Synthesis of Purine-Based Ionic Liquids and Their Applications

Abstract

Bio-based ionic liquids (ILs) are being increasingly sought after, as they are more sustainable and eco-friendly. Purines are the most widely distributed, naturally occurring N-heterocycles, but their low water-solubility limits their application. In this work, four purines (theobromine, theophylline, xanthine, and uric acid) were combined with the cation tetrabutylammonium to synthesize bio-based ILs. The physico-chemical properties of the purine-based ILs were characterized, including their melting and decomposition temperatures and water-solubility. The ecotoxicity against the microalgae Raphidocelis subcapitata was also determined. The ILs show good thermal stability (>457 K) and an aqueous solubility enhancement ranging from 53- to 870-fold, in comparison to their respective purine percursors, unlocking new prospects for their application where aqueous solutions are demanded. The ecotoxicity of these ILs seems to be dominated by the cation, and it is similar to chloride-based IL, emphasizing that the use of natural anions does not necessarily translate to more benign ILs. The application of the novel ILs in the formation of aqueous biphasic systems (ABS), and as solubility enhancers, was also evaluated. The ILs were able to form ABS with sodium sulfate and tripotassium citrate salts. The development of thermoresponsive ABS, using sodium sulfate as a salting-out agent, was accomplished, with the ILs having different thermosensitivities. In addition, the purine-based ILs acted as solubility enhancers of ferulic acid in aqueous solution.

Keywords: ecotoxicity; liquid–liquid equilibrium; solubility; synthesis; thermoresponsive systems.

Figure 1

Aqueous solubility enhancement (S/S₀) of each purine-based salt, relative to the respective free purine (blue bars) and molar solubility (S) of each salt in pure water (orange dots and line). S₀ values were obtained from PubChem [53].

Figure 2

(A) EC₅₀ values (μmol∙L⁻¹), determined after 96 h of exposure time of Raphidocelis subcapitata to the purine-based salts and [N₄₄₄₄]Cl. The error bars correspond to the 95% confidence interval. (B) Correlation between the logarithm of EC₅₀ and the logarithm of the octanol–water partition coefficient (K_ow) of the different salt anions [59]. The coefficient of determination (R²) and the number of experimental points used for the linear regression (n) are also presented.

Figure 3

Binodal curves of the ternary systems, composed of (A) [N₄₄₄₄][Theop], (B) [N₄₄₄₄][Theob], or (C) [N₄₄₄₄]Cl, water, and Na₂SO₄ at 298 K (green), 323 K (orange), or 353 K (blue) (±1 K) and atmospheric pressure (0.1 MPa). [N₄₄₄₄]Cl was not tested at 353 K. (D) Comparison of the binodal curves of [N₄₄₄₄][Theop] (o), [N₄₄₄₄][Theob] (Δ), or [N₄₄₄₄]Cl (◊) at 298 K, using Na₂SO₄ as a salting-out agent.

Figure 4

Binodal curves of the ternary systems, composed of (A) [N₄₄₄₄][Theop] or (B) [N₄₄₄₄][Theob], water, and Na₂SO₄ (blue) or K₃C₆H₅O₇ (orange) at (298 ± 1) K and atmospheric pressure (0.1 MPa).

Figure 5

σ-Profiles and surfaces of ILs anions. Different ILs are represented in different colors: [N₄₄₄₄][Theop] (orange), [N₄₄₄₄][Theob] (blue), [N₄₄₄₄]Cl (green). The molecular surface charge distribution is represented in red (polar segments), blue (apolar segments), and green (neutral segments).

Figure 6

(A) Molar solubility of ferulic acid in IL solutions with different concentrations. (B) Molar aqueous solubility enhancement (S/S₀) of ferulic acid in IL solutions with different concentrations. In both (A) and (B), different ILs are represented in different colors: [N₄₄₄₄][Theop] (blue o), [N₄₄₄₄][Theob] (orange ◊), and [N₄₄₄₄]Cl (green Δ).

Figure 7

(A) Ferulic acid molar solubility, left axis, throughout the pH of the saturated samples with IL concentrations ranging from 0.05 to 1.5 mol∙L⁻¹: water-solubility of ferulic acid at that pH (×); [N₄₄₄₄][Theop] (blue o), [N₄₄₄₄][Theob] (orange ◊), and [N₄₄₄₄]Cl (green Δ). The red, black, and purple curves represent the deprotonation percentage of ferulic acid, right axis (data from Marvin 21.14 [59]). The arrows point to IL concentration increase. (B) Molar aqueous solubility enhancement (S/S₀) of ferulic acid in [N₄₄₄₄][Theop] (blue ○), [N₄₄₄₄][Theob] (orange ◊), and [N₄₄₄₄]Cl (green Δ) solutions at different molar concentrations, with S₀ being the water-solubility of ferulic acid at a similar pH to that of the saturated sample; (C) y-axis zoom of (B).