Emulsions and equilibrium phase behaviour in silicone oil plus water plus nonionic surfactant mixtures

We have investigated several aspects of the emulsion and equilibrium phase behaviour of mixtures of water, nonionic surfactant (C(n)E(m)) and silicone oil (PDMS). Emulsions may be inverted from oil-in-water to water-in-oil on increasing the temperature or the concentration of NaCl electrolyte. The apparent phase inversion temperature (PIT) increases with an increase in the number of oxyethylene groups in the surfactant (E(m)), and decreases with increasing surfactant alkyl chain length (C(n)). The PIT also decreases on adding NaCl but increases on adding tetrabutylammonium bromide as electrolyte. Emulsions were very unstable to coalescence around the conditions of phase inversion using C(12)E(3). For systems containing equal volumes of water and oil at equilibrium, two phase-three phase-two phase transitions are seen for certain surfactants. The extent of solubilisation of PDMS into surfactant aggregates containing these hydrogenated surfactants however is quite low (<3 vol.%). The composition of the third phase changes from mainly surfactant+water in the case of C(12)E(3) to mainly surfactant+oil in the case of the surfactant C(16)P(8)E(1) (P being a propyleneoxide group). Estimates of the monomeric solubility of surfactant in PDMS at different temperatures are given. (C) 1998 Elsevier Science B.V.