Abstract
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, pKa, 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/cm2, adsorbed at the water surface.
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