From molten salts to ionic liquids: a "nano" journey.
ABSTRACT Ionic liquids (ILs), a special group of classical molten salts, are widely used in various fields of science. Historically, researchers have tested ILs out of curiosity or to improve a specific property in a particular system in many areas of chemistry or materials science. However, today, ILs are far from being simple chemical curiosities and sit at the center of various green industrial innovation processes, where they play important roles in materials extraction, reactive catalytic supports, spatial devices, and biotransformations. In this Account, we describe a journey into a nanostructured universe to better understand the unique properties of ionic liquids and their modern applications. Because molten salts have been known for centuries and have found limited uses, we try to explain why modern nonaqueous ILs deserve increased interest and curiosity. We discuss the characteristics that distinguish modern nonaqueous ILs and compare them with classical molten salts. One of the main differences between room temperature ILs, especially those based on imidazolium cations, and simple molten salts, is the molecular asymmetry built into at least one of the ions. This asymmetry in modern, nonaqueous ILs opposes the strong charge ordering due to ionic interactions that normally would cause the system to crystallize. In addition, the presence of a cooperative network of hydrogen bonds between the cations and anions induces structural directionality (the entropic effect). Therefore, modern ILs form preorganized structures, mainly through hydrogen bonding, that induce structural directionality. In contrast, classical salts form aggregates only through ionic bonds. In other words, weak interactions order the structures in modern ILs while charges order the structure within classical salts. ILs cannot be regarded as merely homogeneous solvents. In fact, ILs form extended hydrogen-bond networks with polar and nonpolar nano domains and therefore are by definition "supramolecular" fluids. Thus, ILs are better described as hydrogen-bonded polymeric supramolecules of the type [(DAI)(m)(X)(m_n))](n+)[(DAI)(m_n)(X)(x))](n-). This structural pattern is a general trend for both the solid and the liquid phase and is apparently maintained to a large extent even in the gas phase. This structural organization of ILs can be used as entropic drivers (the "IL effect") for the preparation of well-defined nanoscale structures with extended order, either in the bulk phase or at the gas/vacuum interface.
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ABSTRACT: Using a set of diﬀerent techniques, which included single crystal X-ray, NMR, UV−vis, conductivity measurements, SAXS (small angle X-rays), ESI-MS(/MS) (electrospray (tandem) mass spectrometry), and theoretical calculations, an ample study of the structural organization and supramolecular interaction of the task-speciﬁc ionic liquid 1-methyl-3-carboxymethylimidazolium chloride (named MAI.Cl) was conducted. All techniques allowed for comprehensive investigation in the solid state, solution, and gas-phase behavior of MAI.Cl. Most relevant interactions are demonstrated showing the importance of hydrogen bonding to supramolecular organization of MAI.Cl in diﬀerent states and its tendency to aggregate in aqueous solutions.The Journal of Physical Chemistry C 06/2014; 118:17878−17889. · 4.84 Impact Factor
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ABSTRACT: Rotational dynamics of 1-alkyl-3-methylimidazolium-based ionic liquids has been investigated by monitoring their inherent fluorescence with the intent to unravel the characteristics of the emitting species. For this purpose, temperature-dependent fluorescence anisotropies of 1-alkyl-3-methylimidazolium (alkyl = ethyl and hexyl) ionic liquids with anions such as tris(pentafluoroethyl)trifluorophosphate ([FAP]), bis(trifluoromethylsulfonyl)imide ([Tf2N]), tetrafluoroborate ([BF4]) and hexafluorophosphate ([PF6]) have been measured. It has been observed that the reorientation times (tau_r) of the ionic liquids with ethyl chain scale linearly with viscosity and found to be independent of the nature of the anion. The experimentally measured tau_r values are a factor of 3 longer than the ones calculated for 1-ethyl-3-methylimidazolium cation using the Stokes-Einstein-Debye (SED) hydrodynamic theory with stick boundary condition, which suggests that the emitting species is not the imidazolium moiety but some kind of associated species. The reorientation times of ionic liquids with hexyl chain, in contrast, follow the trend tau_r[FAP] > tau_r[Tf2N] = tau_r[BF4] > tau_r[PF6] at a given viscosity (η) and temperature (T). The ability of the ionic liquids with longer alkyl chains to form the organized structure appears to be responsible for the observed behavior considering the fact that significant deviations from linearity have been noticed in the tau_r versus η/T plots for strongly associating anions [BF4] and [PF6], especially at ambient temperatures.The Journal of Physical Chemistry B 10/2014; · 3.38 Impact Factor
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ABSTRACT: The effect of ultrasound irradiation on the copper catalyzed azide-alkyne cycloaddition between phenylacetylene and 4-azidoquinoline has been studied in solution of different ionic liquids. In particular, we used ionic liquids featuring both aliphatic and aromatic mono- and dications, as well as anions differing in size, symmetry and coordination ability. We also examined the influence of the ionic liquids structural features on the reaction outcomes, finding that under magnetic stirring reactivity is favorably affected by the solvent structural organization, while under sonochemical conditions an opposite trend was observed. In all cases examined, sonochemical activation leads to significant reduction in energy consumption as compared to the reaction conducted under magnetic stirring.Ultrasonics Sonochemistry 08/2014; · 3.82 Impact Factor