Investigation of the Liquid–Vapor Interface of Water–Formamide Mixtures by Computer Simulation and Intrinsic Surface Analysis

Abstract

Molecular dynamics simulations of the liquid− vapor interface of water−formamide mixtures of different compositions, spanning the entire composition range, have been performed in the canonical (N, V, T) ensemble at 300 K. The layer of the surface molecules has been identified and analyzed in detail in terms of the intrinsic identification of the truly interfacial molecules (ITIM) method. The obtained results reveal the strong lateral hydrogen-bonding ability of the surface molecules in these systems. Thus, the surface molecules form a percolating lateral hydrogen-bonding network in every case, which makes the surface layer noticeably more compact than the subsequent subsurface molecular layers. However, the molecules mix with each other even on the molecular scale. Neither strong adsorption, nor lateral self-association of any of the components in the surface layer has been observed, although formamide exhibits a slight ability of being accumulated in the surface layer. Further, in contrast with the aforementioned percolating lateral hydrogen-bonding-mixed network of the surface water and formamide molecules, no such network of the like molecules is observed, even in the case of their large excess at the surface of the mixed systems. The orientational preferences of the surface molecules are also governed by the requirement of maximizing their hydrogen bonds. The main preference of both molecules is found to be the parallel alignment with the macroscopic plane of the liquid surface. The dynamics of the surface molecules is also dominated by the hydrogen bonding of unlike pairs. Thus, water stabilizes the stay of the formamide molecules at the liquid surface, whereas formamide immobilizes the water molecules within the surface layer, preventing them from a considerable lateral diffusion during their entire stay at the liquid surface.

Investigation of the liquid-vapour interface of aqueous methylamine solutions by computer simulation methods

“…This finding reveals that, similarly to DMSO [54] and formamide [55], but in contrast with acetonitrile [49] and HCN [52], the methylamine molecules do not show considerable selfassociation at the surface of their aqueous solution.…”

“…On the other hand, methylamine molecules are noticeably depleted from the second layer, and the composition of only the third subsurface molecular layer agrees reasonably well with that of the bulk liquid phase. In this respect, the aqueous solution of methylamine behaves in a markedly different way from that of acetonitrile [49], HCN [52], and acetone [53], the adsorption of which involves several molecular layers, that of methanol [27] and DMSO [50], the adsorption of which involves also only the first molecular layer, however, no depletion of them occurs in the second layer, and also that of formamide [55], which shows no considerable adsorption even in the first molecular layer. However, in contrast with their clear preference for staying at the liquid surface, no considerable tendency of self-association of the methylamine molecules has been seen in the surface layer.…”

“…However, the surface adsorption of methylamine is strictly limited to the first layer, moreover, methylamine is even noticeably depleted in the second molecular layer, while the composition of the third subsurface layer is already essentially identical with that of the bulk liquid phase. In this respect, methylamine behaves in a markedly different way (i) than acetonitrile [49], HCN [52], and acetone [53], the adsorption of which involves several molecular layers beneath the surface of their aqueous solutions, (ii) than formamide, which is not adsorbed strongly at the surface of its aqueous solution [55], and (iii) also than methanol [27] and DMSO, [50] which are also strongly adsorbed in the first molecular layer, but do not show noticeable depletion in the second layer.…”

“…Among the various methods, the Identification of the Truly Interfacial Molecules (ITIM) [26] was shown to represent an excellent compromise between computational cost and accuracy [31]. Intrinsic surface analyzing methods have been successfully applied in the past 15 years to the liquid-liquid and liquid-vapour interfaces of several neat molecular liquids [26,[28][29][30][31]36,40,[44][45][46][47][48] and their binary mixtures [27,[49][50][51][52][53][54][55] as well as for aqueous electrolyte solutions [56], ionic liquids [57][58][59][60][61], mixtures of ionic and molecular liquids [62], aqueous surfaces covered by amphiphilic polymers [63,64] or surfactants [64][65][66] as well as for lipid membranes [67]. Further, profiles of several physical quantities, such as the density [36,38,45,[68][69][70][71], energy [71], solvation free energy [72,…”

Adsorption of Formamide at the Surface of Amorphous and Crystalline Ices under Interstellar and Tropospheric Conditions. A Grand Canonical Monte Carlo Simulation Study

“…Similar orientational preferences were obtained earlier at the liquid-vapor interface of water-formamide mixtures. 117 The preference of the formamide molecules for these orientations can be understood by 70 At the surface of LDA ice, these preferences are much less sharp, the corresponding peaks of the orientational map are strongly merged, indicating that surface waters can rather largely deviate from these orientations. As a consequence, the co-planar alignment with the macroscopic plane of the surface is also preferred here.…”