Evolution of Equilibrium Pickering Emulsions-A Matter of Time Scales
Evolution of Equilibrium Pickering Emulsions-A Matter of Time Scales
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DOI：
10.1021/jp104662g
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发表时间：
2010-09-30
影响因子：
3.3
通讯作者：
Kegel, Willem K.
中科院分区：
文献类型：
--
作者：
Kraft, Daniela J.;Luigjes, Bob;Kegel, Willem K.
一类新的平衡固体稳定的水包油乳液在不同的时间尺度上具有两个过程的竞争，这两个过程以相反的方式影响平衡液滴尺寸。这项工作的目的是阐明这两个时间尺度的分子起源，并证明它们对乳液液滴尺寸演变的影响。首先，通过油相分子的水解原位生成表面活性分子，自发乳化成颗粒覆盖的液滴。我们表明，在没有稳定的胶体颗粒的油水界面的表面张力连接到这些表面活性分子的浓度，因此也到平衡液滴的大小在胶体的存在下。因此，水解过程设定了这些固体稳定的乳液形成的时间尺度。第二个时间尺度是支配的固体稳定的平衡乳液的最终命运：通过在原位生成的两亲性分子上的胶体颗粒的冷凝，它们的润湿性能的变化，导致从水相到油相通过乳液液滴的生长逐渐转移。通过水相和油相的颜色变化以及在相分离样品中油相聚合后通过电子显微镜观察到这种迁移。令人惊讶的是，相对油体积设定了颗粒转移的时间尺度。相分离成水相和含有胶体颗粒的油相受到乳液液滴沉降的影响。通过水解和在胶体表面上缩合形成表面活性分子的两个过程对液滴尺寸具有相反的影响。通过它们的相互作用，形成了一种动态平衡，其中液滴尺寸总是调整到热力学稳定状态。
A new class of equilibrium solid-stabilized oil-in-water emulsions harbors a competition of two processes on disparate time scales that affect the equilibrium droplet size in opposing ways. The aim of this work is to elucidate the molecular origins of these two time scales and demonstrate their effects on the evolution of the emulsion droplet size. First, spontaneous emulsification into particle-covered droplets occurs through in situ generation of surface-active molecules by hydrolysis of molecules of the oil phase. We show that surface tensions of the oil water interfaces in the absence of stabilizing colloidal particles are connected to the concentration of these surface-active molecules, and hence also to the equilibrium droplet size in the presence of colloids. As a consequence, the hydrolysis process sets the time scale of formation of these solid-stabilized emulsions. A second time scale is governing the ultimate fate of the solid-stabilized equilibrium emulsions: by condensation of the in situ generated amphiphilic molecules onto the colloidal particles, their wetting properties change, leading to a gradual transfer from the aqueous to the oil phase via growth of the emulsion droplets. This migration is observed macroscopically by a color change of the water and oil phases, as well as by electron microscopy after polymerization of the oil phase in a phase separated sample. Surprisingly, the relative oil volume sets the time scale of particle transfer. Phase separation into an aqueous phase and an oil phase containing colloidal particles is influenced by sedimentation of the emulsion droplets. The two processes of formation of surface-active molecules through hydrolysis and condensation thereof on the colloidal surface have an opposite influence on the droplet size. By their interplay, a dynamic equilibrium is created where the droplet size always adjusts to the thermodynamically stable state.