Ionic liquids with special functionalities in the bulk and at interfaces: structural and dynamical properties

This dissertation provides a comparison of certain structural and dynamical properties of typical ionic liquids (ILs) against those where cationic tails are decorated with special functionalities. Comparisons are performed for ionic liquids in their pure liquid form as well as at the vacuum interface. In one of the studies, we explore an ionic liquid which contains a moderately long linear apolar tail on both cations and anions in the pure liquid form and at the vacuum interface. In the liquid phase, we observe a very common sponge-like structure for the charge-network (the repetitive sequence of alternation between cation and anion charge heads) that when exposed to vacuum interfaces changed to a extended lamellar configuration. Such major structural changes due to confinement were not observed for the isoelectronic ionic liquid with diether functionalities on both cationic and anionic tails. In addition, a significant focus in my most recent work has been on the dynamics of ILs, in particular as it pertains to their viscosity. The overarching goal is to link viscosity to the relaxation of structural features in the time-dependent structure function S(q,t). Recent computational studies from us and others, which will be discussed in subsequent chapters, have shown that the relaxation of the time correlation function of the shear stress tensor (which determines the viscosity) is similar to that of S(q,t)² at the specific q value associated with charge alternation. Initial studies discussed this in reciprocal space, but our most recent work in collaboration with Prof. Araque has helped rationalize these findings in the context of the loss of memory about charge locations within the IL charge network. This investigation, which is still ongoing, extends beyond prototypical ILs to those containing the ether functionality. We believe that in-network charge blurring linked with the loss of memory about the position of charged species along a charge network is likely to be very important to the value of the shear viscosity and the time evolution of the stress tensor correlation function for prototypical ionic liquids. However, atypical ionic liquids containing ether functionalities likely have significantly mixed structural contributions to the viscosity. What is certain, is that ether functionalized ILs tend to have lower viscosities than their alkyl counterparts.