Ionic Liquids as High-Performance Lubricants and Lubricant Additives

*Hong Guo and Patricia Iglesias Victoria*

#### **Abstract**

Taking into account the environmental awareness and ever-growing restrictive regulations over contamination, the study of new lubricants or lubricant additives with high performance and low toxicity over the traditional lubes to reduce the negative impact on the environment is needed. In this chapter, the current literature on the use of ionic liquids, particularly protic ionic liquids, as highperformance lubricants and lubricant additives to different types of base lubricants are reviewed and described. The relation between ionic liquids structures and their physicochemical properties, such as viscosity, thermal stability, corrosion behavior, biodegradability, and toxicity, is elaborated. Friction reduction and wear protection mechanisms of the ionic liquids are discussed with relation to their molecular structures and physicochemical properties.

**Keywords:** ionic liquids, friction, wear, tribofilm, additives

#### **1. Introduction**

Friction and wear are inescapable problems in mechanical and electromechanical systems, resulting in massive energy losses. Holmberg and Erdemir [1] have estimated that the energy consumption generated by contacting surfaces in mechanical elements is almost 23% of the total energy consumption in the world, where 20% is used to overcome friction and 3% is used to replace worn surfaces. However, energy losses could be reduced by up to 40% through new advances in lubrication, which can save 8.7% of world energy consumption. Especially, the losses by friction could be decreased by using high-performance lubricants, which cannot only result in economic savings but also in important environmental benefits. In addition, the increase in energy prices leads to high demand for improvement of energy efficiency.

Ionic liquids (ILs) are a class of salts composing of bulky organic cations and organic or inorganic anions. Some of the typical IL molecular structures are shown in **Figure 1**. The large molecular size of the ions and their possible delocalized charge contribute to the uncommonly low melting points of ILs, which are below 100 °C. The first IL, ethylammonium nitrate [(C2H5NH3)NO3], reported by Walden in 1914 is found to have a melting point of 12 °C. Since the 1970s, the research of ILs has become increasingly popular and now ILs have been used for

#### **Figure 1.**

*Typical ionic liquids molecular structures.*

various applications such as effective solvent, catalyst, electrolytes in batteries, and carbon and carbon dioxide capturing.

In the tribological field, ILs have shown great potential as advanced lubricants and "tailor-made" lubricating additives since been explored for lubrication in 2001 [2]. ILs have excellent physicochemical properties including low melting point, low flammability, negligible vapor pressure, and high thermal stability that meet the demands of high-performance lubricants. One of the most important characteristics of ILs is that their properties can be tailored by varying the species of the cations and anions, giving rise to numerous families that can be used across different tribological systems. The superiority of ILs in lubrication can be attributed to their inherent polarity, which can make them form stable ordered layers in the liquid state on metal surfaces to prevent them against contact; and that some elements of ILs can react with the substrate materials to generate a tribofilm to protect the substrate from further wear.

ILs can be conventionally categorized into aprotic ionic liquids (AILs) and protic ionic liquids (PILs), based on the nature of the cation present in the combinations [3]. Since most of the studies in lubrication are focused on AILs, many literature reviews have summarized the research efforts of them. Therefore, this chapter covers more about the progress of PILs in lubrication. Firstly, some important physicochemical properties of ILs will be introduced. Secondly, ILs as neat lubricants, specifically as bulk lubricants, thin-film lubricants, and the surface interactions between ILs and contact surfaces will be discussed. The third part will be focused on the ILs as additives in different base lubricants.

#### **2. Physicochemical properties of ionic liquids**

ILs are highly tunable by changing the cation structures, anion structures or both to satisfy specific engineering and manufacturing requirements. Therefore, to understand the relationship between the chemical structures of ILs and their physicochemical properties, as well as the tribological properties, becomes crucial to the molecular design of the more effective ILs. Their physicochemical properties can be easily influenced by combining different types of cations and anions or varying their alkyl chain lengths.

#### **2.1 Viscosity and thermal stability**

The viscosity behavior will affect the load-carrying capacity of ILs, as well as their formation of boundary lubricating films. In addition, the thermal stability of an IL is also a prerequisite for being used in various tribological systems. Particularly, an outstanding thermal stability would contribute to its application in hightemperature environments. Since thermogravimetric analysis (TGA) has been employed in most of the studies to characterize the thermal stability of ILs, the viscosity and onset thermal decomposition temperature (Td) of some ILs obtained using this method have been summarized in **Table 1**. In general, the molecular structure modification of the cation or the anion will affect the IL's viscosity and thermal stability. ILs having symmetric cations with long alkyl side chains are found to have high viscosity [16–18], which is attributed to the closer packing and enhanced van der Waals interactions between the long alkyl chains. Particularly, the branched ILs are reported to possess higher viscosity than the linear ones [19]. In addition, ILs having high molar mass and ion-interactions such as hydrogen bonds in their molecular structures will get high viscosity [13, 20]. For instance, an imidazolium-based IL with a hydroxyl group (-OH) grafted into the N-1 position of its cation obtained an increase in viscosity, which is attributed to the increased hydrogen bond interactions and the resulting higher molar mass. In the study of Guo et al. [15], the viscosity of the hydroxylammonium PIL is highly dominated by the hydrogen bond interactions among its molecules instead of its molar mass.

The thermal stability of an IL is also closely related to its cation and anion, as can be seen in **Table 1**. Generally, when pairing with the same anion, the imidazoliumbased ILs have a higher thermal stability than the tetraalkylphosphonium-based and the tetraalkylammonium-based ILs [4, 5, 9]. And the imidazolium-based ILs are reported to have a higher thermal stability when their cations have a smaller alkyl chain [21]. In contrast to cations, the anions have more impacts on the thermal stability of ILs. For example, an alkylammonium PIL derived from a stronger acid tends to have a higher thermal stability [22]. Recently, Fadeeva et al. [23] reported that the thermal stability of the alkylimidazolium-based PILs is mainly determined by the anions nature instead of the cation structure. However, with the same triflate anion, the PILs having the cation with larger size and branched chain structure would get a higher thermal stability [24, 25]. In several studies [13–15], the hydroxylammonium PILs with carboxylate anions were found to have low thermal stability. The reason underlaying this phenomenon is related to the reversal proton transfer, leading to the presence of free acid and ethanolamine [15]. Considering the long-term practical applications of ILs in lubrication, the characterization of their long-term thermal stability, through isothermal TGA, should also be concerned [21].

#### **2.2 Corrosion**

Corrosion for most lubricants, such as water-based lubricants, is a complex problem that needed to be solved. The corrosivity of neat ILs or ILs additives are usually evaluated by means of immersion corrosion test, in which the testing specimen, such as copper [26–29], steel [28, 30], or cast iron [29] will be immersed into (or cover the metal surface with) neat ILs or IL containing lubricants. In addition, electrochemical corrosion tests will also be conducted to examine the anticorrosion properties of ILs as well as study the corrosion mechanisms [29, 31]. Through the

#### **Table 1.**

*Kinetic viscosity and thermal stability of some*

 *ILs.*

#### *Ionic Liquids as High-Performance Lubricants and Lubricant Additives DOI: http://dx.doi.org/10.5772/intechopen.96428*

two methods, the anticorrosion performance of four hydroxylammonium phosphate PILs additives was investigated in [28]. Compared to the reference sample immersed in water, the use of neat PILs significantly improved the corrosion resistance of the copper and iron sheets. What is more, the results from the electrochemical test further demonstrated the excellent corrosion inhibition of PILs additives and their attributes of anodic corrosion inhibitors. A protective film generated by the adsorption of PIL molecules, particularly the hydrophilic functional group on steel surface is attributed to anticorrosion performance. Among the four PILs, 2-hydroxypropylammonium di-(2-ethylhexyl) phosphate (TEOAP6) was found to have the best corrosion resistance, and PIL's corrosion inhibition efficiency was related to its functional groups.

In the study of Ma et al. [26], the immersion corrosion test was employed to evaluate the anticorrosion property of PAO when ILs additives were added in different concentrations. Along with ILs, an additive containing Sulfur element was also added to PAO in 1%. It is noted that the addition of only 0.1% ILs can greatly reduce the corrosion tendency of a lubricant, and IL concentration (0.25–0.1%) just slightly affected the corrosion-inhibiting performance. Recently, a PIL 2 hydroxyethylammonium oleate [32], was proved to be an effective corrosion inhibitor for aluminum 1100 in neutral sodium chloride solution. The PIL adsorption layer on the aluminum substrate surface was pointed out to inhibit the diffusion of chloride anions during the electrochemical measurement, where it can also provide a corrosion protection at high chloride concentration for 72 hours.

