PHOTOCHEMICALLY TRIGGERED ISOTHERMAL PHASE-TRANSITIONS IN LYOTROPIC LIQUID-CRYSTALLINE SYSTEMS OF ANIONIC AND CATIONIC SURFACTANTS

Transition temperatures in lyotropic liquid crystalline aqueous potassium myristate (KMy), potassium oleate (KO1), and cetyltrimethylammonium bromide (CTAB) were measured in the presence of small amounts of two photoisomerizable solubilizates: a crown ether bearing azobenzene (AB15C5) and 4-hydroxystilbene (4HS). Transition temperatures THI (hexagonal → isotropic) in the anionic surfactant systems of KMy and KOl rose by up to 15°C with increasing concentration (<1% w/w) of trans-AB15C5 or trans-4HS. In situ trans → cis photoisomerization of 4HS in KMy leads to a further increase of THI by up to 3.5°C. Similarly, in cationic CTAB systems the presence of 4HS caused an increase of TNI (nematic → isotropic), cis-4HS being more efficient than trans-4HS. In the two latter systems isothermal phase transitions can be brought about by exposing samples to light. The origin of the observed effects is connected with a variation of the shape of the solubilizates. This was indicated by measurements of TNI and THI in CTAB and KMy, respectively, in the presence of model compounds differing in shape only. Rate constants of thermal cis → trans isomerizations in the surfactant systems were 1.6 × 10-4 s−1 for AB15C5 at 35°C and 6.1 × 10-5 s-1 for 4HS at 30°C. An activation energy of 89.9 kJ/mol was determined for the thermal cis → trans isomerization of 4HS. © 1994 by Academic Press, Inc.