Some are toxic others like imidazolium based ionic liquids are synthesized from

Some are toxic others like imidazolium based ionic

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could not be applied to all ionic liquids. Some are toxic, others, like imidazolium-based ionic liquids, are synthesized from non-renewable petroleum, and little is known what happens if a spill should occur into the environment. The term green solvent is applicable to some ionic liquids, but not to all of them. 1.1.5 Physicochemical properties Compared to most fields of chemistry, the study and synthesis of ionic liquids is still very new, and as such, our knowledge of the properties of these systems are incomplete. The knowledge base is, however, growing at a fast rate as more and more ionic liquids are synthesized and investigated. One of the more interesting aspects of ionic liquids is the wide range of properties they exhibit. With few exceptions, there are, however, a few properties that they all share. The ionic liquids that are currently being investigated are almost without exception so called room temperature ionic liquids. Ionic liquids remain liquid in a temperature range that is broader than that of other liquids. Most 1-alkyl-3-methyl-imidazolium ionic liquids have a glass transition temperature between -10 C and -60 C and are chemically stable up to between 250 C and 400 C depending on the alkyl chain length and the choice of anion. They thus have a liquid range of over 300 C whereas water has a liquid range of 100 C. Ionic liquids generally form glasses when below their melting point and also show a strong tendency for supercooling [ 17 ]. The high temperatures needed for thermal degradation along with negligible vapor pressure makes ionic liquids ideal for reactions at high temperatures as they do not boil or evaporate at increased temperatures. When looking at the melting point of ionic liquids one can see some trends. The melting point of ionic liquids tends to decrease with increasing alkyl chain length and increase with increasing degree of symmetry. The heat capacity for some commonly used imidazolium based ionic liquids were reported in 2003 [ 18 ] and range from 1.17 to 1.80 J/ K at 100 C which is comparable to most liquids except water which has a heat capacity of 4.18 J/ K at 100 C. Wilkes et al. [ 19 ] have also shown that ionic liquids could be superior to currently used liquids for heat transfer in that they have negligible vapor pressure and are temperature stable over a wider range of temperatures and thus are able to store large amounts of heat. They suggested the use of ionic liquids as heat transfer fluids in large scale solar energy collectors. The lack of vapor pressure for most ionic liquids is a result of the strong Columbic forces between the anions and cations of the liquid. It has however been shown that when operating at high temperature and low pressure some ionic liquids are distillable [ 12 ]. Ionic liquids are generally more viscous then organic solvents, sometimes by several orders of magnitude. The viscosity is highly correlated to the size of the ions of the ionic liquid, mainly the cation as those tend to be larger. The viscosity of an ionic liquid is also highly dependent on the temperature. A temperature increase from 20 C to 25 C for BMIM PF 6 results in a 30% decrease in viscosity.
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