Hamaker constant is a key parameter in the study of particle interactions of materials. Even though there are quite a few methods for measuring the Hamaker constant, the current experimental results showed that the obtained Hamaker constants from different methods for the same material usually had considerable variations. In this paper, a new method for determination of the Hamaker constant was suggested: the Hamaker constant can be easily measured through determination of the swelling pressure of the material in aqueous solution. In this paper, the Hamaker constants of 36 different montmorillonites with different surface potentials were obtained by the method. The results showed that the Hamaker constant of montmorillonite was surface potential dependent. At values of the surface potential larger than -267 mV, the values of the Hamaker constant increased with an increase of the surface potential; nevertheless, at surface potentials lower than -267 mV, the Hamaker constant appeared to decrease with an increase of the surface potential.
The interactions between the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and the polycation poly(diallydimethylammonium chloride) (PDDAC), the aggregations of AOT and PDDAC-bound AOT in PDDAC/AOT aqueous solutions, and the influence of salt on the interactions and aggregations have been studied by isothermal titration calorimetry (ITC), dynamic light scattering (DLS), and negative staining transmission electron microscopy (TEM). The adsorptions of AOT onto PDDAC and the formations of PDDAC-bound AOT micelles, free AOT micelles, and AOT vesicles were examined, and the corresponding critical concentrations were determined. Combining calculations of thermodynamic parameters with the above three experimental techniques, 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. It was also found that 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. Thermodynamic analysis suggested that the adsorption of AOT onto PDDAC and the micellization of PDDAC/AOT in PDDAC/AOT brine solutions are different in mechanism compared with that in corresponding aqueous solutions.
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