Ethylenediamine at air/liquid and air/silica interfaces: protonation versus hydrogen bonding investigated by sum frequency generation spectroscopy

The adsorption process on mineral oxide surfaces is one of the most important factors influencing the migration and distribution of contaminants in the environment. Although there have been numerous studies carried out at the macroscopic scale, there is a lack of molecular-scale interfacial information. The molecular-scale information is often crucial for the determination of sorption mechanisms. In this study, sum frequency generation spectroscopy (SFG), a surface specific vibrational spectroscopy, has been employed to investigate the adsorption of ethylenediamine (H2N(CH2)2NH2, EDA) onto the amorphous SiO2 surface and EDA protonated products at air/liquid interfaces. The SFG spectra of EDA adsorbed on the silica surface and the singly protonated EDA solution are similar in both CH and NH stretching regions. These spectral similarities indicate that EDA molecules are strongly chemisorbed to the silica surface through the protonation of one EDA amine group by surface silanol OH groups, thereby forming a (H2N(CH2)2NH3)+(O-Sitriple bond])- surface complex. The SFG results also indicate that the surface acidity of the silanol OH groups (pKa (HOSitriple bond])) is between the two pKa values of EDA (in the range of 7.56-10.71) at the air/silica interface.