Abstract for presentation at Chemeca 2007

Emulsion film is formed between two oil drops at close contact in water or another aqueous solution. It is an important component of emulsion systems found in many industrial applications, such as food and pharmaceutical processing. Surface active reagents (surfactants) are often used to control the emulsion stability and drainage. Many aspects of surfactant adsorption in emulsion film drainage are not well understood at present. This paper presents the new theoretical and experimental results of our research into quantifying the role of surfactant adsorption and surface properties in emulsion film drainage. The experimental results were obtained using an improved micro-interferometric technique with the Scheludko thin film balance which consists of a short glass capillary with 4 mm inner diameter, called the film holder, connected to a gastight micro syringe system. Thin emulsion films of aqueous SDS (sodium dodecyl sulphate) solutions were formed between two concave water-oil interfaces of two oil (toluene) drops created and attached to the inner wall of the film holder. Initially, two oil drops in an aqueous solution were created in the film holder. The syringe system was then used to pump out the aqueous solution, bringing the interfaces closer and forming a plane-parallel microscopic film with small radius (< 750 microns). An inverted microscope was used to observe and record the transient films using the reflected light. As a result of the interference of the light reflected from the two film surfaces, a set of black and white fringes, called the Newton rings, was observed and digitally recorded by a high-speed video camera system. The film thickness was determined using the Newton rings. The experimental data for the film transient thickness were compared with the standard theory for film drainage, developed based on the Reynolds-Stefan lubrication approximation. The comparison shows significant deviation. The standard film drainage theory was improved by considering the surfactant adsorption, which significantly changes the film surface properties such as the surface tension, surface elasticity, surface viscosity and surface forces. The surface forces considered in the prediction included the van der Waals, electrical double-layer, and non-DLVO (hydrophobic attraction or hydrophilic repulsion) surface forces. A better agreement between the improved theory and the experimental data was obtained. Both the new theory and experiment show important effect of the surfactant adsorption, surface elasticity and surface charge (or potential) of the adsorbed layers. The outcome of this study is significant for a better design of emulsion systems found in the industry.