Formic and acetic acid p Ka values increase under nanoconfinement

Abstract

Organic acids are prevalent in the environment and their acidity and the corresponding dissociation constants can change under varying environmental conditions. The impact of nanoconfinement (when acids are confined within nanometer-scale domains) on physicochemical properties of chemical species is poorly understood and is an emerging field of study. By combining infrared and Raman spectroscopies with molecular dynamics (MD) simulations, we quantified the effect of nanoconfinement in silica nanopores on one of the fundamental chemical reactions-the dissociation of organic acids. The pK_a of formic and acetic acids confined within cylindrical silica nanopores with 4 nm diameters were measured. MD models were constructed to calculate the shifts in the pK_a values of acetic acid nanoconfined within 1, 2, 3, and 4 nm silica slit pores. Both experiments and MD models indicate a decrease in the apparent acid dissociation constants (i.e., increase in the pK_a values) when organic acids are nanoconfined. Therefore, nanoconfinement stabilizes the protonated species. We attribute this observation to (1) a decrease in the average dielectric response of nanoconfined aqueous solutions where charge screening may be decreased; or (2) an increase in proton concentration inside nanopores, which would shift the equilibrium towards the protonated form. Overall, the results of this study provide the first quantification of the pK_a values for nanoconfined formic and acetic acids and pave the way for a unifying theory predicting the impact of nanoconfinement on acid-base chemistry.

Fig. 1. Computational simulation cell (left) containing a nanoconfined, 2D slit pore connected to a bulk solvent reservoir. The transformation of acetic acid to acetate is explored in both nanoconfined (left, top) and in bulk (right, bottom) environments. The presence of the two interconverting species in the nanoconfined and bulk regions here is for demonstrational purposes only. Computational simulations were performed with a single molecule in each region; see the Methods section for further clarification.

Fig. 2. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of bulk and nanoconfined solutions (a) formic acid solutions, (b) acetic acid solutions, (c) nanoconfined formic acid solutions, and (d) nanoconfined acetic acid solutions.

Fig. 3. Experimentally determined speciation curves for (a), (b) formic and (c), (d) acetic acid. Attenuated total reflection Fourier transform infrared (ATR-FTIR) intensities at 1551 cm⁻¹ for the protonated forms, and 1710 cm⁻¹ for de-protonated forms are plotted as a function of pH. Each data point in an average of three measurements; experimental measurement uncertainty is within the size of the symbols.

Fig. 4. Raman spectra of bulk and nanoconfined solutions (a) formic acid solutions, (b) acetic acid solutions, (c) nanoconfined formic acid solutions, and (d) nanoconfined acetic acid solutions.

Fig. 5. Experimentally determined speciation curves for (a) bulk formic acid, (b) bulk acetic acid, (c) nanoconfined formic acid, and (d) nanoconfined acetic acid. Measurement uncertainty is within the size of the symbols for the (a) and (b) plots.