Properties of stabilizers in colloidal dispersions
Surfactants and polymers are frequently used as stabilizers in colloidal dispersions. Depending on their solubility and the interactions in their local environment, their tendency to remain adsorbed on a surface will differ. These conditions will change upon removal of water from the dispersion, which is reflected in the rate of exchange between free and adsorbed species as well as in the bulk concentration. The aim of this study has been to study the properties of stabilizers in colloidal dispersions with respect to adsorption and kinetics, mainly by means of nuclear magnetic resonance relaxometry and diffusometry.
The conformation of adsorbed non-ionic stabilizers was analyzed in terms of NMR relaxation times, where it was found that long oxyethylene chains adsorb in a tail, train, loop conformation, while shorter chains have more rigid conformation. From comparative adsorption studies of nonylphenol ethoxylates with an average of 84 (NP100) and 10 (NP10) oxyethylene units together with polyethylene glycols, the results showed the importance of the hydrophobic group on the conformation. This was also shown from kinetic studies on the rate of adsorption and desorption, where results from NP100 and NP10 showed a 1000 fold difference in residence times, which was a result mainly by the hydrophobic group, but also by the length of the oxyethylene chain. Kinetic studies were also made on concentrated dispersions where it was found that a barrier for desorption existed for NP100, while the adsorption of NP10 was controlled by diffusion, in accordance with the suggested conformation on different surfaces. Furthermore, the mechanisms and the interactions of multiple component adsorption of NP100, sodium dodecylsulfate and polyvinylpyrrolidone on surfaces were investigated. It was found that the associative and segregative behavior of the components in solution also was reflected in the adsorption behavior. Overall, it seemed as if the polymer had less preference to the surface in favor of the mixed combination of the other two components.