New method for determination of surface pKa using second harmonic generation

Abstract

A new method to determine the pKa of chemical species at aqueous insulating interfaces is based on the discovery that the interface potential is linearly proportional to the measured second harmonic electric field. The proportionality constant is the effective third-order susceptibility and is the same for all aqueous interfaces studied, which has its origin in the polarization of bulk water induced by the charged form of the acid or base at the interface. We have determined that CH3(CH2)21NH+3 at the air/water interface has a pKa of 9.9±0.2 versus 10.6 in bulk water.