Dielectric Relaxation of Ion-pairs, Micelles and Hydration in Aqueous Hexyltrimethylammonium Bromide Solutions

Abstract

Through dielectric measurements of aqueous n-hexyltrimethylammonium bromide solutions, the structure and dynamics of ion-pairs, micelles, and water are investigated as functions of concentration and temperature. The frequency range of the study was ∼ 0.2 ≤ν/GHz ≤ 89, and the concentrations were changed from 0.0448 to 0.8476 mol.dm⁻³ at 298.15 K. The effects of temperature (278.15 ≤ T/K ≤ 338.15) on two selected solutions with concentrations above and below the critical micelle concentration were examined. A summation of three Debye processes fits the complex permittivity data of solutions with concentrations below the critical micelle concentration. To fit the spectra above the critical micelle concentration, a summation of five Debye processes is needed. Below the critical micelle concentration, solutes relax due to the tumbling motion of solvent-shared ion pairs. The two micelle relaxation processes that have been seen are the result of free and bound counter ions tangentially and radially diffusing around the charged micelles. Maxwell-Wagner relaxation is equivalent to the high frequency micelle relaxation process. Both the solvent and micelle dispersion provide acceptable volume fractions. Hexyltrimethylammonium behaves like higher n-alkyltrimethylammonium micelles in the context of micelle relaxation. Before critical micelle concentration, the cations, anions, and ion pairs strongly hydrate a total of ∼18 ± 4 non-rotational water molecules. Beyond the critical micelle concentration, there are an additional ∼ 5 ± 2 non-rotational water molecules in addition to ∼ 15 ± 3 slower-moving water molecules in the hydrophobic hydration shell.