Distinctive electrokinetic behavior of nanoporous silica particles treated with cationic polyelectrolyte

In this study we show, for the first time, that the streaming potential of aqueous suspensions of nanoporous silica gel, after treatment with the cationic polyelectrolyte poly-diallyldimethylammonium chloride (poly-DADMAC), can depend very strongly on the concentration of background electrolyte. An increase in the electrical conductivity from 60 to 1000 μS/cm resulted in an approximately 1000-fold increase in the amount of poly-DADMAC that was required to reach an endpoint of zero streaming potential. Results were explained by two contributions to the overall electrokinetic behavior—one due to the outer surfaces and another due to the interior surfaces of nanopore spaces that were inaccessible to the polyelectrolytes. Experiments with cyclical changes in salt content revealed a high degree of reversibility; such observations help to rule out explanations based on salt-induced desorption or enhancement of pore penetration. Supplementary tests with non-porous glass fibers showed no evidence of the distinctive electrokinetic behavior observed in the case of nanoporous particles. Effects of polymer molecular mass and pH, evaluated under similar experimental conditions, agreed with well-established trends.