Ionic Liquids

Room temperature ionic liquids and deep eutectic solvents

The behavior of polymers is complex solvents, such as room temperature ionic liquids or deep eutectic solvents, is interesting from both a fundamental and practical perspective.   The fundamental interest arises from the fascinating sensitivity of polymer behavior to the solvent conditions.  The practical interest arises from the possibility of using the polymer as a scaffold for the working fluid solvent, or from the possibility of fabricating nanostructured materials by tuning the solvent properties.

Ionic liquids (ILs) are usually composed of a large organic cation and a small organic or inorganic anion and are liquid at room temperature (hence they are often referred to as room temperature ionic liquids or RTILs).  They possess a number of interesting and important physical properties such as low volatility, non-flammability, high conductivity, and thermal and chemical stability.  They have generated considerable excitement for their varied potential applications, as solvents for synthesis and catalysis, as electrolytes, as media for separations, as sorption media for gas absorption, and as lubricants.  The fundamental interest and technological importance of ionic liquids has resulted in an exponential growth in studies devoted to understanding their physical chemistry.  A widely studied example of an ionic liquid is 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]).

Deep eutectic solvents (DES) are obtained via the complexation of a quaternary ammonium salt with a hydrogen bond donor.  This results in a depression of the freezing point similar to what is seen in eutectic mixtures of metals.  A classic DES, called reline, is a mixture of choline chloride (ChCl) and urea.  ChCl and urea have melting points of 302oC and 133oC, respectively, but a 1:2 molar ratio (reline) has a melting point of 12oC3a.  DESs have many of the properties of ILs including non-volatility, conductivity, and biodegradability.  They do not suffer from toxicity and cost issues.    Many DES are obtained from natural sources; ChCl is extracted from biomass and used as an additive in chicken feed. reline costs approximately $4/kg.  This is much cheaper than the ionic liquid [BMIM][BF4], which costs $7000/kg.

Force field development

In our group, we have developed a first-principle, physically motivated force field for [BMIM][BF4], reline, and the polyethylene oxide (PEO) based on symmetry-adapted perturbation theory.  For BMIM][BF4], the predictions (from molecular dynamics simulations) of the liquid density, enthalpy of vaporization, diffusion coefficients, viscosity, and conductivity are in excellent agreement with experiment, with no adjustable parameters.  The explicit energy decomposition inherent in the force field enables a quantitative analysis of the important physical interactions in these systems.  We find that polarization is crucial and there is little evidence for charge transfer.  We also argue that the often-used procedure of scaling down charges in molecular simulations of ionic liquids is unphysical for [BMIM][BF4].

We make adjustments to the SAPT force field protocol to treat hydrogen bonding systems such as DES.  We add additional sites to better represent the angular dependence of the interaction, adjust the polarizability to reproduce crystal properties, and adjust short-ranged interactions to better reproduce the liquid structure as observed in first principles molecular dynamics simulations.  The resulting approach gives excellent results, for example, for urea water mixtures.

We develop a hierarchy of coarse-grained models using a “top-down” approach.  We vary the polarization and atomistic resolution of functional groups and parameterize the models using SAPT to the same DFT calculations.   The approach has been successful in the study of room temperature ionic liquids.  A series of coarse-grained models are shown below where the orange sites are Drude particles (for polarizability) and grey sites are hydrogen atoms.  The united atom representation (UA_AA) is as accurate as the fully polarizable atomistic model (AA_AP) while being an order of magnitude more computationally efficient.

Conformational properties and phase behavior

The UA_AA model allows one to perform multi microsecond molecular dynamics simulations. This is necessary because the conformational relaxation correlation times are of the order of 100 ns.  The average conformational properties are in good agreement with experiment and the simulations provide further insight.  For example, there are two conformational motifs in PEO corresponding to ring-like (crown ether) and extended structures.

We have used a deep neural network model to obtain the phase behavior of PEO in various ionic liquids,

Ongoing work involves study the behavior of a variety of polymers in various ionic liquids with an aim of obtaining optimal electrolytes for battery applications.