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Surface Hydrophilicity-Dependent Water Adsorption on Mixed Self-Assembled Monolayers of C7CH3 and C7COOH Residues. A Grand Canonical Monte Carlo Simulation Study Milan Sz€ori,† Martina Roeselova,‡ and Pal Jedlovszky*,§,||,^ †
Department of Chemical Informatics, Faculty of Education, University of Szeged, Boldogasszony sgt. 6, H-6725 Szeged, Hungary Center for Biomolecules and Complex Molecular Systems, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nam. 2, 16610 Prague 6, Czech Republic § Laboratory of Interfaces and Nanosize Systems, Institute of Chemistry, E€otv€os Lorand University, Pazmany P. Stny 1/A, H-1117 Budapest, Hungary HAS Research Group of Technical Analytical Chemistry, Szt. Gellert ter 4, H-1111 Budapest, Hungary ^ EKF Department of Chemistry, Leanyka utca 6, H-3300 Eger, Hungary
)
‡
ABSTRACT: Grand Canonical Monte Carlo (GCMC) simulations are used to study the adsorption and organization of gas-phase water molecules on two-component self-assembled monolayers (SAMs) of eight-carbon alkanethiols bound to a flat virtual carrier, as the character of the SAM surface is changing from completely hydrophobic to completely hydrophilic by randomly replacing methyl-terminated (C7CH3) alkanethiol chains with carboxylic acid-terminated (C7COOH) chains. At low chemical potentials (low relative humidity), a synergistic effect of nearby COOH functional groups on water adsorption is observed. In particular, clusters of nearby surface COOH groups are found to attract considerably more water molecules than the same amount of COOH groups if they are isolated from each other. By promoting lateral waterwater interactions, such surface COOH clusters thus act as condensation nuclei for water. With increasing water chemical potential, the possibility of the formation of new waterwater hydrogen bonds gradually becomes an increasingly important driving force of the adsorption, in addition to the formation of new waterCOOH hydrogen bonds. Further, as the relative humidity increases, the growing number of waterwater hydrogen bonds clearly overcompensates the effect of the decreasing average number of waterCOOH hydrogen bonds per (first layer) water molecules. These findings are supported by the adsorption isotherms, by the preferential orientation of the surface COOH groups and first layer water molecules, and by the binding energy distributions of the water molecules being in direct contact with the SAM surface as obtained from the simulation. The atmospheric relevance of our results is also considered.
1. INTRODUCTION Atmospheric aerosols are known to influence both climate and air quality via scattering visible radiation and by providing sites for condensation and surface-mediated heterogeneous reactions.1 For instance, aerosol surfaces can serve as cloud condensation nuclei on which the water vapor can condense to form cloud droplets.2 The wettability of atmospheric aerosols is sensitive to a wide variety of parameters, such as the size or shape of the particle.35 Chemical composition is also one of these crucial factors, although it can change in time (“aging”) due to the presence of the oxidizing agents in atmosphere.6 Oxidation of the larger volatile organic compounds (VOCs) yields products that can be less volatile and more polar than the precursor molecule.7 Consequently, these organic species, which have sufficiently low volatility, can partition to aerosol particles leading to secondary organic aerosol formation. While the formation of carboxylic (COOH) groups through these processes has been detected by several experimental techniques, such as the tandem mass spectrometry method,8 its maturation mechanism is still the subject of debate. r 2011 American Chemical Society
Because of the presence of interfacial oxygen-containing molecules, these oxidized aerosol particles tend to become progressively more hydrophilic.9,10 Indeed, polar surface groups can dramatically affect the adsorption of polar species, such as water.5,11 In addition, the presence of water initiates additional heterogeneous chemical processes on the surfaces of the particles.12,13 Thus, a detailed description of the changes in the interaction of water with organic surfaces is required for a better understanding of the fundamental atmospheric chemical processes. Because of the complex behavior of aerosol particles in adsorption processes, these effects have to be studied separately to identify their contribution. To elucidate the impact of chemical composition of the surface on water adsorption, a self-assembled monolayer (SAM) can be employed as a model to represent an organic layer adsorbed on an inorganic core.14,15 Received: February 7, 2011 Revised: August 8, 2011 Published: August 16, 2011 19165
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The Journal of Physical Chemistry C By changing the terminal group of the SAM surface, the “age” (i.e., the degree of oxidation) of the organic surface can be mimicked. While the SAM consisting of methyl-terminated residues is a model for hydrophobic (unoxidized) surfaces, the oxidized surface element can be represented, for example, by a carboxyl-terminated residue. Despite the clear atmospheric importance of understanding the water adsorption ability of differently aged organic surfaces, and quantifying the dependence of adsorption on the relative humidity, experimental data concerning the water adsorption isotherm on various, partially oxidized SAMs are scarce. In their early work, Herdt et al. used quartz crystal microbalance to measure the adsorption isotherm of water on a SAM built up solely by COOH-terminated C15 alkyl chains (C15COOH SAM).16 They found that the adsorption capacity of the SAM can reach about three equivalent monolayers before water condensation occurs. However, in their recent ellipsometry measurement, Tiani et al. observed condensed water films of only about one equivalent water monolayer on C11COOH and C11OH SAMs in the presence of saturated water vapor.17 Further, they found no significant amount of condensed water at the surface of the hydrophobic, unaged C11CH3 SAM.17 Similar results were obtained by Moussa et al. using transmission Fourier transform infrared spectroscopy (FTIR) for hydrophilic surfaces, as they detected a water film of about 1.5 equivalent monolayers at the surface of a mixed SAM containing 90% C8COOH and 10% C8CH3 residues at 90% relative humidity.18 However, contrary to the results of Tiani et al.,17 they also found traces of adsorbed water on purely hydrophobic surfaces.18 The wettability of multicomponent SAMs has also been studied experimentally by temperature programmed desorption.1921 Nevertheless, from a purely experimental point of view, it is still impossible to selectively study the influence of the type, concentration, and location of hydrophilic/hydrophobic sites on adsorption, as well as separating these effects from other features (such as surface defects) of the SAM. However, molecular