A new method has been proposed to determine the equilibrium between the bulk and the surface by directly measuring surface concentrations using laser two-photon ionization. This method has been applied to pyrenebutyric acid. The surface concentrations depended on the pH of the solution and were analyzed on the basis of two equilibrium constants and two distribution coefficients. Most pyrenebutyric acid stays on the surface at pH = 2.2. The equilibrium constant, pK a, of pyrenebutyric acid on the water surface was determined to be 7.85 ± 0.13, and this value is shifted to higher value than that in the bulk (4.76). The distribution coefficient of the neutral pyrenebutyric acid was determined as (5.9 ± 2.8) × 10-2 m, and that of pyrenebutyric anion as (4.82 ± 0.1) × 10-5 m. The ratio of the distribution coefficient of the neutral pyrenebutyric acid to that of pyrenebutyric anion was determined to be (1.2 ± 0.6) × 103. These findings indicate that the equilibrium shifts toward the neutral form on the water surface. Laser two-photon ionization was found to be a sensitive and powerful technique to analyze equilibrium on the surface and that between the surface and the bulk.

Laser two-photon ionization technique has been applied to monitor dissolving processes of probe molecules from solution surfaces. The time courses of the photoionization charge decay were measured by changing the properties of the bulk phase after the addition of the amphipathic molecules. The dissolving rate of pyrenebutyric acid, aminopyrene and pyrenesulfonic acid was strongly depended on pH in the bulk solution. The dissociation equilibrium of the conjugated acid-base system governed the solubility of the amphipathic molecules. Effects of surfactants were also observed in the dissolving process of pyrenedecanoic acid.

The properties of the interfacial region between aqueous solution and the air are dissimilar to those of the bulk water. The importance of the interfacial region to many fields of chemistry, biology, and physics has long been recognized, and has demanded an intimate knowledge of the nature of such interfacial microenvironments.Traditionally, the interfacial layer has been investigated using surface tension and calorimetric measurements. It is, however, difficult to determine the surface concentration from these methods, because the surface can not be defined on a molecular level. New spectroscopic methods have recently appeared for studies on the air/water interface.1 Laser two-photon ionization is one of them, and is a very sensitive and effective probe for air-water interface.2-5 When the laser irradiates the water surface, a photoabsorbing molecule on the thin surface layer absorbs a photon and is excited into a real excited state. Then it absorbs a second photon and is photoionized. The water is transparent and is not ionized in a two photon process. The positive electrode, which is located above the water surface, accepts ejected electrons from ionized molecules on the surface. The escape depth of each electron from water can be estimated to be about 1 nm from the escape depth of electrons from D 2 O ice 6 and the Onsager length 7 for water. Thus, a surface layer of water about 1 nm thick can be probed by the laser photoionization method.The solubility of a molecule in water depends on pH. Thus, the surface concentration may depend on the pH of the solution.8 In this paper, the effect of pH and coexisting salt (NaCl) on the surface concentration of pyrenesulfonic acid on water surface was studied using a two-photon ionization technique.

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.