with a production surpassing 100 billion 
 liters per year , biofuels have become an important part of the motor 
 fuel consumption in the world , where ethanol continues to be the most 
 common biofuel worldwide . because of its 
 high energy density , ethanol has also been considered one of the most 
 important renewable resources for the production of hydrogen for transportation 
 and stationary power fuel cell applications .
the steam reforming reaction in which ethanol reacts with water ( c2h5oh + 3h2o  2co2 + 6h2 ) has been the subject of study during the past decade as 
 a promising mechanism to produce hydrogen . although these studies have focused 
 on the search for catalysts that minimize the formation of undesirable 
 intermediates and maximize the conversion of ethanol , the presence 
 of water in biofuels also poses the question of how the effectiveness 
 of additives in fuels containing organic substances is affected .
a crucial step toward an answer is to improve our understanding of 
 the interaction between the organic molecules ( the fuel and the additive ) , 
 water , and metallic surfaces ( the fuel pump , the combustion chamber , 
 etc . ) . for this purpose , various theoretical approaches , such as density 
 functional theory ( dft ) , have proven 
 to be useful as they have been able to calculate adsorption energies 
 as well as configurations of various combinations of organic molecules 
 on metallic surfaces . during the past 
 decade , several functionals have been developed to take van der waals 
 ( vdw ) forces into account and additional studies have shown that it is necessary to use dft 
 functionals that include nonlocal correlation in order to describe 
 the adsorption process correctly .
only a few theoretical studies have analyzed the effects of water 
 on the adsorption of organic molecules , including a rigorous treatment 
 of the van der waals interactions .
on an ab initio level , the effects 
 of a water layer on the adsorption of methanol and formaldehyde on 
 pt(111 ) was studied by bonski and lpez , and more recently , a density functional investigation 
 of the adsorption of a water  ethanol mixture on pt(111 ) was 
 carried out by tereshchuk and da silva .
these studies considered a platinum surface because of its catalytic 
 potential ; however , in many industrial and technological applications 
 where the lubrication additives in fuels play an important role , the 
 involved metallic surface is often iron or some alloy derived from 
 it . in gasolines , several types of organic compounds may be present , 
 such as alcohols , organic acids , and branched alkanes .
the interaction 
 between iron and water is a complex chemical process that may lead 
 to dissociation of the water molecule .
however , earlier experiments 
 and theoretical calculations suggest the presence of adsorbed molecular 
 water at high water coverages . to investigate the effect of water on the adsorption of molecules 
 present in fuels and its additives , we simulated the adsorption of 
 isooctane ( 2,2,4-trimethylpentane ) , ethanol , and acetic acid [ with 
 chemical formulas ( ch3)3cch2ch(ch3)2 , ch3ch2oh , and ch3cooh , respectively ] on a water - covered fe(100 ) surface via 
 first - principles calculations .
these adsorbates were selected because 
 isooctane is a representative aliphatic gasoline compound , ethanol 
 can be found in gasoline as a biocomponent as well as in many other 
 relevant products , and acetic acid may be introduced into fuels as 
 a possible trace contaminant of ethanol .
furthermore , the interaction 
 between the acetic acid and the iron surface is representative of 
 the one occurring between fatty acids , commonly found in fuels as 
 lubricity additives , and metallic surfaces .
therefore , 
 the choice of acetic acid minimizes the effect of vdw forces between 
 aliphatic chains and solely focuses on interactions with the surface . 

the various functional groups contained in the selected molecules 
 facilitate the investigation of their distinct effects on the adsorption 
 process .
the fe(100 ) surface , besides being present in systems of 
 industrial interest , has been considered in various previous studies 
 that investigate the adsorption of water on iron .
therefore , the selection of this surface 
 is advantageous for comparing and validating our results . a formal 
 description of van der waals interactions as implemented in the optb86b - vdw 
 functional , which is limited to pairwise 
 interactions but it is otherwise independent of external input parameters , 
 is applied .
the approaches that go beyond the pairwise additivity 
 and include the vdw energy very accurately , such as the random phase 
 approximation combined with 
 the adiabatic connection and fluctuation dissipation theorem , are computationally prohibitive for the systems treated in this 
 work .
this paper describes the changes in the adsorption energy and 
 equilibrium geometries induced by a water layer and discusses these 
 alterations on the basis of the electronic density .
to perform our simulations , we carried out spin - polarized 
 first - principles calculations within the framework of density functional 
 theory using the vienna ab initio simulation package ( vasp )
. vasp produces an iterative solution of the kohn  sham equations 
 using a plane - wave basis and employing periodic boundary conditions . 

