Probing Graphene-Surfactant Interactions in Aqueous Dispersions with Nuclear Overhauser Effect NMR Spectroscopy and Molecular Dynamics Simulations

V Arunachalam and S Vasudevan, JOURNAL OF PHYSICAL CHEMISTRY C, 121, 16637-16643 (2017).

DOI: 10.1021/acs.jpcc.7b05404

Sonication-assisted exfoliation of graphite in aqueous solutions of ionic surfactants is an attractive, scalable route for the production of defect-free graphene. The interaction of surfactant chains with graphene is crucial both to the process and to the stability of the dispersion. We use H-1 nuclear Overhauser effect (NOE) NMR techniques and classical molecular dynamics (MD) simulations to probe the molecular nature of these interactions in graphene dispersions stabilized by the cationic surfactant cetyltrimethylammonium bromide. We show that the surfactant chains are quasi-bound to the graphene sheets undergoing rapid exchange with the free surfactant ligands in the bulk, but what is surprising is the observation of NOE interactions between groups that are separated by more than 5 angstrom along the chain. MD simulations provide the key to interpreting these observations; these interactions are a consequence of the arrangement of the quasi-bound surfactant chains that allows segments of different chains to come into spatial proximity on the graphene sheet.

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