simulation methods can provide an excellent tool for this purpose.22 Experimental studies can be, in general, well complemented by computer simulation investigations because in a computer simulation a three-dimensional picture of an appropriately chosen model of the system of interest can be obtained at atomistic resolution. Indeed, water films on various partially oxidized mixed SAMs have been simulated several times by several research groups using the method of molecular dynamics.21,2326 However, regarding the adsorption, these simulation studies all suffer from the severe problem that the amount of water that is present at the surface has to be determined a priori, and the stability of these systems consisting of an arbitrary amount of water cannot be guaranteed. In other words, these systems might not correspond to any realistic situation, as the physicochemical parameters controlling water adsorption (i.e., water chemical potential or relative humidity) cannot be controlled this way. To circumvent this problem, the Grand Canonical Monte Carlo (GCMC) method27,28 provides an excellent tool, because in GCMC simulations the chemical potential of the adsorbate (i.e., water) can be controlled and the adsorbed amount of water can be determined as a function of the chemical potential. The adsorption isotherm obtained this way reveals then the water surface density in the stable adsorption layer. Indeed, the GCMC method has already been successfully used to simulate adsorption in model pores of different shapes29,30 as well as on various substrates, such as carbonaceous materials,3,5,3133 covalent organic frameworks,34 ice,3539 and
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metal oxide surfaces including silica,40,41 MgO,42 magnetite,43 kaolinite,4446 and zeolites.47 However, despite this potential of the GCMC method of providing quantitative information on the structure and properties of adsorption layers, which are stable in a broad range of thermodynamic states, to the best of our knowledge, it has never been used to investigate water adsorption on partially oxidized mixed SAMs. In our previous work, we reported GCMC simulations of water on completely hydrophobic and completely hydrophilic SAMs, which are built up exclusively by CH3-terminated and by COOH-terminated alkanethiol derivative molecules, respectively.48 This study revealed important differences between the adsorption ability of these systems, showing that the water adsorption on a fully carboxylterminated (“oxidized”) SAM starts at lower water partial pressure as compared to that of methyl-terminated (hydrophobic) SAM, and an adsorption layer of water is developed on the hydrophilic surface before the condensation.48 However, it could not address the problem of aging, i.e., the fact that the fraction and spatial distribution of the COOH-terminated molecules in the SAM can alter the wettability of the surface. In this Article, we present, for the first time, GCMC simulations of the adsorption of water on binary mixed SAMs consisting of these two types of residues, S(CH2)7CH3 and S(CH2)7 COOH, all of them chemically linked to a flat virtual carrier by their S atom. For completeness, the simulations on the two neat SAMs are also repeated, and hence the surfaces studied here range from completely hydrophobic to completely hydrophilic ones. By changing the composition of the SAM, the hydrophilicity of its surface can be continuously tuned, and its effect on the adsorption properties can be systematically studied. The main goal of this study is to develop a quantitative understanding of the nature of water adsorption on organic surfaces consisting of a different number of randomly distributed polar sites in a hydrophobic background that are exposed to the atmosphere at varying relative water vapor pressure. This is achieved primarily by the calculation of water adsorption isotherms for the above-described set of mixed surfaces, as it provides a way to quantify the amount of water adsorbed at these complex surfaces as a function of their composition and water vapor pressure. In addition, we also report here a detailed analysis in terms of the binding energy distributions that shed light on the energetics underlying the adsorption process at the heterogeneous surfaces. Last but not least, the orientational analysis of the adsorbed water molecules as well as of the COOH groups at the SAM surfaces at the varying level of hydration provides an important input for surface-specific spectroscopic investigations. Finally, we note that because the adsorption isotherm represents a measurable quantity, the predictions based on the present GCMC calculations can be tested experimentally on real mixed SAM surfaces. This Article is organized as follows. Details of the Monte Carlo simulations performed are given in section 2. The obtained results concerning the adsorption isotherms, the distribution of the adsorbed water molecules along the surface normal axis, as well as the surface orientation and energetic properties of the water molecules belonging to the first molecular layer at the SAM surface are presented and discussed in detail in section 3. Finally, in section 4 the main conclusions of this study are summarized.
2. SIMULATION DETAILS Monte Carlo simulations of water adsorbed on SAMs of various compositions have been performed on the grand 19166
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Table 1. Data of the Adsorption Isotherms Calculated on Various Mixed SAMs in the Simulationsa 0% SAM Γ
μ (kJ/mol)
10% SAM
prel
25% SAM
Γ
50% SAM
Γ
75% SAM
Γ
90% SAM
Γ
Γ
Γ
ÆNæ (μmol/m2) ÆNæ (μmol/m2) ÆNæ (μmol/m2) ÆNæ (μmol/m2) ÆNæ (μmol/m2) ÆNæ (μmol/m2) ÆNæ (μmol/m2)
70.47 4.3 1012 67.99 5.2 1011
4.51
0.135
1.52
0.045
6.05
0.180
65.51 6.2 1010
2.29
0.068
3.21
0.096
12.67
0.378
63.04 7.3 109
4.40
0.131
9.38
0.280
26.45
0.789 1.30
60.56 8.7 108
0.56
0.0166
0.83
0.025
0.87
0.026
3.82
0.114
9.71
0.290
20.72
0.618
43.62
58.08 1.04 106
0.56
0.0167
1.37
0.041
1.54
0.046
9.39
0.280
20.01
0.597
34.61
1.03
77.00
55.6 1.24 105 53.12 1.48 104
0.56 0.58
0.0169 0.0174
2.28 4.02
0.068 0.120
3.34 8.91
0.100 0.266
17.45 49.35
0.521 1.47
46.46 116.3
1.39 3.47
74.02 173.5
2.21 5.18
119.4 216.4
64.39
1.92
176.8
5.27
261.2
7.79
318.4
3.27
348.7
51.88 5.11 104
0.60
0.0180
0.283
13.79
0.411
50.64 0.00176
0.63
0.0189
10.8
0.323
28.25
0.843
49.4 0.00610
0.69
0.0206
14.7
0.437
38.28
1.14
48.16 0.0211
0.79
0.0236
27.5
0.822
74.66
2.23
46.92 0.0728
a
100% SAM
0.96
9.48
0.0287 1761
52.5
109.6 399.9
11.9
2346
10.4 70.0
695.5 2630
20.7 78.5
580.2 922.5
2.30 3.56 6.46 9.50 17.3 27.5
5572
166
5615
167
3685
110
3755
112
5812
173
5907
176
5711
170
5777
172
5901
176
45.68 0.252
5986
179
5957
177
5993
179
6026
180
5996
179
5810
173
6038
180
44.44 0.869 43.2
6106 6275
182
6135 6264
183
6000 6238
179
6185 6307
184
6153 6319
184
6103 6300
182
6210 6355
185
Values marked in bold correspond to the systems where 2500 sample configurations have also been saved for the detailed analyses.
