Chemical Physics 129 ( 1989 ) 209-2 12 North-Holland, Amsterdam INTERACTION OF A SODIUM ION WITH THE WATER LIQUID-VAPOR INTERFACE Michael A. WILSON, Andrew POHORILLE Department of Chemistry. University of California. Berkeley, CA 94720, USA and Lawrence R. PRATT Chemical and Laser Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Received 1 April 1988 Molecular dynamics results are presented for the density profile of a sodium ion near the water liquid-vapor interface at 320 K. These results are compared with the predictions of a simple dielectric model for the interaction of a monovalent ion with this interface. The interfacial region described by the model profile is too narrow and the profile decreases too abruptly near the solution interface. Thus, the simple model does not provide a satisfactory description of the molecular dynamics results for ion positions within two molecular diameters from the solution interface where appreciable ion concentrations are observed. These results suggest that the surfaces associated with dielectric models of ionic processes at aqueous solution interfaces should be located at least two molecular diameters inside the liquid phase. A free energy expense of about 2 kcal/mol is required to move the ion to within two molecular layers of the free water liquid-vapor interface. 1. Introduction Presented here is a molecular dynamics observa- tion of the density profile of a sodium ion in a lamella of liquid water in coexistance with its vapor at 320 K. These results describe the free energy of interac- tion between a sodium ion and the liquid-vapor in- terface of a dilute aqueous solution. This interaction is a ba’sic quantity relevant to understanding the structure and thermodynamics of surfactant coatings on aqueous solutions [ 1,2], the stability of micelles composed of ionic surfactants [ 1,3,4], the confor- mational equilibria of macromolecules which can ex- pose ionic groups to aqueous environments [ 5 1, and the processes by which ions are transported between phases [ 6 1. More broadly, the investigations of phys- ical and chemical processes which transport charge within surface regions lead naturally to questions of variation of solvation free energies for charged spe- cies in these inhomogeneous environments. 2. Methods The calculation described here treated a film which was approximately seven molecular layers thick. This thickness represents a compromise so that the soli- tary ion thoroughly samples both interfacial and bulk regions of the solution. If the water film were too thick, then the interesting close encounters of the ion with the interfacial region would be infrequent. If the water film were too thin, then bulk solvation struc- tures would not be observed. The conditions of this calculation were similar to those in our previous calculations on the liquid-va- por interface of pure water [ 7,8]. In particular, the TIP4P pair potential model was adopted for inter- molecular forces between water molecules and a cor- responding potential was used for the interaction between the sodium ion and the water [ 91. Both the water-water interactions and the interactions be- tween the sodium ion and each water molecule were smoothly truncated with a cubic spline as has been discussed elsewhere [ 7, lo]. The locations of the 0301-0104/89/$03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )