New Insight into the Aggregation of Graphene Oxide Using Molecular Dynamics Simulations and Extended Derjaguin-Landau-Verwey-Overbeek Theory.

A comparative experimental and molecular dynamics (MD) simulation study was carried out to investigate the aggregation of graphene oxide (GO). Mechanisms behind the effects of solution chemistries (pH, metal ions, and tannic acid (TA)) and GO topology (carboxyl content, GO size, and GO thickness) were uncovered. For example, MD results showed that more hydrogen bonds formed between GO and water at higher pH, according well with the increased hydrophilicity of GO calculated based on contact angle measurements. Radial distribution functions analysis suggested Ca2+ interacted more strongly with GO than Na+, which explained the experimental observations that Ca2+ was more effective in accelerating the aggregation process than Na+. The adsorption-bridging and steric effects of TA were simulated, and TA was found to be unfolded upon wrapping on GOs, leading to an increased capacity for ion and solvent binding. The evaluations of contributions to GO hydrophilicity, electrostatic energy, and intensities of interactions with metal ions indicated the carboxyl group is the essential functional group in mediating the stability of GO. Overall, by combining MD simulations with experimental measurements, we provided molecular-level understandings toward the aggregation of GO, indicating MD, if used properly, can be applied as a useful tool to obtain insights into the aggregation of nanomaterials.