Monovalent Ion - Graphene Oxide Interactions are Controlled by Carboxylic Acid Groups: Sum Frequency Generation Spectroscopy Studies

Graphene oxide has been extensively researched for use in separating rare earth elements. The presence of monovalent ions during separation processes is inevitable and can impact the adsorption of target ions. To better understand how monovalent ions behave near graphene oxide surfaces, we conducted a systematic investigation using vibrational sum frequency generation spectroscopy to study the effects of alkali metal ions on water organization near graphene oxide thin films at the air/water interface. We used an arachidic acid Langmuir monolayer as a benchmark for a pure carboxylic acid surface. The sum frequency signal showed a nonmonotonic trend with increasing salt concentration, reaching a maximum at 100 µM and 1 mM salt concentrations for graphene oxide and arachidic acid, respectively. Theoretical modeling of the concentration-dependent sum frequency signal from graphene oxide and arachidic acid surfaces revealed that the adsorption of monovalent ions is mainly controlled by the carboxylic acid groups on graphene oxide. An in-depth analysis of sum frequency spectra revealed at least three distinct water populations with different hydrogen bonding strengths. Interestingly, interfacial water structure seemed mostly insensitive to the character of the alkali cation, in contrast to similar studies conducted at the silica/water interface. However, an ion-specific effect was observed with lithium, whose strong hydration prevented direct interactions with the graphene oxide film. Overall, our study provides molecular-scale insights into water structures near interfacial graphene oxide in the presence of monovalent ions, which can aid in the development of graphene oxide-based separations.