#### **2.3 Biodegradability and toxicity**

In terms of the prospective large-scale industrial applications of ILs, it is crucial to examine their biodegradability and toxicity to control the discharge of ILinvolved solvents or lubricants, minimizing the environmental damage. To understand the correlation between the molecular structure and biodegradability and toxicity of ILs is essential to design green IL lubricants. Nowadays, many experimental studies [7, 33] have been conducted to evaluate the environmental impact of ILs. In addition, computational approaches [34–36] are also employed to assess their toxicity. In general, cations and anions do have an influence on the ILs toxicity particularly, cations have a greater impact than anions. ILs having longer alkyl chain length and more branched-chain groups on their cations tend to be more toxic [37]. However, some anions containing fluorine in their structure will cause an increase in toxicity of their corresponding ILs. For example, although the hydroxylammonium and imidazolium cations were evaluated to be less toxic, the toxicity of their ILs increased drastically once NTf2 was incorporated as the anion [38]. Regarding the biodegradability, the IL components 1–Butyl–3–methylimidazolium (Bmim) and bis(trifluoromethanesulfonyl) imide (NTf2) were reported to be non-biodegradable even at low concentration (10 mg/L), while the N,N,N– trimethylethanolammonium (Choline) and acetate (Ac) could be completely degraded with a concentration up to 50 mg/L [38].

In the study of Tzani et al. [39], the biodegradability of a series of carboxylate-PILs were examined and proved to be relevant to the alkyl chain length of the anions. PILs having anions with long alkyl chain length were found to get a decreased biodegradability, except for the one that had an alicyclic ring in its anion, showing an enhanced biodegradability. Lately, Viesca et al. [33] characterized the biodegradability and bacteria toxicity of six PILs derived from alkylhydroxylamine. Owing to the presence of benzenesulfonate aromatic group in anions, the sulfonate-PILs were reported to be less biodegradable compared to the hexanoate-PILs. Regarding the bacterial toxicity behavior, even the hexanoate-PILs exhibited a

better environmental impact, all of them were mild toxic to Vibrio fischeri. Nevertheless, all these PILs were found to outperform the traditional lubricant additive ZDDP concerning the biodegradability and toxicity performance.

#### **3. Ionic liquids as lubricants**

The tribological behavior of ILs as lubricants have been typically evaluated through laboratory bench tests using various macroscopic tribometers, such as the Optimol SRV series tribometers, mini-traction machines, Microtest pin-on-disk tribometer, Plint TE77 high-frequency reciprocating rigs, etc. In addition, the atomic force microscope (AFM) and surface force apparatus (SFA) are usually applied to investigate the nanotribological performance of lubricants. The two main factors, coefficient of friction (COF), and wear volume (or wear rate) of the rubbing materials are normally used to evaluate and compare the lubricating ability and anti-wear performance of IL lubricants.

#### **3.1 Ionic liquids as neat lubricants**

Since Ye et al. [2] initiated the study of ILs in lubrication in 2001, the studies about ILs as neat lubricants for various contact systems such as steel-steel contact [15], steel-ceramic contact [40], and steel-aluminum contact [14] have received considerable attention. **Table 2** summarizes some recent studies of ILs as neat lubricants. Compared to AILs, the use of PILs as neat lubricants has gained more attention than before, owing to their low cost and facile synthesis process.

In Khan et al.'s research, two phosphonium-based PILs with different alkyl chain length in the anions were tested as neat lubricants under steel-steel contact, and a synthetic oil PEG 200 was used as a reference [41]. Since the fatty acid anions of PILs have a better affinity to steel surfaces, the use of PILs showed a significant friction reduction with respect to PEG 200. The tribological performance of the two PILs were found to be determined by the alkyl chain length of their anions and their viscosity, where a PIL with a shorter anion chain length and lower viscosity led to a lower friction coefficient but more material loss. While the results may be inverse once the experiment conditions are changed or other PILs are used. In the study of Vega et al. [14], the effect of anion chain length on the friction and wear behavior of ammonium-based PILs was investigated under steel-aluminum contact. The results revealed that increasing the anion chain length will improve the lubricating ability of PIL with a low friction coefficient. From another study of Vega et al. [42], three oleic-acid derived ammonium-based PILs were evaluated as lubricants in aluminaaluminum contact. In addition to the low friction coefficient, the use of PILs yielded an important wear reduction (98%) compared to the dry condition. Lately, the hexanoate-based PILs were also found to greatly reduce the wear of steel with respect to mineral oil as well as a commercial oil [15]. Tribofilms were detected on the worn steel disks when PILs were used to protect the steel against severe wear.

In addition to the above-mentioned bulk lubricants, ILs can also be employed in the form of thin layers for lubricating micro/nano electromechanical systems (MEMS/NEMS). For example, in Bermúdez's group [40], a PIL - di[bis(2 hydroxyethyl)ammonium] succinate thin layer was created on a steel substrate surface by evaporating water from the PIL + Water mixture, where the PIL thin layer extremely reduced the wear rate of steel compared to the bulk neat PIL.

*Note: a- NH2((CH2)2OH)2; b- NH3((CH2)2OH); c- NH2CH3(CH2)2OH); d- NH(CH3)2(CH2)2OH). #*

*2-ethylhexanoate. δ*

*coefficient of friction at 30 °C. λ coefficient of friction at 80 °C; and the numbers in imidazolium CxCxim, ammonium NHHHx, and phosphonium Px,*

*x,x,x represent the alkyl chain length.*

#### **Table 2.**

*Tribological results of some ILs as neat lubricants (2017–2020).*

#### **3.2 Surface interactions**

As shown in **Figure 2**, it has been widely accepted that when neat ILs or IL additives are introduced between the contacting work pairs, the IL molecules tend to adsorb onto the workpiece surfaces physically or/and chemically and form an ordered boundary lubricating film to protect the moving components from direct contact, leading to low friction. During the sliding frictional process, a protective tribofilm will be subsequently generated on top of the substrate by means of the tribochemical reactions between ILs or their decomposition products and the contacting metal surfaces to reduce mechanical wear.

Although the process of forming the adsorbed boundary lubricating film is still not clear, the IL-adsorption film has been verified through electrical contact resistance (ECR) measurement by Viesca et al. [44]. The results showed that the ILadditive ([C6C1im][BF4]) outpaced the base oil to form a boundary film on the metal surface. The generation of the IL-tribofilm has been demonstrated on various material surfaces [45–47]. But the results from most of the work are relied on the post-analysis of the worn surfaces by employing Scanning Electron Microscopy

*Ionic Liquids as High-Performance Lubricants and Lubricant Additives DOI: http://dx.doi.org/10.5772/intechopen.96428*

**Figure 2.** *Schematic diagram of (A) ILs boundary lubricating film, and (B) IL-induced tribofilm on the metal surface.*

(SEM), Transmission Electron Microscopy (TEM), Energy-dispersive X-ray Spectroscopy (EDS), Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, etc. From the previous research [48], IL decomposition has been demonstrated during the sliding process, but only the anion was found to react with or adsorbed on the steel surface. Particularly, the IL undergoing facile decomposition would interact rapidly with the sliding surface, leading to a low friction coefficient. So the thermal stability of IL can be considered as an index for evaluating the tribo-decomposition behavior on nascent substrate surfaces [49].

Up to now, the characterization of the IL-induced tribofilm thickness, composition, and structure have been intensively investigated. For instance, when phosphonium-phosphate ILs were introduced to the base oils with a small amount (1.04 wt.%), an amorphous-nanocrystalline tribofilm with a 10–200 nm-thick was probed on the worn cast iron surface by TEM, EDS, and electron diffraction [17]. Furthermore, the participation of wear debris in the IL-tribofilm growth was proposed and demonstrated recently by Qu et al. [45, 46] through Atom Probe Tomography (APT) and Scanning Transmission Electron Microscopy (STEM) characterization.

In addition, tribofilm mechanical properties, such as hardness and resistanceto-plastic-deformation (P/S2 ), have also been investigated through nanoindentation measurements [50, 51]. The results revealed that only P/S2 had a correlation with the friction and wear performance, in which a small P/S2 value corresponded to a low friction and wear.

Regarding the growth mechanism of IL-induced tribofilm, a more precise *in situ* characterization is highly desirable in spite of many characterization approaches and spectroscopy techniques have been employed so far. At the same time, the application of the computational methods, such as molecular dynamic simulation, would help to elucidate the generation process of the boundary lubricating film.

#### **4. Ionic liquids as lubricant additives**

Limited to the high cost of being used as neat lubricants (particularly when AILs are used), ILs as additives have gained more and more research attention in recent years. Their highly tunable molecular structures and physicochemical properties make ILs suitable to be added to base lubricants with different nature (polar or nonpolar), such as ester, polyethylene glycol (PEG), PAO, mineral oils (MO), grease, and water-based lubricants.

Until now, ILs have been tested as friction-reducing additives, anti-wear additives, or extreme-pressure additives in many research articles. The tribological performance of IL as additives to non-polar, polar, and water-based lubricants have been summarized in **Tables 3**–**5**, respectively. Different from the traditional friction modifiers, ILs can be strongly adsorbed to the sliding surfaces and generate a resilient boundary lubricating film, leading to important reduction of friction and wear. Some active-elements containing ILs are easy to chemically react with the rubbing surfaces and create an effective tribofilm on top of the workpieces to prevent it against wear or extreme pressure.

