Small-Angle X-ray and Neutron Scattering from Dilute Solutions of Cesium Perfluorooctanoate. Micellar Growth along Two Dimensions

Small-angle X-ray and neutron scattering (SAXS and SANS, respectively) have been measured in H 2 O solutions of cesium perfluorooctanoate (CsPFO) in the concentration range 0.065−0.5 M (much lower than the boundary between isotropic and discotic nematic phases). For 0.1 and 0.3 M solutions, SANS has been measured for various D 2 O/H 2 O mixed solvents. The data have been analyzed with a double-layered ellipsoid form factor combined with the rescaled mean spherical approximation for the intermicellar structure factor. The scattering curves can be fitted well with both prolate and oblate ellipsoid models. However, the semiminor axis of micellar core, R c, obtained for the prolate ellipsoid (15.3 Å) is much longer than the extended length of the fluorocarbon chain (12.6 Å) which is close to the R c obtained for the oblate ellipsoid (13.0 Å). As the surfactant concentration increases, both the aggregation number and the degree of counterion binding increase. These results suggest micellar growth along two dimensions.