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

Abstract

Molecular dynamics simulations of the vapor-liquid interface of water-methanol mixtures of five different compositions were performed on the canonical (N,V,T) ensemble at 298 K. In addition, the vapor-liquid interface of the two neat systems was simulated, as well. The obtained configurations were analyzed by means of the novel identification of truly interfacial molecules method, which provides a full list of the molecules that are right at the surface (i.e., at the boundary of the two phases). The molecular level roughness of the surface, the adsorption of the methanol molecules at the surface layer, the orientation of the surface molecules, the residence time of the molecules at the surface layer, as well as the surface aggregation of the molecules were analyzed in detail. Both the frequency and the amplitude of the surface roughness were found to become larger with an increasing methanol content. This effect was found to be stronger for the amplitude, which falls in the range of 2-4 A, depending on the composition of the system. Methanol was found 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-CH(3) bond remains perpendicular to the macroscopic plane of the surface, pointing the methyl group to the vapor phase. The main orientational preference of the water molecules is to lie parallel to the surface. Methanol was found to remain considerably longer at the surface layer of the mixed systems than water. Thus, contrary to the fact that the residence times of the two molecules were found to be rather similar to each other at the surface of their neat liquids, the residence time of the methanol molecules was an order of magnitude larger than that of water molecules at the surface of their 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 form two-dimensional aggregates, leaving the rest of the surface empty for the major component. The effect of the vicinity of the vapor phase on the properties of the molecules was found to vanish very quickly: the composition of 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.

Properties of the Liquid–Vapor Interface of Acetone–Water Mixtures. A Computer Simulation and ITIM Analysis Study

“…Thus, similar behavior was previously observed in the aqueous solutions of acetonitrile 44 and HCN, 46 in a clear contrast with the adsorption of methanol 43 or dimethyl sulfoxide (DMSO) 45 at the surface of their aqueous solutions, which is strictly restricted to one molecular layer. Further, the behavior of acetone in these aqueous systems is in a marked contrast with that in mixtures with methanol, in which practically no adsorption was observed.…”

“…The results are compared with those obtained previously for other aqueous binary mixtures [43][44][45][46] as well as for mixtures of acetone with methanol. 32 The paper is organized as follows.…”

“…capillary waves. Approximating the interfacial region by a slab parallel with the Gibbs dividing surface was repeatedly shown to lead to a systematic error of unknown magnitude in the structural properties as well as composition of the interfacial layer, [41][42][43][44][45][46] and even propagates to the thermodynamic properties of the binary system. 47 Following the pioneering paper of Chacón and Tarazona, 48 several methods have been proposed to circumvent this problem and to detect the real, capillary wave corrugated, intrinsic liquid surface.…”

Properties of the intrinsic surface of liquid acetone, as seen from computer simulations

“…On the other hand, the neglect of these capillary waves in the analysis, and the use of a slab parallel with the Gibbs dividing surface as the 'surface layer', as done in both previous simulations of the acetone surface [7,40] and in numerous studies of other liquid surfaces, leads to the misidentification of a large set of molecules. Thus, a large number of molecules that are surrounded by like neighbours in all directions are incorrectly identified as 'interfacial' ones, and also many molecules that are at the boundary of the two phases are wrongly identified as 'non-interfacial ones' this way [43][44][45][46][47][48]. This misidentification results in a systematic error not only in the structural properties of the interface [43][44][45][46][47], but also in the calculated thermodynamic properties of the system studied [48].…”

Two-dimensional percolation at the free water surface and its relation with the surface tension anomaly of water

“…The surface of a fluid phase is corrugated by capillary waves, and therefore its surface cannot be defined in terms of an external (i.e., simulation boxfixed) coordinate frame. Although the majority of the early interfacial simulations simply disregarded this problem, the non-intrinsic (i.e., external frame-fixed) treatment of the fluid surface has repeatedly been shown to lead to systematic errors not only in the structure 4, 5 or composition [6][7][8] of the surface layer but also in the thermodynamic properties of the system. 9 Several methods have been proposed in the literature for identifying the truly interfacial molecules, and thus detecting the real, intrinsic (i.e., capillary wave corrugated) surface of a fluid phase.…”