canonical (μ,V,T) ensemble27,28 at the temperature of 300 K. The X, Y, and Z edges of the rectangular basic simulation box have been 90.00, 69.44, and 80.16 Å, respectively, X being the axis perpendicular to the SAM surface. Standard periodic boundary conditions have been applied. The number of water molecules has been left to fluctuate in the simulation by controlling their chemical potential μ. For each SAM surface, simulations have been performed at 1216 different μ values (listed in Table 1), ranging from 70.5 to 43.2 kJ/mol. The SAMs have been built up as a random mixture of two octanethiol derivative components, a methyl-terminated, hydrophobic residue, S(CH2)7CH3, and a carboxyl-terminated, hydrophilic residue S(CH2)7COOH, both of them linked to a virtual carrier by their S atom. These two residues are referred to in this Article as C7CH3 and C7COOH residues, respectively. In this way, the hydrophilic or hydrophobic character of the SAM can be controlled by simply changing its composition. We report here simulations of self-assembled monolayers containing 0%, 10%, 25%, 50%, 75%, 90%, and 100% C7COOH residues. In each system, the SAM has been placed into the YZ plane of the basic simulation box. Each SAM has consisted of 256 alkanethiolate chains, arranged in a defect-free array of 16 16 chains with a surface area of approximately 22 Å2 per molecule. In each case, the positions of the sulfur atoms have been kept fixed at X = 5 Å, as a model for the chemically bound sulfur to the virtual flat carrier. An artificial grid of Lennard-Jones atoms has also been introduced beyond the S atoms (i.e., at X = 4 Å) to avoid penetration of the residues into the virtual carrier of the SAM. The remaining parts of the SAM chains have been allowed to move in the simulations. The molecules building up the SAMs have been described by the all-atom CHARMM22 force field,49 whereas water has been modeled by the SPC/E potential.50 Reasons for this particular choice of potential have been discussed in detail in our previous publication.48 It should be noted that usually the TIP3P51 rather than SPC/E water potential is used in combination with solutes
modeled by CHARMM. We believe, however, that this particular choice of the water potential has an insignificant influence on the results of the present analysis. To confirm this, we have also calculated the adsorption isotherms on the 25% and 75% SAMs using TIP3P instead of SPC/E water. The obtained isotherms always agreed within RT with the ones obtained with SPC/E, and the results of the subsequent detailed analyses also turned out to be insensitive to the particular water model used. All bond lengths and bond angles have been kept fixed in the simulations, while torsional flexibility of the C7CH3 and C7COOH molecules has been allowed. All interactions have been truncated to zero beyond the group-based centercenter cutoff distance of 12.5 Å. The long-range part of the electrostatic interaction has been accounted for by the method of reaction field correction28,52,53 under conducting boundary conditions. The simulations have been carried out using the program MMC.54 Particle displacement and water insertion/deletion attempts have been performed in an alternating order. In a particle displacement step, either, by 50% probability, a randomly chosen water molecule has been randomly translated by no more than 0.25 Å and randomly rotated around a randomly chosen space-fixed axis by no more than 10 or the orientation/ conformation of a randomly chosen SAM residue has been altered. In a SAM residue move, either, by 20% probability, the entire molecule was rotated around a randomly chosen spacefixed axis by no more than 30 (X axis) or 10 (Y or Z axis) or, by 80% probability, one of its dihedral angles has been changed. Residues have been selected for move in a shuffled cyclic order.55 Torsional rotations have been done using the extension biased algorithm; i.e., the maximum angle of rotation has been set to c/(Rmax)1/2, where Rmax is the distance of the farthest rotated atom from the axis of rotation, and c is the stepsize parameter.56 Torsional angles to be changed have been selected in a sequential order, going from the end of the chains toward the S atom, but also subject to a probability filter allowing less frequent changes of the torsions located closer to the outer end of the chains.56 In a 19167
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Figure 1. Equilibrium snapshots of SAMs of three different compositions (i.e., containing 25%, 50%, and 90% C7COOH residues) with the adsorbed water molecules at three different chemical potential values, as taken out from the simulations. The C7CH3 and C7 COOH residues and water O and H atoms are shown by green, blue, red, and white colors, respectively.
water insertion/deletion attempt, either a randomly chosen water molecule has been removed from or a new water molecule has been inserted to the system. Water insertions and deletions have been attempted with equal probabilities. Insertion/deletion steps have been done using the cavity biased algorithm of Mezei;57,58 i.e., insertions have only been attempted into positions located in the middle of pre-existing spherical cavities of the minimum radius of 2.6 Å. Cavities have been searched for along a 90 70 80 grid. The probability of finding a suitable cavity for the insertion attempt in a system containing exactly N water molecules, needed to remove the bias introduced by this insertion strategy in the sampling, has simply been calculated as the ratio of the cavities found and grid points checked.57,58 Starting configurations have been prepared by placing 256 C7CH3 residues in the simulation box in the way described above, and two water molecules randomly above the SAM. Next, the chain terminal CH3 group of the required number of randomly chosen C7CH3 residues has been substituted by a COOH group. The systems created this way have been equilibrated by performing at least 1.5 108 Monte Carlo steps. To check the adequacy of the equilibration run length, we repeated several simulations after a 7 108 Monte Carlo steps-long equilibration period, but no significant change of the results has been observed. Next, in the production phase, the number of adsorbed water molecules has been averaged over 108 equilibrium configurations. Finally, at selected chemical potentials (see Table 1), 2500 equilibrium sample configurations, separated from each other by 2 104 Monte Carlo steps-long trajectories each, have been saved for further analyses. For illustration, an equilibrium snapshot of the system is shown in Figure 1 for three different SAM compositions and three chemical potential values.
3. RESULTS AND DISCUSSION 3.1. Adsorption Isotherms. The adsorption isotherms calculated on the seven SAMs of different compositions are shown in Figure 2, and the corresponding data are also collected in Table 1. The sharp rising part of the isotherms, occurring between 52 and 46 kJ/mol, indicating that the basic box is suddenly filled
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Figure 2. Adsorption isotherms of water obtained on the SAMS of seven different compositions considered. The solid, dashed, dotted, dash-dotted, dash-dot-dotted, and short dotted lines and “O” correspond to the systems containing 0%, 10%, 25%, 50%, 75%, 90%, and 100% C7COOH residues, respectively. The inset shows the isotherms on a logarithmic scale for better visualization of their low μlow ÆNæ part preceding condensation.
corresponds to the condensation of water. As is seen, in the case of the completely hydrophobic 0% SAM, practically no adsorption occurs; the number of water molecules remains close to zero up to the point of condensation. It should be noted that, contrary to the present computer simulation investigation, experimental studies have shown some evidence of prewetting, i.e., water cluster formation before condensation even on completely hydrophobic surfaces.18 This discrepancy probably originates from the fact that, unlike the ideally flat, defect-free model surface used in the simulations, real surfaces are always corrugated by molecular scale defects. Indeed, the formation of such water clusters before condensation is strongly related to the presence of molecular size pores and/or chemical defects that can facilitate the adsorption of the first water molecules at the surface, as these molecules can provide nuclei for prewetting cluster formation.5 With increasing hydrophilicity of the SAM, more water molecules can be adsorbed before condensation, and adsorption also starts to occur at lower chemical potential values. This gradual shift of the low μ part of the adsorption isotherm to lower chemical potential and higher ÆNæ values is clearly seen in the inset of Figure 2, showing the isotherms on a logarithmic scale to enlarge their low μlow ÆNæ part preceding condensation. The observed lack of water adsorption on the completely hydrophobic 0% SAM and the gradual shift of the isotherm to lower μ and higher ÆNæ values with increasing fraction of the hydrophilic C7COOH residues clearly indicate that, at least at low enough surface coverages, adsorption occurs on the surface COOH groups of the SAM. However, it is an important question whether increasing adsorption can solely be attributed to the increasing surface density of the hydrophilic COOH groups. To address this question, we have converted the isotherms obtained on C7 COOH-containing SAMs to the ÆNæ/NCOOH versus μ form, i.e., divided the average number of adsorbed water molecules ÆNæ by the number of the surface COOH sites NCOOH. The ÆNæ/NCOOH versus μ isotherms are shown and compared in Figure 3. As is seen, the ÆNæ/NCOOH (μ) curves obtained for the 10% and 25% SAMs coincide almost perfectly with each other, indicating that the adsorbed amount of water per surface COOH 19168
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Figure 3. Adsorption isotherms of water obtained on the COOH containing SAMs of different composition, normalized by the number of COOH groups. The lines and symbols corresponding to the different systems are the same as in Figure 2. For enlarging their differences, the isotherms are shown on a logarithmic scale.
