MOLECULAR DYNAMICS SIMULATIONS OF IONIC LIQUIDS AND IONIC LIQUID - CONVENTIONAL SOLVENT MIXTURES
Open Access
- Author:
- Conway, Brian
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 13, 2019
- Committee Members:
- Mark Maroncelli, Dissertation Advisor/Co-Advisor
Mark Maroncelli, Committee Chair/Co-Chair
Lasse Jensen, Committee Member
William Noid, Committee Member
Janna Maranas, Outside Member - Keywords:
- molecular dynamics
ionic liquids
classical simulation
spectroscopy
chemistry
physical chemistry
mixtures
ions
diffusion
rotation
solvation dynamics - Abstract:
- This dissertation is comprised of three primary chapters which describe studies of the unique structure and dynamics present in ionic liquids and their mixtures with conventional solvents. Ionic liquids differ from conventional dipolar solvents in that consist of charge-separated ions, which allow for a richer variety of solvation structures than do dipolar solvents. Ionic liquids often have intriguing morphologies and dynamics, which are strongly influenced by the attractive and repulsive components of the constituent ions. When other solvents are added to create binary mixtures, these structural and dynamical peculiarities can be enhanced or largely eliminated. In this dissertation we examine ionic liquid structure and dynamics on a molecular level through molecular dynamics simulations. These simulations both help explain experimentally observed trends and provide new insights into these complex systems. Chapter 2 reports studies of the rotational behavior of 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) mixed with acetonitrile (CH3CN). This study was performed in part to validate the dynamics of a force field later used to study solvation, but also out of interest in understanding the modes of motion by which the solvent or a benzene solute rotates. To do so, we compare experimental 2H T1 NMR relaxation times to simulated rotation times corresponding to various bond axes, which are directly related. The former reports only single time, which encompasses all relaxation mechanisms. In simulation, through calculation of a rotational correlation function, we observe the individual timescales on which various rotational axes relax. Some, like methyl groups, spin and relax very quickly, but slower modes, like those of the cation ring require “tumbling” motion to fully decorrelate from their initial orientation. We consider the timescales of these different types of motion and relate them to the friction on each motion, and reveal how each is related to solution viscosity. Chapter 3 uses the force fields validated in Chapter 2 to study the solvation structure and dynamics of the chromophore Coumarin 153 (C153) dissolved in [Im41][BF4] mixed with either CH3CN or H2O, as prototypical examples of dipolar aprotic and protic cosolvents. These conventional solvents serve to reduce the viscosity of the ionic liquids and make them more suitable for industrial use. Prior spectroscopic studies established the steady-state spectral shifts and time-resolved solvation response in these mixtures as functions of composition. The steady state spectra suggest that C153 is preferentially solvated in H2O in those mixtures, but the solute is insoluble in pure water. Here we focus on calculating analogs to the experimental values and using the atomistic detail of MD simulation to characterize the local solvation environment of C153 in these mixtures and isolate the solvent contributions responsible for the experimental observations, particularly those puzzling shifts in IL + H2O. In the CH3CN mixtures, the spectral shifts are unremarkable with solution composition. In this mixture, the ionic liquid and cosolvent mix essentially ideally, and the spectral shifts and dynamics indicate no preference for the C153 in either mixture component. However, in H2O, C153 is solvated almost entirely by ionic liquid. With increasing water concentration, the changes in the spectral shift can be isolated to contributions owing to H2O hydrogen-bound to a carbonyl on the chromophore. We conclude this work by explaining the motions that the solvent makes about C153 and how it relates to the observed spectral response. Chapter 4 discusses diffusion of small solutes in ionic liquids and dipolar solvents. Their diffusion reaches a curious regime in which the friction experienced by a solute drastically deviates from the Stokes-Einstein prediction. When the solute is much smaller than the solvent, neutral solutes diffuse much faster than expected and charged solutes much slower. Experimentally, this effect has been well documented. Here we try to explain these trends using MD simulation. By reducing our system to spherical single-site solutes, we simplify the interactions experienced. After reproducing the experimental trends, we examine solvation structure and how it gives rise to the observed differences between neutral and charged solute dynamics. Charged solutes typically have solvation shells that are significantly enriched in one of the ionic components, while the neutral solutes possess no such preference. The structurally ordered charged solutes are confined in solvation cages where they undergo in-cage vibrational motions, which persist for long times in the case of the smallest solutes. Likewise, these charged probes have the longest residence times, as a result of the strong attractions between the solute and solvent. Finally, we show that diffusion coefficients are simply related to residence times via a power law relationship. Through this work, we establish that the local ordering plays a pivotal role in slowing diffusion of ionic solutes.