Stability of Various Silicone Oil/Water Emulsion Films as a Function of Surfactant and Salt Concentration

Abstract

There have been reports, originally by the Bristol group, and subsequently by others, of the preparation and properties of emulsions of stable, nearly monodisperse droplets of poly(dimethylsiloxane) (PDMS) in water, where no added surfactant is used. It has been assumed that their stability is due to the high density of surface-ionized hydroxyl groups, similar in fact to the closely related Stöber silica particles. In this study we confirm, from droplet lifetime studies, that droplets, prepared from such synthesized PDMS, are significantly more stable to coalescence than similar-sized droplets prepared from three types of commercially available PDMS, containing HO-, MeO-, or Me3-terminated chains, respectively. It is shown, however, that the zeta potentials of the synthesized PDMS and of the various commercial oils are all very similar (as indeed are their Hamaker constants). So some other explanation must be inferred for the enhanced stability to coalescence of the synthesized PDMS droplets compared to the commercial PDMS droplets. It is shown, for droplets formed from n-hexane and the synthesized oil, that stability to coalescence is conferred at PDMS volume fractions (phiPDMS) around 0.2 in the mixture. The synthesized PDMS is known to consist of mixtures of cyclic PDMS and short-chain linear species, with terminal -OH groups. There is some (indirect) evidence that in the interval 0.25 < phiPDMS < 0.35, the linear PDMS chains may be adsorbed close to a monolayer at the mixed oil/water interface, possibly conferring some enhanced Gibbs elasticity to the interface. This underpins the possibility that, in the synthesized oil droplets themselves, there is also preferential adsorption of the linear chains at the PDMS/water interface, and this leads to a value of the Gibbs elasticity, sufficient to significantly reduce coalescence. Unfortunately, the Gibbs elasticity could not be measured in this case. However, such preferential adsorption is unlikely to occur with the commercial PDMS oils, which are not so heterogeneous. Finally, it is shown that droplets of the three commercial PDMS oils could be stabilized against coalescence, if a sufficient, minimum amount of sodium dodecyl sulfate (SDS) is added. Gibbs elasticity values have been estimated in these cases, from plots of interfacial tension against ln(SDS concentration).