An investigation on ionic solvation by refractometric measurements and electrostatic interaction calculations

Abstract

Lithium bromide, lithium perchlorate and sodium iodide solutions in acetonitrile, water and methanol have been analyzed by refractometry. Refractive index ( n) and density ( d) measurements provide an experimental value of the mean optical polarizability ḡa of solvated ions as a function of the solvation numbers ( n + and n -). The comparison with ḡa values calculated from a simple additivity model indicates the existence of solvation effects. More complete calculations by a dipole induced dipole method, taking into account interactive electrostatic effects, yield more precise information on the solvation. Thus, in acetonitrile, 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 infrared (IR) observations. Moreover, these calculations provide an explanation 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 ḡa values due to solvation in water is very low.