Acid-base equilibrium constants and distribution coefficients of aminopyrene between the surface and bulk of liquid water as studied by a laser two-photon ionization technique

The pH-dependent amount of aminopyrene adsorbed at the water surface was measured using a laser two-photon ionization technique. The distribution coefficients of neutral and cationic aminopyrene between the surface and bulk of liquid water were determined to be 9.6 × 10 ^-6 m and 2.5 × 10 ^-7 m, respectively, The acid-base equilibrium constant, pK _a, of aminopyrene on the water surface was determined to be 2.0, while that of the same molecule in bulk water was 3.6 × 0.1. These results show that neutral aminopyrene tends to stay on the water surface, resulting in equilibrium shifts toward the neutral form. It is demonstrated that laser two-photon ionization is a powerful method to analyze the equilibria of a small amount of molecules, 0.17pmol/cm ², adsorbed at the water surface.