monolayers allow to conclude that the response of the membrane to stimuli is due to the excess of surface tension due to the increase of water beyond the hydration shell, mostly in the acyl chain region. This excess is produced in monolayers by increasing the area per lipid, that is, reducing the surface pressure. In terms of bilayers, the expansion can be achieved by submitting liposomes to a hypotonic swelling. In this condition, membrane expands and liposomes become leaky of solute trapped in the liposome interior in isotonic conditions. Hence, *the modulation of the mechanical properties of the bilayer* by lipid composition, for instance, by including different cholesterol ratios, would allow to have a controlled release. The plots in **Figure 13** help to understand this response. At a given surface pressure, the response decreases with the increase in cholesterol, which can be related to an increase in the membrane rigidity and lower water ratio [16, 51].

Thus, *permeability* (i.e. the ratio between trapped and released solute) depends on the extent of water in the acyl region and in the second shell of phosphate groups. An important derivation of these results is that the properties of permeability can no longer be described by a model that assumes that the membrane is composed of three slabs: *a low dielectric region* (the hydrocarbon chains) and *the two aqueous interphase*s, where the polar groups are located (**Figure 7**). This view predicts that solute permeating the membrane would be those that can partition in a non-polar media [52]. However, *polar amino acids and hydrophilic compounds* such as glycerol, erythritol and urea permeate easily the lipid bilayer. In addition, when swelling is taken place, ions and large molecules such as sucrose trapped in liposomes in isotonic conditions can permeate when they osmotically swell [53, 54].

Therefore, solute permeation involved in the release of trapped solutes is a more complex phenomenon than a simple *partition process*. In terms of water participation at the membrane interphase and in the acyl chain region, the mechanism would involve water reorganization and hence structural changes. To put this complexity into relevance, the interaction of polyarginines (Arg 9) with bilayers of lipids differing in hydration such as phosphatidylcholines and phosphatidylethanolamines will be discussed. In **Figure 15**, the adsorption isotherm of Arg 9 on liposomes composed by PC and PE are shown. Both present typical curves of saturation, denoting a limited number of sites with the affinity to bind Arg 9 are present in each surface [55]. They can be described by Eq. (2):

$$\theta = \frac{\Delta \zeta'}{\Delta \zeta' \max} = \frac{[Arg]^\eta}{K + [\mathcal{A} \eta \lg]^\eta} \tag{2}$$

**21**

lipids from the bilayer.

**Figure 15.**

the CO of the lipids [66, 70].

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems*

**6. Topological effects of osmotic shrinkage. Defects in packing**

*eters* [54]. That is, their volumes increase or decrease as a consequence of the entrance or exit of water driven by osmotic gradients. In the previous section, we show that in swelling conditions, bilayer expansion produces changes in its permeability, that is, osmosis does not only affect the liposome volume but also the membrane density. Now we will see that it also affects the surface properties.

As described above, liposomes and vesicles, as well as cells, respond to *osmom-*

*Adsorption of Arg 9 on DMPC and DMPE liposomes (A) change in zeta potential (ζ) of DMPC and DMPE liposomes with addition of increasing concentrations of Arg 9. θ is calculated by ζ − ζ*

*0*

*the value at saturation. (B) Degree of coverage as calculated from part A versus Arg 9 concentration.*

*0 /ζ*<sup>∞</sup> *− ζ 0 ,*

 *corresponds to liposomes in the absence of Arg 9 and ζ*<sup>∞</sup>

In hypertonic media, liposomes shrink, that is, they *expulse the free water in the internal volume*. However, the volume decrease has a limit due to the repulsive forces put in evidence at short distance as described in **Figure 6**. The overcoming of this repulsive barrier derives in adhesion or fusion of the surfaces and implies *water elimination from the interphase*. The compression process due to osmotic shrinkage in bilayers can be compared with the decrease in area in monolayers. In this system, at high pressures (i.e. low areas), monolayer collapses and lipids are lost. In bilayers, the compression has different consequences due to the impossibility to extrude

