Colloid interaction energies for surfaces with steric effects and incompressible and/or compressible roughness

Authors: Bradford, Scott; SASIDHARAN, SALINI - University Of California; KIM, HYUNJUNG - Jeonbuk National University; GOMEZ-FLORES, ALLAN - Jeonbuk National University; LI, TIANTIAN - China Agricultural University; SHEN, CHONGYANG - China Agricultural University

Submitted to: Langmuir
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2021
Publication Date: 1/20/2021
Citation: Bradford, S.A., Sasidharan, S., Kim, H., Gomez-Flores, A., Li, T., Shen, C. 2021. Colloid interaction energies for surfaces with steric effects and incompressible and/or compressible roughness. Langmuir. 37(4):1501-1510. https://doi.org/10.1021/acs.langmuir.0c03029.
DOI: https://doi.org/10.1021/acs.langmuir.0c03029

Interpretive Summary: Nanoparticles (NPs) that are increasingly used in industrial, agricultural, and environmental applications are commonly coated by adsorbed organic compounds in order to improve their mobility. However, the underlying role of adsorbed organics on NP mobility is not fully understood. Theory was developed in this work to explain the role of rough organic coatings that are compressible on NP transport and fate. Results demonstrate that the adhesive interaction between NPs and surfaces is strongly dependent the NP roughness properties, the compressibility of this roughness, and repulsion between compressed organic coatings. This information can explain experiment observations of retention of coated NPs and increases in the strength of attraction with time. This information will be of interests to scientists and engineers concerned with using coated NPs in various applications.

Technical Abstract: Colloid aggregation and retention in the presence of macromolecular coatings (e.g., adsorbed polymers, surfactants, proteins, biological exudates, and humic materials) have previously been correlated with electric double layer interactions or repulsive steric interactions, but the underlying causes are not fully resolved. An interaction energy model that accounts for double layer, van der Waals, Born, and steric interactions as well as nanoscale roughness and charge heterogeneity on both surfaces was extended, and theoretical calculations were conducted to address this gap in knowledge. Macromolecular coatings may produce steric interactions in the model, but non-uniform or incomplete surface coverage may also create compressible nanoscale roughness with a charge that is different from the underlying surface. Model results reveal that compressible nanoscale roughness reduces the energy barrier height and the magnitude of the primary minimum at separation distances exterior to the adsorbed organic layer. The depth of the primary minimum initially alters (e.g., increases or decreases) at separation distances smaller than the adsorbed organic coating because of a decrease in the compressible roughness height and an increase in the roughness fraction. However, further decreases in the separation distance create strong steric repulsion that dominates the interaction energy profile and limits the colloid approach distance. Consequently, adsorbed organic coatings on colloids can create shallow primary minimum interactions adjacent to organic coatings that can explain enhanced stability and limited amounts of aggregation and retention that have commonly been observed. The approach outlined in this manuscript provides an improved tool that can be used to design adsorbed organic coatings for specific colloid applications or interpret experimental observations.