Properties of water/apolar interfaces as seen from Monte Carlo simulations

Monte Carlo simulations of five different water/apolar interfacial systems have been performed. In four of the systems the apolar phases have been different neat organic liquids, namely n-octane, benzene, CCl4 and 1,2-dichloroethane (DCE), whereas in the fifth system the apolar phase has been vapor. It is found that the structure of water at the interfacial region is remarkably similar in all the five systems. In particular, it is found that the orientation of the water molecules relative to the interface shows a dual preference in all the systems studied. The first of the two preferred orientations is parallel with the interface, whereas in the second preferred orientation the water molecule is perpendicular to the interface and points almost straight toward the apolar phase with one of its O-H bonds. When moving from the aqueous toward the apolar phase, the relative importance of the first orientation decreases. whereas that of the second orientation increases steadily. The solvation free energy profiles of the argon atom and the water molecule, as representatives of apolar and polar solutes, respectively, have been determined across the simulated water/DCE interface by the Cavity Insertion Widom method. It is found that the solvation free energy of both solutes has a minimum at the position of the interface, where the available free volume is about an order of magnitude larger than in any of the two bulk liquid phases. This finding supports recent theoretical and experimental findings suggesting that bulk water is separated from apolar phases by a thin vapor layer of the width of a few Angstroms. (C) 2003 Elsevier B.V. All rights reserved.