Bilayers are soft and dynamic material [56, 57], and as a result, they can bend and deform in response to different stimuli such as dehydration and molecules that may compete with water. So, compression induced by osmotic shrinkage may result in topological changes giving place to regions (domains) of high curvature (invaginations or vesiculations) [58–60]. Thus, bilayer deformation can allow transient defects, when exposed to osmotic gradients (**Figure 16**). These curvature domains have important consequences in the interfacial behavior in the absorption or penetration of peptides composed by different types of amino acids. Many simulation studies showed that defect formation can determine the free energies of many membrane processes [61, 62]. When DPPC LUVs were subjected to hypertonic stress, defects caused by dehydration have more affinity for lytic compounds and amino acids such as phenylalanine [63–66]. The presence of defects in the membrane packing determines the binding and stabilization of peptides containing Arg and Phe motifs [67–69] The disruption of the water network around the phenyl group and the membrane defect has been invoked to explain the *negative free energy* of the formation a PC-Phe (phosphocholine-phenylalanine) complex in the presence of water. An important observation was that a dipole potential decrease was produced in this interaction which was explained by the orientation of the carboxylate opposing to

*DOI: http://dx.doi.org/10.5772/intechopen.85811*

*where ζ is the value at a given Arg 9 concentration, ζ*

where θ is the degree of coverage of the liposome by Arg 9, *K* the dissociation constant and *n* is a stoichiometric coefficient of the binding. In the present case, θ has been calculated by the change in the surface potential at each Arg 9 concentration measured by *electrophoretic mobility* from plots as shown in part A of **Figure 15**.

The fitting of the curves of part B gives an affinity constant for DMPC at 18°C, K = 0.54 × 103 ± 44 M<sup>−</sup><sup>1</sup> and a value n = 1. For DMPE at 18°C, K = 2 × 103 ± 189 M<sup>−</sup><sup>1</sup> and n = 0.74 ± 0.06. According to these results, the affinity of Arg 9 is higher in DMPC than in DMPE. In addition, the interaction of Arg 9 with DMPC is well described by a *Langmuir isotherm*, since n = 1. In contrast, the value of n is different from 1 for Arg 9 in DMPE suggesting different mechanisms of interaction. The strong difference in affinity between PE and PC membrane can be explained recalling the monolayer results. The cut off described in **Figure 12** is much lower in PE than in PC, that is, at pressure above 32 mN/m at which PE is not affected, PC monolayer is still *reactive*. This can be explained by the strong interaction between the polar head groups of the PE molecules that restrict the water excess and hence decreasing responsiveness.

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems DOI: http://dx.doi.org/10.5772/intechopen.85811*

**Figure 15.**

*Liposomes - Advances and Perspectives*

lower water ratio [16, 51].

swell [53, 54].

**Figure 15**.

K = 0.54 × 103

[55]. They can be described by Eq. (2):

± 44 M<sup>−</sup><sup>1</sup>

decreasing responsiveness.

of water beyond the hydration shell, mostly in the acyl chain region. This excess is produced in monolayers by increasing the area per lipid, that is, reducing the surface pressure. In terms of bilayers, the expansion can be achieved by submitting liposomes to a hypotonic swelling. In this condition, membrane expands and liposomes become leaky of solute trapped in the liposome interior in isotonic conditions. Hence, *the modulation of the mechanical properties of the bilayer* by lipid composition, for instance, by including different cholesterol ratios, would allow to have a controlled release. The plots in **Figure 13** help to understand this response. At a given surface pressure, the response decreases with the increase in cholesterol, which can be related to an increase in the membrane rigidity and

Thus, *permeability* (i.e. the ratio between trapped and released solute) depends on the extent of water in the acyl region and in the second shell of phosphate groups. An important derivation of these results is that the properties of permeability can no longer be described by a model that assumes that the membrane is composed of three slabs: *a low dielectric region* (the hydrocarbon chains) and *the two aqueous interphase*s, where the polar groups are located (**Figure 7**). This view predicts that solute permeating the membrane would be those that can partition in a non-polar media [52]. However, *polar amino acids and hydrophilic compounds* such as glycerol, erythritol and urea permeate easily the lipid bilayer. In addition, when swelling is taken place, ions and large molecules such as sucrose trapped in liposomes in isotonic conditions can permeate when they osmotically

