Participants : P. Allongue, J.-N. Chazalviel, C. Henry de Villeneuve, F. Ozanam
PhD student : D. Aureau (2008)
The acid / base equilibrium is the simplest of chemical equilibria. We provide direct evidence that this equilibrium is deeply modified when the carboxylic acid groups are confined with a 2D monolayer (see page). The titration plot in Fig.1 (left) is derived from a quantitative analysis of in situ IR spectra as a function of the solution pH. It indicates the depronotation COOH → COO- + H+ spreads over a wide 6 pH units, starting from pH ∼ 5. The conversion of COOH into COO- is not complete at pH 11.
This phenomenon arises from the creation of a potential drop at the organic layer / solution interface. In fact each leaving proton is giving rise to a COO- surface site, which charge must be compensated by a cation (Fig. 1, right). Protons must thus overcome a potential barrier to leave the surface, which increases the pK (this is the so-called surface pK). Electrostatic repulsions contribute also to slowing down the reaction COOH → COO- + H+. The plot in Fig. 1 (left) follows the law (see solid line):
(where: NS = density of COOH sites of the starting monolayer (determined by IR spectroscopy); α = fraction of COOH sites converted to COO-; CH = capacitance of the layer/electrolyte interface (the only adjustable parameter); e = electron electric charge).
The first two terms in this expression are exactly the same for the reaction in solution. The 3rd term accounts for electrostatic interactions. Mott-Schottky plot measurements confirm these trends: a shift of the flat band potential is observed for a pH > 5-6. The phenomenon is reversible as long as the pH remains below 11.
Figure 1: (Left) Acid / base titration plots for carboxylic groups immobilized on a surface. Data are derived from in situ quantitative IR characterizations of the IR band intensity related to COO- groups. Note that the equilibrium spreads over 6 pH unit and is not complete at pH 11. (Right) Scheme explaining the creation of a potential drop at the acid surface partially deprotonated.
Publications :
[1] D. Aureau, F. Ozanam, P. Allongue, and J. N. Chazalviel, "The Titration of Carboxyl-Terminated Monolayers Revisited: In Situ Calibrated Fourier Transform Infrared Study of Well-Defined Monolayers on Silicon," Langmuir 24 (17), 9440-9448 (2008).