The state of the amphiphilic triblock copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) L64, (EO)13(PO)30(EO)13 (EO ) ethylene oxide, PO ) propylene oxide) in aqueous solution at concentrations below the critical micelle concentration (cmc) or temperatures below the critical micelle temperature (cmt) has been investigated by fluorescence probing using the fluorescent probe pyrene (fluorescence intensity ratio I1/I3, and fluorescence lifetime τ), ultrasonic absorption and conductivity. The effect of addition of sodium dodecyl sulfate (SDS) on the these properties was also investigated. The results suggest that at concentration below the cmc and at temperatures below the cmt, L64 aggregates under the form of small short-lived oligomers that are capable of binding pyrene and SDS. These oligomers probably involve the more hydrophobic components contained in the L64 sample investigated, which is polydisperse in composition like all amphiphilic block copolymers.

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.

The coadsorption from aqueous solutions of an anionic-neutral hydrophilic-hydrophilic diblock copolymer onto a mica-suported surfactant bilayer of a cationic oligomeric surfactant has been investigated. By using an atomic force microscope and a surface forces apparatus nanotribometer, we studied the resulting film morphology, the interactions between two coated surfaces, and the frictional properties of the boundary film. When the coated surfaces were compressed while being fully immersed in an aqueous surfactant solution, the hemifusion of the adsorbed surfactant bilayers could be easily induced. Noticeable friction forces could then be measured between the monolayer-coated surfaces. Coadsorbing poly(acrylic acid)-poly(acrylamide) diblock copolymer with the cationic surfactant changes the cohesion of the adsorbed layers. When the copolymer concentration is sufficiently high, the hemifusion instability of the adsorbed layers can be inhibited, considerably improving its lubricant properties.