Molecular orientation of small carboxylates at the water-air interface.

We study the properties of formate (HCOO - ) and acetate (CH 3 COO - ) ions at the surface of water using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. For both ions we observe a response of the symmetric ( ν s ) and antisymmetric ( ν as ) vibrations of the carboxylate group. The spectra further show that for both formate and acetate the carboxylate group is oriented toward the bulk, with a higher degree of orientation for acetate than for formate. We found that increasing the formate and acetate bulk concentrations up to 4.5 m does not change the orientation of the formate and acetate ions at the surface and does not lead to saturation of the surface density of ions.