Ionic liquids (ILs) offer a wide range of promising applications because of their much enhanced properties. However, further development of such materials depends on the fundamental understanding of their hierarchical structures and behaviors, which requires multiscale strategies to provide coupling among various length scales. In this review, we first introduce the structures and properties of these typical ILs. Then, we introduce the multiscale modeling methods that have been applied to the ILs, covering from molecular scale (QM/MM), to mesoscale (CG, DPD), to macroscale (CFD for unit scale and thermodynamics COSMO-RS model and environmental assessment GD method for process scale). In the following section, we discuss in some detail their applications to the four scales of ILs, including molecular scale structures, mesoscale aggregates and dynamics, and unit scale reactor design and process design and optimization of typical IL applications. Finally, we address the concluding remarks of multiscale strat...

Multiscale Studies on Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Microstructural and Dynamical Heterogeneities in Ionic Liquids.

Inorganic and organic "solvent-in-salt" (SIS) systems have been known for decades but have attracted significant attention only recently. Molten salt hydrates/solvates have been successfully employed as non-flammable, benign electrolytes in rechargeable lithium-ion batteries leading to a revolution in battery development and design. SIS with organic components (for example, ionic liquids containing small amounts of water) demonstrate remarkable thermal stability and tunability, and present a class of admittedly safer electrolytes, in comparison with traditional organic solvents. Water molecules tend to form nano- and microstructures (droplets and channel networks) in ionic media impacting their heterogeneity. Such microscale domains can be employed as microreactors for chemical and enzymatic synthesis. In this review, we address known SIS systems and discuss their composition, structure, properties and dynamics. Special attention is paid to the current and potential applications of inorganic and organic SIS systems in energy research, chemistry and biochemistry. A separate section of this review is dedicated to experimental methods of SIS investigation, which is crucial for the development of this field.

"Solvent-in-salt" systems for design of new materials in chemistry, biology and energy research.

Ultrafast two-dimensional infrared spectroscopy (2D IR) has been advanced in recent years toward measuring signals from only a monolayer of sample molecules at solid–liquid and solid–gas interfaces. A series of experimental methods has been introduced, which in the chronological order of development are 2D sum-frequency-generation (2D SFG), transmission 2D IR, and reflection 2D IR, the latter in either internal, attenuated total reflection (ATR), or external reflection configuration. The different variants of 2D vibrational spectroscopy are based on either the even-order or the odd-order nonlinear susceptibility, and all allow resolving similar molecular temporal and spectral information. In this review, we introduce the basic principles of the different methods of 2D vibrational spectroscopy at surfaces along with a balanced overview on the technological aspects as well as benefits and shortcomings. We furthermore discuss the current scope of applications for 2D vibrational surface spectroscopy, which sp...

https://www.zora.uzh.ch/id/eprint/173927/1/Kraack_and_Hamm%2C_Chem._Rev._2016_RS_fin2.pdf

Surface-Sensitive and Surface-Specific Ultrafast Two-Dimensional Vibrational Spectroscopy

Dynamics and structure of room temperature ionic liquids

The orientational dynamics and microscopic liquid structure of a protic ionic liquid, 1-ethylimidazolium bis(trifluoromethylsulfonyl)imide (EhimNTf2), and its aprotic analogue, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf2), were studied at various water concentrations using optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy, linear infrared spectroscopy, and atomistic simulations. The OHD-OKE experiments essentially measure the orientational relaxation of the Ehim+ and Emim+ cations. The experiments and simulations show a significant dynamical and structural change in EhimNTf2 between the 2:1 ion pair:water and the 1:1 ion pair:water concentrations. The OHD-OKE data show that EmimNTf2/water mixtures exhibit hydrodynamic behavior at all water concentrations up to saturation. In contrast, EhimNTf2/water mixtures deviate from hydrodynamic behavior at water concentrations above 2:1. At the 1:1 concentration, the orientational randomization of the Ehim+ cation is ...

Impact of Hydrogen Bonding on the Dynamics and Structure of Protic Ionic Liquid/Water Binary Mixtures

The dynamics of the room-temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2) were investigated with two-dimensional infrared (2D IR) vibrational echo spectroscopy and polarization selective pump–probe (PSPP) experiments. The CN stretch frequency of a modified Bmim+ cation (2-SeCN-Bmim+), in which a SeCN moiety was substituted onto the C-2 position of the imidazolium ring, was used as a vibrational probe. A major result of the 2D IR experiments is the observation of a long time scale structural spectral diffusion component of 600 ps in addition to short and intermediate time scales similar to those measured for selenocyanate anion (SeCN–) dissolved in BmimNTf2. In contrast to 2-SeCN-Bmim+, SeCN– samples its inhomogeneous line width nearly an order of magnitude faster than the complete structural randomization time of neat BmimNTf2 liquid (870 ± 20 ps) measured with optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments. The orientational c...

