Chemical Engineering, University of Illinois at Chicago , Chicago, Illinois 60607, United States.
ACS Nano. 2018 Feb 27;12(2):1050-1061. doi: 10.1021/acsnano.7b05391. Epub 2018 Jan 17.
Freestanding films of soft matter containing micelles, nanoparticles, polyelectrolyte-surfactant complexes, bilayers, and smectic liquid crystals exhibit stratification. Stepwise thinning and coexisting thick-thin regions associated with drainage via stratification are attributed to the confinement-induced structuring and layering of supramolecular structures, which contribute supramolecular oscillatory structural forces. In freestanding micellar films, formed by a solution of an ionic surfactant above its critical micelle concentration, both interfacial adsorption and the micelle size and shape are determined by the concentration of surfactant and of added electrolytes. Although the influence of surfactant concentration on stratification has been investigated before, the influence of added salt, at concentrations typically found in water used on a daily basis, has not been investigated yet. In this contribution, we elucidate how the addition of salt affects stepwise thinning: step size, number of steps, as well as the shape and size of nanoscopic nonflat structures such as mesas in micellar foam films formed with aqueous solutions of anionic surfactant (sodium dodecyl sulfate (SDS)). The nanoscopic thickness variations and transitions are visualized and analyzed using IDIOM (Interferometry Digital Imaging Optical Microscopy) protocols with exquisite spatiotemporal resolution (thickness ∼1 nm, time <1 ms). In contrast to nanoparticle dispersions that show no influence of salt on step size, we find that the addition of salt to micellar freestanding films results in a decrease in step size as well as the number of stepwise transitions, in addition to changes in nucleation and growth of mesas, all driven by the corresponding change in supramolecular oscillatory structural forces.
含有胶束、纳米粒子、聚电解质-表面活性剂复合物、双层和近晶液晶的软物质的独立膜表现出分层。与通过分层进行的排水相关的逐步变薄和共存的厚-薄区域归因于受限诱导的超分子结构的结构化和分层，这导致超分子振荡结构力。在由离子表面活性剂在其临界胶束浓度以上的溶液形成的自由胶束膜中，界面吸附以及胶束的大小和形状由表面活性剂和添加的电解质的浓度决定。尽管以前已经研究了表面活性剂浓度对分层的影响，但尚未研究在日常使用的水中通常发现的浓度的盐的添加对分层的影响。在本贡献中，我们阐明了盐的添加如何影响逐步变薄：步长、步数，以及纳米级非平面结构（如胶束泡沫膜中的阶地）的形状和大小，该胶束泡沫膜是由阴离子表面活性剂（十二烷基硫酸钠 (SDS)）的水溶液形成的。使用具有精湛时空分辨率（厚度约 1nm，时间<1ms）的 IDIOM（干涉数字成像光学显微镜）协议可视化和分析纳米级厚度变化和转变。与纳米粒子分散体不同，纳米粒子分散体没有盐对步长的影响，我们发现向胶束自由膜中添加盐不仅会导致步长减小，而且还会导致逐步转变的数量减少，此外还会改变阶地的成核和生长，所有这些都由超分子振荡结构力的相应变化驱动。
