STRUCTURES OF METAL BIS(2-ETHYLHEXYL) SULFOSUCCINATE AGGREGATES IN CYCLOHEXANE

The specific identity of the countercation (M(n+)) associated with the amphiphilic anion bis(2-ethylhexyl) sulfosuccinate (AOT-) has an important effect on the structure of the aggregates formed in low-content water-in-oil (w/o) microemulsions. The counterions used here are from the series of Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+. At an AOT- concentration of 0.075 mol dm-3 and w = [H2O]/[AOT-] values < 5.0, the systems are all far from phase boundaries so that inter-aggregate interactions are minimized. Under these conditions, viscosity and small-angle neutron scattering measurements both indicate that for a hydrated metal counterion radius r(h) < 3.0 angstrom (bulk aqueous solution value), spherical surfactant aggregates are present. However, for r(h) > 3.0 angstrom, cylinder-shaped aggregates are favored. These results demonstrate the important effects of counterion variation in fine-tuning the electrostatic forces at the interface in surfactant-containing systems.