The carboxylate anion group plays an important role in
many (bio)chemical
systems and polymeric materials. In this work, we study the orientation
of carboxylate anions with various aliphatic and aromatic substituents
at the water–air interface by probing the carboxylate stretch
vibrations with heterodyne-detected vibrational sum-frequency generation
spectroscopy in different polarization configurations. We find that
carboxylate groups with small aliphatic substituents show a large
tilt angle with respect to the surface normal and that this angle
decreases with increasing size of the substituent. We further use
the information about the orientation of the carboxylate group to
determine the hyperpolarizability components of this group.