Determination of pK, at the air/water interface by second harmonic generation

The asymmetry in forces at an interface due to the different bulk media on either side of it results in unique chemical and physical properties. The chemical composition, the orientational structure, the dielectric and transport properties are among the important properties of the interface that differ from that of the bulk media. One would expect properties such as chemical equilibria, pH, molecular motions, the rates of chemical reactions and physical relaxation processes to differ from that of the bulk media. Despite its importance and widespread interest it remains difficult to get a handle on the chemistry and physics of interfaces using traditional spectroscopic techniques. This is due to the overwhelmingly larger number of solute and solvent molecules in the bulk that dominate the signal originating from the much smaller number of molecules in the interface. An important way to circumvent this difficulty is the application of the nonlinear optical processes of second harmonic (SHG) and sum frequency generation (SFG) to study interfaces [ I]. The key feature of SHG is its intrinsic sensitivity to the non-centrosymmetric interface between two centrosymmetric media. Research in a variety of areas has shown the wide applicability and power of this method [ 2-61, In our laboratory we have focused our attention on equi-