the projector augmented wave ( paw ) method was applied to describe the interaction between the core and the 
 valence electrons .
the vdw interactions were taken into account via 
 the optimized becke86 van der waals ( optb86b - vdw ) 
 exchange 
correlation functional,1where n is the electronic 
 density in the ground state , eoptb86b(x)[n ] is a reparametrized version of the becke86 exchange 
 functional , elda(c)[n ] is a local - density approximation ( lda ) correlation , and enl(c)[n ] is a nonlocal correlation 
 term that approximates the vdw interactions . to model the adsorption 
 of various molecules on an fe(100 ) surface
, we constructed a slab 
 consisting of 10 layers of body - centered cubic ( bcc ) fe and a corresponding 
 molecule placed on one side of the slab at a distance d above it . in modeling the adsorption of water molecules , ( 1  
 1 ) , ( 2  1 ) , and ( 2  2 ) surface cells were studied , placing 
 one , two , and four water molecules , respectively .
this allows for 
 more flexibility in determining the water layer structure by analyzing 
 various possible scenarios , such as those where all the water molecules 
 are constrained to the same geometry [ ( 1  1 ) cell ] and either 
 two [ ( 2  1 ) cell ] or four [ ( 2  2 ) cell ] different orientations 
 of the water molecules are allowed . in the adsorption of organic molecules 
 a (
the vacuum spacing in 
 the z - direction between repeated iron slabs was 29.73 
  .
the z axis is perpendicular to the metallic 
 surface and starts at the bottom layer of the iron slab .
this setup 
 accurately models a bcc fe(100 ) surface , avoids slab  slab interactions , 
 and , in the cases were a single molecule was adsorbed , also avoids 
 intermolecular interactions .
the lattice constant of 2.83  
 used to construct the iron slab was obtained from a bulk calculation 
 for bcc iron using the pbe functional , where the 
 calculation parameters were chosen to keep the accuracy consistent 
 with the rest of the computations .
this value is in good agreement 
 with the experimental lattice constant of 2.86  .
the distance d was defined 
 as the vertical distance between the atom of the molecule with lowest z coordinate and the closest atom to it , whether it is an 
 atom belonging to the water layer ( if present ) or to the iron surface . 
 for the dissociated acetic acid
, the distance d is 
 measured between the acetate and the metallic surface . to ensure 
 sufficiently accurate total energies and forces , we carefully selected 
 and tested our calculation parameters . a tight convergence criterion 
 of 10 ev on the total energy in the self - consistency 
 cycle was used and a cutoff energy of 400 ev was applied for the plane - wave 
 basis set . for the calculations involving a ( 4  4 ) surface cell , 
 the k - space integrations were performed using a 2 
  2  1 monkhorst  pack mesh . for the smaller cells used to model the adsorption of water molecules , 
 a corresponding mesh that generates the same k - point 
 density was applied . the tetrahedron method with blchl corrections 

was employed for the static calculations and a gaussian smearing with 
 a width of 0.2 ev for the relaxations .
test calculations with smaller 
 widths were carried out to ensure that the selected value leads to 
 the ground - state geometry . the conjugate gradient algorithm was used 
 to relax most of the structures , allowing the ions to move until an 
 energy convergence criterion of 10 ev was fulfilled . 
 for those cases where the starting guess was unreliable , a damped 
 molecular dynamics algorithm was preferred . during relaxations , the 
 ions in the top four layers of the slab and the ones constituting 
 the molecule and the water layer ( if present )
were allowed to move 
 in all directions , while the atoms in the six remaining layers of 
 the slab were frozen at bulklike positions . due to the complexity 
 of the system ,
only the relaxations involving the water molecules 
 and the iron slab were verified by performing a simulated annealing . 

the simulation started at a temperature of 300 k and the temperature 
 was gradually decreased to 0 k in 10 ps ( 10  000 time steps ) .
to produce an accurate description of the interaction between the 
 organic molecules treated in this work and the iron surface , we calculated 
 the potential energy curves by varying d and subsequently 
 computing the total energy of the resulting system via static calculations . 

