THE STRUCTURE OF LIQUID WATER.

Modern theories of water structure fall into two general categories--mixture models and continuum models. The Frank-Wen model envisions liquid water as consisting of a mixture of flickering clusters of H-bonded water molecules and more or less free monomeric water. As the temperature increases the clusters melt and the fraction of H-bonded molecules in the liquid gradually decreases, although this fraction is till appreciable even at the boiling point. The clusters are less dense than the free water and as a consequence the application of hydrostatic pressure is very effective at destroying the regions of bound water. The addition of electrolytes may make or break the local water structure while the addition of nonpolar solutes or an interface tends to enhance the adjacent water structure. Two different types of hydration envelope are distinguished temperature and pressure stable, dense, hydrophobic hydration surrounding nonpolar solutes, hydrophobic molecules and hydrophobic segments of biopolymers, and gas-water and solid-water interfaces and coulombic hydration surrounding ions and charge sites. The coulombic hydration atmosphere is itself complex consisting of at least two zones--an inner zone of dense, tightly held electrostricted water and an outer, less dense zone which appears to be indistinguishable from a Frank-Wen cluster. Author