Consistency of Ion Adsorption and Excess Surface Tension in Molecular Dynamics Simulations of Aqueous Salt Solutions

Molecular dynamics simulations of the liquid-vapor interface of aqueous solutions of sodium fluoride and of sodium iodide have been carried out using nonpolarizable force fields for ions and water molecules. Despite the absence of explicit polarizability, the tendency of iodide ions to show an enhanced concentration at the surface that was reported for polarizable force fields (Jungwirth, P.; Tobias, D. J. J. Phys. Chem. B 2001, 105, 10468) is reproduced, while sodium and fluoride ions prefer the interior of the bulk liquid. These observations are confirmed by the contributions of the different species to the surface potential. The systems we study here are much larger than the ones investigated in previous simulations by other authors, which enables us to calculate the adsorption of ions at the interface from the density profiles and subsequently-via Gibbs' adsorption isotherm-the corresponding excess surface tension over that of pure water. The so-obtained values for the surface tension are compared with the results calculated directly from the normal and lateral components of the pressure tensor in the simulation. Consistency is found among the data, but the directly obtained values have significantly larger error bars and are intrinsically more scattered. The Gibbs adsorption isotherm thus not only is a thermodynamic requirement to be met but also offers a reliable and less error-prone way of calculating the surface tension increment from density profiles.