X-ray studies of mixed surfactants at the hexane -water interface and thin aqueous films
The interface between water and mixed surfactant solutions of CH3(CH2)19OH and CF3(CF 2)7(CH2)2OH 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 (m) and for three different values of the molal ratio of fluorinated to total surfactant concentration (X 2 = 0.25, 0.28 and 0.5) determined that the interface can be in three different monolayer phases. X-ray reflectivity study of four different compositions as a function of temperature demonstrates that phases 1 and 2 correspond to an interface fully covered by only one of the surfactants (liquid monolayer of CH3(CH2)19OH in phase I and a solid condensed monolayer of CF3(CF2)7(CH2) 2OH in phase 2). The liquid monolayer of CH3(CH2) 19OH undergoes a transition to the solid monolayer of CF3(CF 2)7(CH2)2OH 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. 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.
Spontaneous thin film formation in a biphase system consisting of PEG, potassium phosphates and water was studied using X-ray reflectivity and off-specular diffuse scattering. Two aqueous phases, PEG-rich and salt-rich phases are formed by varying the overall salt concentration from 12--29 wt/wt% for fixed overall PEG concentration of 13 wt/wt%. Two classes of thin film formation are observed---the thickness of the spontaneously formed film is same as the drop spreading film for systems 12--18% and the thickness of the spontaneously formed film is different from the drop spreading film for systems 23% and 29%. Although system 12% is in the 12--18% class, its behavior is different from the rest. Layer thickness formed by the two methods, spontaneous and drop spreading is the same, although the normalized electron density is different. The liquid-liquid interfacial widths in these systems are smaller than those predicted by thermal fluctuations requiring suppressions due to contributions from van der Waals interaction and bending rigidity of the interface.
0542: Chemical engineering