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

Abstract

We have used second harmonic generation to directly observe chemical changes in a lipid monolayer. We observed that pKa of p-hexadecyl aniline at the air/water interface is 3.6 ± 0.2 versus 5.3 ± 0.1 in the bulk. This fiftyfold increase in acidity is due to the higher free energy of a charged species at the air/water interface, which favors a shift in the equilibrium towards the neutral form.