Steric and electrosteric stabilization of colloids in aqueous salt solutions

Abstract

Steric stabilization of colloidal particles occurs through the presence of a layer of adsorbed polymer, and can be important in a number of activities such as paint formulation, treatment of waste water, and drug delivery. Conventional wisdom dictates that the polymer must be attached well, have good solvency, and be of high molecular weight in order for good stability to be achieved. Although much progress has been made, there are still many unresolved issues concerning stability, especially in the case of aqueous salt solutions. The purpose of this study is to explore two such issues.The first issue concerns the details of the attached polymer's conformation, which is often ignored. In this study, it is shown that only a few long attached "tails" of polymer in the presence of a low molecular weight adlayer can impart stability, despite the dictates of conventional wisdom.The second issue centers around the fact that steric stability should decrease with decreasing polymer solvency. In the case of a thin polymer adlayer overlying charged colloidal particles, the stability should decrease upon the addition of salt due to this decrease in solvency, as well as a decrease in electrostatic repulsion. Surprisingly, it has been observed that certain latices stabilized with low molecular weight, triblock copolymers exhibit an increase in stability upon an increase in salt concentration. This unexpected increase has been defined as restabilization. The purpose of the second part of the study is to systematically characterize when restabilization occurs and its possible causes. It was observed that the phenomena is a strong function of the type of salt, the molecular weight of the polymer and its concentration in solution. It appears that large increases in the adsorbed amount of and ion binding by the polymer are not responsible for the phenomena. One plausible cause is the presence of micelles, which are formed from the unadsorbed triblock copolymer.