MOLECULAR-STRUCTURE OF AQUEOUS INTERFACES

In this review we summarize recent progress in our understanding of th e structure of aqueous interfaces emerging from molecular level comput er simulations. It is emphasized that the presence of the interface in duces specific structural effects which, in turn, influence a wide var iety of phenomena occurring near the phase boundaries. At the liquid-v apor interface, the most probable orientations of a water molecule is such that its dipole moment lies parallel to the interface, one O-H bo nd points toward the vapor and the other 0-H bond is directed toward t he liquid. The orientational distributions are broad and slightly asym metric, resulting in an excess dipole moment pointing toward the liqui d. These structural preferences persist at interfaces between water an d nonpolar liquids, indicating that the interactions between the two l iquids in contact are weak. It was found that liquid-liquid interfaces are locally sharp but broadened by capillary waves. One consequence o f anisotropic orientations of interfacial water molecules is asymmetri c interactions, with respect to the sign of the charge, of ions with t he water surface. It was found that even very close to the surface ion s retain their hydration shells. New features of aqueous interfaces ha ve been revealed in studies of water-membrane and water-monolayer syst ems. In particular, water molecules are strongly oriented by the polar head groups of the amphiphilic phase, and they penetrate the hydrophi lic head-group region, but not the hydrophobic core. At infinite dilut ion near interfaces, amphiphilic molecules exhibit behavior different from that in the gas phase or in bulk water. This result sheds new lig ht on the nature of hydrophobic effect in the interfacial regions. The presence of interfaces was also shown to affect both equilibrium and dynamic components of rates of chemical reactions. Applications of con tinuum models to interfacial problems have been, so far, unsuccessful. This, again, underscores the importance of molecular-level informatio n about interfaces.