THEORETICAL CALCULATION OF THE WATER ION PRODUCT K-W

A dielectric solvation model is applied to the prediction of the equil ibrium ionization of liquid water over a wide range of density and tem perature with the objective of calibrating that model for the study of ionization in water of organic acids, e.g., proteins and nucleic acid s. The model includes an approximate description of the polarizability of the dissociating water molecule. The calculated pK(W) are very sen sitive to the value of the radii that parametrize the model. The radii required for the spherical molecular volumes of the water molecule in order to fit the experimental ion product are presented and discussed . These radii are larger than those commonly used. They decrease with increasing density as would be guessed but the rate of decrease is sli ght. They increase with increasing temperature, a variation opposite t o what would be guessed if radii were strictly viewed as a distance of closest approach. The molecular theoretical principles that might pro vide an explanation of the thermodynamic state dependence of these rad ii are discussed.