Evolution of surfactant-free 'pristine' emulsions

Abstract

The term pristine interface was introduced by Beattie and Djerdjev 20 years ago for emulsions that consist of only water and oil with no surfactant. They are different from Pickering emulsions, which are also surfactant-free but stabilized with colloidal particles. In contrast to previous studies, we monitor the kinetics of the initial stages of emulsion formation. We conducted such tests in an open setup when samples are open to air and CO2 content in the water varies, and in closed setup when samples are isolated with fixed CO2 content. For the open setup, sonication and initial pH > 9 leads to emulsions with high zeta potential and sub-micron droplet size. There are two evolution patterns: short- and long-terms. The short term lasts about 1 day and has changing pH and zeta potential, but almost constant droplet size. The long term is is over several days or even weeks, with droplet size increase toward saturation value (rate dependent on mixing conditions), with pH and zeta potential remaining constant. Emulsification at the closed setup is much less pronounced and pH remains constant. This difference points to the importance of adsorbed CO2 and related carbonate ions in the formation of pristine emulsions and charging droplets interfaces. We hypothesize the existence of structured water molecule layer at the interface, following Eastoe and Ellis. The Electric Double Layer exerts a (dielectrostatic) force on the water dipole moments in this layer that compensates the Kelvins pressure. The droplet size from this model is close to our measurements. Also, there is a repulsion of the water dipole moments, which compensates for the surface tension parallel to the interface. After ruling out alternative hypotheses with our data, we conclude that the model suggested for explaining the stability of nano-bubbles is also consistent with our results for these pristine emulsions.