Two-State Theory of the Structure of Water

Abstract

A new model of the structure of water is proposed. This model is based on the existence in water of puckered hexagonal rings similar to those in ice. In water, these rings are assumed to coexist in two structures: an open-packed icelike structure optimum for forming hydrogen bonds between rings, and a close-packed structure in which the molecules occupy a nearly complete body-centered cubic structure. Mole fractions and volumes consistant with both the specific volume and the observed radial distribution curve of water are obtained. The structure of water is estimated to be approximately 60% icelike at 0°C and 30% icelike at 100°C. The corresponding fractional amount of hydrogen bonding at 0°C is found to be 82% in good agreement with recent estimates from Raman spectral observations. A simple two-state expression for the expansivity is shown to agree with the observed thermal expansion of water over the temperature range 0° to 100°C. The lattice component (and therefore, at 4°C, the magnitude of the relaxational or configurational component) is 1.634×10−3/C°. Using these values, estimates of various other two-state thermodynamic parameters are obtained. Both the specific heat and compressibility of water are found to be approximately 50% structural over the temperature range.