The formation and stability of nanoemulsions

Emulsions are metastable heterogeneous mixtures of one liquid phase dispersed in another immiscible liquid in the form of droplets. Typical emulsions are composed of oil and water with a surfactant to inhibit droplet coalescence. They are ubiquitous in our everyday lives---from food products and cosmetics to oil recovery and pharmaceuticals. The composition and preparation of emulsion systems govern the physical characteristics such as droplet size, optical properties, and rheological behavior. By employing Taylor's prediction for droplet size dependence in the dilute limit, the effects of shear rate, droplet volume fraction, and surfactant concentration have been investigated for an emulsion system composed of silicone oil in an aqueous sodium dodecyl sulfate (SDS) continuous phase. As a result, a method for making a new class of emulsions, nanoemulsions, has been developed. Nanoemulsions are composed of silicone oil droplets that have radii less than 100 nm. At such small droplet sizes, the physical properties of nanoemulsions differ from other microscale emulsion systems. They are optically transparent, become viscoelastic even at low volume fractions, and demonstrate interesting surfactant hysteresis properties. The formulation of nanoemulsions and the effects of surfactant concentration through coalescence studies will be discussed.;The stability of emulsions depends primarily on the surfactant adsorbed to the droplet surfaces. There are very few quantitative experimental techniques for measuring surfactant partitioning to the oil-water interface and those that exist are very complex. We present a new method for measuring the amount of sodium dodecyl sulfate that resides on the droplet surfaces of a silicone oil-in-water nanoemulsion. We adapted a method of filtering emulsion droplets from the bulk continuous phase. The surfactant in the filtrate is then complexed with an ionic dye to obtain accurate concentration measurements with UV-Vis spectrophotometric absorbance. We have successfully measured the amount of SDS that partitions to the oil droplet interface by adapting this ultrafiltration method and expanding it to concentrated nanoemulsions. We have created density profiles of surfactant residing at the liquid-liquid interface in emulsions for varying volume fractions and initial SDS concentrations in the continuous phase.