Structural studies of the phase, aggregation and surface behaviour of 1-alkyl-3-methylimidazolium halide + water mixtures

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• 作者：Ian Goodchild ; Laura Collier ; Sarah L. Millar ; Ivan Prokeš ; Jason C.D. Lord ; Craig P. Butts ; James Bowers ; John R.P. Webster ; Richard K. Heenan
• 刊名：Journal of Colloid and Interface Science
• 出版年：2007
• 期刊代码：125_00219797
• 类别：ch
• 出版时间：15 March 2007
• 卷：307
• 期：2
• 页码：455-468
• 文件大小：362 K

摘要

The surface, phase and aggregation behaviour of mixtures of 1-alkyl-3-methylimidazolium halide, [C_nmim]X, where n is the alkyl chain length, with water has been explored using a variety of methods. Critical micelle concentrations (cmc) and micelle structures have been determined for aqueous [C_nmim]Br solutions for n=2, 4, 6, 8, and 10. Small-angle neutron scattering (SANS) measurements reveal that for the n=8 and 10 systems, at concentrations just above the cmc, small near-spherical aggregates exist, which, after initial growth, possess core radii (aggregation numbers) at intermediate concentrations of (22±2) and (40±3), respectively, for n=8 and n=10. Towards higher concentrations, the aggregates appear to grow, with the aggregates in the [C₁₀mim]Br system becoming increasingly elongated (prolate) with increasing concentration. No evident aggregates are formed in the systems with n=2 and 4. In the n=6 system, it appears that oblate aggregates with radius 9 Å form at the cmc and that the radius increases with increasing concentration. For longer alkyl chain lengths, at high concentrations lyotropic mesophases form in some systems. The mesophase region for the [C₈mim]Cl system has been explored across the composition range using X-ray diffraction and ²H NMR spectroscopy. Both techniques suggest that a major hexagonal phase with lattice parameter of coexists with a minor lamellar phase or possibly a second hexagonal phase . The area per adsorbed molecule at the surface of [C₈mim]Br solutions has been measured as a function of concentration using neutron reflectometry. A minimum in the area per molecule behaviour is coincident with a minimum identified in the surface tension isotherm occurring close to the cmc. The data suggest depletion of [C₈mim]Br from the surface region occurs at concentrations immediately above the cmc.