Osmotic and activity coefficients of monomethyl-, dimethyl-, and trimethylammonium chlorides at 25�C

Abstract

The osmotic and activity coefficients of monomethyl-, dimethyl-, and trimethylammonium chlorides in aqueous solution have been determined at 25°C by the gravimetric isopiestic method. The measurements extend from 0.1 molal (m) to 20, 17, and 15 mol-kg−1, respectively, for the three salts. In the region below 2m, the osmotic coefficients decrease in the order NH4Cl>MeH3NCl>Me2H2NCl>Me3HNCl>Me4NCl, and above 3m the order is reversed. The intermediate members of the series have identical osmotic coefficients at the crossover molality of 3.0. It is suggested that the behavior at low molalities reflects primarily ion-dipole interactions, decreasing as the cation size increases, whereas hydrophobic (structure-making) interactions, increasing in importance with the number of non-polar methyl substituents, are predominant in concentrated solutions of these salts.