Therefore, solute permeation involved in the release of trapped solutes is a more complex phenomenon than a simple *partition process*. In terms of water participation at the membrane interphase and in the acyl chain region, the mechanism would involve water reorganization and hence structural changes. To put this complexity into relevance, the interaction of polyarginines (Arg 9) with bilayers of lipids differing in hydration such as phosphatidylcholines and phosphatidylethanolamines will be discussed. In **Figure 15**, the adsorption isotherm of Arg 9 on liposomes composed by PC and PE are shown. Both present typical curves of saturation, denoting a limited number of sites with the affinity to bind Arg 9 are present in each surface

where θ is the degree of coverage of the liposome by Arg 9, *K* the dissociation constant and *n* is a stoichiometric coefficient of the binding. In the present case, θ has been calculated by the change in the surface potential at each Arg 9 concentration measured by *electrophoretic mobility* from plots as shown in part A of

The fitting of the curves of part B gives an affinity constant for DMPC at 18°C,

and n = 0.74 ± 0.06. According to these results, the affinity of Arg 9 is higher in DMPC than in DMPE. In addition, the interaction of Arg 9 with DMPC is well described by a *Langmuir isotherm*, since n = 1. In contrast, the value of n is different from 1 for Arg 9 in DMPE suggesting different mechanisms of interaction. The strong difference in affinity between PE and PC membrane can be explained recalling the monolayer results. The cut off described in **Figure 12** is much lower in PE than in PC, that is, at pressure above 32 mN/m at which PE is not affected, PC monolayer is still *reactive*. This can be explained by the strong interaction between the polar head groups of the PE molecules that restrict the water excess and hence

and a value n = 1. For DMPE at 18°C, K = 2 × 103

(2)

± 189 M<sup>−</sup><sup>1</sup>

**20**

*Adsorption of Arg 9 on DMPC and DMPE liposomes (A) change in zeta potential (ζ) of DMPC and DMPE liposomes with addition of increasing concentrations of Arg 9. θ is calculated by ζ − ζ 0 /ζ*<sup>∞</sup> *− ζ 0 , where ζ is the value at a given Arg 9 concentration, ζ 0 corresponds to liposomes in the absence of Arg 9 and ζ*<sup>∞</sup> *the value at saturation. (B) Degree of coverage as calculated from part A versus Arg 9 concentration.*

#### **6. Topological effects of osmotic shrinkage. Defects in packing**

As described above, liposomes and vesicles, as well as cells, respond to *osmometers* [54]. That is, their volumes increase or decrease as a consequence of the entrance or exit of water driven by osmotic gradients. In the previous section, we show that in swelling conditions, bilayer expansion produces changes in its permeability, that is, osmosis does not only affect the liposome volume but also the membrane density. Now we will see that it also affects the surface properties.

In hypertonic media, liposomes shrink, that is, they *expulse the free water in the internal volume*. However, the volume decrease has a limit due to the repulsive forces put in evidence at short distance as described in **Figure 6**. The overcoming of this repulsive barrier derives in adhesion or fusion of the surfaces and implies *water elimination from the interphase*. The compression process due to osmotic shrinkage in bilayers can be compared with the decrease in area in monolayers. In this system, at high pressures (i.e. low areas), monolayer collapses and lipids are lost. In bilayers, the compression has different consequences due to the impossibility to extrude lipids from the bilayer.

Bilayers are soft and dynamic material [56, 57], and as a result, they can bend and deform in response to different stimuli such as dehydration and molecules that may compete with water. So, compression induced by osmotic shrinkage may result in topological changes giving place to regions (domains) of high curvature (invaginations or vesiculations) [58–60]. Thus, bilayer deformation can allow transient defects, when exposed to osmotic gradients (**Figure 16**). These curvature domains have important consequences in the interfacial behavior in the absorption or penetration of peptides composed by different types of amino acids. Many simulation studies showed that defect formation can determine the free energies of many membrane processes [61, 62].

When DPPC LUVs were subjected to hypertonic stress, defects caused by dehydration have more affinity for lytic compounds and amino acids such as phenylalanine [63–66]. The presence of defects in the membrane packing determines the binding and stabilization of peptides containing Arg and Phe motifs [67–69]

The disruption of the water network around the phenyl group and the membrane defect has been invoked to explain the *negative free energy* of the formation a PC-Phe (phosphocholine-phenylalanine) complex in the presence of water. An important observation was that a dipole potential decrease was produced in this interaction which was explained by the orientation of the carboxylate opposing to the CO of the lipids [66, 70].

#### **Figure 16.**

*Defects induced by osmotic shrinkage enhance Phe insertion into lipid bilayers.*

As described in **Figure 9**, carbonyl groups are one of the hydration centers in which interfacial water is distributed. The formation of high curvature surfaces is related to changes in the *CO arrangements* [39] (**Figure 17**).

