Dispersion and Stabilization in Water of Droplets of Hydrophobic Organic Liquids with the Addition of Hydrophobic Polymers

An attempt was made to improve the dispersion stability of surfactant-free oil-in-water emulsions (SFEs) with the addition of hydrophobic polymers on the basis of a new hydrophobic approach for dispersion. Benzene (Bz) and cyclohexane (Cy), both with added polystyrene (PS), were dispersed in water. The addition of PS was found to remarkably improve the dispersion stability of Bz SFE, and the turbid dispersion state and 200−300-nm droplet sizes remained unchanged for more than 1 year. Contrary to a preliminary droplet growth observed for PS/Bz, the initial droplet size in PS/Cy SFE was 100 nm, which could not be attained in Cy SFE and showed no change for a long time. The data on the viscosity of the oil phase and the ζ potential of the oil droplets suggested that the increase in the oil viscosity of the PS/Bz droplets and coverage of the surface of the Cy droplets with PS reduce the rates of droplet coalescence and flocculation, respectively. When polyisobutylene (Pib), a viscous liquid polymer, was used instead of PS to disperse alkanes such as Cy and n -hexane as fine droplets in water, the droplets formed could be observed for 1 month. However, both the droplet-stabilizing and ζ-potential deepening effects of Pib varied irregularly with the change in the oil viscosity. On the basis of the stabilization effect at various Pib weight ratios and average Pib molecular weight, this irregularity was suggested to arise from Pib adsorption on the droplet surface, which depresses the droplet flocculation rate by making the surface more hydrophobic and sterically repulsive.