Department of Chemical Engineering, University of California , Santa Barbara, California 93106-5080, United States.
Langmuir. 2017 Sep 26;33(38):9694-9702. doi: 10.1021/acs.langmuir.7b01094. Epub 2017 Sep 15.
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.
表面活性剂在科学和技术的许多领域都起着普遍的作用，并且在各种非平衡情况和过程中，无论是自发的还是有意的，通常都会形成梯度。我们观察和测量了乳胶胶体在阳离子和阴离子表面活性剂梯度下的扩散迁移，这些表面活性剂的浓度既低于也高于临界胶束浓度（cmc）。在 cmc 以下，胶体迁移可以用电解质梯度下扩散电泳的经典理论来描述，尽管会出现细微差别和区别。阳离子表面活性剂吸附在阴离子胶体上，改变表面电荷，从而改变扩散迁移的方向。在 cmc 以上，扩散迁移的迁移率降低了几个数量级。我们认为这是因为带电单体（而不是胶束）主导了胶体扩散电泳。由于在 cmc 以上，单体浓度基本保持不变，因此在 cmc 以上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迁移。我们的研究结果表明了在存在表面活性剂的情况下理解扩散电泳的概念策略，以及预测和设计利用它们的系统的策略。
