CATION-CATION ASSOCIATION OF TETRAMETHYLAMMONIUM IONS IN AQUEOUS MIXED ELECTROLYTE-SOLUTIONS

We have measured the H-1 nuclear magnetic relaxation rates and the self-diffusion coefficients of (CH3)4N+ ions in aqueous solution of (CH3)4NBr as a function of salt concentration and in aqueous mixed electrolyte solution of x 4m (CH3)4NBr + (1-x) 4mMBr, (M = Li, K, Cs) as a function of the mole fraction x of (CH3)4N+. By measuring also the H-2 relaxation rate of the solvent D2O, we could separate the (CH3)4N+ proton relaxation rates into the intra- and intermolecular contribution. This allowed the determination of the so-called association parameter A, which is connected with the integral over the pair correlation function. The results for the cation-cation A-parameter show that in the pure (CH3)4NBr solution no or only weak (CH3)4N+-(CH3)4N+ attraction is present, whereas in the mixed electrolyte solutions containing KBr or CsBr a strong association of the hydrohobic cations occurs. With M = Li, this attractive interaction is also present, however less pronounced. The sequence of the effectiveness of the alkali ions is the same as in the so-called Hofmeister series for salting-out effects on biological macromolecules.