SELF-DIFFUSION OF THE PERCHLORATE ION IN AQUEOUS-ELECTROLYTE SOLUTIONS MEASURED BY CL-35 NMR SPIN-ECHO EXPERIMENTS

We have described the first application of Cl-35 NMR in a pulsed field gradient spin-echo experiment which represents a promising new techni que for determining the self-diffusion coefficient of perchlorate ions in solution. Specifically, we have been able to obtain the first expe rimental data for this diffusion coefficient in aqueous solutions of N aClO4, LiClO4 and Mg(ClO4)(2) at 25 degrees C. We have also reported s elf-diffusion coefficients of water in these solutions obtained by 1(H ) NMR. Including recently published data for Ga(ClO4)(3), at a given c oncentration the self-diffusion coefficient of ClO4- was found to decr ease in the sequence LiClO4 > NaClO4 > Mg(ClO4)(2) > Ga(ClO4)(3). This sequence is irregular for the sodium and lithium salts because, on th e grounds of the lower structure-forming ability of the Na+ ion, one w ould expect a higher mobility for ClO4- in solutions containing Na+ io ns. The latter ('regular') behaviour is indeed found for the self-diff usion 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 v ibrational bands of water, and the unusually strong concentration depe ndence of Na-23(+) relaxation in such solutions. Moreover, it is point ed out that mutual diffusion coefficients and transference numbers rep orted in the literature also behave in a highly anomalous manner. The results support the hypothesis that in the presence of large structure -breaking anions there are distinct cation-anion-water configurations. These appear to be stronger with Na+ as counter-ion than with Li+.