The double-layer interaction between dissimilar ionizable surfaces and its effect on the rate of deposition

Abstract

Rates of deposition were calculated for cases in which sol and collector surfaces are dissimilar and acquire their charge by the dissociation of multiple acidic and/or basic sites. The double-layer interaction between such dissimilar surfaces can induce a sign reversal in the surface charge or the force. These sign reversals are not observed between identical surfaces. A qualitative analysis of the governing equations allows prediction of the double-layer interaction at very small separations without solving the complete equations. Two strongly acidic or basic surfaces practically always repel if the separation is small enough, regardless of the relative signs of their surface charges, whereas two amphoteric surfaces having high buffer capacity practically always attract in the limit of small separation. The lowest rates of deposition are obtained when both particle and collector are strongly acidic or basic and of like charge, whereas the highest rates are obtained when both surfaces are amphoteric. These bounds on the rate are orders of magnitude apart. Of interest was the observation that a strongly acidic particle could be effectively prevented from depositing on a strongly basic collector by double-layer repulsion, even though the charges on the two surfaces have opposite signs. A necessary condition is that the ratio of the two surface potentials at infinite separation must be significantly different from −1.