Roger K, Sparr E, Wennerström H
Laboratoire de Génie Chimique/Université de Toulouse, CNRS, INPT, UPS, Toulouse, France.
Phys Chem Chem Phys. 2018 Apr 18;20(15):10430-10438. doi: 10.1039/c8cp00305j.
Water evaporation from complex aqueous solutions leads to the build-up of structure and composition gradients at their interface with air. We recently introduced an experimental setup for quantitatively studying such gradients and discussed how structure formation can lead to a self-regulation mechanism for controlling water evaporation through self-assembly. Here, we provide a detailed theoretical analysis using an advection/diffusion transport equation that takes into account thermodynamically non-ideal conditions and we directly relate the theoretical description to quantitative experimental data. We derive that the concentration profile develops according to a general square root of time scaling law, which fully agrees with experimental observations. The evaporation rate notably decreases with time as t-1/2, which shows that diffusion in the liquid phase is the rate limiting step for this system, in contrast to pure water evaporation. For the particular binary system that was investigated experimentally, which is composed of water and a sugar-based surfactant (α-dodecylmaltoside), the interfacial layer consists in a sequence of liquid crystalline phases of different mesostructures. We extract values for mutual diffusion coefficients of lamellar, hexagonal and micellar cubic phases, which are consistent with previously reported values and simple models. We thus provide a method to estimate the transport properties of oriented mesophases. The macroscopic humidity-independence of the evaporation rate up to 85% relative humidities is shown to result from both an extremely low mutual diffusion coefficient and the large range of water activities corresponding to relative humidities below 85%, at which the lamellar phase exists. Such a humidity self-regulation mechanism is expected for a large variety of complex system.
复杂水溶液中的水分蒸发会导致其与空气界面处结构和成分梯度的形成。我们最近引入了一种用于定量研究此类梯度的实验装置，并讨论了结构形成如何通过自组装导致一种控制水分蒸发的自我调节机制。在此，我们使用考虑了热力学非理想条件的平流/扩散输运方程进行详细的理论分析，并将理论描述与定量实验数据直接关联。我们推导得出浓度分布根据一般的时间平方根标度律发展，这与实验观察结果完全一致。蒸发速率随时间显著下降，呈(t^{-1/2})关系，这表明与纯水蒸发不同，液相中的扩散是该系统的速率限制步骤。对于实验研究的特定二元体系，它由水和一种糖基表面活性剂（α - 十二烷基麦芽糖苷）组成，界面层由一系列不同介观结构的液晶相组成。我们提取了层状相、六方相和胶束立方相的互扩散系数值，这些值与先前报道的值和简单模型一致。因此，我们提供了一种估计定向中间相传输性质的方法。蒸发速率在相对湿度高达85%时对宏观湿度的独立性表明，这是由极低的互扩散系数以及对应于低于85%相对湿度（此时存在层状相）的大范围水分活度共同导致的。预计这种湿度自我调节机制适用于多种复杂体系。
