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Szoori M.,University of Szeged | Roeselova M.,Czech Institute of Organic Chemistry And Biochemistry | Jedlovszky P.,Eotvos Lorand University | Jedlovszky P.,HAS Research Group of Technical Analytical Chemistry
Journal of Physical Chemistry C | Year: 2011

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 (C7-CH3) alkanethiol chains with carboxylic acid-terminated (C7-COOH) 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 water-water interactions, such surface COOH clusters thus act as condensation nuclei for water. With increasing water chemical potential, the possibility of the formation of new water-water hydrogen bonds gradually becomes an increasingly important driving force of the adsorption, in addition to the formation of new water-COOH hydrogen bonds. Further, as the relative humidity increases, the growing number of water-water hydrogen bonds clearly overcompensates the effect of the decreasing average number of water-COOH 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. © 2011 American Chemical Society. Source

Idrissi A.,Lille University of Science and Technology | Vyalov I.,Lille University of Science and Technology | Vyalov I.,RAS Institute of Chemistry | Kiselev M.,RAS Institute of Chemistry | And 2 more authors.
Physical Chemistry Chemical Physics | Year: 2011

Binary mixtures of CO2 with ethanol and with acetone are studied by computer simulation, including extensive free energy calculations done by the method of thermodynamic integration, at 313 K, i.e., above the critical point of CO2 in the entire composition range. The calculations are repeated with three different models of acetone and ethanol, and two models of CO2. Comparisons of the molar volume of the different systems as well as of the change of their molar volume accompanying the mixing of the two components with experimental data reveal that, among the model pairs tested, the best results are obtained if both components are described by the Transferable Potentials for Phase Equilibria (TraPPE) force field. Around the ethanol/acetone mole fraction of 0.05 all ethanol/CO2 and almost all acetone/CO 2 model pairs considered predict the existence of a sharp maximum of the molar volume. Due to the lack of experimental data in this composition range, however, these predictions cannot be verified/falsified yet. Most of the model pairs considered also predict limited miscibility of these compounds, as seen from the positive values of the free energy change accompanying their mixing, and the miscibility gap is located at the same composition range as the aforementioned molar volume maximum. © 2011 the Owner Societies. Source

Partay L.B.,University of Cambridge | Horvai G.,HAS Research Group of Technical Analytical Chemistry | Horvai G.,Budapest University of Technology and Economics | Jedlovszky P.,HAS Research Group of Technical Analytical Chemistry | Jedlovszky P.,Eotvos Lorand University
Journal of Physical Chemistry C | Year: 2010

The properties of the interface between water and benzene are investigated in detail on the basis of 23 Monte Carlo computer simulations performed at various temperatures and pressures. The interfacial properties are analyzed in terms of the novel identification of the truly interfacial molecules (ITIM) method, by which the intrinsic (i.e., capillary wave corrugated) surface of the two phases can be detected. The obtained results show that the use of a simple, nonintrinsic definition of the interface (made on the basis of the average density profiles of the components) not only leads to a systematic error in determining the list of the truly interfacial molecules, but this error is also reflected in the erroneous calculation of the thermodynamic properties of the system. The obtained results show that the reciprocal width and reciprocal amplitude of both surface layers decrease linearly with the temperature and reach the value of zero (i.e., the corresponding parameters become infinite) at the point of mixing of the two phases. Similar linear relation is observed between these reciprocal quantities and the logarithm of the pressure, but only above a certain temperature. This temperature is thought to be the upper end of the lower critical line of the phase diagram of the system; however, any reliable support of this conjecture would require a considerably larger number of simulations in the temperature range close to this line. The orientational preferences of the surface water molecules, governed by the principle of maximizing the number of their hydrogen-bonded neighbors, are found to be insensitive to the thermodynamic conditions but become weaker with increasing temperature and decreasing pressure. The lateral hydrogen-bonding network of the surface water molecules, spanning the entire water surface at ambient conditions, is found to undergo a percolation transition well, i.e., 200-400 K below the mixing of the two phases, indicating that the existence of such a percolating lateral network is not a universal feature of the water surface but depends also on the thermodynamic conditions. © 2010 American Chemical Society. Source

Szri M.,University of Szeged | Szri M.,Czech Institute of Organic Chemistry And Biochemistry | Jedlovszky P.,Eotvos Lorand University | Jedlovszky P.,HAS Research Group of Technical Analytical Chemistry | Roeselova M.,Czech Institute of Organic Chemistry And Biochemistry
Physical Chemistry Chemical Physics | Year: 2010

Grand canonical Monte Carlo simulations are used to determine water adsorption on prototypical organic surfaces as a function of relative humidity at 300 K. Three model surfaces formed by well-ordered self-assembled monolayers (SAMs) of alkanethiolate chains on gold are investigated: (i) a smooth hydrophobic surface of methyl-terminated C7-CH3 SAM; (ii) a rough hydrophobic surface of randomly mixed two-component SAM, composed of equal fractions of C5-CH3 and C7-CH3 chains (C5/C7-CH3 SAM); and (iii) a smooth hydrophilic surface of carboxyl-terminated C7-COOH SAM. The all atom CHARMM22 force field is used for the SAM chains together with the SPC/E model for water. No noticeable water adsorption is observed on the smooth hydrophobic surface up to saturation. The mild surface roughness introduced by the uneven chain length of the two components constituting the C5/C 7-CH3 SAM has no significant effect on the surface hydrophobicity, and the rough hydrophobic surface also remains dry up to the point when water condensation occurs. In contrast, water readily adsorbs onto the hydrophilic surface by forming hydrogen bonds with the COOH groups of the substrate. In addition, hydrogen bonding with pre-adsorbed water molecules contributes to the mechanism of water uptake. Under low humidity conditions, water is present on the hydrophilic surface as individual molecules or small water clusters and, with increasing relative humidity, the surface coverage grows continuously beyond a monolayer formation. The adsorbed water film is observed to be rather inhomogeneous with patches of bare surface exposed. The amount of water constituting a stable adsorption layer prior to condensation is estimated to consist of about 2-5 molecular layers. Detailed analysis of the simulation results is used to obtain important insights into the structure and energetics of water adsorbed on highly oxidized organic surfaces exposed to ambient air of increasing relative humidity. © the Owner Societies. Source

Jorge M.,University of Porto | Jedlovszky P.,Eotvos Lorand University | Jedlovszky P.,HAS Research Group of Technical Analytical Chemistry | Cordeiro M.N.D.S.,University of Porto
Journal of Physical Chemistry C | Year: 2010

Substantial progress in our understanding of interfacial structure and dynamics has stemmed from the recent development of algorithms that allow for an intrinsic analysis of fluid interfaces. These work by identifying the instantaneous location of the interface, at the atomic level, for each molecular configuration and then computing properties relative to this location. Such a procedure eliminates the broadening of the interface caused by capillary waves and reveals the underlying features of the system. However, a precise definition of which molecules actually belong to the interfacial layer is difficult to achieve in practice. Furthermore, it is not known if the different intrinsic analysis methods are consistent with each other and yield similar results for the interfacial properties. In this paper, we carry out a systematic and detailed comparison of the available methods for intrinsic analysis of fluid interfaces, based on a molecular dynamics simulation of the interface between liquid water and carbon tetrachloride. We critically assess the advantages and shortcomings of each method, based on reliability, robustness, and speed of computation, and establish consistent criteria for determining which molecules belong to the surface layer. We believe this will significantly contribute to make intrinsic analysis methods widely and routinely applicable to interfacial systems. © 2010 American Chemical Society. Source

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