Properties of Free Surface of Water-Methanol Mixtures. Analysis of the Truly Interfacial Molecular Layer in Computer Simulation

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• 作者：L&iacute ; via B. Pá ; rtay ; Pá ; l Jedlovszky ; Á ; rpá ; d Vincze ; George Horvai
• 刊名：Journal of Physical Chemistry B
• 出版年：2008
• 出版时间：May 1, 2008
• 年：2008
• 卷：112
• 期：17
• 页码：5428 - 5438
• 全文大小：487K
• 年卷期：v.112,no.17(May 1, 2008)
• ISSN：1520-5207

文摘

Molecular dynamics simulations of the vapor-liquid interface of water-methanol mixtures of five differentcompositions were performed on the canonical (N,V,T) ensemble at 298 K. In addition, the vapor-liquidinterface of the two neat systems was simulated, as well. The obtained configurations were analyzed bymeans of the novel identification of truly interfacial molecules method, which provides a full list of themolecules that are right at the surface (i.e., at the boundary of the two phases). The molecular level roughnessof the surface, the adsorption of the methanol molecules at the surface layer, the orientation of the surfacemolecules, the residence time of the molecules at the surface layer, as well as the surface aggregation of themolecules were analyzed in detail. Both the frequency and the amplitude of the surface roughness werefound to become larger with an increasing methanol content. This effect was found to be stronger for theamplitude, which falls in the range of 2-4 Å, depending on the composition of the system. Methanol wasfound to be adsorbed at the surface layer, being preferentially at the humps of the molecularly rough surface.Surface methanol prefers to orient in such a way that the O-CH₃ bond remains perpendicular to the macroscopicplane of the surface, pointing the methyl group to the vapor phase. The main orientational preference of thewater molecules is to lie parallel to the surface. Methanol was found to remain considerably longer at thesurface layer of the mixed systems than water. Thus, contrary to the fact that the residence times of the twomolecules were found to be rather similar to each other at the surface of their neat liquids, the residence timeof the methanol molecules was an order of magnitude larger than that of water molecules at the surface oftheir mixtures. A strong lateral microscopic segregation of the molecules was observed at the surface layer;the minor component of the system (irrespective of whether it was water or methanol) was found to formtwo-dimensional aggregates, leaving the rest of the surface empty for the major component. The effect of thevicinity of the vapor phase on the properties of the molecules was found to vanish very quickly: the compositionof the second layer as well as the properties of the molecules of this layer (e.g., dynamics and orientation)did not differ considerably from those in the bulk liquid phase.