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 (1)H 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).