Surfactants play a ubiquitous role in many areas of science and technology, and gradients often form—either spontaneously or intentionally—in a variety of nonequilibrium situations and processes. We visualize and measure the diffusiophoretic migration of latex colloids in response to gradients of cationic and anionic surfactants, both below and above the critical micelle concentration (cmc). Below the cmc, colloidal migration can be described using classic theories for diffusiophoresis under electrolyte gradients, although subtleties and distinctions do appear. Cationic surfactants adsorb onto anionic colloids, changing the surface charge and thus reversing the direction of diffusiophoretic migration. Above the cmc, diffusiophoretic mobilties decrease by orders of magnitude. We argue this to occur because charged monomers (rather than micelles) dominate colloidal diffusiophoresis. Because monomer concentrations remain essentially constant above the cmc, surfactant gradients imposed above the cmc result in very small monomer gradients—and, therefore, very weak diffusiophoresis. Our findings suggest conceptual strategies to understand diffusiophoresis in the presence of surfactants, as well as strategies to predict and design systems that harness them.

中文翻译：

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离子表面活性剂梯度中的扩散电泳

表面活性剂在科学和技术的许多领域中起着普遍的作用，并且在各种不平衡的情况和过程中，梯度是自发地或有意地形成的。我们可视化并测量在临界胶束浓度（cmc）之下和之上的阳离子和阴离子表面活性剂梯度对乳胶胶体的扩散电泳迁移。在cmc下方，尽管出现了细微的区别，但可以使用经典的扩散电泳的理论来描述胶体迁移。阳离子表面活性剂吸附到阴离子胶体上，改变表面电荷，从而改变扩散电泳迁移的方向。在cmc上方，发汗迁移率下降了几个数量级。我们认为这是因为带电的单体（而不是胶束）主导了胶体扩散电泳。因为单体浓度在cmc上方基本保持恒定，所以在cmc上方施加的表面活性剂梯度会导致非常小的单体梯度，从而导致非常弱的扩散电泳。我们的发现提出了在表面活性剂存在下理解扩散的概念性策略，以及预测和设计利用表面活性剂的系统的策略。