Microemulsions formation, stability and their characterisations
This thesis is concerned with aspects of the surface and colloid chemistry of various microemulsion systems stabilised by pure nonionic surfactants and alcohol as well as mixtures of nonionic and anionic surfactants. Phase equilibria and interfacial characteristics of the systems are studied with a view to their potential usefulness for enhanced oil recovery, in which salinity and temperature are important parameters. The equilibrium microemulsion phases are scanned at different temperatures and salinities and thus interfacial boundaries can be determined and optimum salinity scans can be performed accurately using a modified spectrophotometer. Several analytical techniques (e.g., high performance liquid chromatography, gas chromatography, ion-exchange chromatography, mass spectrometry, viscometry, electrical conductivity, photon correlation spectroscopy, UV-spectrophotometry, thermogravimetric analysis, transmission electron microscopy, surface and interfacial tension techniques) have been used to characterise and understand the microchemistry of the microemulsion systems. Ultra-low interfacial tensions (>0.1 µN/m) can be achieved in the microemulsion systems. Surfactant transfer between phases, and phase inversion of micro emulsions are shown to occur around the condition which produces minimum interfacial tension. Adsorption of the surfactants from aqueous and nonaqueous solutions has been investigated and the results show that the extent of adsorption can be reduced significantly in the presence of alcohols (co-solvent). The extent of adsorption increases with increasing temperature and salinity; however, it decreases with an increase in the hydrophilic head group of the surfactant. Adsorption of nonionic surfactants on quartz from the nonaqueous solution (decane) is much greater than from aqueous solution. In microemulsion applications, droplet combustion of w/o microemulsions is also studied for different surfaces (i.e. silica, oxidised Fecralloy and catalyst coated Fecralloy) in the temperature range of 313-573K. Formaldehyde and acetaldehyde are formed as intermediate combustion products. Thus the microemulsion combustion can lead to new oxygenate products. The w/o microemulsion route is used to synthesize colloidal silica of controlled particle size and morphology. The particle size can be varied by changing the molar ratio of water to TEOS using a water pH of 10.5.