Dielectric relaxation of electrolyte solutions in acetonitrile

Complex permittivity spectra in the frequency range 0.95v (GHz)89 for acetonitrile and its solutions of LiBr, NaI, NaClO₄, and Bu₄NBr at 25°C show one Debye equation for the neat solvent whereas the superposition of a Debye process for the solute and a Cole-Cole distribution for the solvent is necessary to account for the dielectric relaxation behavior of the solutions. The reorientation of bulk acetonitrile is diffusive and only weakly coupled to viscosity. The number of solvent molecules irrotationally bound to the electrolyte is in good agreement with conventional solvation numbers for all electrolytes, when kinetic depolarization is assumed to be negligible. The solute relaxation process is dominated by the formation kinetics and reorientation of contact ion pairs. There is evidence for solvent-shared ion pairs in dilute NaClO₄ solutions.