Self-Consistent Ion-Dipole Continuum Models for Ionic Hydration Enthalpies and Conductances

Abstract

Thermodynamic, gaseous vacuum hydration enthalpies for the aqueous alkali metal and halide ions are examined in terms of an ion-dipole interactive force proportional to 1/r id 2. For elements whose ions form highly ionic bonds, the relationship approximates linearity and is sign independent. Calculated enthalpies from the electrostatic dipole interaction are used to assess the extent of ionic bonding and indicate 3 primary waters of hydration. A self-consistent model based on experimental molar conductances and solvent ion radii is developed to explain the behavior of small ionic radii species. It is proposed that the conductance behavior is initiated by field alignment of attached water dipoles at a Gaussian surface radius r id . The different behavior of cations and anions is accounted for by differences in their dipole moment vectors.