Kinetics of Triglyceride Solubilization by Micellar Solutions of Nonionic Surfactant and Triblock Copolymer. 3. Experiments with Single Drops

Abstract

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.

Understanding viscosity reduction of a long-tail sulfobetaine viscoelastic surfactant by organic compounds

“…These organic compounds reduce the viscosity of the surfactant solutions by solubilizing in the core of cylindrical micelles. 5,6,8,[28][29][30] Solubilization disrupts the cylindrical micelle structure, changing longer cylindrical micelles into shorter cylindrical micelles, Kralchvesky and coworkers 31,32 determined the solubilization kinetics of non-polar oils in mixed cylindrical micelles of a nonionic surfactant and a triblock copolymer. The micelles adsorb on the oil-aqueous solution interface before solubilization takes place.…”

“…The micelles adsorb on the oil-aqueous solution interface before solubilization takes place. 31, 32 Shibaev and co-workers 30 showed there are three regimes of micelle change in viscoelastic solutions of potassium oleate at 20 C: (i) the micellar network regime (ii) the transition regime, and (iii) the microemulsion regime. The existence of micellar network regime was attributed to the reduction in length of the cylindrical micelles, disrupting the micelle network.…”

“…The solubilization mechanism of non-polar oils by cylindrical micelles proposed by Kralchevsky and co-workers 31 will be used to interpret the results, as this mechanism has been experimentally veried. 32 Moreover, the formation of cylindrical micelles as soon as the surfactant system was dissolved in the aqueous solution will be taken into account. The effect of temperature explained as follows: temperature increased the average number of transient adsorptions of cylindrical micelles on the oil-aqueous solution interface.…”

Solubilization rates of oils in surfactant solutions and their relationship to mass transport in emulsions

“…This drop-on-fiber technique has been implemented in subsequent solubilization studies [2][3][4][5]. Miller et al [6][7][8][9] and Kralchevsky et al [10,11] used videomicroscopy and microphotography to record the transient behavior of micrometer droplets injected into capillaries filled with aqueous surfactant solutions without using microfibers to fix the position of the drops. Wen and Papadopoulos [12] used a similar technique to measure water transport rates across an oil film.…”

Spontaneous Emulsification of Triolein Induced by Mixed Micellar Solutions of Sodium Polyoxyethylene Alkyl Ether Sulfate and Dodecyldimethyl Amine Oxide

“…proposed that the addition of DDAO to sodium dodecyl dioxyethylene sulfate enhanced the solubilization rate of triolein because of a suppressed electrostatic repulsion between the micelles and the oil/water interface 33 . Based on these reports, we hypothesized that the dilatational elasticity decreases with increasing number of micelles near the drop surface owing to a decrease in the electrostatic repulsion between the micelles and the drop surface.…”