We investigated the kinetics of solubilization of triglycerides (triolein and soybean oil) by observing the diminishing of individual oil drops (radius e 50 µm) 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. 1 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 En-Pm-En 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 fits 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 ns, determined with diminishing oil drops, are compared with the equilibrium values of ns, independently obtained by NMR. 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.
A theoretical model of oil solubilization in micellar surfactant solutions is developed. We consider oils that are practically insoluble in pure water, like triolein and other triglycerides. The nonionic micelles, which are capable to solubilize such oils, are usually rodlike aggregates, composed of surfactant molecules and poly(oxyethylene)-poly(oxypropylene) triblock copolymers, like Synperonic L61 (SL61). The swollen micelles, formed after solubilization, are smaller than the empty ones. The model describes the elementary act of solubilization as a sequence of three steps: (a) adsorption of an empty micelle at the oil-water interface; (b) uptake of oil by the adsorbed empty micelle which then splits into several swollen micelles; (c) desorption of the swollen micelles. Theoretical expressions are derived, which describe the diminishing of an oil drop in the course of solubilization. The parameter values, determined from the best fit of experimental data, imply that the rate-controlling step is the step of micellar adsorption. From the determined rate constant of adsorption we estimate the respective kinetic barrier to micelle adsorption, taking into account the action of surface forces and the hydrodynamic resistance. Our analysis indicates that the triblock copolymer SL61 promotes the solubilization of triglycerides by decreasing the length of the mixed surfactant-copolymer rodlike micelles, which leads to a lowering of the kinetic barrier to their adsorption at the oil-water interface.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.