Effects of self-hydrogen bonding among formamide molecules on the UCST-type liquid-liquid phase separation of binary solutions with imidazolium-based ionic liquid, [Cnmim][TFSI], studied by NMR, IR, MD simulations, and SANS

The upper critical solution temperature (UCST)-type liquid-liquid phase separation of imidazolium-based ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C_nmim][TFSI], where n represents the alkyl chain length of the cation, n = 6, 8, 10, and 12) binary solutions with formamide (FA) was examined as a function of temperature and the FA mole fraction x_FA. The two-phase region (immiscible region) of the solutions is much larger and expands more with the increase in n, in comparison with the previous [C_nmim][TFSI]-1,4-dioxane (1,4-DIO) systems. An array of spectroscopic techniques, including ¹H and ¹³C NMR and IR combined with molecular dynamics (MD) simulations, was conducted on the present binary systems to clarify the microscopic interactions that contribute to the phase-separation mechanism. The hydrogen-bonding interactions of the imidazolium ring H atoms are more favorable with the O atoms of the FA molecules than with 1,4-DIO molecules, whereas the latter interact more favorably with the alkyl chain of the cation. Upon lowering the temperature, the FA molecules gradually self-aggregate through self-hydrogen bonding to form FA clusters. Concomitantly, clusters of ILs are formed via the electrostatic interaction between the counter ions and the dispersion force among the IL alkyl chains. Small-angle neutron scattering (SANS) experiments on the [C₆mim][TFSI]-FA-d₂ and [C₈mim][TFSI]-FA-d₂ systems revealed, similarly to [C_nmim][TFSI]-1,4-DIO systems, the crossover of the mechanism from the 3D-Ising mechanism around the UCST x_FA to the mean-field mechanism at both sides of the mole fraction. Interestingly, the x_FA range of the 3D-Ising mechanism for the FA systems is wider compared with the range of the 1,4-DIO systems. In this way, the self-hydrogen bonding among FA molecules most significantly governs the phase equilibria of the [C_nmim][TFSI]-FA systems.

Decay of hydrogen bonding in mixtures of aliphatic heptanols and bistriflimide ionic liquids

Hydrogen bonding in liquids of the constitution isomers of heptan-1-ol mixed with 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids (ILs), [C_nmim][NTf₂], is investigated using both computational and experimental techniques. All-atom non-polarizable molecular-dynamics (MD) simulations predict that the hydrogen bonds gradually decay with increasing temperature. This decay is more pronounced for the branched alcohols and in the presence of the ionic liquids. The primary and linear isomer, heptan-1-ol, and its tertiary and bulky analogue 3-ethylpentan-3-ol are identified as the opposite extremes of the spectrum of hydrogen bonding stability in the bulk liquid. While neat heptan-1-ol exhibits strong hydrogen bonding at 350 K, 3-ethylpentan-3-ol is prone to hydrogen bonding decay already at 300 K. The presence of ionic liquids is found to affect the hydrogen bonding comparably as a 50 K temperature increase. Since the heat capacities of the associating liquids are very sensitive to any variation in hydrogen bonding strength and to the character of the hydrogen-bonded clusters in the bulk liquid, the calorimetric effort provides useful experimental data to confirm the results predicted by MD simulations. In this work, excess heat capacity is measured for equimolar single-phase mixtures of alcohols and ILs, and it differs largely in its sign and magnitude for individual heptanol isomers. Temperature trends of the excess heat capacities suggest that the stability of hydrogen bonding for individual heptanol isomers is temperature-shifted, based on their capability of hydrogen bonding. The predicted hierarchy of hydrogen bonding in individual alcohols and its impact on the excess heat capacity trends are qualitatively confirmed via thermodynamic modelling of the associative contribution to the excess heat capacities. These terms are found to predetermine the observed non-monotonous excess heat capacity trends.

