Electrostatic Properties of Aqueous Interfaces Probed by Small Solutes

Andrew Pohorille, Michael A. Wilson and Karl Schweighofer, in Simulation and Theory of Electrostatic Interactions in Solution, Pratt, L. R. and Hummer, G., Eds., AIP, in press.

Biomolecular and Cellular Modeling Program
NASA Ames Research Center, Moffett Field, CA 94035
Department of Pharmaceutical Chemistry,
University of California, San Francisco, CA 94143

The excess chemical potentials of methane and its four fluorinated derivatives across the water-hexane, water-octanol, water-glycerol 1-monooleate and water-1-palmitoyl 2-oleoyl sn-glycero 3-phosphatidylcholine (POPC) interfaces are calculated using the particle insertion method. In all cases, the polar species exhibit interfacial minima indicating that these molecules tend to accumulate in the interfacial region, while the nonpolar molecules exhibit no such minimum. The excess chemical potentials are further partitioned into electrostatic and non-electrostatic terms. For polar molecules, the electrostatic term changes nearly linearly over the distance of approximately 10 Å in the interfacial region and appears to depend only weakly on the nature of the interface. Solute molecules are not oriented isotropically at the interface, but tend to align themselves with the excess electric field created by the anisotropic interfacial environment. Using dipoles in a cavity as models, it is further shown that, in the water-POPC system, the electrostatic term changes with the size of the dipole according to the predictions of linear response theory. The less satisfactory performance of this approximation found for other interfacial systems may be an artifact of neglecting long-range effects during simulations of neat interfaces. The non-electrostatic term, dominated by the reversible work of cavity formation, shows interfacially induced structure. In particular, it is responsible for a maximum of the excess chemical potential on the dense, water side of the water-POPC interface. The results of this study provide guidance to developing simple but accurate implicit models of interfacial systems.