The temperature stability of single and mixed emulsions stabilized by nonionic surfactants

Reasonably monodisperse (vegetable) oil-in-water emulsions have been prepared with three different non-ionic surfactants, each of having the same hydrophobic tail group, but with poly(ethylene oxide) (PEO) head groups containing 18, 27 or 48 monomer units. A set of three emulsions, containing similar sized oil droplets (∼400 nm diameter), has been prepared using these three surfactants. In addition, an emulsion with 200 nm diameter droplets has been prepared using the smallest head group surfactant. The critical flocculation temperatures (CFT) of the dilute emulsions have been determined using standard turbidimetric techniques, whilst those of the more concentrated emulsions have been determined using a Turbiscan MA2000 instrument. A systematic study of the effect of surfactant head-group size and concentration (beyond the CMC), droplet size and volume fraction, and added electrolyte (MgSO4), 1,2-propandiol and urea concentrations on the CFT of the single emulsions has been carried out. In addition, the CFT values of binary mixtures of the emulsions have been determined, and the possible effects of oil exchange (Ostwald ripening) and surfactant exchange, between droplets, have been determined.