Adsorption of Guanidinium Cations to the Air-Water Interface

Combining Deep-UV second harmonic generation spectroscopy with molecular simulations, we confirm and quantify the specific adsorption of guanidinium cations to the air-water interface. Using a Langmuir analysis and measurements at multiple concentrations, we extract the Gibbs free energy of adsorption, finding it larger than typical thermal energies. Molecular simulations clarify the role of polarizability in tuning the thermodynamics of adsorption, and establish the preferential parallel alignment of guanidinium at the air-water interface. Guanidinium is the first polyatomic cation proven to exhibit a propensity for the air-water interface. As such, these results expand on the growing body of work on specific ion adsorption.