Osmotic and activity coefficients for the symmetrical tetraalkyl ammonium halides in aqueous solution at 25°

Abstract

The symmetrical tetraalkyl ammonium halides have been employed in a study of the effect of cation size on electrolyte behavior in aqueous solution. Osmotic and activity coefficients were measured by the gravimetric isopiestic vapor pressure technique for tetramethyl-, tetraethyl-, tetrapropyl-, and tetrabutylammonium chlorides, bromides, and iodides. For concentrations below 1 m, the osmotic coefficients for the chlorides increased with the size of the cation: Bu4N+ > Pr4N+ > Et4N+ > Me4N+. The bromides and iodides, however, showed the reverse order. At higher concentrations, the osmotic coefficients of the larger cation salts decreased sharply and some of the sequences were reversed. The dependence of the osmotic and activity coefficients on cation size and concentration is explained as the result of three effects. The order for the chlorides in dilute solution is due to the ability of large organic cations to enforce the water structure, and the more carbon atoms on an ion, the larger this effect will be. However, with the bromides and iodides, "water structure-enforced ion pairing" overshadows the "structure making" ability of the cations, causing an opposite order in the osmotic coefficients. At higher concentrations the tetrabutyl salts form micelles, and hence a sharp lowering of the osmotic coefficient is observed.