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

Structural transformations in cetyltrimethylammonium bromide (CTAB) aggregated systems have been investigated using positron lifetime spectroscopy. Positron lifetime measurements have been carried out in CTAB/water/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, with increasing surfactant concentration, several transformations in the micellar structure are found to occur. The results agree with those obtained from small angle X-ray scattering (SAXS), small angle neutron scattering (SANS) and other conventional techniques. A phase transformation from normal micellar to hexagonal liquid crystalline structure is found to occur at a surfactant concentration of 28% by weight. In CTAB/hexanol solutions, evidence for setting in of molecular aggregation at a surfactant concentration of 0.5 mM has been obtained. With increasing concentration of surfactant, structural transformations in these aggregates are indicated.