Micellization Of Surfactants And Their Interactions With Polymers

The influence of Na2SO4 on the micellization of sodium bis(2-ethylhexyl) sulfosuccinate (AOT), the interactions and the aggregations between AOT and poly (diallydimethylammonium chloride) (PDDAC), and the influence of Na2SO4 on the interactions between AOT and PDDAC were studied by the isothermal titration calorimetry (ITC), dynamic light scattering (DLS), and negative staining transmission electron microscopy (TEM). It was found that hydrophobic interaction is the main force in PDDAC/AOT aquous solutions, while addition of salt enhances the binding of AOT onto PDDAC through the ion-exchange and favors the formations of PDDAC/AOT micelles, free AOT micelles, and free AOT vesicles, but prevents the transition of PDDAC/AOT micelles to the vesicles. It was shown that the micellization of free AOT is driven by entropy gain, while the adsorption of AOT onto PDDAC and the micellization of PDDAC-bound AOT are driven by both enthalpy and entropy.Two kinds of mixed micelles, polyoxyethylene tert-octyl phenyl ether/ dodecyltrimethylammonium bromide (TX100/DTAB) and AOT/DTAB, were studied by ITC. It was shown that the mixtures exhibited synergism favoring the micelle formation. The thermodynamic properties of the systems were found to be dependent on the composition and the total concentration of the surfactants; the micelles might undergo the changes of the composition, from the dominance of one component to the other, and possibly two types of micelles were coexisting in some region. For the system of TX100/DTAB, there was only TX100-rich type of the micelles in the TX100-rich region; while TX100-rich micelles and DTAB-rich micelles were coexisting in the DTAB-rich region. For the system of AOT/DTAB, there was only AOT-rich type of the micelles in the AOT-rich region; while AOT-rich micelles and DTAB-rich micelles were coexisting in the DTAB-rich region. The precipitate was formed when the ratio of anionic ion to cationic ion reached a critical value, however, the precipitation was redissolved when the ratio became lower or higher than the critical value. The titration enthalpy curves were analyzed by an extended Rubingh equation, and the parameter g of the equation and the compositions along the titration enthalpy curves were obtained, which were used to calculate the thermodynamic function of the formation of the mixed micelles and the thermodynamic function of mixing of the two kinds of the micelles.The interactions between the mixed micelles (TX100/DTAB or AOT/DTAB) and sodium polypacrylate (PANa) were studied by ITC and DLS. It was found that as the concentration and the composition of the mixed micelles changed, the surfactants underwent binding to the polymer chains, formation of the polymer/surfactant mixed micelles, the free mixed micelles and the precipitate of the polymer/surfactant by cross links of the polymer chains. For the system of PANa/TX100/DTAB, the cationic headgroups of DTAB molecules bound onto the negatively charged carboxylate sites along the PANa chains in the low DTAB concentration region and the absence of TX100, and the titration peak indicating the crosslink of the polymer chains was exothermic; while DTAB favored to form the TX100/DTAB mixed micelles in the low DTAB concentration region and the presence of TX100, and the crosslink of the polymer chains yielded an endothermic effect. For the system of PANa/AOT/DTAB, the hydrophobic tails of AOT adsorbed onto the polymer backbone at a very low concentration of AOT and produced the enormous heat. When DTAB was titrated into the PANa/AOT brine solution, DTAB favored to form AOT/DTAB mixed micelles firstly, and the formation of precipitate was exothermic; when AOT/DTAB was titrated into the PANa brine solution, the AOT/DTAB mixed micelles bound onto the polymer chains and presented an endothermic effect.