Disjoining pressures, zeta potentials and surface tensions of aqueous non-ionic surfactant/electrolyte solutions: theory and comparison to experiment

Abstract

A self-consistent electrostatic theory is presented to predict disjoining pressure isotherms of aqueous thin-liquid films stabilized by non-ionic surfactants and air/water surface tensions and zeta potentials of electrolyte solutions with and without non-ionic surfactant. The proposed model combines specific adsorption of hydroxide ions at the interface with image charge and dispersion forces on ions in the diffuse double layer. The result is a quantitative description of aqueous solution interfaces as a function of surfactant concentration, ionic strength and pH. Disjoining pressure isotherms of thin-liquid films stabilized by non-ionic surfactants and electrophoresis experiments on air bubbles and oil droplets in aqueous solutions demonstrate that hydroxide ions specifically adsorb at air/water and oil/water interfaces. The surface charge increases with pH, decreases with increasing surfactant concentration, increases slightly with ionic strength, and for n-alkyl polyethylene oxide non-ionic surfactants is not significantly affected by surfactant molecular structure. Concentrated electrolyte-solution surface tensions, however, indicate that ions are repelled from the air/water interface by an ‘image charge’ force, that is parameterized by the ion valence and the ionic strength of the aqueous solution. Additionally, differences in induced–induced dipole forces on an ion near an interface lead to a van der Waals dispersion interaction force that depends on the ion polarizabilites and the molecular properties of the two surrounding bulk phases. Incorporation of these two additional ion free energies into the Poisson–Boltzmann equation along with a simple model for hydroxide-ion specific adsorption at the air/water interface results in a non-linear second-order ordinary differential equation containing two adjustable parameters. The proposed modified Poisson–Boltzmann (MPB) theory accurately predicts newly measured disjoining pressures of thin-liquid foam films stabilized by polyethylene oxide n-alkyl ether surfactants. With no additional adjustable parameters, zeta potentials of nascent air bubbles in water and surface tensions of aqueous electrolyte solutions are successfully predicted. The new electrostatic model also explains the fascinating existence of a surface tension minimum in dilute electrolyte solutions, known as the Jones–Ray effect.