Gas-expanded liquids

CXLs have been used in a variety of roles including separations, particle and polymer processing and catalytic reaction media. They offer several advantages over conventional reaction media (Table 9.1).  

Ease of removal of the CO2 Enhanced solubility of reagent gases Fire suppression capability of CO2 

Milder process pressures (tens of bars) compared to SCCO2 (typically 100 bar) 

Higher gas miscibility compared to ambient condition organic solvents 

Enhanced transport rates due to the properties of dense CO2 Between one and two orders of magnitude greater rates than in neat organic solvents or SCCO2 Substantial replacement of organic solvents with benign dense phase CO2 

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Precipitation/Crystallization to Produce Nano- and Microparticles Because fluids such as C02are weak solvents for many solutes, they are often effective antisolvents in fractionation and precipitation. In general, a fluid antisolvent may be a compressed gas, a gas-expanded liquid, or a SCF. Typically a liquid solution is sprayed through a nozzle into CO2 to precipitate a solute. As CO2 mixes with the liquid phase, it... 

Gas-expanded liquids (GXLs) are emerging solvents for environmentally benign reactive separation (Eckert et al., op. cit.). GXLs, obtained by mixing supercritical CO2 with normal liquids, show intermediate properties between normal liquids and SCFs both in solvation power and in transport properties and these properties are highly tunable by simple pressure variations. Applications include chemical reactions with improved transport, catalyst recycling, and product separation. 

Probing the cybotactic region in gas-expanded liquids (GXLs). 

Anand, M., Odom, L.A. and Roberts, C.B. (2007) Finely controlled size-selective precipitation and separation of CdSe/ZnS semiconductor nanocrystals using C02 gas-expanded liquids. Langmuir, 23 (13), 7338-7343. 

Saunders, S.R. and Roberts, C.B. (2011) Tuning the precipitation and fractionation of nanoparticles in gas-expanded liquid mixtures. Journal of Physical Chemistry C., 115 (2), 9984-9992. 

Hurst, K.M., Roberts, C. and Ashurst, W. (2009) A gas-expanded liquid nanoparticle deposition technique for reducing the adhesion of silicon microstructures. Nanotechnology, 20 (18), 185303. 

Hart, A.E. and Kitchens, C.L. (2011) Reverse micelle synthesis of monodispersed metallic nanopartides via a gas expanded liquid system. Presented at the AIChE 2011 Annual Meeting, Minneapolis, Minnesota, United States. 

Recently the term, gas-expanded liquids (GXLs) [6-8] has been used to describe these unique mixtures while others use the term subcritical mixtures to describe the phase of matter. No matter what term is used to describe these mixtures one point should be clear all of these mixtures are liquids not supercritical fluids. Eurthermore, there is no discontinuity observed in moving from the supercritical condition to a liquid. However, EEL mixtures and supercritical fluids are two different phases of matter. 

Enhanced oil recovery (EOR) using carbon dioxide expansion is the largest scale application of gas expanded liquids. EOR using carbon dioxide aids in the flushing out of oil reservoirs carbon dioxide is injected into the well and displaces the remaining oil. It has several advantages over water, which can also be used in this process. For example, it lowers the viscosity of the crude oil, it... 

Figure 9.3 Preparation of particles using gas expanded liquids.

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. 

Chang CJ, Randolph AD. Solvent expansion and solute solubility predictions in GAS-expanded liquids. R D notes. AIChE J 1990 36(6) 939-942. 

A. M. Scurto, K. Hutchenson and B. Subramaniam, ACS Symposium Series In Gas-Expanded Liquids and Near-Critical Media, 2009, 1006, 3. 

In closing, we would like to mention some applications of the GEMC/CBMC approach and very much related combination of CBMC and the grand canonical Monte Carlo technique to other complex systems prediction of structure and transfer free energies into dry and water-saturated 1-octanol [72], prediction of the solubility of polymers in supercritical carbon dioxide [73], prediction of the upper critical solution pressure for gas-expanded liquids [74], investigation of the formation of multiple hydrates for a pharmaceutical compound [75], exploration of multicomponent vapor-to-particle nucleation pathways [76], and investigations of the adsorption of articulated molecules in zeolites and metal organic frameworks [77, 78]. 

Subramaniam, B. (2010). Gas-expanded liquids for sustainable catalysis and novel materials Recent advances, Coordin. Chem. Rev., 254, pp. 1843-1853. 

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