SMALL-ANGLE NEUTRON-SCATTERING (SANS) STUDY OF AGGREGATES FORMED FROMAQUEOUS MIXTURES OF SODIUM DODECYL-SULFATE (SDS) AND DODECYLTRIMETHYLAMMONIUM BROMIDE (DTAB)

Several different samples of sodium dodecyl sulfate (SDS, anionic surf actant) and dodecyltrimethylammonium bromide (DTAB, cationic surfactan t) in D2O at 40 degrees C in the absence of added salt have been studi ed using small-angle neutron scattering (SANS). The surfactant molar r atios of SDS and DTAB ranges between 15:85 and 40:60 and between 60:40 and 85:15, and the total amount of surfactant ranges between 0.125 an d 5 wt %. The scattering data were fitted by various models for geomet rically differently shaped particles using conventional least-squares methods. Small unilamellar vesicles (350 Angstrom < [R-v] < 650 Angstr om) were found in the very dilute samples of compositions 15:85-25:75 and 75:25-85:15 whereas larger oligolamellar vesicles predominate in t he dilute samples nearer equimolar surfactant composition. When the ov erall surfactant concentration is increased above 0.25-1.0 wt %, depen ding on composition, we observe evident transitions either from small unilamellar vesicles to micellar aggregates or from larger oligolamell ar vesicles to lamellar sheets. In connection with this, the transitio n from rod-shaped micelles, via disk-shaped micelles, to large lamella r sheets is observed as the surfactant composition approaches equimola rity from compositions of both 15:85 and 85:15, at a fixed overall sur factant concentration above the limit where no vesicles form. The expe rimental results are discussed and rationalized from the viewpoint of a simple thermodynamical model. In particular, the theory predicts an abrupt transition from small disk-shaped micelles to almost infinitely large lamellar sheets, as the surfactant composition is slightly chan ged toward equimolarity at a certain overall surfactant concentration, in agreement with the experimental observations.