We use surface-specific intensity vibrational sum-frequency generation (VSFG) and attenuated total reflection spectroscopy (ATR) to probe the ionization state of the amino-acids L-alanine and L-proline at the air/water surface and in the bulk. The ionization state is determined by probing the vibrational signatures of the carboxylic acid group, representing the non-dissociated acid form, and the carboxylate anion group, representing the dissociated form, over a wide range of pH values. We find that the carboxylic acid group deprotonates at a significantly higher pH at the surface than in the bulk.

J. Am. Chem. Soc.
Ultrafast Spectroscopy

Strazdaite, S, Meister, K, & Bakker, H.J. (2017). Reduced Acid Dissociation of Amino-Acids at the Surface of Water. J. Am. Chem. Soc., 139(10), 3716–3720. doi:10.1021/jacs.6b12079