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

Abstract

The pH-dependent amount of aminopyrene adsorbed at the water surface was measured using a laser twophoton 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.

Proton-Coupled, Low-Energy Pathway for Electrocatalytic CO2 Reduction at Re(Diimine) Complexes with a Conjugated Pyrazinyl Moiety

“…Such a trend has been experimentally demonstrated in the basicity of small molecular arylamines. For example, p K a for the conjugate acid of aniline in water is 4.62, for 1-naphthylamine is 3.92, and for amino-pyrene is 3.6 . Thus, for Re complexes with diamine ligands of increasing size, the energetic advantage of the proton-coupled pathway over the unprotonated one will decrease.…”

Estimating pH at the Air/Water Interface with a Confocal Fluorescence Microscope

“…Commonly used experimental methods for studying the water surface pH are spectroscopic methods: second-harmonic generation, [9][10][11] vibrational sum-frequency generation, [12][13][14] heterodyne-detected electronic sum-frequency generation, 15 and two-photo ionization 16 were used to clarify the acidity of the air/ water interface. Mostly, they showed that the water surface pH is more acidic.…”