(183a) Forces Between Emulsions Drops and Charging Behavior in the Presence of Mixtures of Ionic and Non-Ionic Surfactants

The stability of emulsions plays a crucial role in formulation of many food, detergent, pharmaceutical and personal care products. Most of emulsion formulations are stabilized by mixed surfactant system containing a combination of ionic and non-ionic surfactants. There have been many experimental and theoretical works in literature that studied adsorption of mixed surfactants onto air-water interface. However, in many of the emulsions it is the adsorption of mixed surfactants onto oil-water interface that is relevant. In order to understand the stability of oil in water emulsions in presence of mixed surfactants, we experimentally studied the interaction forces between oil drops. In our study, we used surfactant mixture of an ionic (Sodium 1-octanesulfonate monohydrate (C₈)) surfactant and a non-Ionic (Pentaethylene glycol monododecyl ether (C₁₂E₅)) surfactant. We measured forces between colliding oil drops (n-Tetradecane) using Atomic Force Microscope (AFM) in presence of pure surfactants as well as mixture of surfactants of various compositions.  The measured force-distance curves are analyzed using Chan-Dagastine-White model to extract the zeta potential of the oil-water interface. We found that in case of ionic surfactant only system (C₈), zeta potential increases with increase in total surfactant concentration suggesting the ionic surfactant is a highly counter ion condensed state. However, in the case of mixed surfactant system (C₈:C₁₂E₅), zeta potential decreases with increase in total surfactant concentration at mixing ratios of 6:1 and 15:1. This observation is attributed to changes in the counter ion condensation state due to the lower number of ionic dissociation sites. In order to elucidate the effect of competing adsorption of surfactants onto oil-water interface, we measured adsorption isotherms of pure surfactants and mixture of surfactants by pendant drop method. We found that the adsorption of ionic surfactant (C₈) follows Frumkin model whereas the adsorption of non-ionic surfactant (C₁₂E₅) follows Re-Orientation model. The fitted model parameters from pure surfactant adsorption isotherms are used to predict isotherms of mixed surfactants at mixing ratios of 6:1 and 15:1. Surface excess calculations show that adsorption at oil-water interface is dominated by anionic surfactant due to its high diffusivity and adsorption rate constant.  The surface charge density on oil-water interface obtained from theoretical surface excess calculations are compared with that calculated from zeta potential and force measurements. Thus, AFM force measurement combined with adsorption isotherms offers a unique perspective on charging behavior and stability of oil in water emulsions in presence of mixed surfactant system.