Thermodynamics of Adsorption on Graphenic Surfaces from Aqueous Solution

Adsorption of organic molecules from aqueous solution to the surface of carbon nanotubes or graphene is an important process in many applications of these materials. Here we use molecular dynamics simulation, supplemented by analytical chemistry, to explore in detail the adsorption thermodynamics of a diverse set of aromatic compounds on graphenic materials, elucidating the effects of the solvent, surface coverage, surface curvature, defects, and functionalization by hydroxy groups. We decompose the adsorption free energies into entropic and enthalpic components and find that different classes of compounds-such as phenols, benzoates, and alkylbenzenes-can easily be distinguished by the relative contributions of entropy and enthalpy to their adsorption free energies. Overall, entropy dominates for the more hydrophobic compounds, while enthalpy plays the greatest role for more hydrophilic compounds. Experiments and independent simulations using two different force field frameworks (CHARMM and Amber) support the robustness of these conclusions. We determine that concave curvature is generally associated with greater adsorption affinity, more favorable enthalpy, and greater contact area, while convex curvature reduces both adsorption enthalpy and contact area. Defects on the graphene surfaces can create concave curvature, resulting in localized binding sites. As the graphene surface becomes covered with aromatic solutes, the affinity for adsorbing an additional solute increases until a complete monolayer is formed, driven by more favorable enthalpy and partially canceled by less favorable entropy. Similarly, hydroxylation of the surface leads to preferential adsorption of the aromatic solutes to remaining regions of bare graphene, resulting in less favorable adsorption entropy, but compensated by an increase in favorable enthalpic interactions.