Introduction to Special Issue on Water and Associated Liquids

Abstract

In the past 40 years there has been major progress in developing a quantitative as well as qualitative understanding of the structural and thermodynamic properties of dense simple fluids and fluid mixtures both in bulk and at their liquid-vapor interfaces. Relatively simple theories are possible because in bulk systems the slowly varying attractive van der Waals forces on a given molecule tends to cancel and the liquid structure is dominated by the packing of the harshly repulsive molecular cores. At interfaces and in other nonuniform environments attractive forces no longer cancel but their averaged effects can be accurately treated by a simple mean field theory. Associated liquids present new and more difficult problems that are the focus of much current research, as illustrated by work in this special issue. Here we imagine there exist a small set of very strong but directionally localized attractive interactions that can compete efficiently with the core packing forces, generally producing a smaller number of strongly bonded neighbors than are generated by core forces alone. Under appropriate conditions this can lead to fluids with fluctuating networks of nearest neighbor bonds. Workers in this area often use model potentials that incorporate this physics in the simplest possible way and try to arrive at a theoretical understanding of their consequences. Determining typical configurations even in bulk systems is highly nontrivial. Incorporating large solutes or forming an extended interface necessarily disrupts the bulk bond network and this introduces additional theoretical and practical challenges.