these curves will aid in the fitting of potentials required for classical 
 molecular dynamic simulation , allowing the study of these systems 
 in a different time and length scale .
the equilibrium distance was 
 considered to be the value of d that minimizes the 
 total energy of the system .
we stress that this equilibrium distance 
 is obtained from a finite set of energy points calculated for various 
 values of d , and therefore , its accuracy depends 
 on the step size ( d ) between those values . 
 to obtain a first estimate of the equilibrium structure , the potential 
 energy curves of different configurations and adsorption sites were 
 calculated with moderate values of d .
the 
 adsorption energies were calculated according to the equation2where etotmol+fe(100 ) is the total energy of the products adsorbed on the fe(100 ) slab 
 at the equilibrium distance , etotfe(100 ) is the total energy of 
 the clean fe(100 ) slab , and etotmol is the total energy of the 
 isolated gas - phase educts calculated using a cubic box with 15  
 side length . in the presence of a water layer,3was used 
 instead . in this latter equation etoth2o+fe(100 ) is the 
 total energy of the water - covered fe(100 ) slab , etotmol+h2o+fe(100 ) is the total energy of the products adsorbed on the 
 water - covered fe(100 ) surface at the equilibrium distance , and etotmol is the energy of the gas - phase educts as indicated above . because 
 of the magnitude of the step size , the adsorption energies and equilibrium 
 distances obtained in this way are only an approximation , and thus , 
 they will be referred to as such in the rest of this paper .
the resulting 
 most favorable configuration of this analysis was relaxed and used 
 as an input to calculate a more accurate adsorption energy and equilibrium 
 distance using a smaller value of d .
in the equilibrium configuration , the isooctane molecule 
 is oriented with its longest continuous chain of carbon atoms parallel 
 to the fe(100 ) surface ( figure 1a  c ) . 

a similar result was found in our previous investigation , where the effects of vdw interactions on the 
 adsorption of isooctane and ethanol on the same surface were analyzed . 
 in the present study , however , the adsorption geometry plays a more 
 important role , and therefore , we analyzed three orientations of the 
 isooctane molecule ( figure 1a , c , d ) in addition 
 to the ones considered in ref ( 12 ) . for comparison purposes
, we included in our calculations 
 the orientation of the isooctane molecule in the previously determined 
 equilibrium configuration ( figure 1b ) .
in each 
 configuration , we studied the isooctane molecule adsorbed on the top , 
 bridge , and hollow sites .
the difference between the estimated adsorption 
 energies of configurations 13 (  070 mev ) indicates 
 that any of these may be reached at room temperature (  25 
 mev ) and that no particular adsorption site is preferred ( see the supporting information ) .
in contrast , the adsorption 
 energies of configuration 4 are at least 162 mev lower than those 
 calculated for the other configurations , although among them they 
 did not show a preference for an adsorption site either .
this is to 
 be expected since , as we have shown , the 
 binding of the isooctane molecule to the metallic surface is dominated 
 by dispersion forces , and thus , an orientation that maximizes the 
 number of atoms in proximity to the surface is favored .
we selected 
 the adsorption configuration analyzed in our previous study , namely , 
 configuration 2 with isooctane adsorbed on the top site , to carry 
 out the rest of this study , because it is among the possible configurations 
 existing at room temperature and also because we can apply our previous 
 findings of the adsorption process . the lowest energy configuration 
 ( configuration 1 )
, the isooctane molecule is adsorbed at a distance d  2.25  and orients its main c 
its branches 
 constituted by methyl groups are positioned away from the iron slab 
 and above their parent chain ( figure 1a ) .
configurations 
 considered for the adsorption of an isooctane molecule on an fe(100 ) 
 surface , here displayed with the molecule on the top adsorption site 
 at a distance d = 2.00  .
the top row displays 
 the top view of the system , while the bottom row shows the side view . 

the atom marked with an x symbol in the top view of configuration 
 1 is used as a reference to orient the molecule on the top , bridge , 
 and hollow sites , indicated in the same figure with a circle , triangle , 
 and square , respectively .
when ethanol is adsorbed on an fe(100 ) surface , the hydroxyl 
 group of the molecule orients parallel to the surface , while the remaining 
 atoms of the molecule arrange accordingly ( figure 2a , c ) .
just as with isooctane , we extended our previous study by considering three additional configurations 
 of an ethanol molecule adsorbed on an fe(100 ) surface ( figure 2a , b , d ) .
as before , we included the orientation of 
 the ethanol molecule found in ref ( 12 ) ( figure 2c ) and we considered 
 the molecule adsorbed on the top , hollow , and bridge sites in all 
 configurations .
in contrast to isooctane , the calculated adsorption 
 energies are distributed among a larger energy range , and with the 
 exception of configuration 4 , there is notable energy difference between 
 the adsorption sites ( see the supporting information ) .
the largest adsorption energy is achieved when the ethanol molecule 
 is adsorbed on the top site and is oriented as in configuration 3 , 
 in agreement with our previous finding .
our calculations show that this configuration is degenerate in energy 
 with configuration 1 when the ethanol molecule is adsorbed on the 
 top site . for the rest of the study , we considered configuration 2 
 in addition to these equilibrium structures , since a strong interaction 
 between the hydroxyl group and the water molecules is expected .
