AN INVESTIGATION ON IONIC SOLVATION BY REFRACTOMETRIC MEASUREMENTS AND ELECTROSTATIC INTERACTION CALCULATIONS

Lithium bromide, lithium perchlorate and sodium iodide solutions in ac etonitrile, water and methanol have been analyzed by refractometry. Re fractive index (n) and density (d) measurements provide an experimenta l value of the mean optical polarizability alphaBAR of solvated ions a s a function of the solvation numbers (n+ and n-). The comparison with a values calculated from a simple additivity model indicates the exis tence of solvation effects. More complete calculations by a dipole ind uced dipole method, taking into account interactive electrostatic effe cts, yield more precise information on the solvation. Thus, in acetoni trile, n+ values from 3 to 4 for Li+ and about 5 for Na+ are evaluated , an ion-pair structure being favoured, in agreement with previous inf rared (IR) observations. Moreover, these calculations provide an expla nation for the high values of n, observed in previous light scattering experiments. In methanol and water, the expected orders of magnitude are obtained, in particular the change in a values due to solvation in water is very low.