#### **4.1 Ionic liquids as oil additives**

Due to the inherent polarity, the solubility of ILs in oils is a complicated issue. Most imidazolium-ILs are insoluble in the non-polar synthetic oils and mineral oils. So they are always used as lubricant additives in in very low concentrations or in oil-IL emulsions. In 2012, the fully oil-soluble phosphonium-based ILs [P6,6,6,14] [DEHP] and [P6,6,6,14][BTMPP] were explored [74, 75]. These three-dimensional ILs have quaternary structures for both the cations and anions with long alkyl chains, giving rise to a high steric hindrance to screen the ions charge. Inspired by this, ILs having quaternary ammonium and phosphonium cations and halogen-free anions, such as phosphate, sulfonate, orthoborate, and carboxylate have been synthesized and tested as additives to the base oils [76]. Generally, larger cation sizes lead to higher solubilities of IL in nonpolar oils. In addition, ILs having symmetric cations would outperform the ones with asymmetric cations in wear reduction, and the symmetric-cation ILs are hypothesized to have a better mobility in the base oil to interact with metal surfaces and form protective boundary lubricating film [17]. In addition, some phosphonium-based ILs have been examined to show synergistic interactions with traditional additives ZDDP in hydrocarbon oils [77], or GTL base oil [78] to improve the wear resistance of oils.

In contrast to nonpolar oils, ILs have much better solubility in some polar oils, such as PEG200, in which [C6C1C1im][NTf2] can be dissolved up to 40 wt.%. Taher et al. [79] studied the lubricating properties of halogen-free ILs pyrrolidinium bis (mandelato)borate (hf-BILs) as additives to PEG200 in steel-steel contact. The addition of 3 wt.% of hf-BILs in the base oil reduced friction and wear significantly compared to PEG200 and 5 W40 engine oil. It is noted that shorten the length of the longest alkyl chain in this IL cation will improve the friction reduction and wear resistance of the IL-blends under same working conditions.

Recently, Guo et al. [57, 80] examined the tribological properties of three hydroxylammonium hexanoate PIL additives to a nonpolar mineral oil and a polar biodegradable oil. The impact of PILs ionicity and hydrogen bonding on the friction and wear performance was discussed. The results revealed that all PILs improved the lubricity and wear resistance of the biodegradable oil under steel-steel, particularly, the one with the lowest ionicity obtained the least material loss. While, when used as additives to the mineral oil, the three PILs behaved slightly different between steel-steel and steel-aluminum contact. The use of any PIL improved the mineral oil lubricity and wear resistance under both contacts, but PILs had quite different friction behaviors in steel-Al that the one with the highest ionicity presented the best friction.

#### **4.2 Ionic liquids in water-based lubricant**

Water or water-based lubricants can effectively reduce the temperature and clean the contaminants from surface contacts, which leads to a better working

#### *Ionic Liquids as High-Performance Lubricants and Lubricant Additives DOI: http://dx.doi.org/10.5772/intechopen.96428*

**Table 3.**

*Tribological results of ILs as additives for non-polar oils (2017–2020).*

#### *Ionic Liquids as High-Performance Lubricants and Lubricant Additives DOI: http://dx.doi.org/10.5772/intechopen.96428*

**Table 4.**

*Tribological results of ILs as additives for polar oils (2017–2020).* *Ionic Liquids as High-Performance Lubricants and Lubricant Additives DOI: http://dx.doi.org/10.5772/intechopen.96428*

conditions and increase the machine lifetime. Since the high volatile characteristic and high freezing point of water-based lubricants, they are preferable in some specific industrial applications such as cutting and machining. Recent studies about IL additives in water are summarized in **Table 5**.

In the study of Wang et al. [81], N-(3-(diethoxyphosphoryl)propyl)-N,Ndimethyloctadecan-1-ammonium bromide (NP) was investigated as water additive in a steel-steel contact. A lower friction and wear rate, and excellent extremepressure and abrasion resistance were obtained compared to an oil-based lubricant. The superior tribological property was attributed to the physical adsorption of ILs on the steel surfaces and the formation of a protective film due to the tribo-chemical reactions between NP and sliding surfaces.

Bermudez's team [82] reported that water containing 1 wt.% PIL (2-hydroxyethylammonium) succinate (MSu) could reduce the running-in period when lubricating the sapphire-stainless steel contact. It is also noted that a thin PIL boundary film was found on the steel surface once the base water evaporated, leading to an extremely low minimum friction coefficient of 0.0001. In addition, another PIL additive, di[bis(2-hydroxyethyl)ammonium] succinate (DSu) was also investigated under sapphire-stainless steel [40]. The results showed that although the use of 1 wt.% DSu + Water caused a higher running-in friction coefficient compared to that of neat DSu, PIL-mixture received a comparable anti-wear behavior with regards to the neat Dsu, and even got a slightly smaller wear rate of 1.83 105 mm3 /m.

#### **4.3 Ionic liquids and nanoscale additives**

Nanomaterials, such as nanoparticles (NPs), graphene, and carbon nanotubes (CNTs), have been regarded as attractive solid lubricants which can be applied as lubricant additives and components for coatings to achieve good lubricity or superlubricity. In [83], the magnesium silicate hydroxide–based nanoparticles have been studied and proved to be effective anti-wear additives, where the excellent tribological properties can be generally ascribed to the grinding, rolling, filling effects and the tribofilm formation.

However, the poor dispersion and low solubility of nanomaterials in the base lubricants limit their long-term practical applications. Therefore, the nanomaterial surface functionalization becomes necessary to their lubrication performance. The use of an oil-soluble PIL with long-alkyl-chain to incorporate the copper oxide nanoparticles as additives to a base oil PAO was firstly reported in [84]. In this study, the PIL was employed to improve the dispersion of the copper oxide NPs, where the hybrid PIL-NPs additives exhibited an enhanced oil-load capacity and a better anti-wear performance compared to that just using copper nanoparticles as additives. Recently, the friction behavior and wear performance of diamond and ZnO NPs stabilized by trihexyltetradecylphosphonium bis (2, 4, 4-trimethylpentyl) phosphinate were investigated in a steel-ceramic contact [85]. It was found that nanoparticles mixed with IL caused a higher friction coefficient with respect to only IL was used as additive to the gear base oil, where the nanoparticles were regarded as to wear out the film formed by the IL. While the use of diamond/ZnO nanoparticles with IL obtained a smaller wear volume of the ceramic ball compared to that of IL. Particularly, both the hybrid IL-nanoadditives showed effective anti-scuffing properties which revealed their potential to be extreme pressure additives.

*ϕ2water-triethanolamine.*

 *D-gluconate.*

#### **Table 5.**

*Tribological results of ILs as additives for water-based lubricants (2017–2020).*

#### *Ionic Liquids as High-Performance Lubricants and Lubricant Additives DOI: http://dx.doi.org/10.5772/intechopen.96428*

*φwater-glycerol. εmol/L.*

#### **5. Conclusions**

As the aforementioned excellent physicochemical properties and friction and wear performance, ILs not only can be used as neat lubricants, friction-reducing additives, anti-wear additives, extreme pressure additives, but can also be used as corrosion inhibitors. Although IL corrosion inhibitors have been evaluated on many ferrous metals and alloys, their study on non-ferrous metals, such as aluminum is extremely limited, which is worthwhile to discuss. Meanwhile, the relationship between the outstanding corrosion inhibition and high performance of lubrication should be explored, when ILs are used as lubricants and lubricant additives.

Additionally, enormous literature has revealed that the adsorption of the ILs on the metallic surfaces and the tribo-chemical reactions between the active elements of ILs and the surfaces effectively improved the tribological performances of different contacts. However, the adsorption mechanism and tribofilm growth mechanism of ILs are still not clear, and the application of ILs in the tribology field, especially for PILs, should be further explored owing to its efficiency and green nature.

#### **Acknowledgements**

Hong Guo wants to express her gratitude to the Gleason Doctoral Fellowship from the Gleason Corporation.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Acronyms and abbreviations**

*Ionic Liquids as High-Performance Lubricants and Lubricant Additives DOI: http://dx.doi.org/10.5772/intechopen.96428*

### **Author details**

Hong Guo\* and Patricia Iglesias Victoria Mechanical Engineering Department, Kate Gleason College of Engineering, Rochester Institute of Technology, Rochester, USA

\*Address all correspondence to<EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 5**

## Applications of Ionic Liquids in Gas Chromatography

*Umaima Gazal*

#### **Abstract**

The environment offers an enormous innovative panorama of prospects intended for the research of novel biodegradable diluents. Regular composites have been lately recycled to formulate the anionic and cationic fraction of RTIL. Numerous applications of ionic liquids have been explored in segregation discipline. Attributable to the extraordinary polarization as well as exceptional current steadiness, IL-centered immobile segments have been applied to resolution of varied series of critically stimulating complexes frequently extremely polar composites using great boiling points plus physical resemblances comprising elongated sequence fatty acids, essential oils, polycyclic aromatic sulfur heterocycles (PASHs) and PCBs. IL-centered immobile segments facilitated the gas chromatography study for effective as well as precise amount of liquid in the industrialized yields for example pharmaceutical as well as petrochemicals complexes.

