Macroemulsion Stability:  The Oriented Wedge Theory Revisited

Abstract

The correspondence between the equilibrium phase behavior of oil-water-surfactant mixtures and the macroemulsion type and stability is examined. Both the phase behavior and emulsion stability are argued to be dependent 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 stable, whereas, in the balanced state of the surfactant film, an emulsion break usually occurs for a wide variety of systems. To explain the effect of the monolayer bending properties on the macroemulsion stability, the thermally activated rupture of emulsion films is theoretically studied. We consider emulsion films covered by saturated surfactant monolayers, with strong lateral interactions among the adsorbed surfactant molecules. The monolayer at the edge of a nucleation hole in the emulsion film is strongly curved; the bending energy penalty involved leads 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 nucleation hole. At large positive spontaneous curvatures, H 0, the oil-water-oil (O/W/O) films are stable, with a coalescence barrier, in a first approximation, proportional to the bending modulus κ. On the other hand, W/O/W films break without a barrier. Conversely, for large negative values of H 0, W/O/W films are stable, while O/W/O films break without a barrier. In the vicinity of the balanced state, a very steep change in film stability with H0 is predicted. The model reproduces the macroemulsion stability sequence: O/W emulsion-emulsion breakage-W/O emulsion, observed in polyethoxylated nonionic surfactant-oil-water mixtures with increasing temperature. The macroemulsion break is predicted to occur at the balanced (PIT) point, as has been observed experimentally by Shinoda et al. The hole nucleation in multilamellar-stabilized films is shown to be drastically suppressed, in agreement with the experimental findings of Friberg et al. For rigid surfactant monolayers (κ ∼ 100 kT) and multilamellar-stabilized systems, a region of stable multiple emulsions is predicted, while it is prohibited for flexible (κ ∼1 kT) monolayers. Possible extensions of the model to other systems are discussed.

Energetics of Pore Formation Induced by Membrane Active Peptides

“…Understanding the mechanism of pore formation induced by peptides will provide insights into the functions of antimicrobial peptides, which are essential components of the innate immune system, and facilitate the development of new anti-infective therapeutics. Pore formation is also potentially useful for gene and drug deliveries (24).To understand how peptides induce pore formation, consider the simpler case of pores in pure lipid bilayers, which have been extensively studied both experimentally and theoretically (25)(26)(27)(28)(29)(30)(31). In pure lipid bilayers, pores are always produced under tension.…”

Emulsified lipids: formulation and control of end-use properties

“…Its origin is still a matter of debate but it is now well admitted that the activation energy may be determined by several interfacial parameters: surface forces, surface tension, dilatational elasticity, spontaneous curvature, etc. (Kabalnov and Wennerstr€ om, 1996;Leal-Calderon et al, 2007). When coalescence is the main destabilization mechanism, the time evolution of the average droplet size is self-accelerated and the size distribution broadens (Schmitt et al, 2004).…”

“…At a microscopic scale, a single coalescence event between two drops in contact proceeds through the nucleation of a channel that reaches a critical size, above which it becomes unstable and grows. For a coalescence event to occur, an energy barrier has to be overcome (Kabalnov and Wennerstr€ om, 1996). Its origin is still a matter of debate but it is now well admitted that the activation energy may be determined by several interfacial parameters: surface forces, surface tension, dilatational elasticity, spontaneous curvature, etc.…”

Improving Product Quality with Entrapped Stable Emulsions: From Theory to Industrial Application

“…Furthermore, on the premise that the interfaces have been completely covered by emulsifiers, minimizing emulsifier usage can, as mentioned earlier, reduce operating costs and help prevent potential toxicity and/or an allergic response incurred by excessive usage [36]. More in-depth discussions pertaining to micelle formation being detrimental to formation of a stable interface are available elsewhere [77][78][79].…”