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 compoundssuch as phenols, benzoates,
and alkylbenzenescan 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.