STRUCTURAL TRANSFORMATIONS IN CTAB AGGREGATED SYSTEMS INVESTIGATED BYPOSITRON LIFETIME SPECTROSCOPY .1. BINARY-SYSTEMS

Structural transformations in cetyltrimethylammonium bromide (CTAB) ag gregated systems have been investigated using positron lifetime spectr oscopy. Positron lifetime measurements have been carried out in CTAB/w ater/hexanol systems, both binary and ternary. The present paper deals with investigations in CTAB/water and CTAB/hexanol binary solutions. In CTAB/water solutions, the molecular aggregation is found to set in at a surfactant concentration of 0.7 mM (CMC). In these solutions, wit h increasing surfactant concentration, several transformations in the micellar structure are found to occur. The results agree with those ob tained from small angle X-ray scattering (SAXS), small angle neutron s cattering (SANS) and other conventional techniques. A phase transforma tion from normal micellar to hexagonal liquid crystalline structure is found to occur at a surfactant concentration of 28% by weight. In CTA B/hexanol solutions, evidence for setting in of molecular aggregation at a surfactant concentration of 0.5 mM has been obtained. With increa sing concentration of surfactant, structural transformations in these aggregates are indicated.