Hydrophile-Lipophile-Balance and Interfacial Tensions in Water-Hydrocarbon-Surfactant Systems
Three-component systems consisting of water, a hydrocarbon oil, and a nonionic polyoxyethylene surfactant of the form CnH2n+1 (OCH2CH2) mOH (denoted CnEm) exhibit a rich phase behaviour with liquid one-, two- and three-phase regions (and several liquid-crystal phases at higher surfactant concentrations). The capability of a surfactant to solubilize oil in water and water in oil, and to reduce the w/o interfacial tension σ depends on a balance of hydrophilic and hydrophobic interactions and on temperature (Shinoda and Friberg 1986). A well-balanced Cn Em surfactant is predominantly water-soluble at low temperatures and predominantly oil-soluble at higher temperatures. This “phase inversion” usually involves the formation of a surfactant-rich third liquid phase which is described as a microemulsion, or organized surfactant phase (Shinoda and Lindman 1987), although from a phenomenological point of view these systems are closely analogous to liquid three- or four-component systems with smaller amphiphilic molecules (which are not forming micellar solutions or liquid-crystal phases). The aqueous phase α, surfactant-rich phase β, and oil-rich phase y can coexist only over a limited temperature range between a lower critical end-point temperature T1 (at which α and β become identical in the presence of ϒ) and an upper critical end-point temperature Tu (at which β and ϒ become identical in the presence of α) (Kahlweit 1982; Kahlweit et al. 1983, 1984; Kahlweit and Strey 1985).
KeywordsInterfacial Tension High Surfactant Concentration Limited Temperature Range Surfactant Phase Plait Point
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