Aranberri I, Binks B P, Clint J H, Fletcher P D I
Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom.
Langmuir. 2004 Mar 16;20(6):2069-74. doi: 10.1021/la035031x.
We have investigated the rate of water evaporation from concentrated oil-in-water (o/w) emulsions containing an involatile oil. Evaporation of the water continuous phase causes compression of the emulsion with progressive distortion of the oil drops and thinning of the water films separating them. Theoretically, the vapor pressure of water is sensitive to the interdroplet interactions, which are a function of the film thickness. Three main possible situations are considered. First, under conditions when the evaporation rate is controlled by mass transfer across the stagnant vapor phase, model calculations show that evaporation can, in principle, be slowed by repulsive interdroplet interactions. However, significant retardation requires very strong repulsive forces acting over large separations for typical emulsion drop sizes. Second, water evaporation may be limited by diffusion in the network of water films within the emulsion. In this situation, water loss by evaporation from the emulsion surface leads to a gradient in the water concentration (and in the water film thickness). Third, compression of the drops may lead to coalescence of the emulsion drops and the formation of a macroscopic oil film at the emulsion surface, which serves to prevent further water evaporation. Water mass-loss curves have been measured for silicone o/w emulsions stabilized by the anionic surfactant SDS as a function of the water content, the thickness of the stagnant vapor-phase layer, and the concentration of electrolyte in the aqueous phase, and the results are discussed in terms of the three possible scenarios just described. In systems with added salt, water evaporation virtually ceases before all the water present is lost, probably as a result of oil-drop coalescence resulting in the formation of a water-impermeable oil film at the emulsion surface.
我们研究了含有不挥发油的浓缩水包油（o/w）乳液中的水分蒸发速率。水连续相的蒸发会导致乳液压缩，油滴逐渐变形，分隔油滴的水膜变薄。从理论上讲，水的蒸气压对液滴间相互作用敏感，而液滴间相互作用是膜厚度的函数。考虑了三种主要的可能情况。首先，在蒸发速率由穿过静止气相的传质控制的条件下，模型计算表明，原则上，液滴间的排斥相互作用可以减缓蒸发。然而，对于典型的乳液滴尺寸，显著的延迟需要在大间距上作用非常强的排斥力。其次，水的蒸发可能受乳液内水膜网络中的扩散限制。在这种情况下，从乳液表面蒸发的水分损失会导致水浓度（以及水膜厚度）出现梯度。第三，液滴的压缩可能导致乳液滴聚结，并在乳液表面形成宏观油膜，这会阻止进一步的水蒸发。我们测量了由阴离子表面活性剂十二烷基硫酸钠稳定的有机硅o/w乳液的水质量损失曲线，该曲线是水含量、静止气相层厚度和水相中电解质浓度的函数，并根据上述三种可能情况对结果进行了讨论。在添加了盐的体系中，在所有存在的水都未损失之前，水的蒸发实际上就停止了，这可能是油滴聚结导致在乳液表面形成不透水油膜的结果。
