COUNTERION-DRIVEN SPHERE TO CYLINDER TRANSITION IN REVERSE MICELLES -A SMALL-ANGLE X-RAY-SCATTERING AND CONDUCTOMETRIC STUDY

SAXS and conductivity measurements are used to investigate reverse agg regates in isooctane formed by AOT in which the sodium counterion is e xchanged by a trivalent counterion, the aluminum cation Al3+. The corr esponding surfactant is denoted by AlA(3) in contrast to NaA for AOT. The phase behavior of the system AlA(3)/water/isooctane is determined in the dilute region, While NaA surfactant film in reverse micelles ca n be balanced toward zero spontaneous curvature close to room temperat ure and shows gradual transformation of spheres into a connected netwo rk upon swelling by water, AlA(3) aggregation behavior is dominated by a sphere to cylinder transition leading to liquid-gas coexistence of cylindrical aggregates, Spontaneous curvature is strongly directed tow ard water while the area per molecules remains constant at any water c ontent. At water content higher than approximate to 3.5 water molecule s per surfactant molecules, long and rigid cylinders are observed and produce an increase in the sample conductivity.