Dependence of Alkyl Chain Asymmetry on the Phase Equilibria of Three Catanionic Surfactant Mixtures Containing Dodecyltrimethylammonium Chloride−Sodium Alkylcarboxylate−Water

Abstract

The pseudoternary phase behavior of three aqueous systems with catanionic surfactant mixtures was studied at 20 °C, with emphasis on the effect of the level of asymmetry between the parent surfactants. The three mixtures all contain the cationic surfactant dodecyltrimethylammonium chloride (DoTAC), combined with one of the surfactants sodium hexanoate (SH), sodium octanoate (SO), or sodium decanoate (SD). As could be expected, all three systems form a variety of phases deriving from the binary DoTAC−water axis, and the extension of those phases vary with the anionic surfactant chain length. The binary sodium alkylcarboxylate/water systems do not show as rich phase behavior as DoTAC−water at this temperature, and the small hexagonal liquid crystalline phase present in the binary SO−water are destabilized by DoTAC in the pseudoternary system. This can be explained in terms of asymmetry between the surfactant chain lengths, which causes packing problems. Neither SH nor SD forms a hexagonal phase at room temperature, the former due to its short alkyl chain and the latter due to its high Krafft temperature. The DoTAC−SD−water phase diagram is dominated by a lamellar liquid crystalline phase, whereas the range of existence of the lamellar phases in the SO and SH systems are much more limited. The hexagonal phases deriving from the binary DoTAC system can incorporate varying amounts of alkyl carboxylate. The largest extension is found in the SO system, indicating a favorable level of asymmetry between the two surfactants. All three systems form a large isotropic solution phase with water. In the SD system, the solution phase has a discontinuity around the equimolar line at high water content. Up to 13 wt % of total surfactant, there is coacervation resulting in two translucent solutions, one dense and one dilute. In the most dilute region, the solutions within and close to the small two-phase area have a bluish appearance. All solution phases were studied by 1H self-diffusion NMR. The results show that all systems are most likely to form mixed aggregates at anionic/catanionic molar ratios <1. In the DoTAC−SH and −SO systems, there are indications of micelles and anionic surfactant monomers coexisting in the monophasic isotropic solutions at anionic/catanionic molar ratios ≥1. In the DoTAC−SD system, on the other hand, there seems to be coexistence of two different types of aggregates at the molar ratios larger than unity. The phase boundaries of the pseudoternary phase diagrams in this work were constructed on the basis of 2H NMR experiments and visual observations, and the microstructures of the one-phase regions were clarified by SAXS.