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

Abstract

Complex permittivity spectra of aq. solns. of Me4NBr, Et4NCl, Et4NBr, Pr4NBr, Bu4NBr, and Pe4NBr were detd. in the frequency range 0.2 ≤ ν/GHz ≤ 89. At 25°, data cover electrolyte concns. up to the satn. limit. Addnl., selected concns. of Pr4NBr and Bu4NBr were investigated as a function of temp. 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 mols. For Pr4NBr, Bu4NBr, and Pe4NBr, an addnl. 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 av. no. of hydrogen bonds is increased from ‾nHB = 2.5 to ‾nHB = 3.5.