configurations 
 considered for the adsorption of an ethanol molecule on an fe(100 ) 
 surface , here presented analogously to figure 1 .
the atom marked with an x symbol in the side view of configuration 
 1 ( the oxygen atom of the hydroxyl group , colored in green ) is used 
 as a reference to orient the molecule in the adsorption sites .
if the carboxylic hydrogen of 
 the acetic acid dissociates , the resulting acetate radical is adsorbed 
 in a bidentate configuration .
experimental studies have suggested 
 that the carboxylic hydrogen of the acetic acid molecule gets easily 
 detached in the vicinity of clean metal , and metal oxide , surfaces .
for the dissociated molecule , these studies together with theoretical 
 calculations for the adsorption on ge(001 ) and fe(110 ) surfaces have proposed 
 monodentate and bidentate adsorption configurations . moreover , in 
 the latter study , no appreciable adsorption of the nondissociated 
 acetic acid on an fe(110 ) surface was found because dispersion forces 
 were not included in the calculations . on the basis of these results
, 
 we considered the bidentate ( figure 3b ) and 
 the monodentate ( figure 3c ) configurations 
 for the adsorption of acetate , and since we are employing a vdw density 
 functional for all computations , we also included the adsorption of 
 the nondissociated acetic acid ( figure 3a ) . 
 as was done with the previous compounds ,
the adsorption energies of 
 the molecule adsorbed on the top , hollow , and bridge sites in each 
 configuration were calculated .
depending on the nature of the carboxylic 
 hydrogen dissociation , an acetate radical ( homolytic cleavage ) or 
 an acetate anion ( heterolytic cleavage ) can be formed . a previous 
 investigation on the adsorption of acetic acid on an fe(110 ) surface showed that , in the adsorbed state , the acetate 
 radical is energetically favored over the anion . the results of our 
 calculations show that this is also valid for the adsorption on the 
 fe(100 ) surface .
the estimated adsorption energy of the acetic acid 
 of 452 mev indicates that the molecule binds to the surface .
this 
 binding is nevertheless weak in comparison to those of the dissociated 
 acetic acid species ( table 1 ) .
the highest 
 adsorption energy is reached when the acetate radical is adsorbed 
 in the bidentate configuration on the top site of the fe(100 ) surface 
 ( see the supporting information ) , and thus , 
 we selected this configuration to carry out further calculations .
configurations 
 considered for the adsorption of acetic acid and acetate on an fe(100 ) 
 surface , here presented analogously to figure 1 .
the atom with an x symbol in the side view of configuration 1 is 
 used as a reference to orient the molecule on the adsorption sites . 

the two initial positions considered for the dissociated hydrogen 
 are indicated as h1 ( top ) and h2 ( hollow ) . before introducing the water layer ,
a better approximation 
 to the equilibrium structures and adsorption energies was obtained 
 by relaxing the selected configuration of each system mentioned in 
 this section .
each resulting structure was then used to calculate 
 the potential energy curve by displacing the molecule in smaller steps . 

no large variations from the relaxed structure are expected at separations 
 equal to or greater than the equilibrium distance .
indeed , further 
 relaxations carried out for a few selected distances ( d ) to verify this assumption show variations on the order of 10  in bond lengths and 10 deg in angles .
since after the extended analysis no new configuration 
 of isooctane and ethanol with a considerably higher adsorption energy 
 was found , here we simply repeat the previously reported equilibrium 
 distances and adsorption energies ( table 1 ) 
 calculated in a previous study .
this 
 facilitates the comparison of these values with those calculated in 
 the presence of a water layer . after relaxing the acetate system , 
 the oxygen atom on the top adsorption site
is displaced by 0.08  
 toward the bridge site ( moving the rest of the molecule accordingly ) 
 and the distance between the oxygen atoms is reduced by 0.03  . 
 the iron atoms situated directly below the two oxygen atoms of the 
 acetic acid are displaced 0.05  in the direction of the molecule . 
 in equilibrium
, the molecule is adsorbed on the slab at a distance 
 of 2.00  , with an energy of 4.49 ev . at first sight
, our results 
 may suggest that the slab ( and not the molecule ) determines the equilibrium 
 distance , since all the molecules are adsorbed at a distance of 2.00 
  .
however , as mentioned before , these distances are an approximation 
 that depends on the length of the step in d used 
 for the static calculations .
if the energy varied rapidly with d , we used a value of d = 0.125 
  ; otherwise , we set d = 0.25  . to study the dissociated acetic acid ,
the adsorption of the acetate radical considered so far is useful 
 for the construction of potentials that can be applied in classical 
 molecular dynamics calculations , as stated before .
this molecule fragment , 
 however , does not exist under ordinary experimental conditions , and 
 therefore , the hydrogen atom can not be neglected . in a recent study , 
 where the adsorption of acetic acid on tio2 was analyzed , it is indicated that there are three possible 
 reaction paths for the dissociated hydrogen : formation of hydrogen 
 gas , adsorption on the substrate , or reaction with a third substance . 

