Self-assembled structure and dynamics of imidazolium-based protic salts in water solution

Abstract

Protic ionic liquids containing cations with long alkyl chains can form self-assembled micelles, vesicles, microemulsions, and lyotropic liquid crystal structures in water, acid water or tetrahydrofuran, etc. As a result of this unique property, they are regarded as a novel category of amphiphiles, and are gaining importance in the field of colloid and interface chemistry. The critical micelle concentration (CMC) of protic salts, e.g., alkyl-ammonium nitrates in water, was found to increase with decreasing chain length. It is generally believed that a long alkyl chain length is essential for the formation of self-assembled structures. So far, no self-assembled structure has been reported for protic ionic liquids with an alkyl chain length of n < 4. This paper reports on the structure and dynamics of two imidazolium based protic organic salts with no alkyl chain or a methyl group (n = 1) attached to the cation in water solution, determined through a detailed analysis of NMR spectra and pulsed-field gradient NMR data. We demonstrate that these imidazolium cations with no or a short alkyl chain (n = 1) can form a self-assembled clustering structure in water solution, which has a strong influence on the diffusion behavior of imidazolium molecular ions. It is speculated that this self-assembled structure is likely to be present in other similar solutions of ionic liquids with short alkyl chains.