Langmuir. 2018 Jul 24;34(29):8560-8570. doi: 10.1021/acs.langmuir.8b00800. Epub 2018 Jul 11.
Cationic and anionic surfactant mixtures can form viscous films that dominate the rheology and stability of micrometer-sized droplet suspensions. In this work, we use micropipette aspiration to study the mechanical properties of mixed surfactant surface films of anionic sodium dodecyl sulfate (SDS) and cationic dodecylamine hydrochloride (DAH) on alkane and lipid droplets. For octane droplets, SDS was found to decrease the surface tension until a minimum of 5 ± 1 mJ/m was reached after the critical micelle concentration (cmc). The surface viscosity of the droplets was found to be on the order of 10 mN s/m at an SDS concentration of 10 mM. An addition of 0.2 mM of DAH was found to increase this viscosity to a peak of 0.24 ± 0.01 mN s/m. Similar to octane, the surface tension of dodecane decreased to a value of 7.7 ± 0.4 mJ/m at SDS concentrations above cmc. Unlike with octane, however, the dodecane droplets had a significant surface viscosity of 0.37 ± 0.01 mN s/m when only the 10 mM SDS film was present. An addition of DAH caused a decrease in this viscosity initially, before rising to a peak viscosity of 0.45 ± 0.01 mN s/m at a DAH concentration of 0.15 mM. We speculate that the peaks in viscosities were the result of the completions of a phase change associated with microcrystalline SDS/DAH grains growing in the film at the surface of the droplets. Fluorescence microscopy and visual observations provided further evidence that these films can show rigid microcrystalline-like structure. Further work done with soybean oil in the same conditions and with a lipid film, simulating biological lipid droplets, confirmed that lipid droplets behave rheologically similar to alkanes in the presence of these mixed surfactant and lipid films. These results imply that droplet mechanics may be heavily influenced by the presence of microcrystalline grains in the oil-water systems with complex surfactant mixtures.
阳离子和阴离子表面活性剂混合物可以形成粘性膜，从而控制微米级液滴悬浮液的流变学和稳定性。在这项工作中，我们使用微管吸吮技术研究了阴离子十二烷基硫酸钠（SDS）和阳离子十二烷基胺盐酸盐（DAH）在烷烃和脂质液滴上混合表面活性剂表面膜的机械性能。对于辛烷液滴，发现 SDS 降低表面张力，直到在临界胶束浓度（cmc）之后达到 5 ± 1 mJ/m 的最小值。在 SDS 浓度为 10 mM 时，发现液滴的表面粘度约为 10 mN s/m。添加 0.2 mM 的 DAH 会将此粘度提高到 0.24 ± 0.01 mN s/m 的峰值。与辛烷类似，当 SDS 浓度高于 cmc 时，十二烷的表面张力降低到 7.7 ± 0.4 mJ/m 的值。然而，与辛烷不同的是，当仅存在 10 mM SDS 膜时，十二烷液滴具有显著的表面粘度，为 0.37 ± 0.01 mN s/m。添加 DAH 会导致初始时粘度降低，然后在 0.15 mM 的 DAH 浓度下上升到 0.45 ± 0.01 mN s/m 的峰值粘度。我们推测，粘度峰值是由于液滴表面的薄膜中 SDS/DAH 微晶颗粒生长引起的相变化完成的。荧光显微镜和肉眼观察提供了进一步的证据，表明这些薄膜可以显示刚性的类似微晶的结构。在相同条件下用大豆油和脂质膜（模拟生物脂质液滴）进行的进一步研究证实，在存在这些混合表面活性剂和脂质膜的情况下，脂质液滴在流变学上表现出与烷烃相似的行为。这些结果表明，在具有复杂表面活性剂混合物的油水系统中，微晶体颗粒的存在可能会严重影响液滴的力学性能。
