Tyagi Gunjan, Sharratt William N, Erikson Sofia, Seddon Dale, Robles Eric S J, Cabral João T
Department of Chemical Engineering, Imperial College London, London, SW7 2AZ, UK.
The Procter & Gamble Company, Newcastle Innovation Centre, Newcastle-Upon-Tyne NE12 9TS, UK.
Langmuir. 2022 Jun 14;38(23):7198-7207. doi: 10.1021/acs.langmuir.2c00527. Epub 2022 Jun 3.
We examine the solution structures in a mixed surfactant system of sodium dodecyl sulfate (SDS) and ,-dimethyldodecylamine -oxide (DDAO) in water, on both sides of the two-phase boundary, employing dynamic light scattering, small-angle neutron scattering, and Fourier transform infrared spectroscopy. The precipitate phase boundary was accessed by lowering pH to 8, from its floating pH 9.5 value, and was experimentally approached from the monomeric and micellar regions in three ways: at fixed DDAO or SDS concentrations and at a fixed (70:30) SDS:DDAO molar ratio. We characterize the size, shape, and interactions of micelles, which elongate approaching the boundary, leading to the formation of disk-like aggregates within the biphasic region, coexisting with micelles and monomers. Our data, from both monomeric and micellar solutions, indicate that the two phase structures formed are largely pathway-independent, with dimensions influenced by both pH and mixed surfactant composition. Precipitation occurs at intermediate stoichiometries with a similar SDS:DDAO ratio, whereas asymmetric stoichiometries form a re-entrant transition, returning to the mixed micelle phase. Overall, our findings demonstrate the effect of stoichiometry and solution pH on the synergistic interaction of mixed surfactants and their impact on phase equilibrium and associated micellar and two-phase structures.
我们采用动态光散射、小角中子散射和傅里叶变换红外光谱法，研究了水相中十二烷基硫酸钠（SDS）和α,ω-二甲基十二烷基氧化胺（DDAO）混合表面活性剂体系在两相边界两侧的溶液结构。通过将pH值从其漂浮时的9.5降至8来达到沉淀相边界，并且从单体和胶束区域以三种方式通过实验来接近该边界：在固定的DDAO或SDS浓度下以及在固定的（70:30）SDS:DDAO摩尔比下。我们表征了胶束的大小、形状和相互作用，胶束在接近边界时会伸长，导致在双相区域内形成盘状聚集体，与胶束和单体共存。我们从单体和胶束溶液中获得的数据表明，形成的两种相结构在很大程度上与路径无关，其尺寸受pH值和混合表面活性剂组成的影响。在具有相似SDS:DDAO比例的中间化学计量比下会发生沉淀，而不对称化学计量比则形成再入转变，回到混合胶束相。总体而言，我们的研究结果证明了化学计量比和溶液pH值对混合表面活性剂协同相互作用的影响，以及它们对相平衡和相关胶束及两相结构的影响。
