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

A new method has been proposed to determine the equilibrium between the bul k 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 anal yzed on the basis of two equilibrium constants and two distribution coeffic ients. Most pyrenebutyric acid stays on the surface at pH = 2.2. The equili brium constant, pK(a), of pyrenebutyric acid on the water surface was deter mined to be 7.8(5) +/- 0.1(3), and this value is shifted to higher value th an that in the bulk (4.76). The distribution coefficient of the neutral pyr enebutyric acid was determined as (5.9 +/- 2.8) x 10(-2) m, and that of pyr enebutyric anion as (4.8(2) +/- 0.1) x 10(-5) m. The ratio of the distribut ion coefficient of the neutral pyrenebutyric acid to that of pyrenebutyric anion was determined to be (1.2 +/- 0.6) x 10(3). These findings indicate t hat the equilibrium shifts toward the neutral form on the water surface. La ser two-photon ionization was found to be a sensitive and powerful techniqu e to analyze equilibrium on the surface and that between the surface and th e bulk.