PHASE-BEHAVIOR AND LYOTROPIC-LIQUID CRYSTAL-STRUCTURE OF ALKYLTRIMETHYLAMMONIUM BROMIDE-WATER MIXTURES AROUND FREEZING TEMPERATURE OF WATER

Freezing and melting behaviors of concentrated cationic surfactant-wat er binary systems were investigated by using differential scanning cal orimetry and polarized light microscopy, and the phase diagrams around the freezing temperature of water were constructed. The surfactants s tudied were decyl-and octyl-trimethylammonium bromide (abbreviated as DeTAB and OTAB). The OTAB-water system forms a lyotropic-liquid crysta l in highly concentrated region of ca. 80 wt% OTAB. The head groups of OTAB strongly interact with water molecules and reduce freezing point of water down to -33 degrees C. To clarify the structure of the liqui d-crystalline phase, small angle X-ray diffraction study was carried o ut on DeTAB-water system as a function of composition at various tempe ratures. The observed diffraction patterns indicate the formation of t he hexagonal phase, H-alpha, and suggests that the H-alpha phase consi sts of finite cylinder micelles with length increasing with surfactant content. (C) 1998 Elsevier Science B.V.