Mechanism and Thermodynamic Decomposition of Guanidinium (Gdm+) Adsorption at the Air–Water Interface

Determine the detailed molecular mechanism by which guanidinium (Gdm+) cations adsorb to the air–water interface, and quantify the enthalpic (ΔH_ads) and entropic (ΔS_ads) contributions to this adsorption, in order to ascertain whether the adsorption mechanism parallels the previously elucidated mechanism for thiocyanate (SCN−) at aqueous interfaces.

Background

This study uses deep-UV second harmonic generation (DUV-SHG) spectroscopy and molecular dynamics simulations to demonstrate and quantify the adsorption of guanidinium cations at the air–water interface, extracting a Gibbs free energy of adsorption comparable to that of the prototypical surface-active anion SCN−. While the adsorption propensity is established, the authors highlight that the mechanistic basis of Gdm+ adsorption remains unresolved.

Prior work on SCN− established an enthalpy-driven adsorption mechanism associated with solvent repartitioning, accompanied by an entropic penalty, but the authors state that it is uncertain whether Gdm+ follows the same pathway. They explicitly note the need for further theory and experiments to determine ΔH_ads and ΔS_ads for Gdm+ and to complete the description of its interfacial behavior.

References

The exact underlying mechanistic details of Gdm$+$ interfacial adsorption remain to be determined, but future studies will undoubtably provide new insight that may even extend to intriguing Gdm$+$-protein interactions. It is not unreasonable to suggest that Gdm$+$ may follow a similar mechanism, but we cannot be certain without further theory and experiments to uncover additional features of Gdm$+$ ion adsorption ($\Delta H_{ads} , \Delta S_{ads}$). Considering that experiments have shown SCN$-$ adsorbing to both oil-water and graphene-water interfaces with similar $\Delta G_{ads}$ to have underlying mechanistic differences , and new studies highlighting the likelihood of like-charge contact ion-paring of Gdm$+$ ions in solution , we clearly do not yet have a complete description of Gdm$+$ behavior at the air-water interface.

— Adsorption of Guanidinium Cations to the Air-Water Interface  (2408.15423 - Bernal et al., 2024) in Conclusion