**Keywords:** ionic liquids, gas chromatography, static stages, Zwitterionic liquids, polymeric ionic liquids

#### **1. Introduction**

Ionic liquids are the utmost promising liquid green solvents with wide applications in separation science. The effect of the IL organic configuration as well as the stimulus of tributary factors, for example the IL temperature, pH, concentration, analysis time and voltage, are compatibly rationally talked concerning the accomplished parting enlargements. Gas chromatography is unique and extreme proficient, dependable, as well as stout methods for the study of unstable plus semivolatile composites. Effective along with rapid gas chromatographic investigation of objective analytes is mostly reliant on the enactment of the gas chromatography column. Though here have be present main active developments, there quiet a solid claim of extremely choosy, indolent, polar also thermally constant gas chromatography pillars intended for critically stimulating composites for example polychlorinated biphenyls, unrestricted fatty acids and unstable amines [1]. Furthermore, the physicochemical characteristics for instance surface tension, viscosity and melting point are too acute to yield extremely proficient gas chromatography columns. The viscosities of furthermost ionic liquids are frequently 1–3 remits of scale greater than outmoded biological diluents [2]. In demand for an ionic liquid to be measured as a immobile stage, the solid must have great viscosity that rests fixed above a comprehensive high temperature choice. Van der Waals as well as Hydrogen bonding kind interfaces amongst the anion plus cation of Ionic liquids rule the viscosity-properties. Furthermore, it is imperative to ruminate the surface tension

#### *Ionic Liquids - Thermophysical Properties and Applications*

#### **Figure 1.** *Applications of ionic liquids in various fields.*

of the Ionic liquids. Its values extending as of 30 to 50 dyne/cm usually display bigger wettability on the barrier of unprocessed tube pillars [2]. Ionic liquids establish an assembly of biological salts which are fluid lower than 100 °C, moreover, the ionic liquids that are fluid at room temperature are generally recognized as room temperature ionic liquids [3]. Ionic liquids are easy to manufacture, thermally steady, flameproof, chemically inactive, retain small vapor density, polar, and their discernment can be simply regulated by means of fluctuating the component anion or cation; and from now they have been extensively recycled as static stages in conservative gas chromatography (**Figure 1**) [4–10].

Ionic liquids can also be recycled as diluents for the suspension of various resources for example fiber [11], chitin [12], etc. The outstanding solubility of biological/inert composites in ionic liquids as well as a extensive variety of it as the fluid state brand them noble diluents for several responses. Furthermore, they displayed modest produces when related with conservative biological diluents [13, 14]. Thermodynamic factors of these ionic liquids were studied through chromatographic methods. ILs are centered on the numerous method, in company with the utmost extensively considered are N-alkylpyridinium, alkylammonium, N'N-dialkylimidazolium, and alkylphosphonium.

#### **2. Stationary phase in gas chromatography using ILs**

Owing to the ever-developing mandate for the great determination, high sympathy, as well as statistics amusing investigation of composite models for instance aromas, smells, petrochemicals, plus pharmacological uncooked supplies, continuous expansions of gas chromatography supports through exclusive discrimination, squat bleed, high dullness, and in addition varied high temperature operational series are desirable. Because of the high polarization then exceptional thermal constancy, IL-centered stationary stages have been employed to decide an extensive series of methodically stimulating complexes typically precise polar complexes with high steaming facts also fundamental resemblances comprising lengthy sequence oily acids, vital oils and polycyclic aromatic sulfur heterocycles [15, 16]. ILs are

#### *Applications of Ionic Liquids in Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.96702*

characteristically organized with a phosphorous- or nitrogen-comprising biological cation as well as an inorganic or else organic anion. Meanwhile the chief outline of IL-centered GC supports in 1999, ILs have been effectively engaged as stationary stages because of their trivial high polarity, tunable selectivity, high thermal stability and vapor pressure [17]. The ILs can be altered through diverse efficient collections to endure countless solvation interfaces in addition to display exclusive chromatographic discernment such as GC immobile phases. ILs have been exposed to have exclusive solvation competences plus discernment's on the distinctive solvent/solute interfaces [18]. Imidazolium-centered ILs be able to as per contrived to discrete equally non-polar and polar analytes [17]. As, by means of varying the anionic lot of the imidazolium IL since chloride [Cl]- to hexafluorophosphate [PF6]-, a substantial variance in discernment was perceived for polar analytes equaled to the non-polar complexes (**Figure 2**).

Additional examination of dissimilar modules of ILs, containing monocationic imidazolium, pyridinium, as well as pyrrolidinium exposed that the hydrogen contributor capability of the IL immobile phases was subjugated through IL cation. In contrast, the anionic lot was create to adopt the part of hydrogen acceptor anion as of proton giver analytes for example carboxylic acids plus alcohols [2]. Consequently, dicationic [19], tricationic [20], as well as phosphonium-centered cations [21] were oppressed to expand great thermal constancy plus fluid variety of ILs equated to customary monocationic static levels. Lately, in an effort to extend the applicability of IL static phases, task-specific ionic liquids (TSILs) were familiarized by functionalizing the IL cation with numerous agents [22]. For instance, the integration of aromatic segments in the IL cation improved the discernment for scented complexes, for example polycyclic aromatic hydrocarbons (PAHs). This is owing to enriched π-π sort communications amongst analytes as well as the aromatic clusters of the IL cation [4]. Overview of polar efficient clusters, for example hydroxyl segments, can effect in enlarged discernment for hydrogen compliant analytes [23]. Consequently, tweaking the IL-centered GC static phase configuration might augment choosiness essential for parting of precise compound model elements with comparable polarizations.

**Figure 2.** *Common cations and anions.*

One of the important physical property of static stage is melting point as it basically commands the least effective high temperature of the ensued GC column. ILs with short melting points are extremely required as well as are usually acquired via integrating proportion-flouting sections also alkyl sideways chains with diverse dimensions [24–26]. Analytes naturally intermingle using IL-centered stationary segments over moreover partition- or adsorption-kind contrivance [6, 27–28]. Better parting efficacies existed usually providing through the partition-kind retaining contrivance. Once the furnace temperature is lesser than the melting point of the IL-centered static stage, the molecular interface amongst the analytes as well as static stage is to be expected to be controlled by means of adsorption. Variance perusing calorimetry is usually operated to define the melting point of IL-centered stationary stages [25].

#### **3. Incorporation of ionic liquids in multidimensional gas chromatography**

Multidimensional gas chromatography is an influential method to accomplish progressive parting of impulsive as well as quasi-impulsive composites in compound environments [29–31].  The paramount requirement to effectively enhance peak capacity in the composite system is to employ a combination of GC stationary phases possessing different selectivities. It was presented to the chromatographic state compromises advanced ultimate ability than conservative one-dimensional gas chromatography, permitting on behalf of the determination of model elements by means of comparable polarizations otherwise instabilities [32]. In this method, analytes are evaporated then exposed to a sequence of gas chromatography supports by means of chemically diverse static stages attached over an edge. In multidimensional gas chromatography analyte parting is preserved on both column, prominent to an upsurge in the parting control associated to the one [33]. Two types of multidimensional gas chromatography are usually engaged, specifically, core-wounding also inclusive. In core-wounding multidimensional gas chromatography, merely a choice rare portions of overflow after the first support are transported to the second support for extra parting [34].

Diverse support selectivity, which can be characterized using a liberated parting procedure, is the significant requisite to acquire advanced top capability in multidimensional gas chromatography methods [35]. Several customary non-ionic gas chromatography static segments are categorized as both non-polar and polar segments. These supports display a deficiency of variety in positions of solvation abilities, which can bound their capacity to decide composite models through gas chromatography× gas chromatography. Because of this hitch, ionic liquids centered supports have appeared as alternate gas chromatography × gas chromatography static stages. Utilizing ionic liquids centered stakes can permit exclusive solvation abilities in addition selectivities, moreover to advance thermal constancies comparative to customary segments. Ionic liquids have been applied as existing segments combined with customary non-ionic segments in numerous gas chromatography× gas chromatography partings [36]. Meanwhile maximum gas chromatography × gas chromatography partings can be controlled constructed on analyte instability in the first measurement monitored through involvement of dissimilar relations in the second measurement, it is collective to practice ionic liquids supports as the second measurement to estimate their enactment in relations of retaining contrivances. Presently, a huge amount of profitably accessible ionic liquids supports, for example the Supelco Low Bleed community, comprise numerous phosphonium and imidazolium grounded di cations which are typically combined with frequently tri

*Applications of Ionic Liquids in Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.96702*

fluoromethanesulfonate and bis[(trifluoromethyl)sulfonyl]imide anions [2, 20]. Economical ionic liquids static stages have been engaged in the parting of total of analytes for instance savor plus fragrance amalgams [36], aromatic hydrocarbons [37], alkyl halides [38], alkyl phosphonates [39], fatty acid methyl esters [40], as well as additional polar analytes (nitrogen, sulfur as well as oxygen-comprising composites). These analysis specify that ionic liquids supports establish considerable advanced selectivity plus retaining in the direction of commonly polar analytes associated to non-polar analytes owing to hydrogen-bonding interaction, electrostatic interactions and dipole–dipole relations, amongst ions [41].

