Effect of salts on the diffusion of dissolved non-electrolytes

Abstract

Experimental measurements of the diffusion coefficient D_1m of non-electrolytes in electrolyte solutions are reported at 25°C. The systems studied are methane in aqueous NaCl, NaBr, NaI, CaCl₂ and LaCl₃, neopentane in NaCl and CaCl₂, and benzene in (Me)₄NBr, (Et)₄NBr and (n-Bu)₄NBr solutions. In all cases, D_1m was reduced by the addition of the salt. This decrease became greater as the ionic charge and ionic radius increased for a given ionic concentration. The fractional decrease in D_1m was greater for neopentane than for methane. Experimental results are compared with the multicomponent rigid sphere kinetic theory. The model is able to account for qualitative trends in the behaviour of D_1m for all of the systems except those involving tetra-alkyl-ammonium salts; the failure for these latter systems suggests that strong attractive forces are present, and that these forces have a pronounced influence on the collision dynamics.