Self-Assembly in Systems of Didodecyldimethylammonium Surfactants: Binary and Ternary Phase Equilibria and Phase Structures with Sulphate, Hydroxide, Acetate, and Chloride Counterions

Abstract

The phase equilibria for four binary (temperature vs composition) surfactant systems didodecyldimethylammonium sulphate (DDAS), hydroxide (DDAOH), acetate (DDAAc), and chloride (DDAC) with water and one isothermal ternary system (DDAS-water-dodecane) have been studied by water deuteron NMR and polarizing microscopy methods. Both DDAS and DDAC surfactants are practically insoluble in water (≈3.5 × 10 -3 m DDAS and ≈ 1.4 × 10 -2 m DDAC) (m, moles of surfactant per kg of water) at 298 K, but they swell in water, each giving rise to a lamellar liquid crystalline phase. On the other hand, DDAOH and DDAAc are easily soluble in water (1.1 m surfactant) forming extended isotropic micellar solution phases which are followed by a lamellar liquid crystalline phase. The single lamellar phase formed in the surfactant system with divalent SO 2- 4 counterion can incorporate significantly smaller amounts of water ( n ≈ 14.4 moles of water per mole surfactant ion) between their lamellae compared to the corresponding system with monovalent Cl - ( n ≈ 102), OH - ( n ≈ 37), and CH 3COO - ( n ≈ 42) counterions, whereas the minimum amounts of water ( n = 3-5) necessary to form the lamellar phase does not differ significantly among the systems. Moreover, all systems yield only one lamellar liquid crystalline phase compared to the bromide (DDAB) system in which two lamellar phases are obtained in the aqueous system. Unlike other systems, the DDAS system, on heating, forms two isotropic solution phases: one, water-rich L 1 phase exists below ≈5 wt% of surfactant and the other, L 2 phase, above 45 wt% of surfactant and the two phases coexist above 353 K. Thermal stability of the lamellar phase increases in the order DDAAc ≈ DDAS < DDAC < DDAOH ≈ DDAB. The lamellar phase formed in the binary sulphate system has very limited swelling capability with dodecane (≈3%). In addition, in the ternary DDAS system three new homogeneous phases are found to exist; a cubic liquid crystalline phase in the oil-rich region (≈49% oil), a hexagonal liquid crystalline phase between ≈30-38% of water and ≈20-30% of oil and a narrow isotropic solution phase with few percent of oil and extending between ≈62-almost 100% of water. The phase behavior of the ternary DDAS system is almost identical to that of the ternary DDAB system, but the location of single phases in the triangle and microstructure of some phases are drastically different between the two systems. The phase behavior and phase stability may be understood in terms of electrostatic effects and surfactant packing parameters.