Contributions of nanoscale roughness to anomalous colloid retention and stability behavior

Authors: Bradford, Scott; KIM, HYUNJUNG - Chonbuk National University; SHEN, CHONGYANG - China Agricultural University; SASIDHARAN, SALINI - University Of California; SHANG, JIANYING - China Agricultural University

Submitted to: Langmuir
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2017
Publication Date: 8/28/2017
Citation: Bradford, S.A., Kim, H., Shen, C., Sasidharan, S., Shang, J. 2017. Contributions of nanoscale roughness to anomalous colloid retention and stability behavior. Langmuir. 33:10094-10105. http://pubs.acs.org/doi/abs/10.1021/acs.langmuir.7b02445.

Interpretive Summary: An understanding of factors that influences interactions of microorganisms and other colloids with surfaces is needed for many industrial and environmental applications. An approach was developed to predict the interaction of colloids on natural or engineered surfaces with different amounts of roughness and surface charge variability. Results reveal that roughness controlled the interaction between the colloid and the surface. Furthermore, the influence of roughness on colloid interactions was found to vary with the colloid size and the solution chemistry. These observations can explain anomalous colloid retention and aggregation behavior that has frequently been reported in the literature. This information will be of interest to scientists and engineers concerned with colloid interactions in the environmental and industry.

Technical Abstract: All natural surfaces exhibit nanoscale roughness (NR) and chemical heterogeneity (CH) to some extent. Expressions were developed 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 NR and CH. The influence of heterogeneity type, roughness parameters, solution ionic strength (IS), mean zeta potential, and colloid size on predicted interaction energy profiles was then investigated. The role of CH was enhanced on smooth surfaces with larger amounts of CH, especially for smaller colloids and higher IS. However, predicted interaction energy profiles were mainly dominated by NR, which tended to lower the energy barrier height and the magnitudes 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 net electrostatically unfavorable surfaces, especially when roughness occurred on both surfaces and for conditions that produced small energy barriers (e.g., higher IS, lower pH, lower magnitudes in the zeta potential, and for smaller colloid sizes) on smooth surfaces. The combined influence of roughness and Born repulsion frequently produced a shallow primary minimum that was susceptible to diffusive removal by random variations in kinetic energy, even under electrostatically favorable conditions. Calculations using measured zeta potentials and hypothetical 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 IS; reversible colloid retention under favorable conditions; and diminished colloid retention and enhanced colloid stability due to adsorbed surfactants, polymers, and/or humic materials).