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Imaging Ellipsometry Study on the Effect of Electrolyte on the Drainage of an Aqueous Film Trapped Between a Plane Hydrophilic Silica Surface and an Approaching Organic Droplet

Pardon K. Kuipa and Olga Kuipa

An imaging ellipsometer is used to study the drainage of an aqueous film trapped between a hydrophilic silica equilateral prism surface and a heptane or butylacetate droplet. The interfacial droplet profile on approach to the hydrophilic silica surface is such that the droplet is dimpled at its center with the periphery of the droplet (the barrier ring) being the region of closest approach to the hydrophilic silica surface. The time it takes for these drops to either achieve an equilibrium film thickness at the barrier ring or to coalesce with the macroscopic hydrophilic silica surface was experimentally determined; drainage time increases whilst equilibrium film thickness decreases with increasing salt concentration in the continuous phase. The increase in drainage time is attributed to hindered drainage of the aqueous film due probably to an increase in the aqueous film viscosity, which should translate into a decrease in the film elasticity. Compression of the double layer may also qualitatively explain the observed decrease in equilibrium film thickness at the barrier ring as the salt concentration increases. When aluminum chloride is used as the electrolyte the equilibrium film thickness is relatively constant (around 90 ± 10 nm) for the range of concentrations studied. This may be due to the fact that the aluminum ion has a valence of three and may screen electrostatic interactions between the oil droplet and the hydrophilic silica surface at relatively lower aluminum chloride concentrations.