Contributions of Nanoscale Roughness to Anomalous Colloid Retention and Stability Behavior.

Expressions were presented to determine the mean interaction energy between a colloid and a solid-water interface (SWI), as well as for colloid-colloid interactions, when both surfaces contain binary nanoscale roughness and chemical heterogeneity. The influence of heterogeneity type, roughness parameters, solution ionic strength, mean zeta potential, and colloid size on predicted interaction energy profiles was then investigated. Nanoscale roughness was found to dominate the predicted interaction energy profiles, and tended to lower the energy barrier height and the depths of both the secondary and primary minima, especially when the roughness fraction was small. This dramatically increased the relative importance of primary to secondary minima interactions on electrostatically unfavorable surfaces, especially when roughness occurred on both surfaces and for conditions that produced small energy barriers (e.g., higher ionic strengths, lower pH, lower magnitudes in the zeta potential, and for smaller colloid sizes) on smooth surfaces. In many cases, the combined influence of roughness and Born repulsion produced a shallow primary minimum that was susceptible to diffusive removal by random variations in kinetic energy. Calculations using measured zeta potentials and roughness properties demonstrated that roughness provided a viable alternative explanation for many experimental deviations that have previously been attributed to electrosteric repulsion (e.g., a decrease in colloid retention with an increase in solution ionic strength; reversible colloid retention under favorable conditions; and diminished colloid retention and enhanced colloid stability due to adsorbed surfactants, polymers, and/or humic materials).