Globular and bicontinuous phases of nonionic surfactant films

• Published in: Advances in colloid and interface science Vol. 49; pp. 113 - 146
• Main Authors: Olsson, Ulf, Wennerström, Håkan
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 29.04.1994; Elsevier
• Subjects: Chemistry; Colloidal state and disperse state; Emulsions. Microemulsions. Foams; Exact sciences and technology; General and physical chemistry; Surface physical chemistry; Surface-active agents: properties; Monolayer; Interface tension; Phase diagram; Temperature effect; Thin film; Hexadecane; Aggregation; Ethylene oxide polymer; Isotropy; Phase transformation; Micellar critical concentration; Liquid liquid interface; Polarity; Microemulsion; Autodiffusion coefficient; Liquid film; Crystalline phase; Stability; Hydrocarbon; Non ionic surfactant; Theoretical study; Hard sphere model; Free energy; Adsorption; Cyclohexane; Models; Thermodynamic properties; Physicochemical properties
• ISSN: 0001-8686; 1873-3727
• DOI: 10.1016/0001-8686(94)80014-6

Nonionic surfactants of the alkyloligoethylene oxide type form, with water and oil, a range of isotropic a liquid crystalline phases. We analyse the phase behaviour using the flexible surface model and argue that the strong temperature dependence is caused by the fact that the monolayer spontaneous curvature decreases strongly with increasing temperature. This is exemplified with the behaviour of bicontinuous microemulsions, showing a symmetric behaviour around the balanced state, globular microemulsions, behaving as hard spheres near the emulsification failure boundary, and sponge phases appearing when the monolayer spontaneous mean curvature is towards the abundant solvent. It is argued that there is a hierarchy of free energy contributions determining the preferred aggregate shape/phase. With a given oil-water ratio and a surfactant concentration that fixes the polar/apolar interfacial area, the most important free energy contribution comes from having a mean curvature close to the spontaneous curvature. The Gaussian curvature and the entropy terms become important when selecting between structures of similar mean curvature. At higher concentrations, surface forces and higher order elastic terms become significant.