Kinetics of Triglyceride Solubilization by Micellar Solutions of Nonionic Surfactant and Triblock Copolymer. 2. Theoretical Model

Abstract

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.

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

“…However, the observed behavior can be rationalized in terms of the theory proposed by Kralchevsky et al [39], which suggests that there are two rate constants representing "adsorption" and "desorption" of oil-free micelles by the oil drop and a third constant describing the rate of incorporation of oil from the drop into adsorbed micelles that are subsequently emitted. If the adsorption and desorption rates of oil-free micelles are the same for all the alkanes but the rate of oil uptake is slower for the longer-chain oils, the latter will have larger interfacial resistances ℜ I as observed.…”

“…Electrical repulsion between micelles and drop greatly reduces the likelihood of collision between them or even close approach. A recent demonstration of this effect with triolein, where mechanisms involving molecular dissolution could be ruled out, was provided by Kralchevsky et al [11,38,39].…”

“…Kinetic studies aim at determining the limiting step(s) within such mechanisms that dictate mass transfer rates and transient behavior. Most kinetic studies have been interpreted in terms of the same basic mechanisms as discussed above for individual drops: (a) the diffusion of the transferred solutes in the bulk as individual molecules [28,48] or incorporated in micellar aggregates [26,29]; (b) the interfacial resistance to mass transfer, which may refer to transport across a surfactant monolayer or some other physical barrier when molecular dissolution prevails [30][31][32] or to the adsorption and emission of micelles that act as carriers for the transported compounds [1][2][3][4][5]7,8,11,38,39,49]; or (c) the molecular diffusion of the solute in the continuous phase and subsequent capturing by surfactant micelles [10,18]. Recent advances in the understanding of the role of solubilization kinetics on the emulsion destabilization mechanisms as described above are discussed below.…”

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.…”