Influence of the molecular structure on the aggregation of imidazolium ionic liquids in aqueous solutions

The self-aggregation of ionic liquids (ILs) in aqueous solutions was studied with electrical conductivity, density and surface tension versus concentration measurements. Aqueous solutions containing the following ILs were investigated: 1-butyl-3-methyl-imidazolium hexafluorophosphate, [bmim][PF₆], 1-butyl-3-methylimidazolium methylsulfate, [bmim][CH₃SO₄], 1-decyl-3-methyl-imidazolium chloride, [C₁₀mim][Cl], and 1-dodecyl-3-methylimidazolium chloride, [C₁₂mim][Cl]. Two later compounds have surfactant-like structure and aggregate easily in aqueous solutions. The cation of two former ILs is substituted with a short alkyl chain that does not favour a micellar self-assembly. In spite of this, the inspection of data assessed with [bmim][CH₃SO₄] and [bmim][PF₆] solutions indicates the appearance of the micellar aggregation onset. The aggregation behaviour of the [bmim][CH₃SO₄] is similar to this observed with previous systems. In the case of [bmim][PF₆] the aggregation is strongly influenced by the presence of the voluminous, hydrophobic cation. The surface activity of this compound is week due probably to the ion pair formation in the bulk solution.