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 aggregates in isooctane formed by AOT in which the sodium counterion is exchanged by a trivalent counterion, the aluminum cation Al3+. The corresponding 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 can be balanced toward zero spontaneous curvature close to room temperature and shows gradual transformation of spheres into a connected network 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 toward water while the area per molecules remains constant at any water content. At water content higher than approximate to 3.5 water molecules per surfactant molecules, long and rigid cylinders are observed and produce an increase in the sample conductivity.