Dilution of room temperature ionic liquids leads to size-controlled nanoscale association

Significance 

Room temperature ionic liquids (RTILs) have drawn significant research attention owing to their attractive properties, including low vapor pressure, wide electrochemical window and their abilities to solubilize both polar and non-polar compounds. Despite their wide application in different fields, the dynamical and structural properties of RTILs are yet to be fully understood. For example, understanding the extent of their dissociation is central to evaluating other properties like dielectric response and conductivity. Previous studies have shown that RTILs generally exhibit structural order over a wide range of length scales in excess of liquid phase solvents. Besides the structural order, an induced free charge density gradient persisting over length scales was identified.

In RTILs, the free charge density gradient exists in the form of discrete ionic species concentration. This can be attributed to the contact between RTILs and charged surfaces, which can be sensed through induced birefringence and rotational diffusion experiments. The length scale of this gradient not only differs from organization reported in other studies but the structural gradient does not have to necessarily correspond to the charge density gradient. To this end, it is of fundamental interest to establish the relationship between the free charge density gradient and the local and longer-range organization in RTILs.

Herein, Mr. Md. Iqbal Hossain and Professor Gary Blanchard from Michigan State University studied the effects of dilution on the induced free charge density gradients in three imidazolium RTILs diluted with acetonitrile and methanol. The changes in length scale and magnitude were investigated. Time- and depth-resolved fluorescence measurements of cresyl violet rotational diffusion were carried out to evaluate the persistence of free charge density gradients in RTILs. The work is currently published in the journal, Physical Chemistry Chemical Physics.

The authors observed that for all the studied cases, the functional form of the free charge density gradients was not smooth and monotonic diminution with an increase in diluent, but rather a stepwise collapse in both persistence length and magnitude. This is a key finding because it implies persistent heterogeneity in the liquid phase. The collapse is correlated with the onset of formation of the RTIL ion pair dimers, a phenomenon that was found to play a fundamental role in the dissociation of RTIL ion mobility in the binary system. The findings suggested the existence of persistent compositional heterogeneity in diluted RTIL systems.

The existence of molecular-scales aggregates in both acetonitrile and methanol RTIL solutions was observed even at higher dilution beyond the point where free charge density gradients were not seen. The persistence of free charge density gradients in RTILs existed in varying degrees depending on the type of diluent and RTIL. The rotating entity in bulk RTILs displayed a larger effective hydrodynamic volume than could be obtained using bulk viscosity data of the diluted RTILs. The excess volume was illustrated by aggregating the RTIL ion pairs in the diluted system, where the aggregate size was dependent on the diluent and RTIL identity, and it increased with an increase in the dilution.

In summary, Mr. Hossain and Professor Blanchard investigated the dependence of persistence length and magnitude of free charge density gradients in imidazolium RTILs diluted with aprotic and protic solvents. The RTILs used in this study gauged the significance of anion and cation identified and characterization of the free charge density gradients. The findings agreed with the free charge density gradient dependence on dilution data. In a statement to Advances in Engineering, Professor Gary Blanchard, the lead author explained that their findings provided greater insight into the short-range organization in RTILs and its relationship with longer-range charge density gradient and would contribute to advanced studies on RTIL-solvent binary systems, including control over nanoscale structure in liquid phase systems.

Reference

Hossain, M., & Blanchard, G. (2022). The effect of dilution on induced free charge density gradients in room temperature ionic liquidsPhysical Chemistry Chemical Physics, 24(6), 3844-3853.

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