Thermodynamics of Ion Hydration as a Measure of Effective Dielectric Properties of Water

Abstract

An improved method was developed for evaluating the changes in thermodynamic properties associated with hydration of individual species of gaseous monatomic ions. Application of the method to entropy data leads to consistent results only by neglecting the contributions observed during solvation of neutral atoms of the same size. The partial molal entropy of hydrogen ion at 25 deg is estimated to be -3.3 cal/mole deg in satisfactory agreement with thermocell calculations. For cations having the electronic structure of an inert gas, the free energies of hydration indicate effective dielectric constants that depend only on size of ion and are virtually independent of charge. These effective dielectric constants can be used to estimate local dielectric constant as a function of distance from the center of a cation, but the data are not sensitive enough to permit extrapolation beyond the radius of the largest cation studied. Cations not having an inent gas structure show larger absolute enthalpies and free energies of hydration as predicted by ligand field theory, The singly charged d/sup 10/ ions Cu/sup +/, Ag/sup +/, and Au/sup +/ exhibit extreme solvation effects that are not observed for any other ions, including the isoelectronic species Zn/sup 2+/, Cd/sup 2+/, andmore » Hg/sup 2+/. Thermodynamic properties change more during the hydration of anions than for cations of the same size, and the few available data do not exhibit the monatonic variation with ionic size that is observed with cations. (auth)« less