RATES OF SOLUBILIZATION OF TRIOLEIN INTO NONIONIC SURFACTANT SOLUTIONS

Videomicroscopy was used to measure the rates of solubilization of tri olein drops injected into thin, rectangular glass cells containing sol utions of nonionic surfactants and their mixtures with alcohols at var ious temperatures. In most cases it was observed that after an inducti on 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 interfac e. Instead it suggests that processes occurring at the interface contr ol the rate of solubilization. The rates of solubilization of triolein by the secondary alcohol ethoxylate Tergitol 15-S-7 at 30 degrees C a nd 35 degrees C, the latter temperature being just below the cloud poi nt, were several times the corresponding rates for the pure linear alc ohol ethoxylate C12E6 with enough n-dodecanol added to give it nearly the same cloud point. The difference in rates was much greater than co uld be explained by the relatively small differences in equilibrium so lubilization of triolein and apparently stemmed from the smaller thick ness and increased disorder in the hydrocarbon chain region of Tergito l 15-S-7 films which produced greater film flexibility. In contrast, t he rates of solubilization of n-hexadecane by the two surfactant solut ions were about the same and several times greater than the triolein s olubilization rates. Addition of short-chain alcohols to the surfactan t solutions increased the rates of solubilization of both triolein and n-hexadecane for both linear and secondary alcohol ethoxylates. (C) 1 997 Elsevier Science B.V.