Impact of alkyl chain length of temperature-responsive ionic liquids on the aggregation behavior in ionic liquid microemulsions

Abstract

As is known to all, the alkyl chain is a perfect tool to regulate the microstructure of ionic liquid(IL) microemulsion directly. However, it is still a great challenge to design a new type of temperature-responsive IL with different alkyl chain length to construct the stimulus microemulsion. In this study, a new class of temperature-responsive IL microemulsions was built for the first time, which is composed by [P444,n]Br (n = 8,10,12,14,16,18), [C12mim]Br and H2O. The phase diagram and dynamic light scattering analysis revealed that the size of IL microemulsion droplets can be effective regulated by the alkyl chain length of temperature-responsive IL, so as to gain a series of organized structures. The molecular dynamic(MD) simulations were performed by a series of [P444,n]Br/[C12mim]Br/H2O mixture systems to illustrate this unique aggregation behavior of droplet. Simulation results are presented and discussed via the data of interaction energies, aggregation numbers, radial distribution functions (RDFs) and coordination numbers. The results indicate that the aggregation sizes of [C12mim]+ become smaller with the increase of alkyl chain length of tetrabutylphosphonium cation, which agrees well with experimental results. This is because that the weakened interaction of temperature-responsive IL and water molecules by the long alkyl chain could reduce the electrostatic repulsion of the polarity groups of the surfactants, which may decrease the size of the aggregate. These finding can help in further understanding the structure variation law of temperature-responsive IL microemulsions and thereby improve microemulsion theory.