other possible phenomena , such as the diffusion of hydrogen into the 
 bulk ( hydrogen embrittlement ) , are outside the scope of the present 
 study , and thus , they are not discussed further .
the formation of 
 hydrogen gas was considered by introducing a h2 molecule 
 in the supercell shown in figure 3b , followed 
 by a relaxation of the ionic positions . in the equilibrium geometry , 

the hydrogen molecule is adsorbed on a top site with its molecular 
 axis parallel to the surface .
the adsorption of the dissociated hydrogen 
 on the substrate was simulated by introducing a single hydrogen atom 
 at the positions h1 and h2 marked on figure 3b ( one at a time ) and relaxing the cell afterward .
the hydrogen atom , 
 initially in the h1 position , was displaced to a bridge site during 
 the relaxation , while the one initially on the h2 position remained 
 close to the hollow site , with a displacement of 0.4  away from 
 the molecule . with an energy difference of 0.09 ev , the h atom on 
 the bridge site is energetically more favorable than on the hollow 
 site .
depending on the availability of a second hydrogen atom , 
 the dissociated hydrogen of the acetic acid may be adsorbed on the 
 substrate or released as hydrogen gas .
the comparison between the 
 binding energies ( table 1 ) of these systems 
 suggests that the adsorption of hydrogen on the substrate is favored 
 over the formation of molecular hydrogen .
the adsorption energy , 2.15 
 ev , shown in table 1 , was calculated by assuming 
 that each h atom comes from a dissociated acetic acid molecule ( i.e. , 
 2ch3cooh  2ch3coo + h2 ) . 
 to facilitate the comparison with the other adsorption energies , the 
 resulting value is reported in electronvolts per acetic acid molecule . 

if , however , we assume that a h atom is freely available ( i.e. , ch3cooh + h  ch3coo + h2 ) , the 
 adsorption energy increases to 4.79 ev . on a clean surface , a hydrogen 
 atom can be first adsorbed on the substrate . in this case , in order 
 for the formation of a h2 molecule to take place , the energy 
 gained from the formation of the h 
h bond ( 4.52 ev ) and the adsorption of the resulting h2 molecule has to outmatch the energy of the fe 
h bonds ( 2.60 
 ev ) . since the absorption of h2 is weak ( 0.11 ev ) and two fe 
this idea was further investigated by constructing a supercell with 
 the acetate , as in figure 3b , and two hydrogen 
 atoms , one on the h2 position and a second one in the closest bridge 
 site to it . during the relaxation of this system
the distance between 
 these two hydrogens increased until both atoms were adsorbed on hollow 
 positions .
moreover , the total energy of this system is 0.55 ev lower 
 than the energy of the acetate + h2 system , indicating , 
 once again , that in this case the adsorption of h on the metallic 
 surface is favored over the formation of hydrogen gas . on the other 
 hand , if a second hydrogen is available at a relatively low energy 
 cost , for example , from a hydronium cation , the considerable adsorption 
 energy mentioned above indicates that the formation of hydrogen gas 
 is plausible .
the calculated 
 adsorption energy strongly depends on the source of h. see the text 
 for further details .
for the adsorption of molecular water on an fe(100 ) 
 surface , the formation of a monolayer where the water molecules alternate 
 orientations to form a  zigzag  pattern ( figure 4a ) is energetically favorable .
the water molecules 
 are adsorbed on the top sites of the metallic surface ( but slightly 
 shifted toward the bridge site ) , in agreement with previous studies 
 where the adsorption of a single water molecule was considered .
this suggests that the adsorption site preference is not modified 
 by this high water coverage .
this may be caused by the reasonably 
 good match between the calculated lattice constant of bcc fe ( 2.83 
  ) and the average distance between oxygens in water ( 2.81  ) . in this configuration
there are two nonequivalent 
 water molecules , one with both o  h bonds nearly parallel to 
 the surface ( from here on , referred to simply as parallel ) and the 
 other with one o 
h bond pointing to the slab ( referred to as 
 upward ) . for the former ,
the distance between the oxygen and the closest 
 iron atom is 2.28  and for the latter , 3.35  . besides 
 the orientation , the only additional difference between the upward 
 and parallel water molecule is the bond angle .
