The role of surface ionisation in the hydration-induced swelling of graphene oxide membranes

Abstract

Graphene oxide (GO) membranes are hydrophilic materials that swell in the presence of water either in a humid environment or when in contact with liquid water and the magnitude and mechanism of the swelling is dependent on the degree of ionisation of the functional groups present on the surface of the GO flakes. In this work, using Grand Canonical Monte Carlo and Molecular Dynamics simulations we investigate the effect that the surface charge, resulting from the ionisation process, has on both the amount of water adsorbed in the membranes at various humidity conditions and the swelling of the membranes in contact with liquid water. Three models with increasing surface charge from −63 mC/m2 to −177 mC/m2 and a neutral one are used. We show that by incorporating ionised functional groups, the onset of adsorption is shifted to a lower chemical potential and the internal membrane pressure increases due to the repulsive interactions between the graphene flakes. We suggest that for a fairly ordered membrane as the ones modelled here a surface charge of −120 mC/m2 is the upper limit before membrane delaminate. Our simulations also show that the presence of ions in the channels reduced the swelling due to screening effect but also increases the amount of water adsorbed when the membrane is immersed in liquid water. At low chemical potential, instead, the amount of water adsorbed is determined by the number of ionised groups. The charged model is able to qualitatively reproduce experimental data showcasing the importance of including surface charge in GO model to predict hydration and swelling mechanism. These findings are crucial in underpinning the future development of GO membranes in simulation and experimental study for aqueous separations since hydration-induced swelling is widely known to lead to significant deterioration in performances.