Levich Institute and Department of Chemical Engineering, The City College of New York and The Graduate Center of The City University of New York, New York, USA.
Langmuir. 2010 Oct 19;26(20):15761-78. doi: 10.1021/la102438h.
This study examines the transport of surfactant that occurs when an aqueous micellar phase is placed in contact with a clean oil phase in which the surfactant is soluble. Upon contact with oil, surfactant monomer on the aqueous side of the interface adsorbs onto the oil/water interface and subsequently desorbs into the oil and diffuses away from the surface. The depletion of aqueous monomer underneath the interface disturbs the monomer-micelle equilibrium, and aggregates break down to replenish the monomer concentration and accelerate the interfacial transport. The depletion of monomer and micelles drives the diffusive flux of these species toward the surface, and the combined effects of diffusion and aggregate kinetic disassembly, alongside kinetic adsorption and desorption at the interface and diffusion away from the interface into the oil, determine the interfacial transport rate. This interfacial transport is examined here in the quasi-static limit in which the diffusion of monomer and micelles in the aqueous phase is much slower than the time scale for micelle disassembly. In this limit, when the initial bulk concentration of micelles in the aqueous solution is small, the micelle diffusive flux to the surface cannot keep up with the micelle breakdown under the interface, and a micelle-free zone forms. This zone extends from the surface into the aqueous phase up to a boundary that demarcates the beginning of a zone, containing micelles, that extends further into the aqueous phase. Micelles diffuse from the micelle zone to the boundary, where they break down, causing the boundary to retreat. Released monomer diffuses through the micelle-free zone and partitions into the oil phase. The focus of this study is to verify this transport picture by visualizing the micelle-free zone and comparing the movement of the zone to predictions obtained from a transport model based on this two-zone picture. A small hydrophobic dye molecule (Nile red) is incorporated into the micelles; the dye fluoresces only in the hydrophobic environment of the micelles, providing visual contrast between the two zones. Through spatial mapping of the fluorescence using confocal microscopy, the movement of the micelle-free zone boundary can be measured and is shown to compare favorably with simulations of the transport model.
本研究考察了当水胶束相置于可溶有表面活性剂的清洁油相中时发生的表面活性剂传递。与油接触时，界面上水相中的表面活性剂单体吸附到油水界面上，然后解吸到油中并从表面扩散。界面下的水相单体耗尽会干扰单体-胶束平衡，聚集物分解以补充单体浓度并加速界面传递。这些物质的单体和胶束耗尽驱动它们向表面扩散，扩散和聚集物动力学解体的综合效应，以及界面上的动力学吸附和解吸以及扩散到油中，决定了界面传递速率。在准静态极限下，这里研究了这种界面传递，其中水相中的单体和胶束扩散速度远慢于胶束解体的时间尺度。在这种极限下，当水相中的初始胶束浓度较小时，表面的胶束扩散通量跟不上界面下的胶束分解，形成无胶束区。这个区域从表面延伸到水相，直到一个边界，该边界划分出一个进一步延伸到水相中的胶束区。胶束从胶束区扩散到边界，在边界处分解，导致边界后退。释放的单体通过无胶束区扩散并分配到油相。本研究的重点是通过可视化无胶束区并将该区域的运动与基于该两区域图的传输模型的预测进行比较来验证该传输图。将一小部分疏水性染料分子（尼罗红）掺入胶束中；染料仅在胶束的疏水环境中发光，为两个区域提供视觉对比。通过使用共焦显微镜对荧光进行空间映射，可以测量无胶束区边界的运动，并与传输模型的模拟结果进行比较。