the parallel water 
 molecule has a bond angle of 105.36 , and in the upward molecule , 
 the bond angle is equal to 104.20. to determine this structure , 
 several configurations ( figure 4 ) were relaxed 
 with diverse algorithms . in these configurations ,
the orientation 
 of the water molecules and their respective adsorption sites were 
 varied . in a different approach , a simulated annealing in a supercell 
 containing four water molecules adsorbed in a ( 2  2 ) surface 
 cell results in an almost  zigzag  configuration which 
 is , nonetheless , higher in energy ( + 266 mev ) .
the main differences 
 in geometry are a higher distance between the slab and the upward 
 water molecules ( 4.30  ) as well as the tilting of one o 
some of the various configurations 
 considered for the water layer and their difference in energy with 
 respect to the lowest energy configuration .
the various supercells 
 employed for the simulations are here replicated along the axes parallel 
 to the surface to facilitate their comparison . in the proposed equilibrium water layer ( figure 4a ) , the average adsorption energy of each water 
 molecule is 0.51 ev .
the average adsorption energy per molecule was 
 calculated according to the following equation4where etoth2o+fe(100 ) is the total energy of the water molecules 
 adsorbed on the fe(100 ) slab , etotfe(100 ) is the total energy of 
 the clean fe(100 ) surface , and etoth2o is the total energy 
 of the isolated water molecule .
the difference between the average 
 adsorption energy per water molecule of this layer and the one calculated 
 for the single water molecule with gga ( 0.39 ev ) can be attributed to the lateral intermolecular interactions 
 and to the fact that dispersion forces are included in our calculation .
the previously found equilibrium geometry of isooctane 
 ( figure 1b ) was placed on top of the fe(100 ) 
 + zigzag water system ( figure 4a ) .
because 
 of the alternating orientation of the water molecules in the zigzag 
 layer , there exists two possible top adsorption sites , one covered 
 with a parallel water molecule and the other with an upward water 
 molecule .
the difference in unrelaxed adsorption energies ( 75 mev ) 
 between these sites shows no significant preference for one site over 
 the other .
nevertheless , the equilibrium geometry of isooctane adsorbed 
 on a water - covered fe(100 ) surface was obtained by positioning the 
 longest continuous carbon chain of the molecule along the slightly 
 more stable site , which in this case is the one covered with a parallel 
 water molecule , and relaxing the system afterward .
the resulting equilibrium 
 structure ( figure 5 ) exhibits small geometrical 
 changes in the isooctane molecule .
the variation of bond lengths are 
 on the order of 10  , and in angles the largest 
 changes are of the order of 10 deg .
there is , 
 however , a more notable displacement of the water layer and the first 
 layer of the slab away from the molecule (  0.15 and 0.10 
  , respectively ) .
calculated adsorption energy of the isooctane and ethanol 
 molecules adsorbed on an fe(100 ) surface ( with and without a water 
 layer ) as a function of the vertical distance .
a water layer reduces the adsorption energy of isooctane 
 on fe(100 ) by 0.19 ev ( table 1 ) . in the absence 
 of a water layer
, the isooctane molecule induces a nonisotropic accumulation 
 of charge between the slab and the molecule and a deficit of charge 
 in the region surrounding the molecule ( figure 7a ) .
the forces 
 arising from this redistribution of charge lead to binding . with the 
 introduction of a water layer ,
the charge redistribution in the slab 
 caused by the proximity of the isooctane molecule is considerably 
 reduced ( figure 7b ) . the water molecules
the interaction between them , and consequently , the region surrounding 
 the molecule also presents a lesser charge deficit , leading to weaker 
 binding . the lesser charge redistribution in the water layer combined 
 with its screening effects allows the isooctane molecule to form a 
 shorter bond to the water layer than to the clean iron surface ( figure 6 ) .
comparison of the charge 
 density difference ( diff ) of isooctane adsorbed 
 on the bcc fe(100 ) surface at the equilibrium distance with and without 
 a water layer . in the absence of a water layer ,
the charge density 
 difference is defined as diff =   
 ( isooctane + fe(100 ) ) , where  
 denotes the charge density of the isooctane adsorbed on fe(100 ) , while 
 isooctane and fe(100 ) represent 
 the charge densities of the isolated molecule and clean fe(100 ) slab , 
 respectively . in the presence of a water layer ,
the definition changes 
 to diff =   ( isooctane + fe(100)+wl ) , where fe(100)+wl is the charge density of the water - covered fe(100 ) slab .
the charge 
 difference is plotted in a plane perpendicular to the surface for 
 values between 5  10 electrons /  ( solid blue , deficit ) and 5  10 electrons /  ( solid red , accumulation ) .
