PREDICTION OF ION ASSOCIATION IN MIXED-CRUSTAL FLUIDS

A dual-solvent-region model has been developed which permits calculation of the electrostatic free energy of solvation for ions and dipoles in mixtures of water with organic compounds or others volatiles. In this model, an ion (or a dipole) of radius r(i) is surrounded by an organized solvent layer of thickness b-r(i) with a dielectric constant epsilon-1; beyond this layer exists the bulk solvent, assumed to be a dielectric continuum. Taking r(i) equal to the ionic crystal radius, b - r(i) equal to the water molecule radius, and epsilon-1 = 2, excellent agreement is found between ion-association constants measured in mixed solvents and those calculated from the dual-solvent model over wide ranges of temperature and pressure. To account for dielectric saturation, inverse radial exponential functions were incorporated into the model. These functions lead to only a small correction to the initial model, which suggests that dielectric saturation is insignificant for monovalent ions. Similar calculations indicate that dielectric saturation is not significant for multivalent ions, if it is assumed that each ion is surrounded by two layers of organized water. Because the dual-solvent model does not require the fitting of input parameters, it can be readily used to calculate ion-association constants of many species in a large number of systems by adopting the Kirkwood equation and the Looyenga mixing rules to approximate the dielectric constant of the solvent mixture.