Hydration peculiarities of graphene oxides with multiple oxidation degrees

Hydration properties of graphene oxide (GO) are essential for most of its potential applications. In this work, we employ atomistic molecular dynamics simulations to investigate seven GO compositions with different levels of oxygenation. Two atomic charge models for GO are compared: (1, a simplified model) sp² carbons are purely Lennard-Jones sites; (2, a CHELPG model) sp² carbon charges are consistent with the CHELPG scheme. Structural properties were found to depend insignificantly on the charge model, whereas thermodynamics appeared very sensitive. In particular, the simplified model provides systematically stronger GO/water coupling, as compared to the more accurate model. For all GO compositions, hydration free energies are in the range of -5 to -45 kJ mol^-1 indicating that hydration is thermodynamically favourable even for modest oxidation degrees, thus differing drastically from the cases of pristine graphene and graphite. In general, it has been observed that as R increases the high oxidation degree obstructs the formation of new hydrogen bonds, which considerably affects their hydration properties. Although both the used charge models are qualitatively equivalent, the energy and number of hydrogen bonds have been shown to be sensitive to the charge set employed. In particular, the comparison shows that the simplified model tends to overestimate the GO/water interaction energy. The results and discussion presented herein provide a physical background for modern applications of GO, e.g. in electrodes of supercapacitors and inhibitors in processes involving biological molecules.