Phase behaviour of microemulsions stabilised by double chain cationic surfactants and alcohol co-surfactants

We have investigated the phase behaviour of microemulsions stabilised by mixtures containing a strongly amphiphilic double chain cationic surfactant with a weakly amphiphilic short chain alcohol. For systems containing di-hydrogenated tallow di-methyl ammonium chloride (2HT) as the primary cationic surfactant, squalane as oil and 0.4 M NaCl as the aqueous phase, the Winsor II–III–I microemulsion inversion phase sequence is induced by addition of propan-2-ol (IPA) co-surfactant. Using a pseudo-ternary phase diagram representation of the different microemulsion phase regions, the adsorbed monolayer corresponding to zero preferred curvature is estimated to contain 6.7 mol of IPA per mol of cationic surfactant. Compositions of the microemulsion phases within the Winsor III multi-phase region are consistent with a bicontinuous microemulsion structure with an average domain size of 2–5 nm. Observed shifts in the microemulsion phase boundaries with changes in alcohol chain length, oil chain length, cationic surfactant structure, electrolyte concentration and temperature have been rationalised in terms of the effects of these variables on the preferred curvature of the adsorbed surfactant monolayer. Decreasing the chain length of the oil from squalane to hexadecane to dodecane causes a progressive increase in the IPA concentration required for zero preferred monolayer curvature. In turn, the increased alcohol content causes a progressive loss of microstructure in the microemulsions as evidenced by the shrinkage of the Winsor III phase region for hexadecane and its absence with dodecane. Thus, for these microemulsions stabilised by mixtures of a strong (2HT) and weak (IPA) amphiphile, both the preferred monolayer curvature and the extent of microstructure can be tuned.