#### **4. Polymeric ionic liquids centered static stages in gas chromatography**

Polymeric ionic liquids are stimulating family of composites that can be recycled as sorbent coverings in solid phase micro extraction. Polymeric ionic liquids are artificial polymers manufactured after ionic liquids monomers. Furthermore, Polymeric ionic liquids can be basically modified to display greater sensitivity and selectivity nearby diverse section of analytes. Polymeric ionic liquids are characteristically manufactured through functionalizing a polymerizable practical cluster on the cationic component of the ionic liquid by free radical polymerization in the attendance of a thermal originator. Polymeric ionic liquids reveal greater thermal constancy in addition to a confrontation to viscosity decrease at greater temperatures. These valuable structures can develop fiber lifespan, toughness plus eligibility of Polymeric ionic liquids while retentive the discrimination relics the intrinsic to ionic liquids. Polymeric ionic liquids have been displayed to extant extraordinary possessions as well as exhibitions [42–48], assisting novel plus stimulating parting procedures [49–51]. Ionic liquids have drawn abundant consideration in latest centuries as constituents for static stages in gas chromatography, because of stuffs similar their capability to create concurrent nonpolar as well as polar interfaces with the analytes, their extraordinary thermal constancy, before their insignificant air compressions then extensive fluid series [52–56]. Also, it is price revealing that these things can be effortlessly welladjusted over minor fluctuations in the assembly of either the anion or cation, which, also, can intensely modify the choosiness or else the parting capability for the analyte of concern [36, 57–59]. The concern in ionic liquid-covered gas chromatography supports has enlarged afterward their marketable outline in 2008, also today, numerous ionic liquids glazed supports with dissimilar features are viably accessible. Though, one main task for the growth of static stages built on ionic liquids is the research of extremely standardized coverings, which would service decent ultimate regularities as well as extremely active complex partings, also concurrently, deliver extraordinary thermal constancies for the subsequent gas chromatography supports [60–62]. At extraordinary temperatures, identical ionic liquid-glazed silica supports can practice flick commotion prominent to a diminution in the analyte retaining periods as well as efficacy. In this respect, polymerized ionic liquids can offer the compulsory replies, preserving the outstanding thermal constancy of the supports, in addition to uniting the chief structures of an ionic liquid as well as the distinctive polymer characteristics for example better automated constancy plus development capability [62–65].

#### **5. Preference of Zwitterionic liquids in gas chromatography**

Zwitterions, consequent after ionic liquids have inimitable characteristics for example reasonably small crystal conversion temperature, slight ion conductivity as well as exclusive stage conduct afterward partying with water. Moreover, the combination of

convinced zwitterions plus negligible quantity of water can be observed as an exceptional liquid ideal of cell tissues. Zwitterionic liquids can be chemically precise like to predictable aprotic ionic liquids, excluding that the negative as well as positive charges exist in on the similar particle. Zwitterionic complexes are elements that have an entire clear charge of zero as well as are therefore electrically impartial. They transmit proper electrical charges of reverse symbol contained on diverse particles as well as formerly can be measured as internal salts. The utmost collective zwitterionic-kind ionic liquids are nitrogene heterocycles with sulfonate component. These Zwitter ionic Liquids have been competently considered as designable electrolyte constituents for fuel cells [66] as well as lithium batteries [67]. Not as much of discovered Zwitter ionic Liquids are configurations founded on imidazoliums with carboxylate occupations. They have been cast-off as forde sulfurization of fuels [68], Bronsted acidic catalytic agent [69], as liquid crystals [70] or for metallic oxides solubilization [71]. Therefore amino acids be existent typically as per zwitterions in a definite variety of pH then the pH at which the regular charge is zero is known as the particle's isoelectric fact (**Figure 3**).

Through zwitterionic composites, anions as well as cations are roped covalently. Imidazolium sulfonate is one of the example of zwitterionic liquids. The production of room temperature zwitterionic liquids in which both sulfonate anion as well as imidazolium cation elements attribute to the parental particle was conveyed in literature (**Figure 4**) [72, 73].

Motivated via means of the proclamation that these composites can be organized as fluids at room temperature, three zwitterionic liquids integrating alkyl side chain and oligoether substituents were intended then inspected as gas chromatography static stages. The fundamentally-regulated zwitterionic liquids -centered static stages offer distinctive choosiness, robust retaining, exceptional top regularity, also a reasonably widespread employed series appropriate for the study of volatile carboxylic acids. This comprising volatile fatty acids for instance lactic acid as well as butyric acid are significant for the construction of cosmetics, pharmaceuticals and fuels [74–76]. Gas chromatography is furthermost usually recycled for the quantification as well as parting of specific acids in acylated lipids. Derivatization of volatile carboxylic acids by means of numerous approaches for example alkylation plus acylation is characteristically accomplished to upsurge the explosive nature of these composites in addition to mark their investigation viable through gas chromatography (**Table 1**).

```
Figure 3.
Zwitter ionic liquids.
```
**Figure 4.** *Imidazolium sulfonate Zwitterionic liquids.*

**Table 1.**

*Use of ionic liquids as superficial-integrated static segments.*

#### **6. Green aspects of ionic liquids**

The adjustable physicochemical properties of ionic liquids have prolonged their usage addicted to a wide variety of diverse uses. Ionic liquids have an abundant prospective in biological amalgamation, electrochemistry, mass spectrometry, green chemistry as well as partings [79, 80]. In the field of analytical chemistry, ionic liquids have been recycled as static segments as well as diluents for headspace gas chromatography [80], movable segment extracts plus external-attached static segments in liquid chromatography [79, 81] for liquid–liquid abstractions as well as solid-phase micro abstraction [79, 80, 82]. Several "green-engrossed" manufacturing have originate that ionic liquids are outstanding applicants for their uses because of their precise small vapor compression [83, 84]. The overview of ionic liquids such as static segments has released up and around novel outlooks in this arena by means of their exclusive solvation features outcome in unusual discernment, which is entirely dissimilar to that of typical polyethylene glycol as well as poly dimethyl siloxane centered supports. Since of their atypical discernment plus extraordinary unresponsiveness, ionic liquids centered supports have previously establish numerous solicitations in the normal item for consumption ground in and multidimensional as well as mono gas chromatography in addition to preparative gas chromatography, prominent to the comprehensive investigation of composite sections (containing aqueous resolutions), plus the parting of stimulating sets of complexes. The speedily growing usage of Ionic liquids equally in educational plus manufacturing arenas have created an increasing apprehension approximately their effect on the environs. Meanwhile Ionic liquids are extremely solvable in liquid however are not continuously ecofriendly, a discharge of ionic liquids into the atmosphere might clue to substantial water contamination complications. Furthermore, Ionic liquids could develop insistent contaminants in discarded water seepage because of their great constancy in water. Intended for this purpose, research inspecting Ionic liquids biodegradability are of inordinate significance. The rising character of Ionic liquids in production as well as study plus the growing alarm nearby their green influence have advised a requisite for the progress of profligate, dependable as well as reproducible techniques for the classification plus investigation of Ionic liquids [85].

#### **7. Conclusion**

Ionic liquids have solicitations in various areas in chemistry. The applications of ionic liquids as extracts in chromatography displays abundant rewards equated to further extracts. Ionic liquids have been realistic in diverse extents of parting, for instance ionic liquid sustained tissues, as moveable segment extracts as well

as external-joined static stages in chromatography partings also as the abstraction diluent in model provisions, since they can be collected from numerous anions in addition to cations that alter the things as well as stage conduct of fluids. The inflammable, non- explosive environment of ionic liquids marks them an outstanding optimal for the expansion of nontoxic methods. A substantial benefit of ionic liquids-centered stationary segments is their capability to have adequate to high updraft solidity though similarly unveiling a comprehensive host of solvation proficiencies, specific of their inimitable selectivities. In spite of their attainment, viable Ionic liquids-centered immobile segments dearth the solving authority for non-polar analytes, predominantly unsaturated as well as saturated hydrocarbons, cycloalkanes. This nonexistence of discernment has diminished fervor amongst certain parting experts who might modulate the feature of fundamental fine-tuning (in relations of anion/cation combining in addition to operational structures of every constituent) while emerging ionic liquids to display great discernment also robust solving influence. Ionic liquids have strained substantial consideration as gas chromatography immobile segments as of their tunable chemical plus physical properties. Conversely, profitable Ionic liquids-centered gas chromatography supports have not reconnoitered entirely of the solvation characteristics that can be obtainable through ionic liquids.

Moreover, their polarization, viscosity, hydrophobicity plus further physical and chemical properties can be designated by means of selecting the anionic and cationic component. Ionic liquids are considered as "exclusive diluents" as of this adjustable environment, which rises their prospective solicitations. The consumption of ionic liquids is maiden innovative prospects in diverse regions of parting discipline, with novel countless solicitations. Additional uses in partings are linked to the ecological, pharmacological, biomedical as well as various manufacturing trades. Ionic liquids have been discovered in partings for abstraction, reinforced fluid membranes, as extracts then as static segments in chromatography.

#### **Acknowledgements**

The author received no financial support for writing this article. I want to acknowledge my mother **Mrs. Nargis Sultana** and husband **Mr. Safdar Hussain** for his encouragement and support for writing this article.

