The surface, phase and aggregation behaviour of mixtures of 1-alkyl-3-methylimidazolium halide, [C-n mim]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(n)mim]Br solutions for n = 2, 4, 6, 8, and 10. S,all-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 10.5 +/- 10.5 angstrom (22 +/- 2) and 13.2 +/- 10.5 angstrom (40 +/- 3), respectively, for n = 8 and 17 = 10. Towards higher concentrations, the aggregates appear to grow, with the aggregates in the [C(10)mim]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 similar to 9 angstrom form at the cinc 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 [C-8 mim]Cl system has been explored across the composition range using X-ray diffraction and H NMR spectroscopy. Both techniques suggest that a major hexagonal phase with lattice parameter of 29.5 0.5 A coexists with a minor lamellar phase (23.5 +/- 0.3 angstrom) or possibly a second hexagonal phase (27.1 +/-0.4 angstrom). The area per adsorbed molecule at the surface of [C(8)mim]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 [C(8)mim]Br from the surface region occurs at concentrations immediately above the cmc. (c) 2007 Elsevier Inc. All rights reserved.