The role of solvents

Green Chemisrty

  • Solvents in chemistry
  • Chemistry is dominated by the study of species in solution. Although any liquid may be used as a solvent, relatively few are in general use. However, as the introduction of cleaner technologies has become a major concern throughout both industry and academia, the search for alternatives to the most damaging solvents has become a high priority.
    Solvents are high on the list of damaging chemicals for two simple reasons:

    • they are used in huge amounts
    • they are usually volatile liquids (VOCs) that are difficult to contain.3


    Fused salts are liquids containing only ions, ionic liquids. It is possible, by careful choice of starting materials, to prepare ionic liquids that are liquid at and below room temperature. RTILs are different than molecular solvents, water and organic solvents, and chemically analogous to molten salts. Conventional molten salts exhibit a high melting point (i.e., 801 °C for sodium chloride and 614 °C for lithium chloride), which greatly limits their use as solvents in most applications; RTILs, however, remain liquids at or below room temperature.5


  • Solvent properties of ILs
  • Some simple properties of these room-temperature ionic liquids that make them interesting as potential solvents for synthesis are the following3:

    1. they are good solvents for a wide range of both inorganic and organic materials, and unusual combinations of reagents can be brought into the same phase;
    2. they are often composed of poorly coordinating ions, so they have the potential to be highly polar yet noncoordinating solvents;
    3. they are immiscible with a number of organic solvents and provide a nonaqueous, polar alternative for two-phase systems. Hydrophobic ionic liquids can also be used as immiscible polar phases with water;
    4. ionic liquids are nonvolatile, hence they may be used in high-vacuum systems and eliminate many containment problems. They do not evaporate!

    One of the most distinct advantages of RTILs that has been the rationale for their characterization as “Green Solvents” is their negligible volatility. This characteristic renters them promising replacements to volatile organic compounds (VOCs), which are used in large quantities in chemical and engineering industries and are a source of major environmental problems.5 Moreover, many ILs can be recycled and reused repeatedly.


Polarity 3

Solvent polarity is the most commonly used solvent classification. Even when considering molecular solvents it is poorly understood and often confused. Terms such as polar, apolar, and nonpolar are used indiscriminately to apply to values of dielectric constants, dipole moments, and polarizabilities, even though none of these are directly correlated in a simple way.
The simplest qualitative definition is that a polar solvent is one that will dissolve and stabilize dipolar or charged solutes. It is widely thought, though yet to be generally demonstrated, that under this definition, ionic liquids will be highly polar solvents.

Liquid-crystalline properties 10

Long-chain IL salts have attracted some interest due to their liquid crystalline (LC) properties. The origin for these can be found in the formation of domains, "Coulombic" layers where the ionic head-groups interact with the counterions, and "van der Waals" layers built from (anti)parallel stacking of the alkyl chains.
Hexafluorophosphate salts with cations up to C20MIM have been investigated by differential thermal analysis (DTA) and show one or more LC transitions. Melting to isotropic liquids occurs at rather high temperatures ( > 100 °C).


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