Graphene oxide hydration and solvation: an in situ neutron reflectivity study

Abstract

Graphene oxide membranes were recently suggested for applications in separation of ethanol from water using a vapor permeation method. Using isotope contrast, neutron reflectivity was applied to evaluate the amounts of solvents intercalated into a membrane from pure and binary vapors and to evaluate the selectivity of the membrane permeation. Particularly, the effect of D₂O, ethanol and D₂O–ethanol vapours on graphene oxide (GO) thin films (∼25 nm) was studied. The interlayer spacing of GO and the amount of intercalated solvents were evaluated simultaneously as a function of vapour exposure duration. The significant difference in neutron scattering length density between D₂O and ethanol allows distinguishing insertion of each component of the binary mixture into the GO structure. The amount of intercalated solvent at saturation corresponds to 1.4 molecules per formula unit for pure D₂O (∼1.4 monolayers) and 0.45 molecules per formula unit (one monolayer) for pure ethanol. This amount is in addition to H₂O absorbed at ambient humidity. Exposure of the GO film to ethanol–D₂O vapours results in intercalation of GO with both solvents even for high ethanol concentration. A mixed D₂O–ethanol layer inserted into the GO structure is water enriched compared to the composition of vapours due to slower ethanol diffusion into GO interlayers.