SURFACTANT WATER/OIL SYSTEM WITH WEAKLY CHARGED FILMS - DEPENDENCE ONCHARGE-DENSITY/

The effects on the microstructure and phase equilibria of a nonionic s urfactant/water/oil system when replacing one, four, and six per hundr ed of the nonionic surfactant molecules by the ionic surfactant sodium dodecyl sulfate (SDS) have been studied. The nonionic surfactant syst em consists of pentaethylene glycol dodecyl ether (C(12)E(5)), water, and decane, at a constant surfactant to oil weight ratio of 51.9/48.1. The phase equilibria and the corresponding microstructures were inves tigated as a function of temperature and water content. As a function of temperature, the initial phase sequence for this composition range was L(1) phase-lamellar phase-L(3) phase. Three effects on phase behav ior are observed when introducing electrostatic interactions. (i) Wate r rich reverse hexagonal (H-2) and reverse micellar (L(2)) phases repl ace the two-phase region water + L(3) phase. (ii) The crystallization of micellar cubic (I-1) and normal, hexagonal phases moves to lower pa rticle concentrations. (iii) Phase boundaries at higher water contents are strongly shifted to higher temperatures. The latter effect is ana lyzed in detail at the emulsification failure boundary where spherical oil-swollen micelles are in equilibrium with excess oil. Using the be nding energy approach together with Poisson-Boltzmann calculations wit hin the cell model, we have analyzed the charge dependence of the emul sification failure boundary. The SDS effects can be reproduced with a cationic surfactant. We also show that the electrostatic effects are r emoved upon addition of salt. The microstructure in the various phases was studied by small angle X-ray scattering (SAXS). A neutral surface located at the polar/apolar interface is identified where the area pe r surfactant molecule is constant irrespective of the curvature.