the presence of a water layer favors a different 
 adsorption configuration of the ethanol molecule ( figure 8) . since a relatively strong interaction between 
 the hydroxyl group of the ethanol and the water layer
is expected , 
 an adsorption analysis was carried out taking configurations 13 
 ( figure 2a  c ) into account .
the ethanol 
 molecule of each configuration was placed above a water - covered fe(100 ) 
 surface and subsequently relaxed ( figure 9 ) . 
 in all cases
the top adsorption site covered with a parallel water 
 molecule was preferred . in the presence of water ,
the adsorption energy 
 of configuration 2 is approximately 5 mev larger than the corresponding 
 one for configuration 3 , which is one of the equilibrium configurations 
 in the absence of water .
the stabilization of this structure is likely 
 caused by the hydrogen bond between the oxygen of the water molecules 
 and the hydroxyl group of ethanol .
the small energy difference suggests , 
 nevertheless , that no particular orientation is favored at room temperature .
configurations considered for the adsorption 
 of an ethanol molecule on a water - covered fe(100 ) surface , here displayed 
 with the molecule on the top adsorption site at a distance d = 2.00  .
the top row displays the top view of the 
 system , while the bottom row is the side view .
the adsorption energy of ethanol on fe(100 ) decreases by 
 0.51 ev in the presence of a water layer ( table 1 ) .
a previous study showed 
 that ethanol binds to the fe(100 ) surface via dispersion forces and 
 a weak electrostatic interaction between the oxygen of the hydroxyl 
 group and the fe(100 ) surface .
these interactions , together with the 
 pauli repulsion , lead to a charge redistribution , where the electron 
 density in a region surrounding the hydroxyl changes ( figure 10a ) . with the introduction of a water layer , the 
 size of this region decreases significantly ( figure 10b ) .
furthermore , since the redistribution of charge in the 
 top layer of the slab was mainly caused the hydroxyl group of the 
 ethanol molecule , the water layer screens its effect more efficiently . 
 as a consequence ,
only the iron atom directly below the hydroxyl group 
  feels  the interaction . the combination of these effects 
 results in a weaker bond .
comparison of the charge density difference 
 ( diff ) of ethanol adsorbed on the bcc fe(100 ) surface 
 at the equilibrium distance with and without a water layer .
the charge 
 density difference is defined and plotted analogously to figure 7 . with the introduction 
 of a water layer ,
the adsorption configuration of the acetate molecule 
 is moderately changed ( figure 11 ) . as a first 
 approximation to determine the preferred adsorption site ,
the energy of acetate 
 adsorbed on the top site covered with a parallel water molecule is 
 0.21 ev lower than the one covered with an upward water molecule . 
 relaxing the more stable structure produced notable changes in both 
 the acetate molecule and the water layer .
o angle 
 is reduced by 0.17 , and the c  c bond is tilted by 12.13 
 with respect to the surface normal . on the water layer ,
h bonds to 
  point  toward the closest oxygen of the acetate molecule . 
 this can be attributed not only to the tendency of hydroxyl and oxygen 
 to form hydrogen bonds but also to the higher chemical reactivity 
 of the acetate caused by the odd number of valence electrons .
equilibrium 
 structure of acetate adsorbed on a water - covered fe(100 ) surface . a water layer covering an fe(100 ) 

surface increases the adsorption energy of the acetate molecule by 
 0.69 ev ( table 1 ) . without the water layer
, 
 the region surrounding the oxygen atoms of the acetate molecule accumulates 
 a significant amount of charge ( figure 12a ) 
 while decreasing the charge in the region above the slab .
the attractive 
 forces produced by the formation of regions of accumulation and deficit 
 of charge are responsible for the binding between the acetate and 
 the fe(100 ) surface .
the binding mechanism remains essentially the 
 same after introducing the water layer , with the only difference being 
 that the region where the charge is decreased now lies on the water 
 molecules below the acetate molecule ( figure 12b ) .
moreover , the introduction of a water layer allows the formation 
 of hydrogen bonds between the oxygen atoms of the acetate and the 
 hydroxyl group of the water molecules , which may account for the increase 
 in the adsorption energy and decrease in the equilibrium distance 
 ( figure 14 ) .
comparison 
 of the charge density difference ( diff ) of acetate 
 adsorbed on the bcc fe(100 ) surface at the equilibrium distance with 
 and without a water layer .
the charge density difference is defined 
 and plotted analogously to figure 7 but with 
 the charge density difference between values of 5  10 and 5  10 electrons /  , which are 1 order of magnitude larger than the plotted diff of isooctane and ethanol .
