Effect of didodecyldimethylammonium bromide on the phase behavior of nonionic surfactant-silicone oil microemulsions

The effects of adding didodecyldimethylammonium bromide (DDAB) to mixtures of n-alkyl polyglycol ether (CiEj), water, and silicone oil are systematica lly studied. In mixtures of C8E3 and C12E6 with water, small amounts of DDA B cause the upper miscibility gap to vanish and be replaced by a high-tempe rature lamellar phase. In mixtures of C12E5 with octamethylcyclotetrasiloxa ne (D-4), the addition of DDAB makes the surfactant mixture more hydrophili c and expands the lower miscibility gap by increasing the (pseudo)critical temperature. In ternary mixtures of CiEj, D-4, and water, adding DDAB incre ases the surfactant efficiency by up to a factor of three, expanding the si ngle-phase microemulsion region to higher temperatures and lower surfactant concentrations. The temperature limits of the single-phase microemulsion c orrelate with changes in the pseudo-binary phase diagrams upon addition of DDAB. Additionally, the surfactant mixture stabilizes liquid crystalline re gions, but at temperatures that do not obscure the gains in surfactant effi ciency offered by the ionic-nonionic surfactant mixture. Similar results ar e reported for two linear silicone oils, hexamethyldisiloxane and decamethy ltetrasiloxane.