Acid dissociation under hydrostatic pressure: Structural implications for volumetric parameters

• Published in: Journal of molecular liquids Vol. 328; p. 115512
• Main Authors: Miyagawa, Akihisa, Fukuhara, Gaku, Okada, Tetsuo
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.04.2021
• Subjects: Acid dissociation; Hydration; Hydrostatic pressure; Molar compressibility difference; Molar volume difference; Hydration; Molar compressibility difference; Hydrostatic pressure; Acid dissociation; Molar volume difference
• ISSN: 0167-7322; 1873-3166
• DOI: 10.1016/j.molliq.2021.115512

Here, we evaluate the pressure dependencies of the acid dissociation constants (pKa) 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 (ΔV°) and the molar compressibility difference (Δκ) 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 ΔV°, whereas a molecule has positive ΔV° 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 ΔV° and Δκ, particularly for carboxylic acids and phenols. In contrast, when deprotonation causes the hydration sphere to expand, both ΔV° and Δκ are positive. Thus, a positive correlation between ΔV° and Δκ data is interpreted based on the hydration structural change upon deprotonation. [Display omitted] •Spectroscopic measurement under hydrostatic pressure.•Change in acid dissociation constant by pressurization.•Elucidation for change in molar volume difference from hydration.•Revealing relationship between difference in molar volume and molar compressibility.