The Electroviscous Effect at Fluid-Fluid Interfaces

Interfacial shear viscosity is one of the most important properties of surfactant monolayers. It plays an important role in many chemical processes involving soft materials such as emulsions and foams. The presence of electrolytes is common in many of these applications. This work reports an experimental study on the effect of electrolytes on interfacial shear viscosity at gas liquid and liquid liquid interfaces in the presence of ionic surfactants. The electrostatic double layer formed at the fluid-fluid interfaces by the adsorption of ionic surfactants was responsible for the electroviscous effect. The electrolytes reduced the thickness of the diffuse double layer, and hence, reduced the interfacial shear viscosity. The role of the valence of ions present in these salts was according to that expected from the Gouy-Chapman theory of double layer. The free energy of activation for viscous flow at the interface was calculated from the Eyring's theory of interfacial shear viscosity.