Nanoconfined imidazolium ionic liquids

Doctoral thesis, 2022

Long-chain ionic liquids are attracting interest as alternatives to their short-chain analogues as electrolytes. At the same time, they are also considered as alternatives to mesogens and conventional surfactants as templates. Interestingly, the nanoconfinement of long-chain ionic liquids in a solid and porous matrix (to result in a macroscopically solid hybrid material) can in some cases result in improved properties. This is due to the different chemical and steric environment to which the ionic liquid is exposed in confinement.

In this work, the effect of nanoconfinement on the mechanism of proton transport and the capacitive behaviour of the protic ionic liquid HC8imTFSI (1-octylimidazolium bis(trifluromethane-sulfonyl)imide) was studied. With this aim, proton conduction was investigated by broadband dielectric spectroscopy and pulsed-field gradient nuclear magnetic resonance spectroscopy, considering a mixture of imidazole and HC8imTFSI inside hydrophobised mesoporous silica. Results show that even inside narrow pores only ca 5 nm wide, the self-diffusion of the confined ionic liquid is unrestricted, due to weak interactions between the cations and the pore walls. By contrast, when pure HC8imTFSI was confined in the pores of hydrophilic silica, ca 3.5 nm wide, results from electrochemical impedance spectroscopy combined with equivalent circuit modelling indicated a high in-pore resistance. A further important finding is that at the in-pore protic ionic liquid/electrode interface an increased specific capacitance was measured, suggesting a higher charge density than for the interface between the electrode and the bulk protic ionic liquid.

Another aspect that has been considered in this thesis, is the ability of long-chain ionic liquids to function as a templating agent. The templating mechanism, the limitations associated to the chemical structure and the resulting pore morphology have been investigated. Results have revealed that tetradecyl- and hexadecyl-methylimidazolium chloride are suitable soft-templates for the formation of vertically aligned, uniform, channel-like pores, running through the entire thickness of a silica film deposited with the electrochemically assisted self-assembly method. By contrast, dodecyl-methylimidazolium chloride as well as the protic analogue hexadecyl-imidazolium chloride do not show a templating function under the investigated synthetic conditions. In all cases studied, the mechanism of pore formation is critically discussed.