### **Author details**

Umaima Gazal Department of Chemistry, Raja Bahadur Venkata Rama Reddy Women's College, Affiliated to Osmania University, Hyderabad, India

\*Address all correspondence to<EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Applications of Ionic Liquids in Gas Chromatography DOI: http://dx.doi.org/10.5772/intechopen.96702*

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#### **Chapter 6**

Ancient and Contemporary Industries Based on Alkali and Alkali-Earth Salts and Hydroxides: The Historical and Technological Review

*Rina Wasserman*

### **Abstract**

Although sodium, potassium, calcium, and magnesium were isolated *as the chemical elements* by Sir Humphry Davy for the first time at the beginning of the 19th century, alkali salts and hydroxides have been widely known and used since the very ancient time. The word "alcali" & "alkali" was borrowed in the 14th century by literary Roman-Germanic languages from Arabic *al-qalī*, *al-qâly* ou *al-qalawi* ( ), which means "calcinated ashes" of saltwort plants. These ashes are characterized nowadays as mildly basic. They have been widely used in therapy, cosmetics, and pharmacy in Mediaeval Europe and the Middle East. However, the consumption of these alkali containing ashes, as well as natron salts and calcined lime-based materials used for different customer purposes, like therapy, pharmacy, cosmetics, glass making, textile treating, dyes, brick making, binding materials, etc., was commonly known since the very ancient times. The current review of the archeological, historical, and technological data provides the readers with the scope of the different everyday life applications of alkali and alkali-earth salts and hydroxides from ancient times till nowadays. The review obviously reveals that many modern chemical manufacturing processes using alkali and alkali-earth salts and hydroxides have a very ancient history. In contrast, there has been a similarity of targets for implementing alkali and alkali-earth salts and hydroxides in everyday life, from the ancient past till the modern period. These processes are ceramic and glass making, binding materials in construction, textile treatment, metallurgy, etc. So, this review approves the common statement: "The Past is a clue for the Future."

**Keywords:** alkali, caustic, lime, pH, natural cement, Portland cement

#### **1. Introduction**

The alkali metals Na and K reside in the first column of the periodic table. Sir Humphry Davy, a prominent English scientist of the 19th century, electrolyzed sodium, Na, and potassium, K, and named them in 1807 [1]. At first, Davy called metallic potassium and sodium "the basis of potash" and "basis of soda,"

respectively. Consistently, he renamed these new metals potassium and sodium. Dmitry Mendeleev designated this discovery as " … one of the greatest discoveries in Chemistry … " [2]. However, after discovering these alkali elements that play an essential role in modern life, they have been little known by non-scientists for many years [3]. Although potassium and sodium as metals entered human life only ca. 200 years ago, humans have been familiar with their substances for thousands of years. Usage, treatment, and conscious transformations of alkali substances, which are almost six thousand years old, could be called the first advanced chemical technology in humankind's Big history.

The word "alcali" & "alkali" was borrowed in the 14th century by literary Roman-Germanic languages from Arabic al-qalī, al-qâly ou al-qalawi (القلوي(, which means "calcinated ashes" of saltwort plants. These ashes are chemically characterized nowadays as mildly basic. They have been widely used in therapy, cosmetics, and pharmacy in Mediaeval Europe and the Middle East. However, the consumption of these alkali containing ashes, as well as natron salts and calcined lime-based materials used for different customer purposes, like therapy, pharmacy, cosmetics, glass making, textile treating, dyes, brick making, binding materials, etc., was commonly known since the very ancient times.

The current article intends to review those technological processes of alkali substances, modernly called 'chemical technology,' and track these processes' ancient and historical roots revealed by archeological findings and historical descriptions.

Undoubtedly, the ancient civilizations were not aware of the contemporary "chemical language" and did not carry out any scientific investigations or testing the chemical and technological procedures before their implementation. However, the archeological findings have revealed in the last 100 years a vast amount of builtin chemical knowledge possed by the prominent ancient civilizations in their everyday life. Alkali salts played an essential role in human health and body care during ancient times. The ancient texts' interpretations have revealed alkalis substances' conscious usage as detergent and hygienic remedy throughout human history. Furthermore, the ancient texts have distinguished between alkaline salts' mineral and botanical origin, although emphasizing similar usage. Let us get down to some examples of alkali-based substances' knowledge and use in ancient and historical times.

#### **2. Use of alkaline salts in ancient and historic cosmetics, food, cleaning, and medicine**

#### **2.1 Alkaline salts as the most initial raw materials of the ancient Mesopotamian pharmacology**

The first documented use of ordinary table salt and soda could be related to Sumer and Akkadian Empires in Mesopotamia (3500–2000 BCE). **Figure 1** presents the map of the ancient Near East in the fourth millennium DC. At the beginning of the 20th century, the University Museum's archeological expedition, Philadelphia, the USA, to Nippur (a lower part of modern Iraq) excavated the cuneiform tablet aged ca. 2100 BCE [5]. In the tablet (**Figure 2**) decrypted at the half of the 20th century, the Sumerian script described the pharmacological processes involving alkaline substances of mineral and botanical origin. The mineral salts of alkali metals mentioned in the tablet are sodium chloride and potassium nitrate. Sumerians obtained alkalis also from soda ash, which they called Td-Gaz. This soda ash had a botanical origin by burning halophytic (high salinity) and alkaline plants, like glassworts (most likely the *Salicornia fruticosa* L.) rich in sodium carbonate, Na2CO3 *Ancient and Contemporary Industries Based on Alkali and Alkali-Earth Salts and Hydroxides… DOI: http://dx.doi.org/10.5772/intechopen.99739*

#### **Figure 1.** *Map of ancient Mesopotamia [4].*

[5, 7]. Sumerians had an abundance of designations for these alkaline plants and alkali ashes and salts according to their origin and manufacturing processes [8–10]:

Glassworts are hardy to high alkaline environments and store absorbed alkaline salts in their tissues during growth [11–13]. According to the modern analytical tests, sodium and potassium carbonate content in ashes obtained from the Near East halophytic plants could be 38.5% - 93% [11]. According to the Sumerian script, the further treatment of soda ash included its multistage purification and pulverization processes. Sumerians widely implemented this "halophytic ash" soda in pharmacology, making simple detergents and soaps for body cleansing and religious purifying:

#### **Figure 2.**

*Picture of cuneiform (clay tablet) with the pharmacological inscription, Nippur, c. 2100 B.C. University of Pennsylvania, Philadelphia USA. [6].*

"With water I bathed myself. With soda I cleansed myself. With soda from a shiny basin I purified myself." [14].

#### *2.1.1 The rise of soapmaking*

Another Sumerians' pharmacological use of sodium salts (soda ash and regular salt, NaCl) was making a medicated ointment soap as a rubbing remedy for ailments. The preparation process was based on thorough mixing of sodium salts, i.e., sodium ash and regular salt, with various natural organic ingredients. [7]. Generally, the earliest Sumerians'soaps were made for medical purposes and wool washing but not for general cleansing purposes. To extract alkali from the plants, Sumerians put into use the following technological stages [14]:

*Ancient and Contemporary Industries Based on Alkali and Alkali-Earth Salts and Hydroxides… DOI: http://dx.doi.org/10.5772/intechopen.99739*

This method was based on a long-time, slow and thorough process to assure the high yield of alkalis' extraction and, therefore, the more expensive product was obtained. The Sumerian elites used this product for ritual purifying. Common Sumerians got a more simple leaching process in everyday life. They stirred the plant ash in water and filtered the suspension before using it to remove the insoluble impurities. The resulting basic lixivium (alkali leachate) was widely used for everyday cleansing and washing purposes.

#### *2.1.2 Advanced technologies for table salt manufacturing*

The Old and New Babylonian languages, which are Sumerian and Akkadian, respectively, used the same logogram for table salt , [9, 10, 15], but had the different pronunciation of this term: *mu-n*(u) in Sumerian (Old Babylonian) vs. *ṭab-tu(m)* in Akkadian (New Babylonian) [8, 16].

Table salt, which is sodium chloride, NaCl, is described in the decrypted Sumerian (Old Babylonian) texts as an essential ingredient of the human's diet, food preservative, and the pharmacological salting-out ingredient to separate a medicated ointment soap from the glycerin, excess of water, and impurities [7, 14, 17, 18].

A great deal of salt treatment in various parts of the Ancient World, since the prehistoric times, till the 19th century C.E., was the uniformity of salt production techniques [19]:

The utilization of salines for salt production was probably the most crucial method of salt superiority in antiquity. The leather sacks were used in Ancient Mesopotamia to transport large quantities of salt from salines to villages and towns. Also, salt molds made of porous ceramic material or reeds were used to transport the precipitated "salt cakes" to consumers grinding them for daily use. Wooden bowls served as salt containers in the home [17]. In the Sumerian and Akkadian Empires, salt served as a reward for work as a state servant. According to the decrypted Akkadian texts, unskilled workers and high-quality artisans employed at the reconstruction of the Ékur Temple (**Figure 3**) in Nippur under the Kings Naram-Sin (reigned 2261–2224 BCE) and Shar-Kali-Sharri (ruled 2217–2193 BCE) were given 0.421 and 0.842 liters of salt per capita per month, respectively [20].

Those amounts' mass equivalent comprises an average of 10.4 and 20.8 gr salt per person per day in a 30-day month, respectively [17]. According to the U.S. Food and Drug Administration, the recommended daily sodium intake is less than

**Figure 3.** *Photograph of Ekur, the ziggurat of Enlil at Nippur [4].*

2,300 mg per day [21]. In the context of salt intake, one gram of sodium equals approximately 2.5 gr of salt [22]. Thus, by present-day measurement, the ancient Accadian workers obtained a surplus of salt as salary. Perhaps, this was done because of the extensive use of salt as an animal food' preservative in a hot climate. Also, salt played a high role in many medical remedies to help afflictions of the soul, psyche, male virility, and magic rituals in the Ancient Sumer and Accadian empires [17]. Considering the Akkadian Empire's high population during the third millennium BCE, massive salt consumption could be imagined. Therefore, large-scale logistics of salt gathering, a well-established delivery system, massive salt loading and unloading operations, commodity distribution, and numerous material equipment could be considered for that ancient society [17].

