Self-diffusion of the perchlorate ion in aqueous electrolyte solutions measured by 35Cl NMR spin–echo experiments

Abstract

We have described the first application of ³⁵Cl NMR in a pulsed field gradient spin–echo experiment which represents a promising new technique for determining the self-diffusion coefficient of perchlorate ions in solution. Specifically, we have been able to obtain the first experimental data for this diffusion coefficient in aqueous solutions of NaClO₄, LiClO₄ and Mg(ClO₄)₂ at 25 °C. We have also reported self-diffusion coefficients of water in these solutions obtained by ¹H NMR. Including recently published data for Ga(ClO₄)₃, at a given concentration the self-diffusion coefficient of ClO₄^– was found to decrease in the sequence LiClO₄ > NaClO₄ > Mg(ClO₄)₂ > Ga(ClO₄)₃. This sequence is irregular for the sodium and lithium salts because, on the grounds of the lower structure-forming ability of the Na⁺ ion, one would expect a higher mobility for ClO₄^– in solutions containing Na⁺ ions. The latter (‘regular’) behaviour is indeed found for the self-diffusion coefficients of water in these systems. These irregularities are discussed in relation to other anomalies in solutions of perchlorates and other structure-breaking anions, including the splitting of the vibrational bands of water, and the unusually strong concentration dependence of ²³Na ⁺ relaxation in such solutions. Moreover, it is pointed out that mutual diffusion coefficients and transference numbers reported in the literature also behave in a highly anomalous manner. The results support the hypothesis that in the presence of larger structure-breaking anions there are distinct cation–anion–water configurations. These appear to be stronger with Na⁺ as counter-ion than with Li⁺.