Ionic liquids solution characterization

Quite recently, a series of N-alkyl substituted imidazolium salts has been evaluated for additive effects on the mesomorphic behavior and ensuing optical properties of HPC aqueous solutions, followed by characterization of the thermotropicity of novel cellulose derivatives with such an ionic liquid structure in the side-chains [193]. 

As discussed below, ionic liquids often behave comparably to conventional polar organic solvents [6, 8, 10]. But the physics underlying solvation are entirely different. As noted above, ILs are characterized by considerable structural and dynamic inhomogeneity, and even simple concepts, such as the dipole moment, cannot be productively applied. We are therefore in the unusual position of needing to explain how an exotic microscopic environment produces conventional macroscopic behavior. To this end, we will review empirical characterizations of the ionic liquid environment, and then turn our attention to the underlying physics of solute-solvent interactions. 

A potentially unique form of specific solute-solvent interaction has been proposed for ionic liquids. Blanchard and Brennecke [234] note that the solubilities of aromatic species are anomalously high in an imidazolium-based IL when compared to solutes of comparable molecular weight and dipole moment. This cannot be explained purely by ji-ji interactions, because while ji-ji interaction energies can be significant [235], the solubilization of a pure aromatic liquid must disrupt at least as many ji-ji contacts as it creates. However, work by Holbrey and co-workers [171] characterizes a cocrystalline clathrate form of an imidazolium-based IL with benzene, which shows distinctive ji-ji stacking. 

The theory is based on two observations. First, solute-solvent interactions are characterized by dipole-ion interactions, and so are much weaker than the ion-ion interactions between solvent species. Thus, the presence of the solute dipole should not greatly perturb the liquid from the electrostatic structure of the neat liquid. Second, because the ionic liquid is a conductor, the electric field of the solute must be screened by the solvent. This observation has been confirmed... 

The preparation of a precursor solution followed by the construction and characterization of HEC is presented in Scheme 4. To a solution of OxA in TEG (1 1 wt ratio), the inorganic iodide salt (MI M = Li, K, Na, NH4) or ionic liquid (EtPrlm ) and I2 were added (I l2 = 10 1 mol ratio). Both silica precursors (ICS-2ME or ICS-DEM) and TEOS (1 1 wt ratio) were then admixed to form a sol, which was used for the construction of the HEC. The combinations of silica precursors with different inorganic iodides and EtPrlm F are shown in Table 1. 

An important common feature of macroion solutions is that they are characterized by at least two distinct length scales determined by the size of macroions (an order up to lOnm in the case of ionic micellar solutions) and size of the species of primary solvent (water molecules and salt ions, i.e. few Angstroms). Considering practical colloidal macro-dispersions, like foams, gels, emulsions, etc., usually we are dealing with as many as four distinct length scales molecular scale (up to lnm) that characterizes the species of the primary solvent (water or simple electrolytes) submicroscopic or nano scale (up to lOOnm) that characterizes nanoparticles or surfactant aggregates called micelles microscopic or mesoscopic scale (up to lOO m) that encompasses liquid droplets or bubbles in emulsion and foam systems as well as other colloidal suspensions, and macroscopic scale (the walls of container etc). 

The solution chemistry of nonaqueous solvents is very different from that of water-rich mixed solvents. pH measurement in nonaqueous solvents is difficult or impossible. Salts often show a limited degree of dissociation and limited solubility (see [132] for solubility of salts in organic solvents). Ions that adsorb nonspecifically from water may adsorb specifically from nonaqueous solvents, and vice versa. Therefore, the approach used for water and water-rich mixed solvents is not applicable for nonaqueous solvents, with a few exceptions (heavy water and short-chain alcohols). The potential is practically the only experimentally accessible quantity characterizing surface charging behavior. The physical properties of solvents may be very different from those of water, and have to be taken into account in the interpretation of results. For example, the Smoluchowski equation, which is often valid for aqueous systems, is not recommended for estimation of the potential in a pure nonaqueous solvent. Surface charging and related phenomena in nonaqueous solvents are reviewed in [3120-3127], Low-temperature ionic liquids are very different from other nonaqueous solvents, in that they consist of ions. Surface charging in low-temperature ionic liquids was studied in [3128-3132]. 

Liquid crystal phases, or mesophases, are characterized by a partial order, intermediate between the full orientational and translational disorder of the isotropic liquid phase and the full orientational and translational order of the crystalline phase. Thermotropic liquid-crystal phases are obtained for a given compound (or possibly a mixture) as a function of temperature, while the so-called lyotropic liquid-crystal phases are obtained as a function of the concentration of a given solute in a solvent Typical examples of the latter systems are the various types of aggregates formed by amphiphilic molecules either in water or in organic solvents. In this chapter we will be interested only in thermotropic systems. An interesting review on lyotropic ionic liquid crystals can be found in Ref. [2],... 

Polarity is the most common classification for solvents. There is no absolute polarity scale in a first approximation, it can be considered that polar solvents are characterized by their ability to dissolve charged solutes. Since ionic liquids are themselves salts, they are expected to be very polar. 

---