Kinetics of triglyceride solubilization by micellar solutions of nonionic surfactant and triblock copolymer. 3. Experiments with single drops

We investigated the kinetics of solubilization of triglycerides (triolein and soybean oil) by observing the diminishing of individual oil drops (radius less than or equal to 50 mum) in a micellar surfactant solution. We used two solubilization cells. Cell no. 1 is a centimeter-sized thermostated vessel. containing the investigated micellar solution and a few oil drops. Cell no. 2 represents a set of horizontal glass capillaries filled with the solution in each of them an oil drop was injected by a syringe. Cell no. I is more easy to operate, whereas cell no. 2 allows a quantitative interpretation of the results, We carried out experiments with solutions of two nonionic surfactants and investigated the effect of various additives on the solubilization rate, The addition of an anionic surfactant is found to suppress the solubilization due to an enhanced electrostatic repulsion between the micelles and the oil-water interface. This inhibitory action can be partially removed by addition of an amphoteric surfactant. Highest solubilization rates have been achieved by Addition of E-n-P-m-E-n triblock copolymers (Synperonics) to the nonionic surfactant solutions. The experimental data for diminishing drops agree very well with the theoretical time dependence of the drop radius, From the Pits we determined the solubilization rate, the compound kinetic constant of solubilization, and the number of oil molecules/swollen micelle, n(s). The kinetic values of n(s), determined with diminishing oil drops, are compared with the equilibrium values of n(s), independently obtained by NAIR. The data confirm that, in the considered case, the time-limiting step is the adsorption of empty micelles at the oil-water interface. In particular, each swollen micelle desorbs from the triolein-water interface after taking 5-20 triolein molecules, depending on the solution's composition. The reported results can be helpful for the analysis and control of the solubilization kinetics of triglycerides and other water-insoluble oils.