groups is independent from the overall surface COOH density in the surfaces of low COOH concentration. It is also seen, however, that above 25% C7COOH of the SAM, even the isotherm normalized by NCOOH shifts gradually to lower μ and higher ÆNæ/NCOOH values with increasing surface hydrophilicity. These findings clearly indicate that in SAMs where the fraction of the hydrophilic C7COOH residues is low, and hence these residues are isolated from each other, adsorption indeed occurs on the individual surface COOH groups independently from each other. However, with increasing fraction of the C7COOH residue, the surface COOH groups can be in the vicinity of each other with increasing probability, leading to an increasing deviation of the water adsorption from the scheme of independent adsorption on isolated COOH sites. Instead, nearby COOH groups seem to have a synergistic effect on water adsorption: a cluster of nearby surface COOH groups can adsorb considerably more waters at lower chemical potentials than the same amount of COOH groups if they are isolated from each other. This synergistic effect of the close-by COOH groups is related to the fact that the water molecules bound to the COOH groups are also close to each other and, hence, able to form hydrogen bonds also with each other, in addition to those formed between water and the COOH groups. Thus, the lateral waterwater interaction contributes substantially to the driving force of the adsorption. In this sense, clusters of surface COOH groups can act as nuclei initiating the adsorption of a relatively large amount of water. The synergistic effect of the nearby COOH groups on water adsorption is illustrated in Figure 4, showing a cluster of 38 water molecules adsorbed on four nearby COOH groups of the SAM surface, as taken out from our simulation. This cluster, having the shape of a small droplet, is clearly kept together by an extended network of waterwater hydrogen bonds and is anchored to the SAM surface by several waterCOOH hydrogen bonds. It is also seen from Figure 2 that the point of condensation gradually shifts to lower chemical potential values with increasing fraction of the hydrophilic C7COOH residue. This finding is rather surprising because, in principle, the point of condensation is characteristic solely of the adsorbate molecule, and, given that the surface is flat, as in the present case, it should be independent of the surface above which it occurs. Although the observed point of condensation scatters between about 46 and 48.5 kJ/mol, i.e., within a μ range of comparable width with RT at 300 K, this
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Figure 4. Snapshot showing a cluster of 38 water molecules adsorbed at a portion of a SAM surface that consists of four nearby COOH groups, as taken out from the simulation performed at the chemical potential value of 49.40 kJ/mol. C, O, and H atoms are marked by blue, red, and white colors, respectively. Red dotted lines indicate hydrogen bonding.
difference, in particular, the clear trend that the point of condensation shifts to lower μ values with increasing hydrophilicity, needs to be explained. It should be noted that condensation occurs when the adsorbed water layer becomes infinitely (i.e., macroscopically) wide. However, in the simulation, we can only detect the point where the adsorption layer gets as wide as the simulation box. Because the driving force of water adsorption is largely coming from the formation of hydrogen bonds with the already adsorbed water molecules, the adsorption layer of water may consist of many molecular layers, and hence it can be considerably broader than our simulation box. Thus, the seeming shift of the point of condensation can be explained by the finite size effect, namely that this shift is in fact that of the point where the adsorbed water layer becomes as wide as the basic simulation box rather than where it becomes infinitely wide. To confirm this, we also determined the point of condensation of water in our basic box without any SAM present. Condensation of water in the SAMfree box occurred at μ = 44.3 kJ/mol, a value even higher than that observed above the completely hydrophobic 0% SAM. To further confirm that the observed shift of the point of condensation can be attributed to finite size effect, and hence it should be regarded as an artifact, we repeated several simulations of different mixed SAMs around the point of condensation by increasing the surface normal axis of the basic box by a factor of 2. The point of condensation of the isotherms obtained in the large box clearly shows the same trend as in the original box; however, the difference between the points of condensation above SAMs of different compositions became noticeably smaller by enlarging the basic box. Therefore, we took the value of μ = 44.3 kJ/mol, obtained in the SAM-free box, as the real point of condensation of water, μ0. Having the value of μ0, the isotherms can also be converted to the more conventional Γ(prel) form, where the water surface density Γ can simply be given as Γ¼
ÆNæ YZ
ð1Þ
whereas prel, i.e., the pressure p normalized by the pressure of the saturated vapor p0, can be calculated as prel ¼ 19169
p expðμÞ ¼ p0 expðμ0 Þ
ð2Þ
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Figure 5. Density profiles of the adsorbed water molecules on SAMs of three different compositions as obtained at five different chemical potential values. Top panel, SAM of 25% C7COOH content; middle panel, SAM of 75% C7COOH content; bottom panel, SAM of 100% C7COOH content. Dotted, dashed, solid, dash-dotted, and dash-dotdotted lines correspond to the chemical potential values of 48.16, 50.64, 51.88, 55.60, and 60.56 kJ/mol, respectively. The short dashed vertical line at X = 19.3 Å shows the boundary of the first molecular layer as regarded in the present study.
Obviously, such a conversion can only be done up to the point of condensation, i.e., to the prel value of 1, and hence up to the μ value of μ0 = 44.3 kJ/mol. The Γ and prel data corresponding to the obtained isotherms are also included in Table 1. 3.2. Density Profiles. The molecular number density profile of the adsorbed water molecules along the surface normal axis X is shown in Figure 5 as calculated for the 25%, 75%, and 100% SAMs at five different chemical potential values. As is seen, at low chemical potentials, the obtained profile has a single, narrow peak. Up to a certain point, the increase of the chemical potential leads only to the increase of the height of this peak without its substantial broadening. In this μ range, adsorption occurs in the first molecular layer. Once this layer becomes saturated, the water molecules start building up outer molecular layers, giving thus rise to multilayer adsorption. In accordance with the observed behavior of the adsorption isotherms, saturation of the first molecular layer occurs at lower chemical potentials above more hydrophilic surfaces. Thus, for instance, at μ = 48.16 kJ/mol, multilayer adsorption occurs on the 75% and 100% SAMs, but the adsorption layer is still monomolecular on the 25% SAM (see Figure 5). It is also evident that multilayer adsorption does not occur in a layer by layer manner (which would result in gradually broadening density peaks of the same height), but involves simultaneously several molecular layers, covering a rather broad X range. This finding is also in clear accordance with our previous claim that the adsorption layer can easily be of comparable width with the basic simulation box size, leading to a systematic overestimation of the point of condensation above hydrophilic surfaces.
