Factors affecting diffusion coefficient of the hydroxide ion in aqueous tetraalklylammonium nitrate solutions

Abstract

The effect of OH concentration on its diffusion coefficient in aqueous Me4NN03 and BU4NN03 solutions at total ionic strength of 0.50 mol dm-3 was studied. The diffusion coefficient was found to increase from a Iow value and reached a steady-state. The possibility of ion-pair formation is dis-cussed. The mobility of the hydroxide ion is less in aqueous BU4NN03 than in the corresponding solutions of alkali metal nitrates. This was attributed to the hydrophobic hydration of BU4N+ ion. In the concentration range of 0.10 to 0.50 mol dm-3 Bu4NN03, the rapid decrease of the diffusion coefficient of the hydroxide ion with increasing solute concentration was found to be mainly due to the obstruction effect of the cations aggregates. Bu4N+ ions tended to interact in the form of an assembly of three ions with their hydrophobic hydration spheres overlapping with each other. No structural effects on the water molecules were found to be necessary to explain this behaviour of the hydroxide ion.