Experimental and DFT studies on surface properties of sulfonate-based surface active ionic liquids

Abstract

Two anionic surface active ionic liquids (SAILs), 3-methyl-1-ethoxycarbonylimidazolium decanesulfonate ([Etmim][C10H21SO3]) and 3-methyl-1-ethoxycarbonylimidazolium dodecanesulfonate ([Etmim][C12H25SO3]) were synthesized and studied. The surface properties, aggregation properties and thermodynamic parameters of [Etmim][C10H21SO3] and [Etmim][C12H25SO3] aqueous solutions were determined using surface tension and electrical conductivity. Micelle formation of [Etmim][C10H21SO3] and [Etmim][C12H25SO3] was spontaneous and entropy-driven.Density functional theory (DFT) calculation was used to optimize the configuration of the anions (C10H21SO3, and C12H25SO3), imidazolium cation ([Etmim]+), and SAILs ([Etmim][C10H21SO3] or [Etmim][C12H25SO3]) with different number of water molecules (1–3) using Gaussian 16 at B3LYP/6-31G(d,p) level.Typical hydrogen bonds, bond length changes, bond angle changes, and charge changes of typical atoms in hydrated complexes were analyzed. For imidazolium cation hydrates, H2O formed two types of hydrogen bonds with cation ([Etmim]+). For sulfonate anion hydrates, H2O formed two types of hydrogen bonds with anions ([C10H21SO3]- or [C12H25SO3]-). Hydrogen atoms in H2O formed hydrogen bonds with oxygen atoms of polar sulfonate anions in three ways: 1:1, 2:1, and 2:1. The typical electron withdrawing atoms (N and O) in imidazolium cation was beneficial for the micelles formation. The electrostatic repulsion and electrostatic attractive interaction between polar groups of SAILs affected the CMC values of [Etmim][C10H21SO3] or [Etmim][C12H25SO3] aqueous solutions. Binding energy (D0), difference of binding energy (ΔD0) of sulfonate ion hydrates and SAIL hydrates confirmed the tendency of [Etmim][C12H25SO3] micelle formation was higher than that of [Etmim][C10H21SO3].