Acid dissociation under hydrostatic pressure: Structural implications for volumetric parameters

Here, we evaluate the pressure dependencies of the acid dissociation constants (pK(a)) of pH indicator dyes and amino acids by spectroscopic methods, including chemometric-assisted direct and indirect absorption spectrometry. These data can be useful for considering molecular properties under extreme conditions, such as in deep sea. We confirmed a positive correlation between the molar volume difference (Delta V degrees) and the molar compressibility difference (Delta kappa) data for the deprotonation of molecules examined here and in literature. This relationship is discussed based on structural changes in the molecules upon deprotonation and associated hydration changes. Deprotonation from a carboxyl or phenolic hydroxyl group results in a negative Delta V degrees, whereas a molecule has positive Delta V degrees when it loses a positive charge by deprotonation. This can be interpreted as a deprotonation-induced change in the size of the hydration sphere around the molecule. The contraction of the hydration sphere by deprotonation leads to negative Delta V degrees and Delta kappa, particularly for carboxylic acids and phenols. In contrast, when deprotonation causes the hydration sphere to expand, both Delta V degrees and Delta kappa are positive. Thus, a positive correlation between Delta V degrees and Delta kappa data is interpreted based on the hydration structural change upon deprotonation. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study evaluated the pressure dependencies of the acid dissociation constants (pK(a)) of pH indicator dyes and amino acids, confirming a positive correlation between the molar volume difference (ΔV°) and the molar compressibility difference (Δκ) data. The relationship is discussed based on structural changes in molecules upon deprotonation and associated hydration changes, suggesting a contraction of the hydration sphere by deprotonation leads to negative ΔV° and Δκ.