Observation of adsorption-induced expansion in the interlayer spacing of graphene oxide frameworks during supercritical adsorption

During physisorption it is generally assumed that the solid remains inert. However, as the adsorbate molecules go into the pores it is possible that the adsorbant will experience structural changes. This is intriguing, as well as possibly important for improvements in the sorption characteristics of the material. In the past, changes have been observed in subcritical conditions, e.g. the Metal Organic Frameworks "gate opening transitions". Here we focus on structural changes under supercritical conditions in Graphene Oxide Frameworks (GOF). Using in situ neutron scattering, we have observed an increase of the interlayer spacing of GOFs during adsorption of three supercritical gases (hydrogen, methane, xenon) in the 0-150 bar range. (The GOFs were synthesized by the insertion of diboronic acid (DBA) molecules between layers of graphene oxide.) We observe an approximate law of corresponding states where the layer expansion can be mapped into a quasi universal curve (vs. molar excess or absolute adsorption) when adjusted by the critical temperature of the adsorbed gas. Comparison of the experimental findings with molecular dynamics simulations suggest possible structures for the pores.