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Figure 6. Definition of the local Cartesian frames fixed (a) to the individual surface water molecules and (b) to the individual COOH groups, and of the ϑ and ϕ angular polar coordinates of the surface normal vector, pointing, by our convention, from the adsorbed water layer to the SAM, X, in these frames, as used in the orientational analyses.
It is also seen from Figure 5 that in the case of multilayer adsorption the density peak corresponding to the first molecular layer is followed by a small but clear minimum. The position of this minimum at X = 19.3 Å can serve as an estimate of the boundary of the first molecular layer (see Figure 5). Thus, in the following, all analysis done for the first molecular layer of adsorbed water involves water molecules located at X e 19.3 Å. 3.3. Orientation of the COOH Groups and Water Molecules at the Interface. To investigate the structure of the adsorption layer in more detail, we have calculated the orientation of the adsorbed water molecules belonging to the first molecular layer in contact with the SAM surface and also that of the COOH groups of the SAM relative to the surface plane. The orientation of a rigid body (e.g., a water molecule or a COOH group) relative to an external direction (e.g., the interface normal) can, in general, be described by two independent orientational variables. Thus, the orientational statistics of (rigid) molecules or groups relative to a planar interface can be fully characterized by the bivariate joint distribution of these two orientational variables.59,60 It has been shown that the angular polar coordinates ϑ and ϕ of the interface normal vector in a Cartesian frame fixed to the individual molecules or groups represent a suitable choice of such a parameter pair.59,60 However, because the polar angle ϑ is formed by two general spatial vectors (i.e., the interface normal vector and the z axis of the molecule-fixed reference frame), but ϕ is an angle of two vectors (i.e., the projection of the interface normal vector to the xy plane of the local frame and the x axis of this frame) that are restricted to lay in a given plane (i.e., the xy plane of the local frame) by definition, uncorrelated orientation of the molecules with the interface results in a uniform bivariate distribution only if cos ϑ and ϕ are chosen to be the two independent variables.59,60 In this study, we use the convention that the interface normal vector, X, is directed from the adsorbed water layer toward the SAM. The local Cartesian frame fixed to the water molecules and COOH groups is defined in the following way. For water, axis x 19170
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The Journal of Physical Chemistry C of this frame is perpendicular to the molecular plane, axis z coincides with the main symmetry axis of the molecule, and axis y is perpendicular to the above two, pointing along the two H atoms. Axis z is directed from the O toward the H atoms, whereas, due to the C2v symmetry of the water molecule, axes x and y can always be directed in such a way that the relation 0 e ϕ e 90 holds. In the case of the COOH groups, axis z of the local frame is perpendicular to the plane of the group, axis x points from the C to the H atom, and axis y is perpendicular to these two axes, directed in such a way that the y coordinate of the carbonyl O atom is positive. Because of the planar symmetry of the COOH group, axis z is always directed in such a way that the relation cos ϑ g 0 holds. The definition of these local Cartesian frames and that of the polar angles ϑ and ϕ are illustrated in Figure 6. The P(cos ϑ,ϕ) orientational maps of the SAM COOH groups and of the first layer water molecules are shown in Figures 7 and 8, respectively, as obtained for SAMs of four different compositions (i.e., for those of the 25%, 50%, 75%, and 90% C7COOH content) at four different chemical potential values. As is seen, the SAM COOH groups have three different orientational preferences, as the corresponding orientational maps exhibit three distinct peaks. The first orientation corresponds to the cos ϑ and ϕ values of about 0.7 and 180, respectively. In this orientation, marked here as ICOOH, the plane of the COOH group is tilted by about 4050 relative to the plane of the interface, and its H atom gets as close to the adsorbed water layer as possible. The other two preferred orientations are characterized by the ϕ values of about 120 and 240, respectively, whereas the value of cos ϑ falls between 0 and 0.5 in both cases. In these orientations, referred to here as IICOOH and IIICOOH, respectively, the plane of the COOH group is tilted by 6090 relative to the plane of the interface in such a way that in orientation IICOOH the OH bond lies roughly parallel with the interface, while in orientation IIICOOH it sticks as straight away from the SAM as possible. It is also seen that orientations markedly different from these three ones are clearly dispreferred, as they occur with vanishingly small probabilities. The three preferential COOH orientations are illustrated at the bottom of Figure 7. It should also be noted that, although the three preferred alignments are, in most cases, reflected in three distinct peaks of the P(cos ϑ,ϕ) orientational maps, these three peaks are rather close to each other, indicating that the COOH group can be brought from one preferred alignment to the others by relatively small rotations. Further, with increasing C7COOH content of the SAM, the corresponding peaks are increasingly overlapping, and in the case of the pure C7COOH SAM, the individual peaks can no longer be resolved (see Figure 5 of ref 48). It is also seen that in all of the three preferred alignments, the COOH group points to the water layer by at least one of the lone pair directions of one of its O atoms (i.e., by that of the carbonyl oxygen in orientations ICOOH and IIICOOH, and by that of the hydroxylic oxygen in orientation IICOOH); however, this lone pair direction never declines strongly from the surface plane. Because hydrogen-bonding H atoms can be accepted from these directions, this finding indicates that the SAM COOH groups can accept hydrogen bonds from the adsorbed water molecules in all three preferred alignments. Further, in orientations ICOOH and IIICOOH, the OH bond points also toward the layer of the adsorbed waters in such a way that it is only modestly tilted from the surface plane; hence, in these alignments, the COOH group can also act as a H-donor partner in hydrogen bonds with the
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Figure 7. Orientational maps of the COOH groups at the SAM surface as obtained in SAMs of four different compositions at four different chemical potential values. Lighter colors indicate higher probabilities. The preferred COOH orientations corresponding to the ICOOH IIICOOH peaks of the maps are shown at the bottom of the figure. C, O, and H atoms are marked by blue, red, and white colors, respectively. X is the surface normal vector pointing from the adsorbed water layer to the SAM.
