Micellization behavior of an imidazolium surface-active ionic liquid within aqueous solutions of deep eutectic solvents: a comparative spectroscopic study

Abstract

Ionic liquids and deep eutectic solvents are topics of immense importance and are attracting many researchers worldwide owing to their green nature and broader application potential. These green solvents have been widely used in synthesis, catalysis and biocatalysis, nanoscience, pharmaceutics, etc. Hence, it is exciting to see how ionic liquids behave within deep eutectic solvents. Here, we examined the aggregation behavior of an imidazolium-based surface-active ionic liquid (SAIL), 1-decyl-3-methylimidazolium tetrafluoroborate [Dmim][BF₄], within aqueous solution of 5 wt% DES choline-based deep eutectic solvents (DESs). Two choline-based DESs (ChCl-urea and ChCl-Gly) were prepared by heating a mixture of 1 : 2 molar ratios of an ammonium salt (choline chloride) with hydrogen bond donors (urea and glycerol). The DESs were characterized using FTIR and ¹H-NMR spectroscopic techniques. Micellization behavior of the SAIL [Dmim][BF₄] within these aqueous DESs media was investigated using fluorescence, FTIR, dynamic light scattering (DLS), ¹H-NMR, and NOESY. The information about the local microenvironment surrounding the probe molecules and size of the aggregates of [Dmim][BF₄] in the presence of 5 wt% of aqueous DESs were obtained from fluorescence and DLS, respectively. DLS results showed that IL [Dmim][BF₄] forms relatively larger micelles within aqueous solutions of DES ChCl-urea (avg. hydrodynamic radii = 94.6 nm) compared with ChCl-Gly (avg. hydrodynamic radii = 82.8 nm). A significant decrease in the critical micellar concentration and an increase in aggregation number (N _agg) were observed, clearly indicating that micellization of IL [Dmim][BF₄] is greatly favored in the DES solutions. FTIR study depicts the strength of intermolecular interactions such as hydrogen bonding, ion-ion pair interactions, and dipole-dipole interactions between the ILs and DESs. The ¹H-NMR data showed that differences in chemical shifts can provide significant indication about the IL-DES interactions. ¹H-NMR and 1H-1H 2D NOESY spectroscopy were employed to gain insights into these IL-DES interactions that are responsible for the aggregation behavior of the IL [Dmim][BF₄] within aqueous DES solutions. It was observed that IL [Dmim][BF₄] forms self-assembled structures within the aqueous DESs media. The current results are expected to be useful for colloidal aspects of ILs and DESs and their mixtures with water.

Scheme 1. Structures of IL 1-decyl-3-methylimidazolium tetrafluoroborate, choline chloride, glycerol, urea, cetylpyridinium chloride, pyrene and 1-pyrene carboxyaldehyde.

Fig. 1. FT-IR spectra of synthesized ChCl-based DESs: (A) ChCl–urea and (B) ChCl–Gly.

Fig. 2. ¹H-NMR spectra of [A] ChCl–Gly- and [B] ChCl–urea-based DESs.

Scheme 2. Synthesis diagram of DESs based on glycerol, urea and choline chloride.

Fig. 3. Graph plot between F₁/F₃ intensity ratios versus log[DmimBF₄] within the two deep eutectic solvents ( water, ChCl–Gly and ChCl–urea, 5 wt%) in aqueous medium at ambient conditions. The lines indicate sigmoidal fitting of the experimental data.

Fig. 4. Plot of PyCHO intensity versus log[DmimBF₄] (M) concentration within 5 wt% aqueous DESs solution [A] native DmimBF₄, [B] DmimBF₄–ChCl–urea, [C] DmimBF₄–ChCl–Gly. The lines indicate sigmoidal fitting of the experimental data.

Fig. 5. Plot of ln(F₀/F_Q) versus concentration of CPC (mM) in aqueous [DmimBF₄] (0.01 M) IL solution in the presence of different DESs (5 wt%) at ambient conditions. The lines indicate linear fitting of the experimental data.

Fig. 6. Size distribution obtained from DLS for aqueous [Dmim][BF₄] in the presence and absence of deep eutectic solvents i.e., [A] pure [DmimBF₄] in water; (B) [DmimBF₄] in ChCl–Gly aqueous solution; and (C) [DmimBF₄] in ChCl–urea aqueous solution.

Scheme 3. {1H–1H} interactions of ChCl with glycerol and urea.

Scheme 4. Schematic representation of the {1H–1H} interactions of the following DESs with [Dmim][BF4]: [A] glycerol and [B] urea.