EFFECT OF COUNTERION RADIUS ON SURFACTANT PROPERTIES IN WINSOR-II MICROEMULSION SYSTEMS

The effects of varying the surfactant counterion radius r on the oil-w ater interfacial tensions gamma(ow) and water-in-oil (w/o) microemulsi on droplet radii R are reported. The surfactants are derivatives of Ae rosol-OT(AOT-is bis(2-ethythexyl)sulfosuccinate) with quaternary ammon ium counterions [NH4+], [(CH3)4N+], and [(C2H5)4N+]; in this series r increases from 15 to 2.8 angstrom. We have used spinning drop tensiome try to measure gamma(ow) as a function of electrolyte concentration [N aCl] with n-heptane as oil and small-angle neutron scattering (SANS) t o determine the w/o droplet spontaneous radii R(o) in cyclohexane syst ems. Apparently the counterion radius only has a small effect on the g amma(ow) vs [NaCl] behavior. For example, for NH4+ the electrolyte con centration at the tension minimum [NaCl]min is approximately 0.0425 an d for [(C2H5)4N+] 0.0525 mol dm-3. Similarly, the effect of the ion ra dius r on R(o) in the Winsor II system is also small; for NH4+ R(o) ap proximately 21 angstrom and with [(C2H5)4N+]R(o) approximately 30 angs trom. The weak dependence of these parameters on ion radius r is discu ssed in the light of recent work by Binks et al. demonstrating a much stronger dependence on alkyl chain length.