Phase behavior of alkanolammonium carboxylates

Abstract

Partial binary phase diagrams for different alkanolammonium carboxylates with water have been determined. The alkanolamines employed were monoethanolamine, triethanolamine and triisopropanolamine, while the fatty acid has been varied with respect to chain length (C8–C22) and saturation.

The phase diagrams show two features that distinguish them from ordinary soap-water phase diagrams. First, multi-phase regions and miscibility gaps occur at low concentrations of surfactant, in particular, for triethanolammonium carboxylates. Secondly, the stability regions of the liquid crystalline phases are different, and in general smaller,compared to those formed by the corresponding alkali carboxylates. It is suggested that the first feature is due to that alkanolamines are weak bases and, thus, that any surfactant aggregate comprises a few percent of unhydrolyzed acid, while the second feature is attributed to the larager size of the counterion compared to alkali ions, which tends to destabilize ordered structures.