water molecules. Finally, it should be noted that although two neighboring COOH groups can certainly form hydrogen bonds with each other, their preferred alignments are such that in these alignments they cannot form cyclic dimers with each other, even in the case of only a small amount of adsorbed water on SAMs of high COOH surface densities. The orientational maps of the adsorbed water molecules (Figure 8) show a rather complex orientational behavior. In the case of SAMs of low C7COOH content, water molecules prefer alignments close to the cos ϑ and ϕ values of 1 and 90, respectively. In this alignment, denoted here as Iwat, the water molecule stays perpendicular to the interface pointing away from the SAM by both hydrogens, and toward the SAM by the two lone pair directions. In the case of more hydrophilic SAMs, this peak shifts to larger cos ϑ values, and a marked orientational preference is seen around the {cos ϑ = 0.5, ϕ = 90} point of the P(cos ϑ,ϕ) map. In this alignment, marked here by IIwat, the plane of the water molecule is still perpendicular to the interface, but now it sticks by one of its OH bonds straight away from the SAM, whereas by the other OH bond as well as by the two lone pair directions in a tilted way toward the SAM. At low chemical potentials, another peak, denoted here by IIIwat, is seen at cos ϑ ≈ 0.5 and ϕ = 0. With increasing amount of adsorbed water, this peak gradually shifts to lower cos ϑ values down to about 0.3. The corresponding orientation is marked here as IVwat. In these two orientations, the water molecule lies 19171
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Figure 8. Orientational maps of the water molecules belonging to the first molecular layer at the surface of SAMs of four different compositions as obtained at four different chemical potential values. Lighter colors indicate higher probabilities. The preferred water orientations corresponding to the IwatIVwat peaks of the maps are shown at the bottom of the figure. O and H atoms are marked by red and white colors, respectively. X is the surface normal vector pointing from the adsorbed water layer to the SAM.
nearly parallel with the interface, tilting by only about 2030 in such a way that the two H atoms point toward the SAM in orientation IIIwat, and away from that in orientation IVwat. The preferred water orientations IwatIVwat are illustrated in Figure 8. As is clear, in orientations IwatIIIwat, the water molecule points in a tilted way toward the SAM by two or three of its hydrogen-bonding directions (i.e., by the two lone pairs in orientation Iwat, by one H atom and the two lone pairs in IIwat, and by the two H atoms and one lone pair in IIIwat). Therefore, in these alignments, they can easily form hydrogen bonds with the surface COOH groups. On the other hand, in orientation IVwat, only one lone pair among the four hydrogen-bonding directions points toward the SAM. Further, this lone pair direction is aligned roughly perpendicular to the interface, while the COOH groups do not prefer orientations in which their OH bond points straight to the water layer (see Figure 7). This means that water molecules in orientation IVwat cannot form strong hydrogen bonds with the surface COOH groups of the SAM. Understanding the physical reasons lying behind the preference for orientation IVwat, it should be noted that alignments IIwatIVwat are also preferred at the free water surface61 as well as at the water/apolar liquidliquid interfaces.6264 In particular, the reason behind the preference for orientations similar to IVwat at apolar interfaces is that in this orientation the water molecule can maintain three strong waterwater hydrogen bonds at the
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expense of sacrificing the fourth one, i.e., what could have been formed in the lone pair direction that sticks to the other phase.64,65 Considering this, and also the fact that in our case orientation IVwat becomes preferred only in the case of a large enough amount of adsorbed water, i.e., when the adsorption is no longer limited to one molecular layer, it is clear that the reason for the preference of this orientation is the possibility of the formation of new waterwater rather than waterCOOH hydrogen bonds. The obtained results show that the orientational preferences of the adsorbed water molecules and surface COOH groups are determined by several factors. First, the constraints imposed by the conformation and ordering of the hydrocarbon chains in the SAM restrict the orientations available for the surface COOH groups. This factor is likely the cause of the observed lack of COOH orientations in which the hydrogen-bonding directions of the COOH group would point straight to the water layer. At low coverages, water molecules prefer orientations such as Iwat or IIwat, in which they can form hydrogen bonds with more than one COOH groups. This preference is particularly strong at strongly hydrophilic SAMs (i.e., when the COOH surface density is high). On the other hand, at high enough coverages when the adsorption involves more than one molecular layer, the surface water molecules adopt an orientational pattern similar to that at hydrophobic fluid interfaces.6164 In this case, besides the formation of new waterCOOH hydrogen bonds, that of new waterwater hydrogen bonds becomes another driving force of the adsorption, and also another factor that determines surface orientation. In this case, the orientation of the COOH groups is also largely dictated by that of the water molecules. These results are illustrated by Figure 9, showing several small hydrogen-bonded clusters of COOH groups and water molecules at the SAM surface, as taken out from equilibrium configurations of the simulations. Thus, panels (a) and (b) show hydrogen-bonded COOHwater pairs, panels (c) and (d) illustrate the cases when one water molecule is hydrogen bonded to two COOH groups (at low surface coverages) and when one COOH group is hydrogen bonded to two water molecules (in SAMs of low C7COOH content), respectively, panels (e) and (f) show two and three water molecules, respectively, bound to two neighboring COOH groups (in panel (e) these COOH groups are also H-bonded to each other, while in panel (f) they are linked by a bridging water molecule), whereas panel (g) shows a cluster of three COOH groups and four water molecules. The discussed orientational preferences can also be observed in the cluster shown in Figure 4. 3.4. Energetic Background. To shed some light onto the energetic background of the adsorption, we have also calculated the distribution of the binding energy Ub, of the adsorbed water molecules belonging to the first molecular layer for the different SAMs. The binding energy, Ub, is the energy required to bring an adsorbed water molecule to infinite distance from the system; hence, it equals the total energy of the interactions of this molecule with the rest of the system. Further, besides the P(Ub) binding energy distributions, we have also calculated the disand Uwat contributions to the binding tribution of the USAM b b energy, coming from the interactions of the given water molecule with the SAM and with the other water molecules in the system, respectively. ), and P(Uwat The P(Ub), P(USAM b b ) distributions of the first layer water molecules in the different COOH containing SAMs are shown in Figure 10 as obtained at four different chemical 19172
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Figure 9. Equilibrium snapshots of small hydrogen-bonding clusters of COOH groups and water molecules at the SAM surface, as taken out from the simulations. (a) A water molecule of orientation IIIwat bound to a COOH group of orientation IICOOH, (b) a water molecule of orientation Iwat bound to a COOH group of orientation IIICOOH, (c) a water molecule of orientation IIwat bound to two COOH groups, (d) a COOH group of orientation ICOOH binds two water molecules, (e) a cluster of two COOH groups H-bonded to each other binds two water molecules, (f) a cluster of two COOH groups water bridged to each other binds three water molecules, and (g) a cluster of three COOH groups and four water molecules. Color coding is the same as in Figure 4; the red dotted lines indicate hydrogen bonding.
potential values. At the lowest chemical potential considered, i.e., μ = 55.60 kJ/mol, the P(Uwat b ) distributions are always bimodal, having a sharp and high peak at zero energy, and a second one around 25 kJ/mol. The first peak reflects water
molecules that are isolated from the other waters, whereas the second one is due to water molecules having one hydrogenbonded water neighbor. This peak is considerably higher in the case of SAMs of at least 75% C7COOH content than for the 19173
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Figure 10. Distribution of the total binding energy of the adsorbed water molecules belonging to the first molecular layer (bottom panels) as well as that of its contributions coming from the interactions of the water molecule with the SAM (middle panels) and with the other water molecules in the system (top panels) in the six different COOH containing SAMS at the chemical potential values of (a) 55.60 kJ/mol, (b) 51.88 kJ/mol, (c) 50.64 kJ/mol, and (d) 48.16 kJ/mol. The lines and symbols corresponding to the different systems are the same as in Figure 2.
