Small angle neutron and X-ray scattering study of the formation and structure of micelles of CTAB in formamide

Small angle neutron and X-ray scattering was used to study micellization of cetyltrimethylammonium bromide (CTAB) in partially deuterated or hydrogenated formamide from the absolute values of scattered intensities. Highly charged aggregates of around 6 monomers were observed at CTAB concentrations above the cmc (2.8% wt at 60degC). These aggregates along with spherical particles (2 nm radius) containing 29 monomers were consistently observed at concentrations above 8%. These particles were considered to be micelles as they had similar structure, albeit of smaller size to those observed in water (2.7 nm, aggregation number 90). They also had a higher charge in formamide than in water (degree of ionization 0.55 in formamide and 0.14 in water). With increase in surfactant concentration, the micelles elongated, although the radius of the cylinders in the two dimensional hexagonal phase remained close to 2 nm. The importance of interactions of polar head with solvent molecules of high dipole moment and dielectric constant is discussed. The less spontaneous self-association of surfactant molecules in formamide than in water poses the problem of the definition of the cmc. (orig.)