ISOTHERMAL SWITCHING OF PHASES IN LYOTROPIC LIQUID-CRYSTALLINE SURFACTANT SYSTEMS VIA PHOTOREACTIONS OF SOLUBILIZED MOLECULES

Light induced phase transitions were found in lyotropic liquid crystalline systems of aqueous cetyltrimethylammonium bromide (CTAB) and of Aerosol-OT(AOT)/decane/water: the isothermal transitions isotropic --> nematic and isotropic --> hexagonal in CTAB systems were triggered by the photocyclization of a solubilized naphthyl vinyl ether, the transition isotropic --> inverse-hexagonal in the ternary AOT system was induced by the trans-cis photoisomerization of a solubilized stilbazolium salt. The transition time (between starting irradiations and detecting anisotropic phases) was found to increase with the difference between irradiation temperature and transition temperature in the non irradiated case; transition times were shorter by two orders of magnitude in the AOT-system as compared to CTAB. The position of isotropic/liquid crystalline phase borders produced by partial irradiation of layers of aqueous CTAB were more stable in hexagonal as compared to nematic systems.