Assessment of the UCST-type liquid-liquid phase separation mechanism of imidazolium-based ionic liquid, [C8mim][TFSI], and 1,4-dioxane by SANS, NMR, IR, and MD simulations

Liquid-liquid phase separation of binary systems for imidazolium-based ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C_nmim][TFSI], where n represents the alkyl chain length of the cation), with 1,4-dioxane (1,4-DIO) was observed as a function of temperature and 1,4-DIO mole fraction, x_1,4-DIO. The phase diagrams obtained for [C_nmim][TFSI]-1,4-DIO systems showed that the miscible region becomes wider with an increase in the alkyl chain length, n. For n = 6 and 8, an upper critical solution temperature (UCST) was found. To clarify the mechanism of the UCST-type phase separation, small-angle neutron scattering (SANS) experiments were conducted on the [C₈mim][TFSI]-1,4-DIO-d₈ system at several x_1,4-DIO. The critical exponents of γ and ν determined from the SANS experiments showed that phase separation of the system at the UCST mole fraction occurs via the 3D-Ising mechanism, while that on both sides of UCST occurs via the mean field mechanism. Thus, the crossover of mechanism was observed for this system. The microscopic interactions among the cation, anion, and 1,4-DIO were elucidated using ¹H and ¹³C NMR and IR spectroscopic techniques, together with the theoretical method of molecular dynamics (MD) simulations. The results on the microscopic interactions suggest that 1,4-DIO molecules cannot strongly interact with H atoms on the imidazolium ring, while they interact with the octyl chain of the cation through dispersion force. With a decrease in temperature, 1,4-DIO molecules gradually aggregate to form 1,4-DIO clusters in the binary solutions. The strengthening of the C-H⋯O interaction between 1,4-DIO molecules by cooling is the key to the phase separation. Of course, the electrostatic interaction between the cations and anions results in the formation of IL clusters. When IL clusters are excluded from 1,4-DIO clusters, liquid-liquid phase separation occurs. Accordingly, the balance between the electrostatic force between the cations and anions and the C-H⋯O interaction between the 1,4-DIO determines the 3D-Ising or the mean field mechanism of phase separation.

Microstructural and Dynamical Heterogeneities in 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.

Revisiting the Liquid-Liquid Phase Behavior of n-Alkanes and Ethanol

Mixtures of alkanes and ethanol are important in many areas, for example, as fuel blends. This paper describes new experimental data obtained for the liquid-liquid equilibrium phase behavior of normal alkanes (n-alkanes; C_nH_2n+2; 9 ≤ n ≤ 24) with ethanol. The results were obtained by applying the cloud point method in a temperature range of T = 230-423 K at ambient pressure. All systems are partially miscible with an upper critical solution point. The two phase regions of the phase diagrams show no indication of any obvious optical irregularities, like birefringence, coloring, formation of schlieren, or remarkable turbidity, except critical opalescence. With increasing length of the molecular chain of the n-alkanes, the (liquid-liquid) critical point is shifted to higher temperatures and higher ethanol content. The data are analyzed numerically implying Ising criticality. The nonsymmetric shape of the phase body is considered in different approaches for describing the diameter by presuming (a) the validity of the rectilinear diameter rule, (b) a nonlinear diameter predicted in the theory of complete scaling, and (c) combining both concepts. The numerical analysis yields the critical temperature, the critical composition, the width, and the diameter of the phase diagrams. The results are compared with literature data sets from similar mixtures; these data are also evaluated in terms of the models applied here. Phase diagrams of 13 different sets of mixtures are measured and analyzed to extract general aspects of the behavior of the normal alkane-ethanol mixtures. A simple Flory-Huggins-like approach allows a semiquantitative description of the experimental results of the critical temperatures. Therefore, it confirms the picture of molecular ordering within the solutions.

Vapor Liquid Equilibria of 1-Ethyl-3-methylimidazolium Triflate (C2mimTfO) and n-Alkyl Alcohol Mixtures

The isobaric vapor liquid equilibria (VLE) of different binary mixtures of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (C₂mimTfO) with the n-alkyl alcohols, methanol, ethanol, propan-1-ol, and butan-1-ol, are studied at the pressures of p = 500, 700, and 1000 mbar, covering a composition range 0.25-0.35 ≤ x(solvent) ≤ 1.0. Complementarily, the experimental results are compared with calculations by the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS). For deriving suitable PC-SAFT parameters, experimental liquid densities were determined for the neat IL C₂mimTfO and its longer homologues, 1-butyl-3-methylimidazolium trifluoromethanesulfonate (C₄mimTfO) and 1-hexyl-3-methylimidazolium trifluoromethanesulfonate (C₆mimTfO), in a temperature range of 288.15 K ≤ T ≤ 363.15 K (C₂mimTfO) and 293.15 K ≤ T ≤ 363.15 K (C₄mimTfO and C₆mimTfO), respectively. The PC-SAFT EoS is found to be suitable for describing the VLEs under study with good accuracy (AARD_VLE ≤ 0.4%).

