Phase equilibria and structures in ternary systems of a cationic surfactant (C₁₆ TABr or (C₁₆ TA)₂SO₄), alcohol, and water

Abstract

The dependence of alcohol chain length on the isothermal phase behavior of the ternary systems hexadecylrrimethylammonium bromide/alcohol/water has been investigated. A liquid crystalline phase (the normal hexagonal one) occurs in the phase diagrams along the surfactant/water axis and this phase extends in the interior of the diagrams.

When the alcohol is methanol, ethanol or butanol, there is in the ternary phase diagram a continuous solution region from the water to the alcoholic corner, and in the butanol case, in addition, a small region of lamellar liquid crystalline phase in the interior of the diagram. When the alcohol chain length is increased, the continuous solution region is divided into two subregions, an aqueousL ₁ and an alcoholicL ₂. The lamellar phase occupies the center of the phase diagrams and has the capability to incorporate large amounts of water under one-dimensional swelling. On the alcoholic side of the lamellar phase occur a reversed hexagonal liquid crystalline phase and a cubic liquid crystalline phase in the octanolic system; in the decanolic system the cubic phase is missing, but instead another liquid crystalline phase, presumably with rod-structure, occurs in addition to the reversed hexagonal phase.

In a decanolic system where the monovalent bromide ion is replaced by the divalent sulphate ion there are the same solution regionsL ₁ andL ₂, and phase regions with liquid crystalline normal hexagonal and lamellar structures. The lamellar phase has lost much of its capability of incorporating water. That is in analogy with the conditions in anionic systems where the counterion charge has been increased. There is no reversed hexagonal phase, but on the alcoholic side of the lamellar phase, there is the same “foreign” liquid crystalline phase with a presumed rod-structure as in the monovalent system.