Ion and water interactions near graphene oxide thin films are fundamentally interesting and relevant to a range of applications, including heavy metal separations, water desalination, and radionuclide decontamination. Graphene oxide is an excellent candidate for these membrane applications because it is durable and contains useful oxygen functional groups. There is little interfacial, molecular-scale, experimental information about these systems, as most studies utilize bulk spectroscopy techniques. Nanoscale, interfacial information about ion and water behavior is critical for membrane advances, including material regeneration, and improved selectivity and permeability.<br/>              To help remediate this knowledge gap, we used interfacial-specific x-ray and non-linear spectroscopy techniques to probe lutetium and water interactions near model, self-assembled graphene oxide thin films at the air-water interface. Previously, we demonstrated preferential trivalent ion adsorption onto graphene oxide and noted that adsorption was not governed only by electrostatics.<sup>1</sup> In this work, we varied the local surface charge of the graphene oxide film by changing the subphase pH and quantify both the electric field within the interfacial region and the relevant pKa values for our graphene oxide using vibrational sum frequency generation spectroscopy. This non-linear technique is limited to the interface and also provides information about the local water network. Additionally, our x-ray fluorescence near total reflection measurements reveal a dramatic change in Lu ion adsorption as probed over aqueous subphase pH. X-ray reflectivity data show a corresponding change in the thin film graphene oxide structure as well. We posit the lutetium changes speciation from a typical point charge in acidic conditions to a lower-charged hydroxide molecule in basic conditions while the graphene oxide film functional groups undergo protonation concurrently. The resulting change in the local surface charge strongly affects both ion adsorption and water organization. The adsorbed ion densities suggest the lutetium hydroxide molecules are not adsorbing via charge compensation. Taken together, these results highlight the importance of detailed, nanoscale, interfacial-specific experiments on model graphene oxide systems.

Impact of local surface charge on trivalent ion adsorption to graphene oxide films

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.