Macroemulsion stability: The oriented wedge theory revisited
Macroemulsion stability: The oriented wedge theory revisited
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DOI：
10.1021/la950359e
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发表时间：
1996-01-24
期刊：
影响因子：
3.9
通讯作者：
Wennerstrom, H
中科院分区：
文献类型：
--
作者：
Kabalnov, A;Wennerstrom, H
考察了油-水-表面活性剂混合物的平衡相行为与粗乳液类型和稳定性之间的对应关系。相行为和乳液稳定性都被认为是依赖于表面活性剂单分子层在油-水界面处的弯曲弹性。在正的自发曲率下，O/W乳液是稳定的;在负的自发曲率下，W/O乳液是稳定的，而在表面活性剂膜的平衡状态下，对于各种各样的系统，通常会发生乳液破裂。为了解释单层膜的弯曲性质对粗乳液稳定性的影响，从理论上研究了乳液膜的热活化破裂。我们认为乳液膜覆盖的饱和表面活性剂单分子层，与吸附的表面活性剂分子之间的强横向相互作用。在乳液膜中的成核孔的边缘处的单层强烈地弯曲;所涉及的弯曲能罚导致单层自发曲率的符号和绝对值上的聚结势垒的强烈依赖性。与早期的空穴成核理论相比，允许乳液膜厚度变化，以使成核空穴的自由能最小化。在大的正自发曲率，Ho，油-水-油（O/W/O）膜是稳定的，与聚结障碍，在第一近似，成比例的弯曲模量kappa。另一方面，W/O/W膜在没有屏障的情况下破裂。相反，对于大的负值的Ho，W/O/W膜是稳定的，而O/W/O膜破裂没有障碍。在附近的平衡状态，一个非常陡峭的变化，在膜的稳定性与Ho的预测。该模型再现了宏观乳液的稳定性序列：O/W乳液-乳液破裂-W/O乳液，观察到在聚乙氧基化非离子表面活性剂-油-水的混合物随着温度的升高。预测宏观乳液破裂发生在平衡（PIT）点，如Shinoda等人的实验所观察到的。多层稳定膜中的空穴成核被示出被显著抑制，与Friberg等人的实验结果一致。（kappa类似于100 kT）和多层稳定的系统，预测稳定的多重乳液的区域，而对于柔性（kappa类似于1 kT）单层是禁止的。可能的扩展模型到其他系统进行了讨论。
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, Ho, the oil-water-oil (O/W/O) films are stable, with a coalescence barrier, in a first approximation, proportional to the bending modulus kappa. On the other hand, W/O/W films break without a barrier. Conversely, for large negative values of Ho, 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 Ho 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 (kappa similar to 100 kT) and multilamellar-stabilized systems, a region of stable multiple emulsions is predicted, while it is prohibited for flexible (kappa similar to 1 kT) monolayers. Possible extensions of the model to other systems are discussed.