Vapor Liquid Equilibria of Binary Mixtures of 1-Butyl-3-methylimidazolium Triflate (C4mimTfO) and Molecular Solvents: n-Alkyl Alcohols and Water

Isobaric vapor liquid equilibria (VLE) of binary mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium trifluoromethanesulfonate (C₄mimTfO) with either water or short chained n-alkyl alcohols (methanol, ethanol, propan-1-ol, and butan-1-ol) are described in this study. Two different microebulliometers and a classical VLE apparatus were compared and the VLEs were determined in the composition range 0.4 ≤ x(solvent) ≤ 1 at three different pressure levels ( p = 500 mbar, 700 mbar, and 1000 mbar). The experimental data were modeled using the soft-SAFT equation of state, which was able to accurately describe the nonideal behavior of these mixtures. The combined experimental-modeling results obtained contribute to establish the structure-property relationship between the C₄mimTfO and n-alkyl alcohol molecules and to infer about its influence on the phase behavior of these solvents.

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

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.

Chain length effects on the vibrational structure and molecular interactions in the liquid normal alkyl alcohols

Alkyl alcohols are widely used in academia, industry, and our everyday lives, e.g. as cleaning agents and solvents. Vibrational spectroscopy is commonly used to identify and quantify these compounds, but also to study their structure and behavior. However, a comprehensive investigation and comparison of all normal alkanols that are liquid at room temperature has not been performed, surprisingly. This study aims at bridging this gap with a combined experimental and computational effort. For this purpose, the alkyl alcohols from methanol to undecan-1-ol have been analyzed using infrared and Raman spectroscopy. A detailed assignment of the individual peaks is presented and the influence of the alkyl chain length on the hydrogen bonding network is discussed. A 2D vibrational mapping allows a straightforward visualization of the effects. The conclusions drawn from the experimental data are backed up with results from Monte Carlo simulations using the simulation package Cassandra.

Influence of the alkyl side-chain length on the ultrafast vibrational dynamics of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (CnmimNTf2) ionic liquids

Probing the vibrational dynamics of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C_nmimNTf₂) ionic liquids (ILs) using femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) has indicated the ultrafast vibrational energy transfer between counter ions which is governed by interionic interactions and facilitated by hydrogen bonds. In this study, fs-CARS is used to investigate the ultrafast dynamics of the vibrational modes of the C_nmimNTf₂ ILs with n = 6, 8, 10, and 12 in a spectral region, which involves the imidazolium ring and the alkyl side-chain vibrations. The vibrational Raman modes with wavenumbers around 1418 cm^-1 are excited through the CARS process and the ultrafast time evolution of the consequently excited vibrational modes is monitored. The investigation of the life times of the fs-CARS transient signals indicates that the time scale of the dynamics becomes much faster when the alkyl side-chain length of the C_nmimNTf₂ is longer than n = 8. This observation suggests an increase in the hydrogen bonding interactions due to the nano-structuring of the ionic liquids, which became evident with an increasing length of the alkyl side-chain. This behavior is also found in molecular dynamics simulations. There, an increase of the oxygen density around the C(2)-H moiety of the imidazolium ring, which is the predominant site for hydrogen bond formation, is observed. In other words, the longer the alkyl side-chain, the more reorganization of the ionic liquid into polar and non-polar domains occurs and the higher the probability of finding interionic hydrogen bonds at the C(2)-H position becomes.

Mesoscopic organization in ionic liquids

We discuss some published results and provide new observations concerning the high level of structural complexity that lies behind the nanoscale correlations in ionic liquids (ILs) and their mixtures with molecular liquids. It turns out that this organization is a consequence of the hierarchical construction on both spatial (from ångström to several nanometer) and temporal (from fraction of picosecond to hundreds of nanosecond) scales, which requires joint use of experimental and computational tools.

Alcohols as molecular probes in ionic liquids: evidence for nanostructuration

A comprehensive study of the solution and solvation of linear alcohols (propan-1-ol, butan-1-ol and pentan-1-ol) in ionic liquids (ILs) is presented.