Dynamics in a Room-Temperature Ionic Liquid from the Cation Perspective: 2D IR Vibrational Echo Spectroscopy

The dynamics of four 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide room-temperature ionic liquids (RTILs) with carbon chain lengths of 2, 4, 6, and 10 were studied by measuring the orientational and spectral diffusion dynamics of the vibrational probe SeCN(-). Vibrational absorption spectra, two-dimensional infrared (2D IR), and polarization-selective pump-probe (PSPP) experiments were performed on the CN stretch. In addition, optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments were performed on the bulk liquids. The PSPP experiments yielded triexponential anisotropy decays, which were analyzed with the wobbling-in-a-cone model. The slowest decay, the complete orientational randomization, slows with increasing chain length in a hydrodynamic trend consistent with the increasing viscosity. The shortest time scale wobbling motions are insensitive to chain length, while the intermediate time scale wobbling slows mildly as the chain length increases. The 2D IR spectra measured in parallel (⟨XXXX⟩) and perpendicular (⟨XXYY⟩) polarization configurations gave different decays, showing that reorientation-induced spectral diffusion (RISD) contributes to the dynamics. The spectral diffusion caused by the RTIL structural fluctuations was obtained by removing the RISD contributions. The faster structural fluctuations are relatively insensitive to chain length. The slowest structural fluctuations slow substantially when going from Emim (2 carbon chain) to Bmim (4 carbon chain) and slow further, but more gradually, as the chain length is increased. It was shown previously that K(+) causes local ion clustering in the Emim RTIL. The K(+) effect increases with increasing chain length. The OHD-OKE measured complete structural randomization times slow substantially with increasing chain length and are much slower than the dynamics experienced by the SeCN(-) located in the ionic regions of the RTILs.

Alkyl Chain Length Dependence of the Dynamics and Structure in the Ionic Regions of Room-Temperature Ionic Liquids.

In a previous study of room temperature ionic liquid/water mixtures, the first clearly observed biexponential decays in optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments on a liquid were reported, (Sturlaugson, A. L.; Fruchey, K. S.; Fayer, M. D. J. Phys. Chem. B 2012, 116, 1777), and it was suggested that the biexponential behavior is indicative of the approach to gelation. Here, new OHD-OKE experiments on mixtures of the room temperature ionic liquid 1-methyl-3-octylimidazolium chloride (OmimCl) with water are presented. The OmimCl/water system is shown to gel over the water mole fraction range of 0.69–0.81. In the OHD-OKE decays, the biexponential behavior becomes more distinct as the gelling concentration range is approached from either high or low water concentrations. The biexponential decays are analyzed in terms of the wobbling-in-a-cone model, and the resulting diffusion constants and “relative” order parameters and cone angles are reported. Comparison of the OmimCl/water data...

/pdf/orientational-dynamics-in-a-lyotropic-room-temperature-ionic-28pjbyww3v.pdf

Orientational dynamics in a lyotropic room temperature ionic liquid.

The isotropic phase of nematogenic liquid crystals has nanometer length scale domains with pseudonematic ordering. As the isotropic to nematic phase transition temperature (TNI) is approached from above, the orientational correlation length, ξ, of the pseudonematic domains grows as (T - T(*))(-1/2), where T(*) is 0.5-1 K below TNI. The orientational relaxation, which is a collective property of the pseudonematic domains, was measured with optical heterodyne detected-optical Kerr effect (OHD-OKE). The orientational relaxation obeys Landau-de Gennes theory, as has been shown previously. To examine the environmental evolution experienced by molecules in the pseudonematic domains, two-dimensional infrared (2D IR) vibrational echo experiments on the CN stretching mode of the non-perturbative vibrational probes 4-pentyl-4(')-selenocyanobiphenyl (5SeCB) and 4-pentyl-4(')-thiocyanobiphenyl (5SCB) in the nematogen 4-cyano-4(')-pentylbiphenyl (5CB) were performed. The 2D IR experiments measure spectral diffusion, which is caused by structural fluctuations that couple to the CN vibrational frequency. Temperature dependent studies were performed just above TNI, where the correlation length of pseudonematic domains is large and changing rapidly with temperature. These studies were compared to 2D IR experiments on 4-pentylbiphenyl (5B), a non-mesogenic liquid that is very similar in structure to 5CB. The time constants of spectral diffusion in 5CB and 5B are practically identical at temperatures ≥5 K above TNI. As the temperature is lowered, spectral diffusion in 5B slows gradually. However, the time constants for spectral diffusion in 5CB slow dramatically and diverge as T(*) is approached. This divergence has temperature dependence proportional to (T - T(*))(-1/2), precisely the same as seen for the correlation length of pseudonematic domains, but different from the observed orientational relaxation times, which are given by the Landau-de Gennes theory. The data and previous results show that spectral diffusion in 5CB has no contributions from orientational relaxation, and the structural dynamics responsible for the spectral diffusion are likely a result of density fluctuations. The results suggest that the correlation length of the density fluctuations is diverging with the same temperature dependence as the pseudonematic domain correlation length, ξ. The isotropic-nematic phase transition in liquid crystals is described in the context of the slowing of orientational relaxation associated with divergent growth of the orientational correlation length. The results presented here show that there is another divergent dynamical process, likely associated with density fluctuations.

/pdf/new-divergent-dynamics-in-the-isotropic-to-nematic-phase-24o6nq8xqh.pdf

Heather E. Bailey

Papers

Impact of Hydrogen Bonding on the Dynamics and Structure of Protic Ionic Liquid/Water Binary Mixtures

Dynamics in a Room-Temperature Ionic Liquid from the Cation Perspective: 2D IR Vibrational Echo Spectroscopy

Alkyl Chain Length Dependence of the Dynamics and Structure in the Ionic Regions of Room-Temperature Ionic Liquids.

Orientational dynamics in a lyotropic room temperature ionic liquid.

New divergent dynamics in the isotropic to nematic phase transition of liquid crystals measured with 2D IR vibrational echo spectroscopy