#### *2.1.3 Saltpeter green manufacturing*

Another alkali substance described by Sumerians pharmacological text [5] was niter or saltpeter, K2NO3. Sumerians called saltpeter *mú-nu* (*mú*, 'to make grow,' + *nu*, 'fire'), whereas the Akkadian word for saltpeter was *marru* (bitter) [17]. Nev-

ertheless, also, in this case, they used for this alkaline salt the same logogram

[10]. The Old Babylonian (Sumerian) word used for saltpeter in the 4th millennium DC demonstrates the knowledge of potassium nitrate ignitability. It should be emphasized that the last millennium civilization has begun using the saltpeter as the oxygenating ingredient in gunpowder only since 9th century AD [23, 24].

 This intermedial crystallized substance contained a mixture of alkali salts (sodium chloride, potassium nitrate, and others). According to this ancient script, Sumerians harnessed fractional crystallization processes to purify obtained niter. The text,

*Ancient and Contemporary Industries Based on Alkali and Alkali-Earth Salts and Hydroxides… DOI: http://dx.doi.org/10.5772/intechopen.99739*

although, emphasized the yield of the processes. Our days, we call such processes of 'green' innovative and emerging technologies [25–29].

#### **2.2 Descendants of the knowledge and experience of sodium alkaline salts**

The knowledge and experience of sodium alkaline salts' use were "inherited" by Babylon (2000–144 BCE) and the Hittite (1900–700 BCE) empires (both in Mesopotamia). Alkaline substances based on wood or plant ash were in high demand there. A ca.1300 BCE text described the Hittites using sodium salt, possibly sodium soda, and another alkali compound from plants, for washing the hands in a religious practice [14]. In ancient Babylonia, many halophytic plants were used for their alkaline substances. Babylonians used the same operations as Sumerians to extract alkalis from the plants for cleansing, hygienic, ritual, cosmetical, and medical purposes.

The usage of table salt, NaCl, as a food preservative agent was well known in ancient Mesopotamia in the first millennium BCE [18]. Table salt was an essential product in the food allowance paid by Ancient Mesopotamian rulers to workers and was considered a necessity for regular human maintenance.

#### **2.3 Alkaline salts in the ancient Egyptian pharmacology (3000 BCE: 395 CE)**

Another civilization distinguished by a conscious understanding of alkalis salts' importance and their broad medical and pharmacological usage was Ancient Egypt (3000 BCE – 395 CE). Alkalis salts used by the Egyptians since very ancient times were mineral soda (natron), Na2CO3NaHCO3xH2O, regular salt, NaCl, and sodium sulfate, Na2SO4 [30–32]. Natural Soda occurs in Egypt principally in the Wadi Natrun in the Libyan desert (**Figure 4**), and to less extent, at El Barnugi, in Lower Egypt, and at Mahamid, in Upper Egypt.

The Wadi Natrun deposits have been probably the oldest known source of natural soda globally, and they served to supply that commodity for thousands of

#### **Figure 4.**

*(A) Location map of Wadi El Natrun (Western Nile Delta), (B) inland saline lakes including Lake Hamra at Wadi El Natrun, (C) the swamp beside Lake Hamra about 5 m above the lake level. (picture copyright © [33]).*

years. In ancient times there were two soda lakes, which became united when water was most abundant [30]. The soda was found there in three forms:

a. as a solution in the lakes' water;

b. in а solid form at the bottom of the lakes;

c. as a crust on the ground.

The archeological excavations of the 19th and 20th centuries revealed (among others) seventeen ancient Egyptian papyri dealing with medical, pharmacological, and body purification issues and covering the period of more than two millennia, dating from the Middle Kingdom (2040–1640 BCE) to the end of the Greco-Roman Period (332 BCE-395 CE) until the Roman Empire's break up [34]. These papyri proved the importance of sodium salts in Ancient Egypt. However, sodium salts' usage in medicine, mummification, and ancient Egypt's hygiene began much earlier, five millennia ago, during the Early Dynastic Period (2920–2575 BCE) [31, 32].

The papyri mentioned plenty of times usage of sodium-based minerals as medication commodity:

Furthermore, the salt and natron's mentions are more than one-third of the papyri's total mineral references. No other mineral was mentioned so frequently in the medical. These figures confirm the importance of sodium salts for health treatments in Ancient Egypt.

According to the papyri, the most common causes of salt usage in Ancient Egypt were treating wounds and mummifications (sodium chloride is well known as a putrefaction inhibitor), cure against diarrhea and dehydration, and cosmetics [31]. The interesting fact is that the use of salt is still quite common among traditional folk remedies.

In Ancient Egyptian medicine, natron's use was very similar to salt, mostly externally for wound treatments, skin curing, purification, mummification and embalming, and washing. M. Sapsford carried out the analytical tests to reveal and estimate salt and natron's role in the papyri's skin-curing prescriptions [31]. This study showed that salt served a moisture's retainer's role in anti-wrinkle skin creams, and natron possessed the highly desiccant ability. For the purification, salt and natron were used after an illness as a ritual cleansing means to be re-accepted as a fully functioning person in Ancient Egyptian society. Natron was also used as a means of unique purifying oneself after a period of "uncleanliness." There is some papyri' evidence of salt and natron' use for laundry'state-provided service. The exceptional importance of salt and natron for Ancient Egyptian society could be

*Ancient and Contemporary Industries Based on Alkali and Alkali-Earth Salts and Hydroxides… DOI: http://dx.doi.org/10.5772/intechopen.99739*

demonstrated by the fact that the village's households received a part of their monthly ration package by salt and natron [35].

#### **2.4 Standard practices with alkaline salts adopted by ancient Hebrews, Greeks, and Romans (ca. 1150 BCE: 500 CE)**

The Old Testament texts describe the wide use of salt by ancient Hebrews for a variety of purposes [36]:

Ancient Hebrews distinguished between alkali sodas of different origins. They called natron, the mineral sodium carbonate, *neter*. In comparison, the word *borit* was used for sodium carbonate of vegetable origin (halophytic plant ash). The Ancient Hebrews commonly used *neter* and *borit* as laundry and body-cleaning agents [37].

The ancient Greeks and Romans eventually adopted these ancient practices of alkali salts' usage as laundering and body-cleaning agents and distributed them on the European continent [37]. However, since 600 BCE, the above-mentioned technological practices were changed to obtain solid soap, whereas wood ash became the main alkali-containing constituent, and animal fats were the binders in solid soaps. Pliny the Elder attributed the invention of alkali containing solid soap to one of the northern Celtic tribes [38]. Galen (129–199/216 CE), Greek physician, writer, and philosopher, wrote: "Soap is made by cooking beef, she-goat, or wether fat, mixed in with lye and quicklime." [39].

#### **2.5 The Middle East as the disseminator of the ancient pharmacological crafts and knowledge of alkaline salts to Western and Southern Europe (since 7th CE)**

Around 700–800 CE, the craft industry of soapmaking containing alkali plant ashes, animal fats, and the different plant oils became abundant in the Western and Southern Mediterranean, especially in Italy and Spain. This fact was mentioned in detail by Abū Mūsā Jābir ibn Ḥayyān, the Arab savant who lived in the 8th-century C.E. [40].

It should be emphasized that the English word "alkali" was "borrowed" from the Arabic language in the Middle Ages. The Arabic word al-qaly is the most common word for ash obtained from the alkaline saltworts plants [41]. Since ancient times, the caustic ashes'specific properties were well-known among the Middle East population. The Middle Eastern craftsmen inherited this time-honored traditional knowledge of the Ancient civilizations, whereas the Crusaders and the Arabic-speaking merchants helped disseminate this craft knowledge in Mediaeval Europe.

#### **2.6 Declining the use of the natural alkali-bearing minerals since the industrial revolution in Europe**

Until the Industrial Revolution in the 18th and 19th centuries, the caustic plant ashes had a primary use in semy-boiled body cleansing soaps possessing mild alkali pH. The natural mineral natron was the raw material for caustic washing and laundry soaps. At the end of the 18th century, Nicolas Leblanc, a French chemist, and surgeon invented an industrial process of converting the ordinary table salt, NaCl, into sodium carbonate, Na2CO3, to address the growing demand of the traditional industries in soda [42, 43]. Since that time, the soapmaking craft based on natural alkali-bearing minerals was gradually ousted by growing industrial technologies with chemically obtained detergents as raw materials. Industrial cleaning products of most of the 20th century have proven themselves very effective detergents. However, these artificial chemicals were found as highly allergenic and causing other unintended deleterious effects. Thus, the current trend to turn back to the traditional soapmaking crafts using the alkali-containing plant ashes has become very popular in the last years [44–46]. Thus, one more time in human history, the ancient knowledge of body cleaning and washing agents containing the plant ashes has been resurfaced and rejuvenated, being back now in human life's place.

#### **3. Soda and potash-based glassmaking**

As was previously mentioned, Leblanc's invention of artificial soda production at the end of the 18th century addressed a growing need of the European population and industry for caustic raw materials used in a) textile manufacturing as a bleaching agent; b) glassmaking as a soda-lime flux; and c) soapmaking for saponification of fats and oils [42].

