Electrostatic Effects on Interfacial Film Formation in Emulsion Systems

Abstract

The adsorption at silica from dilute emulsion systems was studied within situellipsometry. In particular, the effects of electrostatic interactions on the adsorption rate and the adsorbed layer structure and formation were investigated by varying the emulsion droplet and surface charge, as well as the electrostatic screening, accomplished by varying pH and the excess electrolyte concentration. Electrostatic interactions were found to markedly affect the adsorption rate, but not the adsorbed layer structure or the mechanism for the adsorbed layer formation. For all cases investigated, the adsorbed layer thickness corresponds to emulsion droplets or multilamellar liposomes, and the adsorbed layer formation proceeds through attachment of emulsion droplets and/or multilamellar liposomes at the surface without extensive droplet spreading or liposome collapse. When the droplets and the surface are similarly charged, the adsorption is facilitated by increasing the electrostatic screening or by decreasing the emulsion droplet and surface charge, accomplished by increasing the excess electrolyte concentration and decreasing pH, respectively. When the droplets and the surface are oppositely charged, the adsorption rate is much higher than that observed when the droplets and the surface are similarly charged, although the adsorbed layer structure and the mechanism for the adsorbed layer formation are similar. Qualitatively, these effects may be understood by considering only electrostatic and van der Waals interactions.