less hydrophobic SAMs, reflecting the fact that at a given chemical potential value more hydrophilic SAMs adsorb a larger amount of water (see Figures 2 and 3, and Table 1). Further, in the case of SAMs of at least 75% C7COOH content, this peak exhibits a shoulder around 47 kJ/mol, reflecting the presence of water molecules with even two hydrogen-bonded water neighbors. The SAM contribution to the total binding energy, USAM , b shows a much simpler, unimodal distribution at this low chemical
potential value in every case. For the most hydrophobic SAMs (i.e., the 10% and 25% COOH ones), this peak is located around 43 kJ/mol, indicating that the water molecules form, on average, two hydrogen bonds with the C7COOH molecules, and these waterCOOH hydrogen bonds are slightly, by 24 kJ/mol, weaker than waterwater ones. In the case of the more hydrophilic SAMs, this peak gradually shifts to slightly lower energies, down to about 50 kJ/mol. This shift reflects the fact that, for each water molecule, as the COOH fraction in the 19174
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The Journal of Physical Chemistry C SAM surface increases, there is an increasing number of nearby COOH groups to which the water molecule is not hydrogenbonded, but which also contribute somewhat to the UbSAM interaction energy. Thus, the position of this peak still corresponds to the average number of two hydrogen-bonded COOH neighbors of the adsorbed water molecules, in accordance with the fact that the ratio of the adsorbed water molecules and surface COOH groups never exceeds 0.5 at this low chemical potential value (see Figure 3 and Table 1). The P(Ub) total binding energy distributions are rather similar contributions, but they are shifted to to those of the USAM b somewhat lower energies (due to the lateral interaction of the adsorbed waters), and, obviously, this shift is larger for more hydrophilic SAMs containing more adsorbed water molecules. The increase of the chemical potential leads to several changes SAM ), and P(Ub) distributions. In general, in the P(Uwat b ), P(Ub wat while the P(Ub ) distributions shift to lower energies, the ) distributions shift to higher energies, in accordance P(USAM b with the finding that besides the formation of new water COOH hydrogen bonds, that of new waterwater hydrogen bonds also becomes an increasingly important driving force of the water adsorption. Thus, at μ = 51.88 kJ/mol, the shoulder of P(Uwat b ) of the most hydrophilic SAMs at 43 kJ/mol becomes more pronounced than that at μ = 55.60 kJ/mol. At higher chemical potentials, even the P(Uwat b ) distributions of the less hydrophilic SAMs exhibit this shoulder, while that of the SAMs of at least 75% C7COOH content becomes unimodal, having its peak at 60 kJ/mol in the case of μ = 50.64 kJ/mol, and around 75 kJ/mol in the case of μ = 48.16 kJ/mol. These Uwat b values indicate that in these systems the first layer water molecules have, on average, 2.5 and 3 hydrogen-bonded water neighbors, respectively. All of these changes are in accordance with the increasing amount of adsorbed water molecules as their chemical potential as well as the COOH content of the SAM increase. ) distributions show an opposite trend of changes The P(USAM b with increasing water chemical potentials, in particular, in the case of the hydrophilic SAMs. Thus, while in the case of the 10% ) distribuand 25% COOH SAMs the main peak of the P(USAM b tion at about 43 kJ/mol remains apparent in the entire chemical potential range investigated, in the case of the more hydrophilic systems, this peak gradually reduces to a shoulder, and a new peak emerges around 6 to 8 kJ/mol, indicating that an increasing number of water molecules do not form any hydrogen bond with the SAM. In interpreting this finding, it should be recalled that our analysis is limited here to the water molecules belonging to the first molecular layer, i.e., to the ones that are in direct contact with the SAM, and even the majority of such water molecules are not directly hydrogen bonded to the SAM surface. This finding is in clear accordance with our previous observation about the increasing dominance of orientation IVwat (i.e., an alignment in which the water molecule cannot form any hydrogen bond with the SAM COOH groups) with increasing surface hydrophilicity and increasing water chemical potential and stresses again the increasing importance of the formation of new waterwater hydrogen bonds in the adsorption process. The changes of the total binding energy distributions, P(Ub), with increasing water chemical potential indicate that the increasing number of waterwater hydrogen bonds overcompensates the effect of the decreasing average number of water COOH hydrogen bonds per (first layer) water molecule. Thus,
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the main peak of the P(Ub) distributions of the hydrophilic SAMs gradually shifts from about 60 kJ/mol (at μ = 55.60 kJ/mol) to about 90 kJ/mol (at μ = 48.16 kJ/mol), indicating that in this latter case a first layer water molecule has, on average, 3.5 hydrogen-bonded neighbors (irrespective of whether these are waters or COOH groups). In the case of the less hydrophilic SAMs, this shift is somewhat weaker; however, even in the case of the 10% and 25% COOH SAMs, the main peak of P(Ub), being the only peak at μ = 55.60 kJ/mol, is accompanied by another peak at about 65 kJ/mol at the highest chemical potential considered, indicating the appearance of water molecules with three hydrogen-bonded neighbors even in these systems. Further, these P(Ub) distributions extend down below 90 kJ/ mol, indicating that even water molecules with four hydrogenbonded neighbors are present in these systems. Concerning the P(Ub) total binding energy distribution, its sharp peak in the 10% SAM system, occurring at zero energy in the case of high enough water chemical potentials, needs to be explained. This peak is given by water molecules that are isolated from the rest of the system, i.e., being in the vapor phase. A careful inspection of the equilibrium snapshots of the system suggests, in accordance with the fact that this peak is only apparent in the case of the most hydrophobic mixed SAM, that these water molecules are typically moving from one COOH group to another one. Because in the 10% COOH SAM these groups are typically located rather far from each other, such a “hopping” water molecule has to move to rather large distances, and hence it has to lose all of its hydrogen bonds during the course of such moves. Finally, it is worth comparing our results with those of similar studies. Recently, Grimm et al. reported21 a combined theoretical and experimental investigation of the D2O adsorption on C3 COOH, C4COOH, C15COOH, and C16COOH SAMs. Although the ordering of the SAM clearly depends on the length of the constituting alkanethiol chains, which alters also its water adsorption properties, and hence direct comparison of our results with those of Grimm et al. is not possible due to the differences in the systems studied, it is worth noting that the distributions of the total binding energy as well as those of its SAM and water contributions obtained here for the neat COOH-terminated C8COOH SAM fit well to the trend seen by Grimm et al.,21 taking also into account the fact that the water surface densities they considered correspond to our 100% SAM system in the chemical potential range between about 53.1 and 51 kJ/mol.