*The curvature domains*, in fact, increases the bilayer free energy surface by exposing hydrophobic region and carbonyl configuration as observed in part B of **Figure 17**. The stabilization of peptides or amino acid is the result of the decrease of free energy at expense of the bending modulus, that is, the energy cost of topological changes [67, 71, 72]. This last quantity is an important parameter that governs a membrane's tendency for defect formation. Many kinds of defects may be formed, including vesicle budding and fusion, depending on the presence of non-bilayer lipid phases, cholesterol and interactions between lipid bilayers and other biomolecules [19, 73].

**23**

**Figure 18.**

*organization.*

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems*

*The lipid matrix in cell membranes presents a variety of lipids*. As shown in **Figure 18**, all of them present a diacyl glycerol structure with acyl chains differing in saturation and length, a phosphate group in position 3 linked to a proton, choline, ethanolamine, glycerol, serine or inositol residues. It is difficult to accept that within the economicity principle of biology, this wide variation does not play a

It must be noticed that all lipids may be aligned along a plane containing the glycerol backbone and the carbonyl groups. On one side of this plane, the hydrocarbon region can vary in terms of chain length, saturation and ramification. Each of these variations may give a complex matrix in which water cannot be excluded and constitute a media of a wide range of dielectric properties. On the other, polar groups protrude into the aqueous phase at different magnitudes. Importantly, also these groups are able in different extents to form hydrogen bonds between its lipid neighbors and with water. *In conclusion, it is an oversimplification to reduce the bilayer structure in which*

*lipids organize facing the polar groups to water and segregating the nonpolar chains*.

is saturated with works in which only "try and error" strategies are employed.

• the interface is a bidimensional solution of hydrated polar groups.

The lack of details in these regions in relation to their emergent properties is a major limitation in the design of biomimetic systems that, as liposomes and vesicles, may be used for biotechnological and medical purposes. As a consequence, literature

• excess water beyond the hydration shell has solvent properties for additives in the bulk water phase and confers free energy that is excess for binding of

• Dissolution in the water membrane phase changes the water activity (aw) and

*Protrusion of the groups esterified to the phosphate in the surface of the membrane promotes different water*

*DOI: http://dx.doi.org/10.5772/intechopen.85811*

role in *functional properties of the membrane.*

In this chapter, we have briefly discussed that:

amino acids and peptides.

affects the surface pressure.

**7. Concluding remarks: design and modulation**

**Figure 17.** *Orientation of CO groups at the bilayer interphase in relation to curvature.*

*The Role of Water in the Responsive Properties in Lipid Interphase of Biomimetic Systems DOI: http://dx.doi.org/10.5772/intechopen.85811*


**1. Introduction**

Photocatalysis has wide applications in environmental, fuel production, and chemical synthesis [1, 2]. Photocatalytic materials that can convert photon energy to chemical energy are employed to split water to produce hydrogen and also produce highly reactive intermediates for chemical synthesis and reactive oxygen radicals for the degradation of organic pollutants [3]. Light absorption generates holes and electrons within the valence and conduction bands in heterogeneous photocatalysis. Those charges may migrate within the semiconductor particle and be trapped at surface sites. They'll also participate in the interfacial electron transfer processes involving the molecules of electron acceptor (A) and donor (D). within the photocatalytic organic degradation process, oxygen act as A and water as D, creating anion (O2 •–) and chemical group (OH• ) radicals. Those reactive radicals are liable for most of the oxidation of organic substances. Oxidation under these conditions is sometimes complete, giving H2O and CO2 because of the final products.

