Molecular Dynamics Simulations of the Liquid/Vapor Interface of Aqueous Ethanol Solutions as a Function of Concentration

Abstract

Molecular dynamics computer simulations are utilized to study the structural and thermodynamic properties of the liquid/vapor interface of aqueous ethanol solutions as a function of concentration. In addition, the free energy profile for inserting a single ethanol molecule into a 0.059 mole fraction aqueous ethanol solution is calculated using statistical mechanical perturbation theory. The calculated free energy for solvation of an ethanol molecule in the bulk solution, the surface tension as a function of ethanol concentration, and the average orientation of ethanol molecules at the solution/vapor interface are in agreement with the corresponding experimental data. The calculated equilibrium free-energy profile, however, exhibits a barrier to solvation that is considerably smaller than that predicted by the resistance model for mass accommodation.

An overview of current issues in the uptake of atmospheric trace gases by aerosols and clouds

“…In the uptake experiments by (Jayne et al, 1991) the gas phase density of ethanol was about 5×10 13 molecules cm −3 , so that at saturation the aqueous phase concentration is on the order of 2×10 17 molecules cm −3 (about 4×10 −4 M or a mole fraction of 7×10 −6 ) and surface concentration is about 1×10 18 molecules cm −3 (about 2×10 −3 M or a mole fraction of 4×10 −5 ). Stewart et al (2003) performed simulations of the free energy profile for ethanol uptake into a 0.059 mole fraction aqueous ethanol solution and found the same qualitative behavior as in the infinitely dilute case. Changes of solvation and adsorption free energies with concentration were less than the uncertainties in the computed values.…”

“…Molecular simulations of aqueous interfaces provide detailed information about the structure, energetics, and dynamics of interface and molecular processes occurring at the boundary between vapour and liquid water (Pohorille and Benjamin, 1993;Pratt and Pohorille, 2002;Garrett et al, 2006). Many interaction potentials for water (Rahman, 1974;Lemberg and Stillinger, 1975;Rahman et al, 1975;Lie et al, 1976;Matsuoka et al, 1976;Stillinger and Rahman, 1978;Berendsen et al, 1981Berendsen et al, , 1987Jorgensen et al, 1983;Carravetta and Clementi, 1984;Sprik and Klein, 1988;Niesar et al, 1989Niesar et al, , 1990Benjamin, 1991;Zhu et al, 1991;Caldwell and Kollman, 1995;Dang and Chang, 1997) have been used in simulations of aqueous interfaces (Townsend et al, 1985;Wilson et al, 1987a;Pohorille and Wilson, 1993;Taylor et al, 1996). The results from these simulations vary quantitatively from simulation to simulation, but they provide a consistent qualitative picture of the interface:…”

Marangoni flow revisited

“…2 that the change of surface tension with alcohol concentration, dγ /dc, is more than 6 times greater for a ∼5% solution than it is for a 40% solution (and see also Refs. [15,16]). Since dγ /dc is higher at lower values of c one would expect the Marangoni effect to be more pronounced at lower alcohol concentrations.…”

“…The fraction, c, is V Et /V Tot which stands for the ethanol volumetric fraction. The line is a guide to the eye; data is taken from[15,16].…”

A new modified parachor model for predicting surface compositions of binary liquid mixtures. On the importance of surface volume representation

“…There are several approaches that have been proposed to determine surface composition. This includes direct experimental measurements [5][6][7][8][9][10], calculation methods employing the Gibbs adsorption isotherm, the classical thermodynamics model, the semi-empirical/empirical models, density functional theory (DFT) [11] as well as simulations using molecular dynamics [12,13]. It should be mentioned that the direct experimental measurement of surface composition is generally quite difficult and only relatively few systems have been investigated so far.…”