THERMAL TRANSITIONS IN SURFACTANT-BASED LYOTROPIC LIQUID-CRYSTALS

The state of water and several thermal transitions were examined in se veral surfactant/water lyotropic liquid crystalline systems. The surfa ctants studied were Aerosol OT (AOT), didodecyldimethylammonium bromid e (DDAB), dioctadecyldimethylammonium bromide (DODAB) and n-decylphosp honic acid (DPA). In the AOT/water system, three types of water were d etected: ''bulk-like'' water, interfacial water and hydration water (a bout six molecules of water per molecule of AOT). This system does not exhibit the gel-liquid-crystal transition, probably because the branc hed, short tails of AOT do not allow for crystallization upon cooling because of steric hindrance. In the DDAB/water and DODAB/water systems , only bulk-like water and two molecules of hydration water per molecu le of surfactant were observed. The gel-liquid-crystal transition was detected at 15-degrees-C for DDAB and at 50-degrees-C for DODAB. In th e DPA/water system, only bulk-like water was seen. The gel-liquid-crys tal transition for this surfactant was detected at about 12-degrees-C.