MACROEMULSION STABILITY - THE ORIENTED WEDGE THEORY REVISITED

The correspondence between the equilibrium phase behavior of oil-water -surfactant mixtures and the macroemulsion type and stability is exami ned. Both the phase behavior and emulsion stability are argued to be d ependent on the bending elasticity of the surfactant monolayer at the oil-water interface. At positive spontaneous curvatures, O/W emulsions are stable; at negative spontaneous curvatures W/O emulsions are stab le, whereas, in the balanced state of the surfactant film, an emulsion break usually occurs for a wide variety of systems. To explain the ef fect of the monolayer bending properties on the macroemulsion stabilit y, the thermally activated rupture of emulsion films is theoretically studied. We consider emulsion films covered by saturated surfactant mo nolayers, with strong lateral interactions among the adsorbed surfacta nt molecules. The monolayer at the edge of a nucleation hole in the em ulsion film is strongly curved; the bending energy penalty involved le ads to a strong dependence of the coalescence barrier on the sign and the absolute value of the monolayer spontaneous curvature. By contrast with earlier hole nucleation theories, the emulsion film thickness is allowed to vary, in order to minimize the free energy of the nucleati on hole. At large positive spontaneous curvatures, Ho, the oil-water-o il (O/W/O) films are stable, with a coalescence barrier, in a first ap proximation, proportional to the bending modulus kappa. On the other h and, W/O/W films break without a barrier. Conversely, for large negati ve values of Ho, W/O/W films are stable, while O/W/O films break witho ut a barrier. In the vicinity of the balanced state, a very steep chan ge in film stability with Ho is predicted. The model reproduces the ma croemulsion stability sequence: O/W emulsion-emulsion breakage-W/O emu lsion, observed in polyethoxylated nonionic surfactant-oil-water mixtu res with increasing temperature. The macroemulsion break is predicted to occur at the balanced (PIT) point, as has been observed experimenta lly by Shinoda et al. The hole nucleation in multilamellar-stabilized films is shown to be drastically suppressed, in agreement with the exp erimental findings of Friberg et al. For rigid surfactant monolayers ( kappa similar to 100 kT) and multilamellar-stabilized systems, a regio n of stable multiple emulsions is predicted, while it is prohibited fo r flexible (kappa similar to 1 kT) monolayers. Possible extensions of the model to other systems are discussed.