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

Abstract

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.

Shifted equilibria of organic acids and bases in the aqueous surface region

“…the local hydronium and hydroxide concentrations, in the aqueous surface region upon investigation of indicator species and their ability for protonation/deprotonation at the aqueous surface region. 6 , 8 , 10 , 11 , 13 , 17 Although acid–base equilibria in the aqueous surface region have been investigated for several chemical compounds by various experimental and theoretical techniques, no study has been conducted investigating proton acceptors and donors using the same approach. Results of separate studies on individual compounds show that neutral forms of carboxylic acids 7 and amines 10 are more abundant at the surface region in comparison to their respective charged forms, i.e.…”

Influence of Cations on the Fluorescence Quenching of an Ionic, Sterically Congested Pyrenyl Moiety by Iodide in Water

“…In that regard, the titration curve of Py4C (Figure ) shows only one equivalence point, a p K a ≈ 4.5, indicating that the carboxylic groups of Py4C behave nearly independently (rather than synergistically); they do not interact significantly with each other. This p K a is slightly lower than that of PyBu (4.6) , and of other aliphatic carboxylic acids (∼4.6), perhaps as a result of an inductive effect by the nearby oxygen atom of the linker groups. The independence of the four carboxylic acid groups of Py4C was anticipated also based on its rigidity and the long distance between the carboxylic groups.…”

“…The spectroscopic properties of pyrenyl derivatives have been used extensively to examine electrostatic interactions of polyelectrolytes, aggregates, and microheterogenity in aqueous solutions. − Many of these investigations have exploited the fluorescence from pyrenyl groups and its quenching to obtain detailed information about micellar structure, ground-state association constants, and residual charges on aggregates. − However, pyrene itself can be solubilized in significant concentrations only within the heterogeneous, hydrophobic microenvironments within aqueous media (e.g., the interiors of micelles); the use of unsubstituted pyrene as a probe for ionic and electrostatic interactions directly within aqueous environments is very difficult. Thus, the solubility issues are typically overcome by the derivatization of pyrene with polar groups …”

Solution chemistry in the surface region of aqueous solutions

“…The mechanisms of these models and the proposed different protolytic properties of the aqueous surface region have been subject to intensive discussions. The experimental and theoretical approaches applied and the conclusions drawn from these have been contradictory [3,[49][50][51][52][53][54][55][56][57][58].…”

“…The definition of pH requires the presence of free hydrated or solvated hydronium and hydroxide ions, and their spatial distribution at the water-air interface have created an intense debate [3,50,[53][54][55]59]. The hydronium and hydroxide ions are very and fairly small, respectively, with relatively high charge density, and thereby strongly hydrated as seen in their large hydration enthalpies, ΔH hydr o , of −1091 (H + ) and −460 (OH − ) kJ/ mol [60], being monovalent ions.…”