Pulsed-field gradient 1H NMR study of diffusion and self-aggregation of long-chain imidazolium-based ionic liquids

Diffusion and self-aggregation (micellization) processes of the neat 1-decyl-3-methyl-imidazolium chloride ([C(10)mim][Cl]) and its aqueous solutions have been studied using pulsed-field gradient H-1 NMR. Different from many RTILs, whose temperature dependencies of the diffusion coefficient in log D vs. 1/T plot exhibit curved profiles and thus obey the Vogel-Fulcher-Tammann (VFT) equation, those in the neat [C(10)mim][Cl] have been found to be perfect Arrhenius-type. This finding has been discussed in terms of the classification of compounds into 'fragile'-, 'Intermediate'- and 'strong glass formers'. The Arrhenius-type behavior can be explained by the presence of the long and flexible alkyl chains, which soften the system making it more plastic and less 'fragile'. The obtained activation energy values can be compared with those available in the literature for other RTILs and for some long-chain fatty acids (e.g. heptadecanoic (C-17) and nonadecanoic (C-19)) close to the melting point in the orientationally disordered solid phases. It can be assumed that the diffusion processes in the neat long-chain RTILs, similar to other long-chain molecular systems, go through the vacancies and the boundaries of highly dynamic mesoscopic structures. The critical aggregation concentration and the aggregation number have been determined from the dependence of diffusion coefficient on concentration in an aqueous solution. (C) 2015 Elsevier B.V. All rights reserved.