The reactions can therefore be used in water, air, or surface purification. But, the recombination of photogenerated charge carriers competes with this photocatalytic degradation process, which is a critical factor in limiting the kinetics of photocatalysts and reaction rate. To overcome this limitation, numerous photocatalysts have been developed to enhance efficiency. The nanomaterials with large surface areas, abundant surface states, and specific morphologies have emerged as pioneering photocatalysts for the dye degradation process. The optimal structure-properties relationships are essential for efficient organic pollutant degradation [4]. Significantly, the hierarchical hetero-nanostructured materials called nanoarrays give rise to separating the photogenerated electron-hole pairs to improve the photocatalytic activity further [5]. To reduce the recombination of charge carriers, the photocatalyst surface need suitable and sufficient acceptors. Since these processes involve some complicated steps, but improvement of separation and transportation of photogenerated charge carriers are the main challenges to designing highly effective photocatalysts for practical applications. Another critical issue that induces the photocatalytic activity of a catalyst is the nature of its surface/interface chemistry. The surface energy and chemisorption properties are vital in transferring electrons and energy between substances at the interface. This process allows the overpotential of redox reactions on the photocatalyst surface, which reduces photo-corrosion. Previous studies have mainly focused on the reactivity of the catalyst. Moreover, the less diffusion rate of oxygen in water, limits the photocatalytic reactions even under high light intensity. In contrast to conventional double-phase photocatalytic systems, which consist of catalysts immersed in a bulk liquid phase, triple-phase catalytic systems by supporting catalysts at the gas-liquid boundaries have been developed and exhibited outstanding performance [6].

The wettability modification of a catalyst surface includes a vital role in improving the charge transfer ability. Superwetting behavior could be a unique wetting phenomenon that always depends upon the phases. Superwettable surfaces, like super-hydrophilic and superhydrophobic surfaces, exhibit unique transport dynamics and providing exceptional prospects for reinforcing chemical process efficiency. Therefore, these materials are dramatically different from traditional materials. Superwetting catalysts have enhanced catalytic activity when introducing an air layer between the catalyst and liquid. The charge carriers from reacting interface are very fast to radicals by oxygen, and enough oxygen within the air layer can effectively capture electrons and minimize the electron-hole recombination. Superwetting materials are commonly designed by controlling surface energy, chemical compositions, and geometric structures of solid surfaces. This chapter discussed the photocatalytic organic pollutants supported superwetting materials. The discussion mainly contains widely investigated photocatalytic reactions involving gas and water molecules as reactants and products for organic pollutant degradation.


**2. Intrinsic properties of TiO2**

Titanium belongs to the IVA group of elements and, as many other metals, is able to form a wide range of oxides. Titanium(IV) dioxide (titania) exists in three common crystalline phases: rutile, which is the thermodynamically stable phase, and the metastable phases anatase and brookite. Anatase and rutile have more extensive applications because they are more stable than brookite (**Figure 1**). In all three forms, titanium (Ti4+) atoms are coordinated to six oxygen (O2−) atoms, forming "TiO6 " octahedra. Anatase and rutile have a tetragonal

geometry, differently from brookite, which has an orthorhombic geometry. In rutile, the

polymorphs.

Titania is an n-type semiconductor since, like other metal transition oxides, the oxygen

ous structural defects; for example, point defects, which are empty sites (vacancies), where constituent atoms are missing within the structure and interstitial atoms occupy the space between the regular atomic sites. The point defects contribute to the electrical conductivity in two ways: they can provide ionization, and they can also move in response to an electric field producing an ionic current. An oxygen vacancy is formed by the transfer of an oxygen atom on a normal site to the gaseous state. These oxygen vacancies act as electron donors, so the material contains an excess of electrons resulting in an increase of the electri-

exhibits photocatalytic oxidation (PCO) and photoinduced superhydrophilicity (PSH)

" octahedron is slightly distorted, while anatase consists of strongly distorted octahedral units. In the rutile structure, each octahedron is surrounded by ten close octahedrons; instead, in the anatase polymorph, each octahedron is in contact with eight neighbors. Interatomic distances also differ between the polymorphs. For anatase, with respect to rutile, the Ti-Ti distances are lengthier and, instead, the Ti-O distances are shorter. These differences in lattice structures cause different mass densities and electronic band structures

. Due to its intrinsic properties, the anatase phase is the most

/ *mnm Pbca*

a = b = 4593 c = 2959

. Indeed, every crystal has vari-

a = 9184 b = 5447 c = 5145

Mesoporous TiO2 Thin Films: State of the Art http://dx.doi.org/ 10.5772/intechopen.74244 59

a good candidate for photocatalyst materials

"TiO6

Volume (Å3

Density (g cm3

between the two forms of TiO2

interesting one for applications.

and self-cleaning surfaces in air.

cal conductivity.

