Solubilisation and emulsification of silicone oils in aqueous surfactant systems
Solubilisation and emulsification of polydimethylsiloxane (PDMS) oils into aqueous surfactant solutions have been investigated with a series of nonionic surfactants and an anionic surfactant (AOT).For nonionic surfactants at oil-water interfaces, the effect of temperature, surfactant structure, electrolyte concentration and PDMS molecular weight on the emulsion phase inversion of water/PDMS/surfactant systems has been studied in detail. For certain nonionic surfactants the equilibrium phase behaviour of equal volumes of water and oil has been studied in Winsor systems. At air-water surfaces, the spreading behaviour of PDMS on aqueous nonionic surfactant solutions has been investigated for a range of surfactant structures. Above the critical micelle concentration (cmc) and with 50 cS PDMS, the initial and equilibrium spreading coefficients have been determined. For C12E5, the dependence of the spreading coefficients on oil molecular weight also was investigated. The complete adsorption isotherms of the volatile PDMS oils on C12E5 solutions have been measured, which yields the composition of the mixed layer of surfactant and oil. The competition for the surface between PDMS and surfactant has been studied by opening up the monolayer of C12E5 and L-77 by dilution below the cmc. The preliminary neutron reflectivity study showed that PDMS forms a thinlayer on top of the surfactant chain region.For the anionic surfactant AOT, the transition of Winsor systems at equilibrium for hexamethyldisiloxane (0.65 cS)+AOT+aqueous NaCI from I- 111-1is1 effected by increasing the electrolyte concentration. In single phase microemulsions, the uptake of oil into aqueous surfactant solutions and the solubilisation of water into surfactant solutions in oil are determined both as a function of salt concentration and temperature. The partitioning of salt between dispersed and excess water phases is considered. The stability of macroemulsions prepared from the coexisting phases in Winsor systems is investigated. Correlation between the emulsion phase inversion, Winsor phase transition and oil-water interfacial tension is discussed. The mixing behaviour of 0.65 cS PDMS oil and AOT at air-water surfaces has been studied as a function of oil activity, surfactant and electrolyte concentrations.