Effect of interfacial charge on micellar structure

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Abstract
We have studied the structure and effective charge of sodium octylphosphate micelles in aqueous solution as a function of concentration and pH. Such variations may be used to alter the structural charge Z0 of the polar headgroup from 0.8 to 2 without altering the surfactant molecule. Small angle neutron scattering coupled with the Hayter-Penfold analytical technique was used to measure the aggregation number N and the effective charge Z. The micelles are small and spherical for Z0 = 2, increasing to larger ellipsoids as Z0 is decreased to unity. For Z0 = 0.8, highly anisotropic micelles are formed. The effective surface charge density values lie in the range of 5 to 8 μC/cm2 and were found to be consistent with a theoretical Poisson-Boltzmann cell model calculation. The area per headgroup increases strongly with Z0, with a concomitant variation of the hydration. Incidence on solubilizing power toward benzene is discussed