Hydration and association in solutions of tetraalkylammonium iodides

Abstract

Conductivity, self-diffusion coefficient and viscosity were measured at 298 K for tetraalkylammonium iodides in aqueous solutions, 0.5 M urea solutions and mixtures of water with tert-butanol. The molar conductivity was analysed via the Fuoss–Hsia equation. The limiting molar conductivity, the separation of ions forming the ion pairs and the association constants were determined. In all studied systems the association of opposite ions was weak, but contact ion pairs were present. The hydrodynamic radii of ions, determined from the self-diffusion coefficients, were used to compute the dynamic hydration numbers. As expected, the dynamic hydration numbers were noticeably smaller than the static numbers, because they represented the water molecules located inside the R₄N⁺ ions, between the alkyl chains. Addition of tert-butanol reduced the hydration of Me₄N⁺ and, probably, Bu₄N⁺ ions, but induced aggregation of Bu₄N⁺ cations only.