Dielectric spectroscopy of ion-pairing and hydration in aqueous tetra-n-alkylammonium halide solutions

Complex permittivity spectra of aqueous solutions of Me4NBr, Et4NCl, Et4NBr, Pr4NBr, Bu4NBr, and Pe(4)NBr were determined in the frequency range 0.2 less than or equal to v/GHz less than or equal to 89. At 25 data cover electrolyte concentrations up to the saturation limit. Additionally, selected concentrations of Pr4NBr and Bu4NBr were investigated as a function of temperature. The spectra of Me4NBr, Et4NCl, and Et4NBr exhibit three dispersion steps which can be assigned with falling relaxation time to the tumbling motion of ion-pairs, the co-operative relaxation of the H-bond network of bulk water and the fast reorientation of mobile H2O molecules. For Pr4NBr, Bu4NBr, and Pe(4)NBr an additional dispersion step emerges, which is assigned to H2O in the hydration shell of the tetraalkylammonium ions. Compared to bulk water the dynamics of this slow water is reduced by a factor of 2.5-3 and the average number of hydrogen bonds is increased from (n) over bar (HB) = 2.5 to (n) over bar (HB) = 3.5.