X-ray Reflectivity and Interfacial Tension Study of the Structure and Phase Behavior of the Interface between Water and Mixed Surfactant Solutions of CH₃(CH₂)₁₉OH and CF₃(CF₂)₇(CH₂)₂OH in Hexane

Abstract

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.

Effect of Molecular Orientation on Monolayer and Multilayer Formations of Fluorocarbon Alcohol and Fluorocarbon-α,ω-diol Mixture at the Hexane/water Interface

“…The curves below X 2 = 0.98 (curve 10) have a break point, and their shapes change regularly from that of the pure FC 10 diol system (curve 1) with increasing X 2 . The curves above X 2 = 0.99 (curve 13) also show a break which changes gradually from that observed in pure FC 10 OH system (curve 15). At X 2 = 0.982 and 0.985 (curves 11 and 12), the curves have two break points at m = 7.5 and 7.85 mmol kg −1 at X 2 = 0.982 and m = 7.45 and 9.75 mmol kg −1 at X 2 = 0.985 (indicated by arrows).…”

“…Three kinds of curves originated from the three types of phase transitions meet at m eq = 7.6 mmol kg −1 , γ eq = 29.8 mN m −1 , and X 2 = 0.981, indicating that in the FC 10 diol−FC 10 OH system, at least three different states appear depending on m and X 2 .…”

“…This is mainly due to the loss of dispersion interaction by mixing of parallel-oriented FC 10 diol with normal-oriented FC 10 …”

“…14 −16 In the course of our systematic studies on the adsorption of molecules at the air/water and oil/water interfaces, the mixing of molecules in the adsorbed film was examined by constructing the phase diagram of adsorption (PDA) from thermodynamic analysis of interfacial tension data and by evaluating the electron density profile from X-ray reflectivity measurement. Among others, in our previous study on the adsorbed films of 1-icosanol (C 20 OH)−1H,1H,2H,2H-perfluorodecanol (FC 10 OH) mixture at the hexane/water interface, 7−10 the PDA indicated that C 20 OH and FC 10 OH molecules are miscible in the expanded state, whereas they are practically immiscible in the condensed state. The mixing of both alcohols in the expanded state accompanied a large positive excess Gibbs energy of adsorption due to weaker molecular interaction between fluorocarbon (FC) and hydrocarbon (HC) chains than between the same ones.…”

“…The mixing of both alcohols in the expanded state accompanied a large positive excess Gibbs energy of adsorption due to weaker molecular interaction between fluorocarbon (FC) and hydrocarbon (HC) chains than between the same ones. Furthermore, it was found that FC 10 OH molecules form a condensed phase domain surrounded by a low density gaseous phase in the expanded state, and the two kinds of domains, FC 10 OH condensed and C 20 OH condensed domains, coexist in the temperature range close to the phase transition between the FC 10 OH condensed and C 20 OH condensed states.…”