TiO2

vacancies represent the predominant defect type in TiO2

**Property TiO2** Molecular weight (g mol−1) 79.88 Melting point (°C) 1825 Boiling point (°C) 2500–3000

Dielectric constant 31 114

Space group *I*41

Lattice constant (Å) a = b = 3784

**Table 1.** Compared intrinsic properties of main TiO2

**Property Anatase Rutile Brookite** Refractive index [18] 2.52 2.72 2.63

Crystal structure [19, 20] Tetragonal Tetragonal Orthorhombic

c = 9515

Molecule/cell 4 2 8

/*amd P*42

) 136.25 62.07 257.38

) 3.79 4.13 3.99

when it is illuminated with UV light, making TiO2

**Figure 1.** Crystal structures of TiO2 : (a) anatase, (b) rutile, and (c) brookite (adapted from Ref. [17]) (**Table 1**).

**Table 1.** Compared intrinsic properties of main TiO2 polymorphs.

to antibacterial applications [12]. However, to design efficient systems and devices for the above-cited uses, stable materials with a well-defined crystalline structure, highly controlled crystallite size and shape, as well as high available surface area and accessible pore networks able to ensure contact with catalytic substrates, polymers, or nanospecies are required [13]. Although the early mesoporous materials were produced in the form of powders, some applications, such as membranes, low-dielectric-constant interlayers, optical sensors, and optoelectronic devices, require ultralow-k dielectrics and low-refractive-index materials with a good mechanical stability and of hydrophobic nature. These requirements led to the preparation of mesoporous thin films [14–16]. After a brief description of the different TiO2 polymorphs and properties, the present chapter will therefore be dedicated to mesoporous

 thin films (MTTFs), reviewing the different preparation methods reported throughout the literature, the main characterization techniques employed to study the structure and the morphology of the prepared thin films, and finally the description of their most successful

Titanium belongs to the IVA group of elements and, as many other metals, is able to form a wide range of oxides. Titanium(IV) dioxide (titania) exists in three common crystalline phases: rutile, which is the thermodynamically stable phase, and the metastable phases anatase and brookite. Anatase and rutile have more extensive applications because they are more stable than brookite (**Figure 1**). In all three forms, titanium (Ti4+) atoms are coordinated

" octahedra. Anatase and rutile have a tetragonal

: (a) anatase, (b) rutile, and (c) brookite (adapted from Ref. [17]) (**Table 1**).

TiO2

applications.

**2. Intrinsic properties of TiO2**

58 Titanium Dioxide - Material for a Sustainable Environment

to six oxygen (O2−) atoms, forming "TiO6

**Figure 1.** Crystal structures of TiO2

geometry, differently from brookite, which has an orthorhombic geometry. In rutile, the "TiO6 " octahedron is slightly distorted, while anatase consists of strongly distorted octahedral units. In the rutile structure, each octahedron is surrounded by ten close octahedrons; instead, in the anatase polymorph, each octahedron is in contact with eight neighbors. Interatomic distances also differ between the polymorphs. For anatase, with respect to rutile, the Ti-Ti distances are lengthier and, instead, the Ti-O distances are shorter. These differences in lattice structures cause different mass densities and electronic band structures between the two forms of TiO2 . Due to its intrinsic properties, the anatase phase is the most interesting one for applications.

Titania is an n-type semiconductor since, like other metal transition oxides, the oxygen vacancies represent the predominant defect type in TiO2 . Indeed, every crystal has various structural defects; for example, point defects, which are empty sites (vacancies), where constituent atoms are missing within the structure and interstitial atoms occupy the space between the regular atomic sites. The point defects contribute to the electrical conductivity in two ways: they can provide ionization, and they can also move in response to an electric field producing an ionic current. An oxygen vacancy is formed by the transfer of an oxygen atom on a normal site to the gaseous state. These oxygen vacancies act as electron donors, so the material contains an excess of electrons resulting in an increase of the electrical conductivity.

TiO2 exhibits photocatalytic oxidation (PCO) and photoinduced superhydrophilicity (PSH) when it is illuminated with UV light, making TiO2 a good candidate for photocatalyst materials and self-cleaning surfaces in air.

The PCO property is activated by absorption of UV photons of energy greater than TiO2 bandgap energy. For anatase phase this energy is 3.2 eV; therefore, UV light (λ ≤ 387 nm) is required, while rutile phase has energy of 3.0 eV (λ = 400 nm). The absorption of a photon excites an electron to the conduction band (eCB −) generating a positive hole in the valence band (hVB +), so that photoexcitation produces electron-hole pairs.

can adsorb as OH groups, giving to the TiO2

unless there are pores opening at the surfaces [26].

tures with aligned vertical pores.

