Phase inversion in nonionic surfactant-oil-water systems
This study has been concerned with the inversion of water in oil (W/0) emulsions, to oil in water (O/W) emulsions and vice-versa. It has been shown that there are two types of emulsion phase inversion that can occur in nonionic Surfactant-Oil-Water (nSOW) systems: (i) A "transitional" inversion, which is brought about by changing the nSOW phase behaviour, by altering the surfactant's affinity for the oil and water phases and, (ii) a "catastrophic" inversion, induced by increasing the dispersed phase fraction and occurs at closest packing of unstable dispersed phase drops. The inversion mechanism of the two inversion types has been characterised. The two inversion types can be represented as boundaries on a "map" relating nSOW phase behaviour with water to oil volume ratio. The form of the map depends on the nature of the oil. At the transitional point, the nSOW system can be 3 phase - an oil phase, a water phase and a surfactant phase microemulsion. Ultra-low interfacial tension exists between the phases - this property is of interest for producing extremely fine emulsions with low energy input. Transitional inversions are sometimes reversible. In nSOW systems, true catastrophic inversions can be induced by moving the water to oil ratio in one direction only. Double emulsion drops (W/O/W or O/W/O) are sometimes produced before inversion and inversion points are dependent on dynamic conditions. A thermodynamic relationship between nSOW phase behaviour, oil type, surfactant type, surfactant concentration and temperature has been derived, based on the partitioning of surfactant between oil, water and a surfactant micelle phase. It has been shown how this can be used to classify nonionic surfactants. The effect of agitation conditions, water addition rate and oil phase viscosity, on the drop types and drop sizes of emulsions present before and after inversion (for each inversion type) has been studied extensively. Surfactant type and concentration also affect drop behaviour and drop sizes. Various drop types have been identified and qualitative and quantitative analysis of the factors controlling the drop sizes of emulsions at each stage of a phase inversion has been developed.