4. SUMMARY AND CONCLUSIONS Water adsorption was studied on self-assembled monolayers consisting of mixtures of various compositions of two types of residues, hydrophobic S(CH2)7CH3 and hydrophilic S (CH2)7COOH, by the grand canonical Monte Carlo technique. By changing the composition, and therefore the hydrophilicity of the SAM, the alteration of the adsorption properties of the organic surface was examined, and the corresponding water adsorption isotherms were calculated. The isotherms show that not only the molar ratio of the two SAM components but also the distribution of the polar sites in the surface play a crucial role in controlling water adsorption, and reveal an interesting nonlinear effect due to synergy between close-lying hydrophilic COOH groups embedded in the hydrophobic CH3 matrix. At low mole fraction of the hydrophilic C7COOH residues, the surface carboxyl groups are mostly 19175
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The Journal of Physical Chemistry C isolated from each other, and hence they provide individual adsorption sites on the surface. At higher C7COOH mole fractions, the COOH groups more likely have nearby COOH neighbors, resulting in clusters of adsorbed water molecules bridging between these COOH sites. Thus, clusters of nearby COOH groups have a synergistic effect on water adsorption, as a cluster of such nearby surface COOH groups can attract considerably more water molecules, in particular at low chemical potentials, than the same amount of COOH groups would if they were isolated from each other. This synergistic effect originates from the lateral waterwater interactions, that the formation of new waterwater hydrogen bonds is at least as important of a driving force of the adsorption as the formation of new water COOH hydrogen bonds. The synergism due to the nearby COOH groups is reflected in a nonlinear shift of the isotherm at the low chemical potential regime with increasing surface hydrophilicity. These findings were also supported by the observed preferential orientations of the first layer water molecules and surface COOH groups. With increasing surface hydrophilicity and increasing humidity (i.e., water chemical potential), such a water orientation was found to become increasingly dominant in which the water molecule cannot form any hydrogen bond with the surface COOH groups, but, similarly to the free water surface61 and waterapolar liquid/liquid interfaces,6264 it can form three strong hydrogen bonds with its water neighbors. In the case of a large amount of adsorbed water (i.e., when the adsorption extends beyond the first molecular layer), water molecules prefer orientations similar to those at the free water surface,61 and, besides the constraints imposed by the conformation and ordering of the C7COOH chains, the orientational preferences of the surface COOH groups are dictated by the preferred water orientations. On the other hand, at low surface coverages, the preferred water orientations are imposed by the requirement of forming at least two hydrogen bonds with the surface COOH groups in their preferred alignments. These conclusions are also in accordance with the results concerning the binding energy distributions. Thus, at low chemical potentials, water molecules form, on average, two hydrogen bonds with the surface COOH groups, while water water hydrogen bonds occur much less frequently. With increasing water chemical potential, the distribution of the SAM contribution to the total binding energy gradually shifts to higher, whereas that of the water contribution to lower energies, indicating the increasing importance of the lateral waterwater interaction with respect to the usual waterCOOH adsorbate adsorbent interaction as the driving force of the adsorption process. The total binding energy distribution also shifts to lower energies with increasing humidity, reflecting that the average number of hydrogen bonds formed by the first layer of water molecules increases from two to above three in the chemical potential range investigated. This finding clearly stresses that the increasing number of waterwater hydrogen bonds overcompensates the effect of decreasing average number of water COOH hydrogen bonds per (first layer) water molecules. Considering the atmospheric relevance of this work, the most important finding is that the occurrence of clusters of nearby COOH sites causes increased water adsorption capacity of the organic surfaces even at low humidity. The probability of the occurrence of such configurations increases nonlinearly with increasing fraction of the hydrophilic surface groups, which clearly stresses the important role of the surface aging process
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of atmospheric aerosols. At the same time, however, surface organic material may undergo substantial reorganization upon oxidation, resulting in polar groups being buried inside the aerosol particles, inaccessible to water vapor. For example, mobility of molecules of an organic surfactant layer on aqueous core makes it possible for the oxidized molecule to reorient or diffuse into the aqueous subphase as a result of increased solubility. Significant structural changes have been reported also for oxidized organic material on a solid substrate, with no surface oxygen-containing polar groups present.2 The findings of the present study are thus more relevant for cases in which organic molecules are immobile on the surface and/or oxidation does not induce large reorganization of the organic material. In such cases, the polar groups introduced by oxidation are more likely to remain exposed at the surface. The synergistic effect of nearby oxygen-containing functional groups on water adsorption will then result in increased water uptake and, therefore, have important implications for heterogeneous chemistry as well as the ability of organic aerosols to act as cloud condensation nuclei. Further studies combining surface-sensitive experimental techniques with methods of computational chemistry and molecular modeling are needed to gain insight into these important atmospheric processes.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +36-1-3722500. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Hungarian OTKA Foundation (Project No. 75328) and the Ministry of Education of the Czech Republic (grants ME09064 and LC512). M.S. and M.R. received support for this work from the AirUCI institute, funded by the National Science Foundation (grants CHE-0431312 and CHE-0909227). The work in Prague was performed within the framework of the research project Z40550506. We thank Mate Labadi for excellent technical support and Professors Barbara Finlayson-Pitts and Pavel Jungwirth for valuable discussions. ’ REFERENCES (1) George, I. J.; Abbatt, J. P. D. Nat. Chem. 2010, 2, 713. (2) McIntire, T. M.; Ryder, O. S.; Gassman, P. L.; Zhu, Z.; Ghosal, S.; Finlayson-Pitts, B. J. Atmos. Environ. 2010, 44, 939. (3) Moulin, F.; Picaud, S.; Hoang, P. N. M.; Jedlovszky, P. J. Chem. Phys. 2007, 127, 164719. (4) Kireeva, E. D.; Popovicheva, O. B.; Persiantseva, N. M.; Khokhlova, T. D.; Shonija, N. K. Colloid J. 2009, 71, 353. (5) Hantal, Gy.; Picaud, S.; Hoang, P. N. M.; Voloshin, V. P.; Medvedev, N. N.; Jedlovszky, P. J. Chem. Phys. 2010, 133, 144702. (6) Izsak, R; Sz€ori, M.; Knowles, P. J.; Viskolcz, B. J. Chem. Theory Comput. 2009, 5, 2313. (7) Kroll, J. H.; Seinfeld, J. H. Atmos. Environ. 2008, 42, 3593. (8) Dron, J.; El Haddad, I.; Temime-Roussel, B.; Jaffrezo, J. L.; Wortham, H.; Marchand, N. Atmos. Chem. Phys. 2010, 10, 7041. (9) Ellison, G. B.; Tuck, A. F.; Vaida, V. J. Geophys. Res. 1999, 104, 11633. (10) Bertram, A. K.; Ivanov, A. V.; Hunter, M.; Molina, L. T.; Molina, M. J. J. Phys. Chem. A 2001, 105, 9415. (11) Holmes, S.; Schwartz, S. Compos. Sci. Technol. 1990, 38, 1. (12) Rudich, Y. Chem. Rev. 2003, 103, 5097. (13) Rudich, Y.; Donahue, N. M.; Mentel, T. F. Annu. Rev. Phys. Chem. 2007, 58, 321. 19176
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