Pingali Sai Venkatesh, Takiue Takanori, Luo Guangming, Tikhonov Aleksey M, Ikeda Norihiro, Aratono Makoto, Schlossman Mark L
Department of Physics, University of Illinois at Chicago, Illinois 60607, USA.
J Phys Chem B. 2005 Jan 27;109(3):1210-25. doi: 10.1021/jp045887q.
The interface between water and mixed surfactant solutions of CH(3)(CH(2))(19)OH and CF(3)(CF(2))(7)(CH(2))(2)OH in hexane was studied with interfacial tension and X-ray reflectivity measurements. Measurements of the tension as a function of temperature for a range of total bulk surfactant concentrations and for three different values of the molal ratio of fluorinated to total surfactant concentration (0.25, 0.28, and 0.5) determined that the interface can be in three different monolayer phases. The interfacial excess entropy determined for these phases suggests that two of the phases are condensed single surfactant monolayers of CH(3)(CH(2))(19)OH and CF(3)(CF(2))(7)(CH(2))(2)OH. By studying four different compositions as a function of temperature, X-ray reflectivity was used to determine the structure of these monolayers in all three phases at the liquid-liquid interface. The X-ray reflectivity measurements were analyzed with a layer model to determine the electron density and thickness of the headgroup and tailgroup layers. The reflectivity demonstrates that phases 1 and 2 correspond to an interface fully covered by only one of the surfactants (liquid monolayer of CH(3)(CH(2))(19)OH in phase 1 and a solid condensed monolayer of CF(3)(CF(2))(7)(CH(2))(2)OH in phase 2). This was determined by analysis of the electron density profile as well as by direct comparison to reflectivity studies of the liquid-liquid interface in systems containing only one of the surfactants (plus hexane and water). The liquid monolayer of CH(3)(CH(2))(19)OH undergoes a transition to the solid monolayer of CF(3)(CF(2))(7)(CH(2))(2)OH with increasing temperature. Phase 3 and the transition regions between phases 1 and 2 consist of a mixed monolayer at the interface that contains domains of the two surfactants. In phase 3 the interface also contains gaseous regions that occupy progressively more of the interface as the temperature is increased. The reflectivity determined the coverage of the surfactant domains at the interface. A simple model is presented that predicts the basic features of the domain coverage as a function of temperature for the mixed surfactant system from the behavior of the single surfactant systems.
通过界面张力和X射线反射率测量，研究了己烷中CH(3)(CH(2))(19)OH和CF(3)(CF(2))(7)(CH(2))(2)OH的水与混合表面活性剂溶液之间的界面。在一系列总本体表面活性剂浓度以及氟化表面活性剂与总表面活性剂浓度的摩尔比的三个不同值（0.25、0.28和0.5）下，测量了张力随温度的变化，确定该界面可以处于三种不同的单分子层相。为这些相确定的界面过量熵表明，其中两个相是CH(3)(CH(2))(19)OH和CF(3)(CF(2))(7)(CH(2))(2)OH的凝聚单表面活性剂单分子层。通过研究四种不同组成随温度的变化，利用X射线反射率来确定液 - 液界面处所有三个相中的这些单分子层的结构。用层模型分析X射线反射率测量结果，以确定头基团和尾基团层的电子密度和厚度。反射率表明，相1和相2对应于仅由一种表面活性剂完全覆盖的界面（相1中为CH(3)(CH(2))(19)OH的液体单分子层，相2中为CF(3)(CF(2))(7)(CH(2))(2)OH的固体凝聚单分子层）。这是通过对电子密度分布的分析以及与仅含有一种表面活性剂（加己烷和水）的系统中液 - 液界面的反射率研究直接比较来确定的。随着温度升高，CH(3)(CH(2))(19)OH的液体单分子层转变为CF(3)(CF(2))(7)(CH(2))(2)OH的固体单分子层。相3以及相1和相2之间的过渡区域由界面处的混合单分子层组成，该混合单分子层包含两种表面活性剂的域。在相3中，界面还包含气态区域，随着温度升高，气态区域逐渐占据更多的界面。反射率确定了界面处表面活性剂域的覆盖率。提出了一个简单模型，该模型根据单表面活性剂系统的行为预测混合表面活性剂系统中域覆盖率随温度变化的基本特征。
