The stability of emulsions plays a crucial role in formulation of many food, detergent, pharmaceutical and personal care products. Most emulsion formulations are stabilized by mixed surfactant systems containing a combination of ionic and non-ionic surfactants. The charging behavior of ionic surfactant on emulsion drops is commonly studied with elector-kinetic or atomic force microscopy (AFM) measurements and has been shown to exhibit counter ion condensation similar to that of a micelle. The charging behavior of non-ionic surfactants is far more complicated as a number of studies have observed residual charge dependent behavior with changes in pH or background electrolyte at for both air-water or oil-water interfaces. This charging behavior is often attributed to the native charge on the air-water or oil-water interface.
We present direct force measurements between two colliding oil droplets in aqueous solutions of non-ionic surfactant (Pentaethylene glycol monododecyl ether (C12E5)) using AFM. These measured forces exhibit a clear dependence on background electrolyte concentration. Quantitative modeling of these forces, which accounts for electrical double layer and retarded van der Waals forces as well as droplet deformation, was used to extract the surface charge as a function of surfactant and electrolyte concentration. We note the importance of including the fully retarded treatment of the van der Waals attraction in these systems as well as accounting for the screening effects of the electrolyte on the determination of the surface potential. We compare these data to existing measurements of the native charge on the oil-water interface. In addition, we discuss the implications of these data when trying to use surface potential to probe the charging behavior in mixed surfactant emulsion system with ionic and non-ionic surfactants.