Counterion effects in the self-assembly of didodecyldimethylammonium halides and sulphate surfactants

Abstract

Binary phase diagrams of the aqueous didodecyldimethylammonium surfactant with four different counterions — Cl− (DDAC), OH− (DDAOH), CH3COO− (DDAAc) and SO 4 2− (DDAS) are determined at 298K. A lamellar liquid crystalline phase is identified for all systems. DDAOH and DDAAc also form normal micellar solution phases in addition to lamellar liquid crystals. The swelling of the lamellar phase at maximum water is much reduced with SO 4 2− counterion compared to that with other monovalent counterions. The ternary phase diagram determined for the system DDAS-water-dodecane at 298K shows that the system forms a water-rich solution phase with oil-in-water type structure, a normal hexagonal liquid crystalline phase at intermediate water contents, and an oil-rich cubic phase with a bicontinuous-type structure, in addition to the lamellar phase formed in the binary system.