A VISCOSITY STUDY ON OIL-IN-WATER MICROEMULSIONS

The viscosity of oil-in-water microemulsions formed from isopropyl myristate, polysorbate 80, sorbitol and water has been studied as a function of oil-volume fraction,θ{symbol}, surfactant and cosurfactant concentration, and surfactant: cosurfactant mass ratio. Phase studies have indicated an optimum polysorbate: sorbitol mass ratio for microemulsion formation of 1:2.5. The variation in relative viscosity, ηrel, with θ{symbol} behaved in conformity with the equation ηrel = exp[aθ{symbol}/(1 - Kθ{symbol})], where a is a constant and K is the hydrodynamic interaction coefficient. It was found that a decrease in polysorbate: sorbitol mass ratio of the microemulsion produced a systematic increase in a and a decrease in K. In addition, a values increased linearly from 3.03 to 4.54 while K values decreased from 1.72 to 1.39 with increase in the polysorbate and sorbitol concentrations in the microemulsion from 30 to 45%. The increase in the constant a was attributed to the greater hydrodynamic volume of the droplets and the bound solvent layer was calculated to increase from 7 to 22% of the droplet core radius. © 1990.