equilibrium structure of the dissociated acetic acid adsorbed on 
 a water - covered fe(100 ) surface .
calculated adsorption energy of acetate adsorbed on an fe(100 ) surface 
 ( with and without a water layer ) as a function of the vertical distance .
the presence of a water layer 
 increases the binding energy of the dissociated acetic acid and promotes 
 the formation of hydrogen gas . to study the acetic acid ,
fe(100 ) system 
 described above and placed on top of an upward water molecule on the 
 position analogous to h1 in the water - free system ( figure 3b ) . during the relaxation of this system , the dissociated 
 hydrogen and a hydrogen of its closest parallel water molecule ( in 
 direction away from the acetate ) react to produce hydrogen gas .
this considerable 
 amount of energy and the fact that this equilibrium state was reached 
 through an ionic relaxation indicate that other reaction paths for 
 the dissociated hydrogen are questionable .
the resulting adsorption 
 energy is 0.70 ev larger than the corresponding system without the 
 water layer ( table 1 ) , where the dissociated 
 hydrogen is adsorbed on the metallic surface . the water molecules 
 of the layer help to stabilize the resulting acetate and hydroxyl 
 group by reorienting themselves to facilitate the formation of hydrogen 
 bonds with them ( figure 13 ) .
the effect of a water layer on the adsorption of organic molecules 
 on an fe(100 ) surface strongly depends on the type of bond between 
 them . in those molecules , where dispersion forces contribute significantly 
 to the binding mechanism , the water layer has a stronger screening 
 effect .
additionally , this effect can be enhanced by the presence 
 of polar functional groups in the molecule .
the type of functional 
 group plays also an important role in defining the geometry of the 
 equilibrium configurations ; if they contribute to the formation of 
 hydrogen bonds , a different adsorption geometry may be stabilized 
 through them , as in the case of ethanol , and an increase in the adsorption 
 energy can take place like for acetic acid . with the introduction 
 of a water layer , there is a moderate reduction of the adsorption 
 energy of isooctane on fe(100 ) but no significant change in the equilibrium 
 geometries . in comparison
, ethanol exhibits a larger decrease of the 
 adsorption energy and a change in the adsorption configuration . on 
 the other hand ,
the adsorption energy of acetic acid is increased 
 and there is a moderate change in the equilibrium geometry .
. 
 for instance , as long as the same corresponding binding mechanism 
 takes place , alkanes , alcohols with short carbon chains , and ( short 
 chain ) carboxylic acids may exhibit similar effects in the adsorption 
 energies and equilibrium geometries with the introduction of a water 
 layer as isooctane , ethanol , and acetic acid , respectively .
the increase in production and use of fuels containing biocomponents 
 has triggered a considerable amount of research focused on improving 
 their combustion efficiency .
as our results demonstrate , it is also 
 important to consider how these substances interact with surfaces 
 and how they affect such interactions of components already present 
 in conventional fuels . in particular
, we showed that the presence 
 of water may have an undesirable ( or desirable ) impact for substances 
 whose usefulness is directly related to their adsorption properties , 
 as in the case of lubricity additives .
we anticipate that the potential 
 energy curves produced will aid in the fitting of accurate surface 
lubricant 
 interaction potentials used by other methods , such as classical molecular 
 dynamics simulations , where the influence of additional variables 
 can be analyzed to improve our description and understanding of these 
 lubricated systems .
 the 
 presence of water in biofuels poses the question of how it affects 
 the frictional performance of additives in fuels containing organic 
 substances . to investigate the effect of water on the adsorption of 
 molecules present in fuel and its additives we simulated within the 
 framework of density functional theory the adsorption of ethanol , 
 isooctane ( 2,2,4-trimethylpentane ) , and acetic acid on a bare and 
 a water - covered fe(100 ) surface . 
 van der waals interactions are taken 
 into account in our computations . in those molecules , where dispersion 
 forces contribute significantly to the binding mechanism 
 , the water 
 layer has a stronger screening effect . 
 additionally , this effect can 
 be enhanced by the presence of polar functional groups in the molecule . 
 
 thus , with the introduction of a water layer , the adsorption energy 
 of isooctane and ethanol is reduced but it is increased in the case 
 of the acetic acid . 
 the adsorption configuration of ethanol is changed , 
 while the one of acetic acid is moderately , and for isooctane only 
 very slightly altered . 
 therefore , the effect of a water layer in the 
 adsorption of organic molecules on an fe(100 ) surface strongly depends 
 on the type of bond and consequently , so do the tribological properties . 
Introduction
Computational
Details
Results
and Discussion
Conclusion