Formic and acetic acid pK_a values increase under nanoconfinement

Area of Science:

• Physical Chemistry
• Nanotechnology
• Environmental Science

Background:

• Organic acids are ubiquitous environmental compounds whose acidity is sensitive to environmental conditions.
• The effects of nanoconfinement on the physicochemical properties of chemical species, particularly acid-base chemistry, are not well understood.
• Understanding these effects is crucial for various applications, including catalysis and environmental remediation.

Purpose of the Study:

• To quantify the impact of nanoconfinement on the dissociation of organic acids (formic and acetic acid).
• To measure the pKa values of organic acids confined within silica nanopores.
• To develop a predictive understanding of how nanoconfinement affects acid-base equilibria.

Main Methods:

• Combined experimental techniques (infrared and Raman spectroscopies) with molecular dynamics (MD) simulations.
• Measured pKa of formic and acetic acids in 4 nm silica nanopores.
• Constructed MD models to calculate pKa shifts for acetic acid in slit pores of varying sizes (1-4 nm).

Main Results:

• Both experimental and simulation results demonstrated an increase in pKa values (decrease in apparent acid dissociation constants) for nanoconfined organic acids.
• Nanoconfinement was found to stabilize the protonated form of the organic acids.
• Observed pKa shifts were attributed to reduced dielectric response and/or increased proton concentration within nanopores.

Conclusions:

• This study provides the first quantitative pKa values for nanoconfined formic and acetic acids.
• Nanoconfinement significantly alters the acid-base properties of organic acids.
• The findings lay the groundwork for a comprehensive theory on the influence of nanoconfinement on acid-base chemistry.