Rates of solubilization of triolein into nonionic surfactant solutions

Videomicroscopy was used to measure the rates of solubilization of triolein drops injected into thin, rectangular glass cells containing solutions of nonionic surfactants and their mixtures with alcohols at various temperatures. In most cases it was observed that after an induction period where drop radius was nearly constant, the rate (-dR/dt) of decrease in radius, which was proportional to the solubilization rate per unit area, was independent of time and of initial drop size. This behavior is not consistent with solubilization limited by diffusion in a stagnant surfactant solution with local equilibrium at the interface. Instead it suggests that processes occurring at the interface control the rate of solubilization. The rates of solubilization of triolein by the secondary alcohol ethoxylate Tergitol 15-S-7 at 30 °C and 35 °C, the latter temperature being just below the cloud point, were several times the corresponding rates for the pure linear alcohol ethoxylate C₁₂E₆ with enough n-dodecanol added to give it nearly the same cloud point. The difference in rates was much greater than could be explained by the relatively small differences in equilibrium solubilization of triolein and apparently stemmed from the smaller thickness and increased disorder in the hydrocarbon chain region of Tergitol 15-S-7 films which produced greater film flexibility. In contrast, the rates of solubilization of n-hexadecane by the two surfactant solutions were about the same and several times greater than the triolein solubilization rates. Addition of short-chain alcohols to the surfactant solutions increased the rates of solubilization of both triolein and n-hexadecane for both linear and secondary alcohol ethoxylates.