Why Does Ethaline Apparently Behave as an Ideal Binary Mixture?

Area of Science:

• Physical Chemistry
• Materials Science
• Spectroscopy

Background:

• The phase diagram of choline chloride (ChCl) in ethylene glycol (EG) suggests ideal nonelectrolyte behavior, which is unexpected for salt solutions.
• Understanding the solvation and aggregation behavior of salts in glycols is crucial for predicting their physical properties.

Purpose of the Study:

• To investigate the solvation and aggregation of choline salts in ethylene glycol using dielectric relaxation spectroscopy (DRS) and quantum-chemical calculations.
• To elucidate the reasons behind the seemingly ideal behavior of choline chloride-ethylene glycol mixtures.

Main Methods:

• Broad-band dielectric relaxation spectroscopy (DRS) was employed to study solutions of choline chloride (ChCl), choline iodide (ChI), and chlorocholine chloride (ClChCl) in ethylene glycol (EG).
• Quantum-chemical calculations were performed to complement the experimental spectroscopic data.
• Measurements were conducted up to saturation at 298.15 K.

Main Results:

• All three solutes exhibited weak solvation in EG, with only one EG OH-group dynamically affected per solute equivalent.
• Contact ion pairs were significant below 1 M, but free cation concentrations remained low across all concentrations.
• A substantial fraction of dipolar cations were undetectable by DRS, suggesting their incorporation into large solute aggregates.

Conclusions:

• The formation of large solute aggregates, rather than ideal mixing, explains the observed phase diagram of ChCl + EG mixtures.
• The extensive aggregation leads to the very low ionicity observed in these choline salt/EG systems.
• The findings challenge the initial assumption of ideal nonelectrolyte behavior for these salt solutions.