(*Im*3*m*), and (d) cubic (*Pm*3*m*) (from Ref. 25).

water contact angle for TiO2

**3. Preparation of MTTFs**

**3.1. Synthetic aspects**

ties of TiO2

TiO2

surface its superhydrophilic character. Indeed,

Mesoporous TiO2 Thin Films: State of the Art http://dx.doi.org/ 10.5772/intechopen.74244 61

was *ca.* 72° (hydrophobic), while UV illumination turned it close

when illuminated, titania surface exhibits amphiphilicity caused by the creation of alternating hydrophilic/hydrophobic domains. It was found that in the absence of UV illumination, the

to 1° (superhydrophilic) [23]. These photoinduced phenomena coupled with the PCO proper-

 nanomaterials can be classified according to their shape and dimension: 0D nanomaterials refer to nearly spherical nanoparticles (quantum dots); 1D refers to nanowires, nanobelts, and nanorods; and 2D materials correspond to thin films, while 3D is used to indicate porous nanostructures.

Mesoporous thin films have very peculiar features, particularly high surface area, controlled porosity, high flexibility in composition, and surface design [24]. For practical applications, mesoporous thin films must possess a number of general features: (i) they must be continuous and free of crack; (ii) crystalline walls are highly desirable in order to process into functional materials; and (iii) pores must be accessible, preferably from the film surface [25]. As a matter of fact, unlike powder, the internal space of a mesoporous thin film may not be accessible

Mesoporous materials can form various pore structures including hexagonal (*p*6mm), cubic (*Ia*3*d*, *Im*3*m*, and *Pm*3*m*), and disorganized structures (**Figure 3**). Although the hexagonal structure is the most encountered one due to its easy synthetic procedure, this structure has a drawback in terms of pore accessibility when obtained as thin films, since the interfacial energy between the substrate surface and the film materials often lead to the pores laying parallel to the substrate surface [25]. For this reason, recent development of synthesis protocols of mesoporous titania thin films has been achieved to access to cubic structures or to hexagonal struc-

**Figure 3.** A representation of the structures found in mesoporous materials: (a) hexagonal, (b) cubic (*Ia*3*d*), (c) cubic

are at the basis of the self-cleaning properties of TiO2

$$\text{TiO}\_2 + \text{h}v \rightarrow \text{h}\_{\text{VB}} "+ \text{e}\_{\text{CH}}\text{"} $$

The charge carriers can migrate to the catalyst surface and initiate redox reactions with adsorbates. The redox potential for hole in VB is +2.53 V, which is sufficiently oxidizing to overcome the binding energy of electron in OH− to form a hydroxyl radical from water. The hydroxyl radical can subsequently oxidize organic species with mineralization producing mineral salts, CO2 , and H2 O. Similarly, an electron in CB (E= −0.52 V) is sufficiently reductive to react with O2 to form O2•− (superoxide radical anion), which can react with H+ to produce hydroperoxide radical contributing to the degradation of organic molecules (**Figure 2**) [21]. The main drawback in the use of titania as photocatalyst is its high electron-hole recombination rate, but several methods to improve photocatalytic activity by promoting separation of the electronhole pair have been developed, such as doping and heterojunction coupling.

The PSH property consists in the alteration of the TiO2 wettability under UV irradiation and in the formation of a highly hydrophilic surface state. One possible explanation of the PSH property is that electrons reduce Ti(IV) cation sites in Ti(III) and holes oxidize O2− anions to molecular oxygen. The expulsion of O2 molecules creates surface vacancies on which water

**Figure 2.** Schematic mechanism of TiO2 photoactivation (adapted from Ref. [22]).

can adsorb as OH groups, giving to the TiO2 surface its superhydrophilic character. Indeed, when illuminated, titania surface exhibits amphiphilicity caused by the creation of alternating hydrophilic/hydrophobic domains. It was found that in the absence of UV illumination, the water contact angle for TiO2 was *ca.* 72° (hydrophobic), while UV illumination turned it close to 1° (superhydrophilic) [23]. These photoinduced phenomena coupled with the PCO properties of TiO2 are at the basis of the self-cleaning properties of TiO2 -covered glasses and walls.