SMALL-ANGLE X-RAY AND NEUTRON-SCATTERING FROM DILUTE-SOLUTIONS OF CESIUM PERFLUOROOCTANOATE - MICELLAR GROWTH ALONG 2 DIMENSIONS

Small-angle X-ray and neutron scattering (SAXS and SANS, respectively) have been measured in H2O solutions of cesium perfluorooctanoate (CsP FO) in the concentration range 0.065-0.5 M (much lower than the bounda ry between isotropic and discotic nematic phases). For 0.1 and 0.3 M s olutions, SANS has been measured for various D2O/H2O mixed solvents. T he data have been analyzed with a double-layered ellipsoid form factor combined with the rescaled mean spherical approximation for the inter micellar structure factor. The scattering curves can be fitted well wi th both prolate and oblate ellipsoid models. However, the semiminor ax is of micellar core, R-c, obtained for the prolate ellipsoid (15.3 Ang strom) is much longer than the extended length of the fluorocarbon cha in (12.6 Angstrom) which is close to the R-c obtained for the oblate e llipsoid (13.0 Angstrom). As the surfactant concentration increases, b oth the aggregation number and the degree of counterion binding increa se. These results suggest micellar growth along two dimensions.