Colloid interaction energies for physically and chemically heterogeneous porous media

Authors: Bradford, Scott; TORKZABAN, SAEED - University Of California

Submitted to: Langmuir
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2013
Publication Date: 2/25/2013
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2425.pdf
Citation: Bradford, S.A., Torkzaban, S. 2013. Colloid interaction energies for physically and chemically heterogeneous porous media. Langmuir. 29(11):3668-3676.

Interpretive Summary: Microbial interactions with porous media play a critical role in determining their environmental transport and fate. Small scale variations in roughness and/or charge on the microbe or soil are known to influence these interactions. In this work we use colloids as a proxy for microbes and predict colloid interactions in heterogeneous porous media. Simulation results demonstrate that roughness and variations in charge can enhance colloid interactions and retention, in comparison to uniform surfaces. However, these interactions will be strong functions of the solution chemistry and the colloid size. The findings from this study will be of interest to scientists and engineers concerned with predicting the fate of microbes, colloids, and nanoparticles in environmental and industrial applications.

Technical Abstract: The mean and variance of the colloid interaction energy (phi*) as a function of separation distance (h) were calculated on physically and/or chemically heterogeneous solid surfaces at the representative elementary area (REA) scale. Nanoscale roughness was demonstrated to have a significant influence on the colloid interaction energy for different ionic strengths. Increasing the roughness height reduced the magnitude of the energy barrier (phi max*) and the secondary minimum (phi 2min*). Conversely, increasing the fraction of the solid surface with roughness increased the magnitude of phi max* and phi 2min*. Our results suggest that primary minimum interactions tend to occur in cases where only a portion of the solid surface was covered with roughness (i.e., isolated roughness pillars), but their depths were shallow as a result of Born repulsion. The secondary minimum was strongest on smooth surfaces. The variance in the interaction energy was also a strong function of roughness parameters and h. In particular, the variance tended to increase with the colloid size, the magnitude of phi*, the height of the roughness, and especially the size (cross-sectional area) of the heterogeneity. Nonzero values of the variance for phi 2min* implied the presence of a tangential component of the adhesive force and a resisting torque that controls immobilization and release for colloids at this location. Heterogeneity reduced the magnitude of phi* in comparison to the corresponding homogeneous situation. Physical heterogeneity had a greater influence on mean properties of phi* than similar amounts of chemical heterogeneity, but the largest reduction occurred on surfaces with both physical and chemical heterogeneity. The variance in phi* tended to be higher for a chemically heterogeneous solid.