STRUCTURE OF COBALT AEROSOL-OT REVERSED MICELLES STUDIED BY SMALL-ANGLE SCATTERING METHODS

The surfactant-cyclohexane-water ternary phase behaviour and reversed micelle and water-in-oil (w/o) micro-emulsion structure of the cobalt (II) derivative of the anionic amphiphile Aerosol-OT (AOT) [Co(water)6 ](AOT)2 have been studied by polarising microscopy, small-angle neutro n and X-ray scattering (SANS, SAXS). The surfactant forms an H2 revers ed hexagonal phase on swelling with up to 25 wt.% cyclohexane. At high er concentrations of oil the fluid L2 reversed micellar phase is prese nt, and a w/o phase forms up to w = 25.0 (w = [water]/[AOT-]). For w > 25.0 at 25-degrees-C a Winsor II system separates cleanly i.e. a w/o droplet system at the 'natural' radius of the monolayer, co-existing w ith an essentially surfactant-free water phase. The SAXS I(Q) profiles show that major changes in aggregate shape occur as a function of w a t constant surfactant concentration. At low surfactant concentrations, [AOT-] = 0.075 mol dm-3, the w = 0 reversed micelles, formed from dil uting the H2 phase, are small near-spherical aggregates. The scatterin g is consistent with cylindrical micelles at low w, 5-10, and spherica l w/o droplets at the Winsor II boundary w = 25.0. The results are exp lained in terms of the influence of parent H2 and co-existing water ph ases on the aggregate shapes in the L2 phase. We have used the SANS co ntrast variation method to investigate the internal cross-section stru cture of the cylindrical, w = 5.0, reversed micelles. The results show that the radius of the polar core, r, is only slightly larger than th e hydrated radius of the [Co(water)6]2+ counterion and that the surfac tant shell thickness, delta, is essentially equal to the length of the AOT hydrocarbon chains. This suggests an open staggered 'string of be ads' structure for the micelles, rather than a polar core that can be significantly swollen with water. This model gives us some insight int o structure of the lyotropic H2 phase.