Effect of pH and salt on the protonation state of fatty aniline investigated by sum-frequency vibrational spectroscopy.

This article focuses on the investigation of Langmuir monolayers of fatty aniline (C16-aniline) using sum-frequency vibrational spectroscopy (SFG) to determine the surface acid dissociation constant (pKa) and protonation behavior under varying pH and sodium chloride (NaCl) concentrations. By applying a multi-peak Lorentzian model and the theory of SFG for charged interfaces combined with the generalized Poisson–Boltzmann equation in the Gouy–Chapman model, the surface pKa of aniline was quantitatively determined to be 4.4 ± 0.3. The study found that the protonation fraction of the aniline headgroup remains below 5% at pH 6 even with up to 1 M NaCl, and that the Gouy–Chapman model effectively explains the observed SFG spectra in the OH region across different conditions. Additionally, the analysis accounted for contributions from water molecules in the bonded interfacial layer (BIL), improving the fit to experimental data. These findings provide insights into the interfacial chemistry of fatty aniline relevant to surface charge and molecular ionization at interfaces.