Non-aqueous high internal phase emulsions. Preparation and stability
Several novel non-aqueous high internal phase emulsions (HIPEs) of petroleum ether dispersed in a number of polar organic solvents [formamide (FA), N,N′-dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO)] as the continuous phase, with internal phase volume ratios (ϕ) of 0.9, have been prepared, employing both polymeric and non-polymeric non-ionic surfactants. A number of factors have been shown to be important in determining HIPE stability. First, the nature of the polar organic phase: the most stable emulsions are formed with formamide. This is attributed to its high polarity and ability to form a water-like hydrogen-bonded network, however, rationalising the relative performance of the other polar solvents is more difficult. Second, the interfacial behaviour of the surfactant employed: interfacial data for aqueous solutions in contact with hydrocarbon do not adequately describe surfactant performance and a much better guide is the hydrophilic–lipophilic balance (HLB) value of the surfactant. Although this may be an empirical guide to surfactant behaviour, in this context it is useful, indeed rather powerful. The third important parameter is the molecular size or complexity of the surfactant. Generally, low-molecular-weight surfactants perform poorly, whereas PEO–PPO–PEO block copolymers of appropriate HLB value allow the formation of stable HIPEs. The PEO block appears to be solubilised in the polar solvent and the PPO block in the hydrocarbon. Presumably, each block attemts to form a (separate) random coil on its own side of the interface. Consequential effects may well include an increase in viscosity and slow desorption kinetics at the interface.