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

Abstract

Complex permittivity spectra of aqueous solutions of Me₄NBr, Et₄NCl, Et₄NBr, Pr₄NBr, Bu₄NBr, and Pe₄NBr were determined in the frequency range 0.2 ≤ ν/GHz ≤ 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 Pe₄NBr 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 _HB = 2.5 to _HB = 3.5.