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

Abstract

Langmuir monolayers of fatty aniline (C16-aniline) were investigated using sum-frequency vibrational spectroscopy at various pH levels and NaCl concentrations. To analyze the sum-frequency generation (SFG) spectra of aniline, a multi-peak Lorentzian model, in accordance with the theory of SFG for a charged interface, was applied. First, SFG spectra of C16-aniline from pH 2 to 6 at a constant ionic strength of 10 mM (where the phase of the complex potential of the dc-induced signal was suppressed to a few degrees) were fitted with the above-mentioned method. The mean-field theory that considers the chemical equilibrium of the aniline headgroup was used to analyze the fitting results to find that the pKa of aniline is 4.4 ± 0.3. The protonation fraction of the aniline headgroup was estimated to be less than 5% at pH 6 and NaCl concentrations were up to 1M. The generalized Poisson–Boltzmann equation in the Gouy–Chapmann model effectively explained the observed SFG spectra in the OH region for fatty aniline at pH as low as 2, even for the systems without addition of any salt.