Switchable solvents

A switchable solvent is one that can be made to change important physical properties, such as polarity or hydrophobicity, under the influence of an external factor, which may be a change of temperature or pressure or the addition or removal of a 

The other example that Pollett et al describe is a class of molecular liquids (hat can be converted to ionic liquids by reaction with CO. The reactions are of the general form of Equations 8.6 (one component) or 8.7 (two components). 

The R groups need to be fairly bulky, for example, R=3-trialkylsilylpropyl or 3-trialkoxysilylpropyl in one example cited. The nitrogenous compound in the two-component example may by heterocyclic, for example, l,8-diazabicyclo[5.4.0] undec-7-ene (DBU, 3). 

They describe a Heck reaction of bromobenzene with styrene to give stilbene, carried out in the ionic form of DBU -1- hexanol -1- CO. The product E-stilbene 

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A significant advantage that these switchable solvents have over many other media is that they can be tailor-made for a particular process and particular properties can be turned on and off as desired. Unfortunately, this means that in most cases they will be considerably more expensive than simple alternatives such as water. Often the switch in these systems is the introduction of a gas such as carbon dioxide, and although the pressures involved are typically lower than those used for supercritical conditions, many users would still be wary about using and containing these gases. Further information on the switching mechanism for several cases is provided below. 

The switching of a switchable solvent (a) Reversible protonation of 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) in the presence of an alcohol and carbon dioxide, (b) Polarity switching in reaction (a), (c) Miscibility of decane with the alcohol-DBU mixture (non-polar) under nitrogen and separation of decane from the ionic liquid (polar) under carbon dioxide. [Reprinted with permission from Nature 2005, 436, 1102. Copyright 2005 Nature Publishing Group.]... 

Jessop and co-workers used the same technology as developed for switchable solvents to obtain switchable surfactants (Scheme 9.1). These have many possible applications, and their use would reduce waste and solvent usage (Table 9.4). 

Clearly this is the least mature field within the solvent alternatives arena however, this also means that, as with tailor-made ionic liquids, it is likely that tailor-made switchable solvent systems will continue to advance and become an increasingly important area of research during the coming decades. As with all areas of clean technology, synergies and overlaps with other areas of sustainable development will increase and lead to new advances. For example, in the area of gas expanded liquids, the focus has so far been on petroleum-sourced VOCs and therefore significant advances could be made by investigating other types of gas expanded media, whether they be renewably sourced VOCs or non-volatile alternatives. 

Possibly the least explored and newest options available to the green chemist are liquid polymer solvents (Chapter 8) and switchable and tunable solvents (Chapter 9). Unreactive low molecular weight polymers or those with low glass transition temperatures can be used as non-volatile solvents. In particular, poly(ethyleneglycols) and poly(propyleneglycols) have been used recently in a range of applications. Probably the most important recent additions to our toolbox are switchable solvents. New molecular solvents have been discovered that can be switched from non-volatile to volatile or between polar and nonpolar environments by the application of an external stimulus. Gas-expanded liquids will also be discussed in Chapter 9, as carbon dioxide can be used as a solubility switch and to reduce the environmental burden of conventional solvents. 

Solvents that can defeat Murphy s Law of Solvents have been identified and are called switchable solvents these are liquids that have one set of properties (that presumably meet the needs of one process step) and that can be reversibly switched to another set of properties to meet the needs of the next step. As an example of the flexibility offered by switchable solvents, consider the copolymerization of CO2 and cyclohexene epoxide catalyzed by a Cr(salen)... 

Philip Jessop Queen s University Carbon dioxide triggered switchable solvents and surfactants... 

The recovery of the ionic liquid l-ethyl-3-methylimidazolium acetate (abbreviated as [C2mim][OAc]) with a solvent mixture containing acetone, 2-propanol, and a small amount of water was demonstrated. This formed a phase switchable solvent system, which could concentrate and separate oleophilic solutes, short chain carbohydrates, and lignin fragments from used ionic liquid in a two-stage extraction procedure. Figure 7.5 shows the whole process of ionic liquid recovery and biomass separation. The research was conducted by pre-treating 100 g of corn stover with 10 wt.% of ionic liquid. After the separation process, the recovered ionic liquid contained few residual solutes, so it could be reused for biomass dissolution and... 

Jessop PG, Mercer SM, Eldebrant DJ (2012) C02-triggered switchable solvents, surfactants, and other materials. Energy Environ Sci 5 7240-7253... 

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