Self-Aggregation Properties of Ionic Liquid 1,3-Didecyl-2-methylimidazolium Chloride in Aqueous Solution: From Spheres to Cylinders to Bilayers

The self-aggregation behavior of the double-chained ionic liquid (IL) 1,3-didecyl-2-methylimidazolium chloride ([C₁₀C₁₀mim]­Cl) in aqueous solution has been investigated with a number of different experimental techniques. Two cmc values (cmc₁ and cmc₂) are obtained from conductivity measurements. The fraction of neutralized charge on the micellar surface suggests that cmc₁ corresponds to the formation of spherical micelles and cmc₂ to the transition from spherical to cylindrical micelles. Data obtained from fluorescence spectroscopy (using pyrene and Nile red as chemical probes), fluorescence anisotropy (using rhodamine B as probe), and chemical shift ¹H NMR (in D₂O) provide a picture that is also consistent with a sphere-to-cylinder transition. This structural change is further confirmed by diffusion-ordered NMR spectroscopy (DOSY), from the self-diffusion coefficients for surfactant unimer and aggregates. Furthermore, a third evolution from cylindrical micelles to bilayer aggregates is proposed from the analysis of diffusion coefficients at high surfactant concentration ([IL] > 0.2 M). Phase scanning experiments performed with polarized light microscopy clearly demonstrate the presence of a lamellar liquid crystalline phase at very high IL concentration, thus confirming the coexistence of bilayer structures with elongated micelles, found at lower concentration. Additionally, [C₁₀C₁₀mim]Cl micelles are proposed as novel reaction media, as evidenced by the solvolysis reaction of 4-methoxybenzenesulfonyl chloride (MBSC)