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

Miki Sato, Takeshi Kaieda, Kohshin Ohmukai, Hirofumi Kawazumi, Akira Harata, Teiichiro Ogawa

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    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, pKa, 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.

    Original languageEnglish
    Pages (from-to)9873-9877
    Number of pages5
    JournalJournal of Physical Chemistry B
    Issue number42
    Publication statusPublished - Oct 26 2000


    All Science Journal Classification (ASJC) codes

    • Physical and Theoretical Chemistry
    • Surfaces, Coatings and Films
    • Materials Chemistry

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