Protonation of headgroups of fatty amines in Langmuir monolayer controlled by surface density

Charge status of the headgroups of the molecules consisting monolayer at different area per molecules is studied by sum-frequency vibrational spectroscopy. Langmuir monolayers consisting of mixtures of 1-hexadecanol (HD) and octadecyl amine (ODA) molecules at different molar ratios were prepared to investigate the effect of surface area/molecule on the charge status of the amine headgroup. Here the fatty alcohol molecules worked as mere spacers to widen the distance between the amine groups. Sum-frequency vibrational spectra in the OH range was larger for the monolayers having intermediate area/molecule, which indicated the surface is more charged even when there are less amine groups in the monolayer. To compare with the above systems, 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP), a surfactant molecule having quaternary amine headgroup was then mixed with 1-hexadecanol (HD) at different molar ratios and investigated by SFVS. Comparison of the OH spectral areas of HD/ODA and HD/DPTAP systems allowed quantitative estimation of the protonation fraction of amine headgroups in the ODA mixture monolayers. Gouy–Chapman theory that took into account the area change followed the observed trend on the protonation of the amine headgroup.