Liquid crystalline phases of 1,2-dimethyl-3-hexadecylimidazolium bromide and binary mixtures with water

Abstract

1,2-Dimethyl-3-hexadecylimidazolium bromide, which lacks an acidic proton at the C-2 imidazolium ring position, exhibits thermotropic SmA 2 phase behavior above its melting point of 90.3 °C and up to 232.1 °C. The corresponding layer spacing and the full width at half maximum from XRD patterns display a temperature dependence. In binary mixtures with water, over a concentration range of 40–75%, lyotropic liquid crystalline phases form between 0.5 and 25 °C. The molecules of the new ionic liquid (IL) in the binary mixtures show three different self-assembly processes, namely, formation of rod micelles, coexistence of rod micelles and a hexagonal phase, and a pure hexagonal phase. The distance between the two centers of adjacent columns of cylinder units remains nearly constant at 4.97 ± 0.04 nm when the IL content exceeds 70%, indicating that the cylinder units attain a dense and highly ordered packing.