Dodecyl sulfate-based anionic surface-active ionic liquids: synthesis, surface properties, and interaction with gelatin

Four dodecyl sulfate-based anionic surface-active ionic liquids were synthesized. Physicochemical parameters of surface-active ionic liquids (SAILs) or SAIL/gelatin aqueous solutions were demonstrated. Critical micelle concentration (cmc) values of [Emim][C₁₂H₂₅SO₄], [Amim][C₁₂H₂₅SO₄], [Bmim][C₁₂H₂₅SO₄], and [Etmim][C₁₂H₂₅SO₄] were 2.10 mmol/L, 1.95 mmol/L, 0.95 mmol/L, and 0.86 mmol/L at 298.15 K, respectively. cmc values of four SAILs followed the decreasing trend: [Emim][C₁₂H₂₅SO₄] ≈ [Amim][C₁₂H₂₅SO₄] > [Bmim][C₁₂H₂₅SO₄] ≈ [Etmim][C₁₂H₂₅SO₄]. The alkyl chain length (ethyl to butyl group) and electron absorption effect (carboxylate ester or allyl group) of substituent in cationic imidazolium groups played important role in micelle formation of SAILs. cmc values of SAIL/gelatin aqueous solutions were lower than those of SAILs. Micelle formation (SAIL or SAIL/gelatin) was spontaneous, exothermic, and entropy-driven. From DFT calculation, typical hydrogen bonds were found between active hydrogen atom of imidazole ring and oxygen, sulfur atoms of sulfate anion group. The information obtained would provide guide for design and synthesis of novel SAILs for wide gelatin application.