X-ray studies of mixed surfactants at the hexane-water interface and thin aqueous films.

摘要

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.