Thermodynamic Model for the Prediction of Contact Angles of Oil Droplets on Solid Surfaces in SDS Solutions

The attachment of a droplet of oil to a solid surface in the presence of an aqueous surfactant solution may be quantified by contact angle measurements. A classical thermodynamic model has been developed to predict this characteristic contact angle. The model minimizes the Gibbs free energy for a system that includes five mass-transfer actions: micellization of surfactant monomer in aqueous solution, adsorption of surfactant at the solid-solution interface, adsorption of surfactant at the oil-solution interface, adsorption of the oil at the solid surface, and adsorption of water at the solid surface. Limitations in the model include empirical values for the energy of steric/restrictive interactions in micellization and the interfacial free-energy term for adsorption of surfactant at the oil-solution interface. In addition, the free energies for adsorption of water, oil, and surfactant at the solid surface are adjustable parameters. The model has been validated by comparison with experimental values of contact angle measured for droplets of hexadecane on a gold surface. This approach allows for the use of published physical property data for the prediction of surfactant distribution and contact angle in a given system and may be useful in guiding aqueous cleaning applications.