Ion Valence and Concentration Effects on the Interaction between Polystyrene Sulfonate-Modified Carbon Nanotubes in Water

P Rama and AR Bhattacharyya and R Bandyopadhyaya and AS Panwar, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 9619-9631 (2018).

DOI: 10.1021/acs.jpcc.7b10467

We use molecular dynamics simulations and the adaptive biasing force method to evaluate the potential of mean force between two carbon nanotubes (CNTs), with each surface modified by an adsorbed sodium- polystyrene sulfonate (Na-PSS) polyanion, in aqueous electrolyte media. Changes in the electrolyte concentration and counter-ion valence can lead to qualitative changes in the interactions between polyelectrolyte- modified CNTs. We show that in the presence of monovalent NaCl salt, a long-range screened electrostatic repulsion exists between CNTs. This repulsion can be described by a generalized Derjaguin-Landau-Verwey- Overbeek interaction that accounts for anisotropy of charged cylindrical colloids. In contrast, an attraction between CNTs is observed in the presence of divalent MgCl2 salt. The attraction is attributed to ion- pair correlations between anionic SO3- groups, on different PSS chains, induced by Mg2+ counter ions acting as bridges between the SO3- groups. However, in the salt-free case where divalent Mg2+ counter ions are considered instead of the Na+ counter ions, condensation of Mg2+ counter ions on the adsorbed PSS chain results in the neutralization of surface charge and leads to a short-range steric repulsion between the CNTs. Thus, our simulations show that qualitatively different interactions, either short-range steric repulsion, long-range repulsion or attraction, can arise between PSS-modified CNTs based on counterion valence and electrolyte concentration.

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