Aggregation behavior of zwitterionic surface active ionic liquids with different counterions, cations, and alkyl chains

Abstract

A group of zwitterionic surface active ionic liquids (SAILs) with different counterions, cations and alkyl chains, 3-(1-alkyl-3-imidazolio)propanesulfonate β-naphthalene sulfonate, (C_nIPS-Nsa, n = 12, 14), 3-(1-dodecyl-3-imidazolio)propanesulfonate benzenesulfonate (C₁₂IPS-Bsa), and dodecyl-N,N-dimethylammonio-3-propane sulfonate β-naphthalene sulfonate (SB-12-Nsa), were synthesized. Their aggregation behaviors in aqueous solutions were systematically investigated by surface tension, dynamic light scattering (DLS) and ¹H NMR spectroscopy. Surface tension and DLS results illustrated that the surface properties, micelle size, and micellization behavior of zwitterionic SAILs in aqueous solutions are significantly affected by the anion type, anionic structure and the hydrophobicity of the alkyl chain. The SAILs with more hydrophobic anions and long alkyl chains are expected to favor the micellization. The steric hindrance and hydrophobicity of the cations, as well as the binding strength of the cations with the anions, also play important roles in the aggregation of zwitterionic SAILs. Additionally, the micelle formation mechanism was acquired by detailed analysis of the ¹H NMR spectra. The existence of π–π stacking between imidazolium and counterions was proved. The enhanced π–π stacking and hydrophobic effect of Nsa⁻ can promote the aggregation of zwitterionic SAILs. Density functional theory (DFT) calculations illustrated that the negative interaction energy of the complexes were C₁₂IPS-Bsa/H₂O > SB-12-Nsa/H₂O > C₁₂IPS-Nsa/H₂O > C₁₄IPS-Nsa/H₂O. It is more difficult to form micelles in complexes with more negative interaction energy. The counterion electronegativity of Nsa⁻ is smaller than that of Bsa⁻, which favors the formation of micelles.