Synergistic stabilization of oil-in-water emulsions using a binary mixture of chitosan and hydroxypropyl cellulose.

A judicious binary mixture of chitosan and hydroxypropyl cellulose (HPC) has been used to prepare a series of oil-in-water emulsions via high-shear homogenization at pH 4.9. Employing either biopolymer alone results in markedly inferior emulsion stability, suggesting a synergistic effect. Laser diffraction studies indicate that an optimum chitosan/HPC mass ratio of 5:5 produces the finest, most stable droplets. As expected, smaller oil droplets are produced when employing either higher shear or higher biopolymer concentration, and the mean droplet diameter can be tuned from 5 to 30 μm by adjusting these two parameters. However, the droplet diameter is surprisingly insensitive to the oil mass fraction. Such emulsions remain stable between pH 2.6 and pH 5.9. This binary stabilizer approach works well for a range of model oils. Aqueous electrophoresis measurements indicate that the chitosan component confers substantial cationic surface charge on the droplets. After homogenization, the mean droplet diameter can be gradually increased in a relatively controlled manner by storing the emulsion at 40 °C for up to 48 h. Given the biodegradable and biorenewable nature of chitosan and HPC, the versatile nature of this dual emulsifier system suggests its suitability for a wide range of formulations.