Salt promotes protonation of amine groups at air/water interface

Interfacial water reorientation caused by protonation/deprotonation of primary fatty amine (octadecylamine, ODA) headgroup was investigated by interface-selective vibrational sum-frequency generation spectroscopy. Sum-frequency intensity from ODA monolayer on neutral pH water was very small implying only small portion of protonated amine group (-NH₃⁺) initially existed. By increasing the ionic strength, however, SF intensity of water OH band was enhanced markedly up to ~1 mM, and then decreased in both NaCl and NaI solutions. By measuring the phase of the sum-frequency spectra, it was found that water dipoles under the ODA headgroup point downward indicating the surfaces were always positively charged. This demonstrated that increasing ionic strength facilitates protonation of primary amine headgroups. Simple model based on Poisson-Boltzmann (PB) theory explained this protonation behavior of primary amines.