Templating and Phase Behaviour of the Long Chain Ionic Liquid C₁₆mimCl

Abstract

In the light of recent studies about the favourable sol–gel templating behaviour of the long chain ionic liquids (ILs) 1-hexadecyl-3-methylimidazolium chloride and -bromide (C 16 mimCl and C 16 mimBr) the phase behaviour of these ILs is investigated in detail and compared to common surfactants such as hexadecyltrimethylammonium chloride and -bromide (CTAC and CTAB). Specially developed small angle X-ray scattering (SAXS) evaluation approaches confirm a lamellar double-layer structure for crystalline C 16 mimCl of the ABACAB type. Presumably, the orientation of the head groups repeats in every second head group layer. Therefore, the imidazolium head group exhibits long range interactions on packing and templating, in contrast to the ammonium head group. The lyotropic phase behaviour is investigated via SAXS, isothermal titration calorimetry (ITC) and surface tension measurements. Interestingly, the phase diagrams of C 16 mimCl and CTAC are quite different, especially at higher concentration of surfactant: instead of a gel phase in the phase diagram of CTAC, a lamellar-hexagonal two phase region as well as a pure lamellar phase is found in the phase diagram of C 16 mimCl. The critical micellar concentration (cmc) and enthalpy of micellization (Δ mic H ) show that the cmc for imidazolium based surfactants is slightly lower and Δ mic H is more exothermic than for the ammonium type surfactants. These results are able to account for the specialties observed in the sol–gel templating behavior.