Should deep eutectic solvents be treated as a mixture of two components or as a pseudo-component?

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Engineering

Background:

• Deep eutectic solvents (DESs) are gaining significant attention due to their unique properties.
• DES dilutions often exhibit deviations from ideal solution behavior.
• Accurate modeling of DES properties, especially viscosity, is crucial for their application.

Purpose of the Study:

• To investigate the influence of different molecular weight considerations on the physical properties of aqueous Deep Eutectic Solvent (DES) dilutions.
• To analyze the excess molar volumes (V^E) and excess viscosities (ln η^E) of choline chloride-urea (ChCl2U) aqueous solutions.
• To determine the most appropriate method for treating ChCl2U as either a two-component mixture or a pseudo-component.

Main Methods:

• Utilized existing density and viscosity data for aqueous dilutions of choline chloride-urea (ChCl2U) in a 1:2 molar ratio.
• Calculated excess molar volumes (V^E) and excess viscosities (ln η^E) under two different molecular weight assumptions for ChCl2U.
• Assessed the impact of treating ChCl2U as a mixture versus a pseudo-component on thermodynamic and transport properties.

Main Results:

• The choice of molecular weight (mixture vs. pseudo-component) did not significantly alter the sign or temperature dependence of excess molar volumes (V^E).
• Differences were observed in the magnitude of V^E deviations and the DES concentration at which minima occurred.
• Excess viscosities (ln η^E) showed opposite signs depending on the treatment: negative for the pseudo-component and positive for the mixture model, with the pseudo-component approach aligning better with hydrogen bonding observations.

Conclusions:

• Treating choline chloride-urea (ChCl2U) as a pseudo-component is more appropriate for accurately describing the excess viscosity of its aqueous dilutions.
• The pseudo-component model better reflects the hydrogen bond network formation in ChCl2U aqueous solutions.
• The study highlights the importance of selecting the correct model for understanding DES behavior in solution.