Microemulsion: prediction of the phase diagram with a modified Helfrich free energy

Abstract

Microemulsions are thermodynamically stable, macroscopically homogeneous but microscopically heterogeneous, mixtures of water and oil stabilised by surfactant molecules. They have unique properties like ultralow interfacial tension, large interfacial area and the ability to solubilise other immiscible liquids. Depending on the temperature and concentration, non-ionic surfactants self assemble to micelles, flat lamellar, hexagonal and sponge like bicontinuous morphologies. Microemulsions have three different macroscopic phases (a) 1phase- microemulsion (isotropic), (b) 2phase-microemulsion coexisting with either expelled water or oil and (c) 3phase- microemulsion coexisting with expelled water and oil.rnrnOne of the most important fundamental questions in this field is the relation between the properties of the surfactant monolayer at water-oil interface and those of microemulsion. This monolayer forms an extended interface whose local curvature determines the structure of the microemulsion. The main part of my thesis deals with the quantitative measurements of the temperature induced phase transitions of water-oil-nonionic microemulsions and their interpretation using the temperature dependent spontaneous curvature [c0(T)] of the surfactant monolayer. In a 1phase- region, conservation of the components determines the droplet (domain) size (R) whereas in 2phase-region, it is determined by the temperature dependence of c0(T). The Helfrich bending free energy density includes the dependence of the droplet size on c0(T) as , where ls is the effective length of the surfactant, and are the bending and Gaussian modulus. However, this approach cannot account for the 3phase region. Therefore, a modified Helfrich equation is proposed which describes the three macroscopic phases. It assumes that within a well defined temperature interval two spontaneous curvatures coexist. The consequences of these assumptions are investigated. As model systems, a moderate and a weak surfactant (short-chain) microemulsion were used. Systematic scattering and calorimetric experiments were conducted to predict the phase behaviour and thereby to investigate the validity of this assumption.rnrnA quantitative prediction of the concentration dependence of the phase transition temperatures for 1phase-, 2phase- and 3phase- microemulsions is one of the highlights of my work. For this purpose, microemulsions with different ratios of the components have been studied. A relation between the temperature dependency of the spontaneous curvature and the hydration of surfactant is facilitated. The generalised description for water-oil-surfactant system using the modified 'Helfrich bending free energy' is shown to be a successful concept. The experiments lead to a clear picture of the validity of the modified Helfrich bending free energy for a wide range of surfactants and phase transitions in these systems.rn

Microemulsions are thermodynamically stable, macroscopically homogeneous but microscopically heterogeneous, mixtures of water and oil stabilised by surfactant molecules. They have unique properties like ultralow interfacial tension, large interfacial area and the ability to solubilise other immiscible liquids. Depending on the temperature and concentration, non-ionic surfactants self assemble to micelles, flat lamellar, hexagonal and sponge like bicontinuous morphologies. Microemulsions have three different macroscopic phases (a) 1phase- microemulsion (isotropic), (b) 2phase-microemulsion coexisting with either expelled water or oil and (c) 3phase- microemulsion coexisting with expelled water and oil.rnrnOne of the most important fundamental questions in this field is the relation between the properties of the surfactant monolayer at water-oil interface and those of microemulsion. This monolayer forms an extended interface whose local curvature determines the structure of the microemulsion. The main part of my thesis deals with the quantitative measurements of the temperature induced phase transitions of water-oil-nonionic microemulsions and their interpretation using the temperature dependent spontaneous curvature [c0(T)] of the surfactant monolayer. In a 1phase- region, conservation of the components determines the droplet (domain) size (R) whereas in 2phase-region, it is determined by the temperature dependence of c0(T). The Helfrich bending free energy density includes the dependence of the droplet size on c0(T) as , where ls is the effective length of the surfactant, and are the bending and Gaussian modulus. However, this approach cannot account for the 3phase region. Therefore, a modified Helfrich equation is proposed which describes the three macroscopic phases. It assumes that within a well defined temperature interval two spontaneous curvatures coexist. The consequences of these assumptions are investigated. As model systems, a moderate and a weak surfactant (short-chain) microemulsion were used. Systematic scattering and calorimetric experiments were conducted to predict the phase behaviour and thereby to investigate the validity of this assumption.rnrnA quantitative prediction of the concentration dependence of the phase transition temperatures for 1phase-, 2phase- and 3phase- microemulsions is one of the highlights of my work. For this purpose, microemulsions with different ratios of the components have been studied. A relation between the temperature dependency of the spontaneous curvature and the hydration of surfactant is facilitated. The generalised description for water-oil-surfactant system using the modified 'Helfrich bending free energy' is shown to be a successful concept. The experiments lead to a clear picture of the validity of the modified Helfrich bending free energy for a wide range of surfactants and phase transitions in these systems.rn