Brønsted acidity in deep eutectic solvents and ionic liquids.

Area of Science:

• Electrochemistry
• Physical Chemistry
• Materials Science

Background:

• Proton behavior in ionic liquids and deep eutectic solvents (DESs) remains poorly understood.
• Accurate measurement of proton activity in non-aqueous media is challenging.
• Understanding acid-base properties in these novel solvents is crucial for their application.

Purpose of the Study:

• To determine acid dissociation constants (pK_a) for organic acids in DESs.
• To investigate the pH behavior of ionic liquids and the influence of water.
• To explore the development of electrochemical pH sensors for these media.

Main Methods:

• Titration of DESs with triflic acid using standard pH indicator solutes.
• Determination of pK_In for bromophenol blue.
• Measurement of pK_In values for two ionic liquids: [Bmim][BF₄] and [Emim][acetate].
• Development and testing of an electrochemical pH sensor.
• Analysis of liquid junction potentials between reference electrodes in different DESs.

Main Results:

• Organic acids exhibited slightly lower dissociation in DESs compared to water (pK_a values 0.2–0.5 higher).
• [Emim][acetate] demonstrated more basic properties than water.
• Water significantly altered the pH of ionic liquids due to phase partitioning.
• An electrochemical pH sensor showed a linear response, albeit with a reduced slope compared to aqueous solutions.
• Liquid junction potential in DESs was pH-dependent.

Conclusions:

• Established a method for assessing acid-base properties in DESs.
• Highlighted the significant influence of solvent type and water content on proton behavior.
• Demonstrated the potential, with limitations, of electrochemical sensors for non-aqueous pH measurements.
• Provided foundational data for understanding proton activity in ionic liquids and DESs.