Autoprotolysis in Modern Solvents

KANZAKI Ryo

doi:10.2116/bunsekikagaku.74.469

<p>Autoprotolysis occurs in solvents when they possess both proton donating and accepting abilities, proving that the solvents act as an acid-base reaction medium. This paper provides an overview of recent studies on autoprotolysis and associated acid-base reactions in protic ionic liquids (PILs), highly concentrated electrolyte solutions, and deep eutectic solvents (DESs), which have recently gained attention as acid-base reaction media. In PILs, which consist of onium salts, potentiometric titrations using a hydrogen electrode revealed that the autoprotolysis constants roughly reflect the acid-base properties of each constituent ion in aqueous solutions. This finding suggests that the precursor acids and bases act as the proton carrier and the entity of the base, respectively, in PILs. Their Brønsted acidity and basicity allowed the estimation of the so-called <pH window> of the PILs. In highly concentrated lithium bis(trifluoromethanesulfonyl)amide aqueous solutions (20 mol kg⁻¹), contrary to Debye-Hückel theory, the activity coefficient of hydrogen ions was found to increase with increasing coexisting electrolyte concentration. Furthermore, calorimetric titrations revealed that this increase in activity coefficient is caused by the enthalpic disadvantage of proton carriers in highly concentrated electrolyte solutions, likely due to electrostatic repulsions from ubiquitous lithium ions. Although the concentration of hydrogen ions under neutral conditions is lower than in pure water, the increased activity coefficient leads to a decrease in the estimated pH. In DES of ethylene glycol and choline chloride, the autoprotolysis is predominantly mediated by ethylene glycol, while the autoprotolysis constant is influenced by the coexisting choline chloride. These findings provide important insights for understanding the acid-base characteristics of novel solvents and offer valuable guidance for rational solvent design.</p>

Autoprotolysis in Modern Solvents

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