Synthesis and Characterization of the Choline-Amino Acid Ionic Liquids [Ch][Ala], [Ch]2[Glut], and [Ch][Threo] and Their Aqueous Binary Solutions

Abstract

In this work, the synthesis of the choline-amino acid ionic liquids (CAAILs) cholinium alaninate ([Ch][Ala]), dicholinium glutamate ([Ch]₂[Glut]) and cholinium threoninate ([Ch][Threo]) was conducted at 313.15 K and 0.1 MPa. Then, after validating the chemical structure of the CAAILs, the binary aqueous solutions containing [Ch]₂[Glut] and [Ch][Threo] were characterized by measuring liquid density, refractive index, electrical conductivity, dynamic viscosity, torque and shear stress. For [Ch][Ala], only refractive index was determined, as the other properties were already addressed in a previous work of the research group. The liquid densities (ρ) of the binary solutions were assessed at 288.15, 293.15, 298.15, 303.15, and 308.15 K and 0.1 MPa, and correlated with the Connors and Wright model, presenting low standard deviations (0.37 < σ_SD\ kg·m^–3 < 1.40). Then, the calculated excess volumes (V_m^E) were correlated with third-degree polynomials (0.0447 < σ_SD·10⁶ / m³·mol^–1 < 0.0729) and Redlich–Kister expansions (0.040 < σ_SD·10⁶ / m³·mol^–1 < 0.082), while dynamic viscosities (μ), torque (τ) and shear stress (σ) were measured at 288.15–308.15 K and 0.1 MPa and described using an Arrhenius linearization (1.14 < σ_SD·10² < 2.32). Finally, the ionicity (or degree of dissociation, α) of each CAAIL in the binary solutions was determined with composition using the Walden plot methodology, at 298.15 K and 0.1 MPa.