2.2. Synthesis of ionic liquids

The 1-alkyl-3-methylimidazolium halide ionic liquids [C₆mim]Cl, [C₈mim]Cl, [C₁₀mim]Cl, [C₁₂mim]Cl, [C₁₂mim]Br, [C₁₂mim]I, [C₁₄mim]Cl, [C₁₆mim]Cl and [C₁₈mim]Cl were prepared from freshly distilled N-methylimidazol and the corresponding alkyl halide, according to literature [31,32]. Solid surface-active ionic liquids, [C_nmim]X with n ≥ 12, were repeatedly crystallized from tetrahydrofuran or ethyl acetate until colorless crystals were obtained. Sulfonate- and phosphonate-based surface-active ionic liquids were synthesized through a two steps procedure involving the alkylation of 1-dodecylimidazole, which was previously synthesized following a procedure reported in literature and distilled before use [33]. 1-Dodecylimidazole was reacted with the corresponding methyl ester to yield 1-dodecyl-3-methylimidazolium mesylate ([C₁₂mim]OMs), 1-dodecyl-3-methylimidazolium tosylate ([C₁₂mim]OTs), and 1-dodecyl-3-methylimidazolium dimethylphosphate ([C₁₂mim]Me₂PO₄). Analytical data were in accordance with literature and details can be found in previous work [31,32,34,35]. In case of 1-dodecyl-3-methylimidazolium acetate ([C₁₂mim]OAc), the procedure given by Ferguson et al. [36] was adapted, which includes the neutralization of the corresponding hydroxide ionic liquid with acetic acid to obtain the acetate counterion. Analytical data were in accordance with literature and details can be found in our previous work [27].The synthesis of ionic liquids with ester-functionalization, namely [C₁₂bet]Cl and [C₁₂COmim]Cl, relied on the pre-formation of dodecyl 2-chloroacetate that was further reacted with trimethyl amine or methylimidazole according to literature protocols [37,38].

1-Methylimidazol (1.19 g, 14.49 mmol) and dodecyl 2-chloroacetate (3.81 g, 14.49 mmol) were stirred at ambient temperature for 24 h. The obtained solid was crystallized from THF, collected via filtration and washed with anhydrous THF and diethyl ether. After drying in vacuum, a colorless solid was yielded in 81% (4.1 g). ¹H NMR (200 MHz, CDCl₃): δ_H = 0.86 (t, J = 6.36, 3 H, -CH₂-CH₃), 1.18–1.36 (m, 18 H, -O-CH₂-CH₂-(CH₂)₉-CH₃), 1.64 (quint, J = 6.55, 2 H, -O-CH₂-CH₂-(CH₂)₉-CH₃), 4.06 (s, 3 H, N-CH₃), 4.16 (t, J = 6.84, 2 H, -O-CH₂-CH₂-(CH₂)₉-CH₃), 5.47 (s, 2 H, N-CH₂-COO-), 7.40 (s, 1H, H-4), 7.51 (s, 1H, H-5), 10.58 (s, 1 H, H-2).

Dodecyl 2-chloroacetate (5.39 g, 20.51 mmol) was dissolved in 15 mL anhydrous THF. A solution of trimethylamine in THF (102.5 mmol) was added dropwise at room temperature. After stirring overnight, the precipitate was collected via filtration and washed with anhydrous THF and diethyl ether. After drying in vacuo (2 × 10⁻² mbar) overnight, [C₁₂bet]Cl was obtained as colorless crystals in 88% yield. ¹H NMR (CDCl₃): δ = 0.81 (3H, t, J = 6.95, -C₁₁H₂₂-CH₃), 1.19 (18H, m, -C₂H₄-C₉H₁₈-CH₃), 1.58 (2H, t, J = 6.74, -CH₂-CH₂-C₁₀H₂₁), 3.60 (9H, s, N-(CH₃)₃), 4.10 (2H, t, J = 6.95, -CH₂-C₁₁H₂₃), 5.01 (2H, s, Cl-CH₂-CO).