#### **3.1 2500 Year-long Ascension of crockery glassmaking based on soda and potash**

Glassmaking has used natron, which is natural soda, since very ancient times. The first regularly produced glass was made in Egypt and the Near East in the sixteenth century BC [47]. The numerous archeological excavations revealed intensely colored glass, simulating precious stones such as turquoise, carnelian, lapis lazuli, amethyst, obsidian, and others, produced during the Late Bronze Age (1600– 1200 BCE) [48]. Manufacturing any glass needs fluxes acting as atoms' network modifiers [49]. The network modifiers in very ancient glasses were the alkali metals and the alkali earths. The alkali metals, particularly sodium and potassium, disrupt the atom's network structure in glass, lower the melting point, and compromise the general stability of the glass (*ibid*). Alkali earths, especially calcium, usually counteract this effect to a certain extent and stabilize the glass. Ancient and historical glasses are alkali-lime-silicate glasses because alkali carbonates, such as plant ashes and natron, were the critical raw materials consciously used by glassmakers [31, 43, 48, 50–64]. It is now widely accepted that during the Late Bronze Age, Soda and potash-rich plant ash enhanced by increased lime content was the primary flux additive used to make glass in the ancient Near East [48, 50, 64, 65].

The use of natron and trona, the natural sodium carbonates, in the glassmaking of the ancient world began to be evident at *circa* 1000 BC [48] and continued almost two millennia [31, 47, 48, 53, 56, 61, 62]. The primary source of natron, for ancient Near Eastern glass manufacturing since 1600–1200 BC, was Wadi-el-Natrun, in

*Ancient and Contemporary Industries Based on Alkali and Alkali-Earth Salts and Hydroxides… DOI: http://dx.doi.org/10.5772/intechopen.99739*

Egypt [31, 48, 53, 61, 62, 64, 66]. However, Pliny the Elder mentioned in his *Natural History* the natron deposits from the al-Barnuj region in the Egyptian Nile Delta, the Lake Van in the eastern region of Turkey, and the al-Jabbul lakes in Syria resources used by the ancient Greek and Roman glassmakers [67]. However, the current archeological research on the glass production in the ancient Near East in 1000 BC – 1000 AC has not provided an unambiguous opinion regarding the possibility of the ancient large-scale exploration of natron from the deposits other than Wadi-el-Natrunor al-Burnuj [54, 61]. Since the Roman era and till the 9th century BC, almost only Egyptian natron deposits supplied the flux raw material for global glassmaking. From the 7th century AC towards the end of the first millennium, the Old World's glassmaking crafts faced a shortage of mineral natron from Egypt and the Levant [51]. This natron shortage led to "re-inventing" the millenniaold alkali flux, i.e., glassmaking in Mediaeval Europe widely adopted the plant potash-ash fluxes [55, 57, 60, 63]. In the 9th and 10th centuries, the art of poly- and monochromatic luster-stained glass became very popular in the Near-Eastern Islamic world [68], see **Figure 5**.

The soda-containing flux used in Egyptian luster-stained glasses of the Islamic period (9th – 10th centuries) was natron, whereas luster-stained glass vessels from the Syria–Palestine region and Mesopotamia were crafted with sodium and potassium rich plant ashes [70].

#### **3.2 Alkali-containing plant ashes catalyzed the invention of colored stained glass for architectural purposes**

Colored stained glass has played a significant role in European architecture since the 12th century AC [60, 71, 72], see **Figure 6**.

From the 12th century up to 1440 AC, the European window glassmaking technique was a broad glass method for producing small rectangular glass sheets

#### **Figure 5.**

*The ceramic dish with blue, green, and manganese-purple glaze, from Raqqa, northern Syria, 12th century AD. Presented in British museum (museum inventory number 1923.2–17.1). Picture-copyright ©*discover Islamic art *(*MWNF) *[69].*

**Figure 6.** *A medieval window at Troyes cathedral, France (14th century). Wikipedia [73].*

[43, 74]. Finally, from the 15th century until the mid-19th century, the primary window glassmaking technique in Europe was the crown glass method of producing sheet glass [74], see **Figure 7**.

Both these techniques use alkali-containing plant ashes as a flux. However, the alkali-containing plant ashes used in glassmaking differed in the different European regions. In the Eastern and Southern Mediterranean regions, the soda-rich halophytic glassworts' ash, pure or blended with natron, was imported from the Syrian-Palestine region and Egypt and widely used in glassmaking since Mediaeval times, thanks to the commercial and technological interconnections with the Islamic East [76, 77].

Since the 13th century, the glassmakers enhanced the raw plant ashes by admixture with higher lime and magnesium oxide content to obtain the glass with good chemical stability and low thermal expansion [78].

In Central Europe in the 12th – 18th centuries, the glassmaking crafts widely used potash and soda-rich wood ashes as fluxes, whereas higher contents of lead oxide and lime in ash were found effective for enhancing the glass durability [74, 79, 80].

#### **3.3 Alkaline salts of different origins have promoted the continuous development of glass technologies for construction purposes**

Towards the 15th century, window glassmaking in Central Europe began using sea salt, NaCl, as an additive to ash flux to control the window glass composition affecting glass mechanical stability [79]. As an additive to wood ash flux, sea salt, soda, and niter were widely used to produce cylinder (hull) glass, a more advanced form of broad-glass manufacture, in Central and Nothern Europe until the 17th century (see **Figures 8** and **9**). The alternative important potash-containing material used as a flux in window glass manufacturing in England till the 19th century was kelp ash [82]. Kelp ash is a substance produced by the burning of seaweed [83]. The use of kelp ash in glass manufacturing was generally declined since the first half of the 19th century because of industrial sodium carbonate manufacturing (see **Figure 8**).

*Ancient and Contemporary Industries Based on Alkali and Alkali-Earth Salts and Hydroxides… DOI: http://dx.doi.org/10.5772/intechopen.99739*

#### **Figure 7.**

*Robert Bénard (French artist, 1734–1777). Crown or window glass making engraving plates. Plates XV & XVI from [75]. The description (*in French*) of particular craft operations is also available online at [75].*

#### **Figure 8.**

*Manufacturing cylinder (hull) glass. Engraving of a German glassworks, 1865. ©* Bildarchiv Preussischer Kulturbesitz *[81].*

#### **Figure 9.**

*1850s Original Manuscript Book of glassmaking Recipes, procedures, and Formulaes. Pictures copyright © 2021 M. Benjamin Katz, fine books/rare manuscripts (https://www.mbenjaminkatzfinebooksraremanuscripts. com/product/4340/1850s-ORIGINAL-MANUSCRIPT-BOOK-OF-GLASSMAKING-RECIPES-PROCEDURES-AND-FORMULAES-UNIDENTIFIED). a)* Text in the red rectangles on the left: *"Flint. Metal made by me at Hull July 1850 … Saltpeter 1½ lbs. … .".* Text in the red rectangles on the right*: "No. 1. Amber … nitre 8 oz. … . No. Amber … nitre 16 lbs"*. *b)* Text in the red rectangles on the left: *"Saltpeter or nitrate of pot ashes it is better than the rough nitre for melting flint glass it is composed of 44% of nitric acid and 51% of pot ash and 4% of water".* Text in the red rectangles on the right*: Soda is very much used in the melting of glass it is often made from sea salt 100 lbs. and sulfuric acid 80 lbs. which is called sulphate of soda. Nitrate of soda is a strong flux it is used by some in place of nitrate of pot ash. It is nitric acid 54 soda 32."*

Using pure industrial carbonate of soda with sand and lime enabled the invention and manufacturing of a) large sheets of polished plate glass since the second half of the 19th century; b) drawn flat sheet glass from early in the 20th century; c) float glass since the late 1950s [74].

#### **4. Conclusions**

Alkali-containing salts have been the essential commodity in human life since very ancient times. Ancient civilizations studied to explore the natural resources of this commodity and developed sophisticated technological crafts and industries based on the specific properties of alkali-containing salts. For millennia this development was not based on scientific research and development but a trial-and-error approach. Nevertheless, the practical results of this empirical approach to the invention of the products based on alkali-containing salts were awe-inspiring.

Since the 19th century, synthetic alkali-containing carbonates have accelerated the industrial revolution in soap and washing detergents' production and window glass manufacturing. The invention of industrial alkali carbonate as a leading chemical commodity is fascinating because of the stages it went through; first, exploitation of natural resources for more than three and half millennia, followed by chemical industrial manufacturing for *ca.* one century, and a return to using the natural natron and plant ashes since the second half of the 20th century [84]. Turning back to the natural soda carbonate and potash resources since the late 90s of the 20th century is an obvious result of the crucial ecological approach to soap and glassmaking.

The Big History's holistic approach allows concluding that alkali-containing salts always have been essential for human well-being and highly appreciated raw materials. Thus, the millennia-old knowledge and use of this commodity have always been an authentic technological heritage.

*Ancient and Contemporary Industries Based on Alkali and Alkali-Earth Salts and Hydroxides… DOI: http://dx.doi.org/10.5772/intechopen.99739*

### **Author details**

Rina Wasserman Department of Conservation of Sites and Monuments, Western Galilee College, Acre, Israel

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