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J. of Supercritical Fluids 43 (2007) 150180

Review article

A review of ionic liquids towards supercritical fluid applications

Seda Keskin, Defne Kayrak-Talay, Ugur Akman , Oner HortacsuDepartment of Chemical Engineering, Bogazici University, Bebek 34342, Istanbul, Turkey

Received 2 August 2006; received in revised form 8 May 2007; accepted 29 May 2007

bstract

Ionic liquids (ILs), considered to be a relatively recent magical chemical due their unique properties, have a large variety of applications inll areas of the chemical industries. The areas of application include electrolyte in batteries, lubricants, plasticizers, solvents and catalysis inynthesis, matrices for mass spectroscopy, solvents to manufacture nano-materials, extraction, gas absorption agents, etc. Non-volatility and non-ammability are their common characteristics giving them an advantageous edge in various applications. This common advantage, when consideredith the possibility of tuning the chemical and physical properties of ILs by changing anioncation combination is a great opportunity to obtain

ask-specific ILs for a multitude of specific applications. There are numerous studies in the related literature concerning the unique properties,reparation methods, and different applications of ILs in the literature. In this review, a general description of ILs and historical background areiven; basic properties of ILs such as solvent properties, polarity, toxicology, air and moisture stability are discussed; structure of ILs, cation, anionypes and synthesis methods in the related literature are briefly summarized. However, the main focus of this paper is how ILs may be used in thehemicals processing industries. Thus, the main application areas are searched and the basic applications such as solvent replacement, purificationf gases, homogenous and heterogeneous catalysis, biological reactions media and removal of metal ions are discussed in detail. Not only thedvantages of ILs but also the essential challenges and potentials for using ILs in the chemical industries are also addressed. ILs have becomehe partner of scCO2 in many applications and most of the reported studies in the literature focus on the interaction of these two green solvents,.e. ILs and scCO . The chemistry of the ILs has been reviewed in numerous papers earlier. Therefore, the major purpose of this review paper is

2o provide an overview for the specific chemical and physical properties of ILs and to investigate ILscCO2 systems in some detail. Recovery ofolutes from ILs with CO2, separation of ILs from organic solvents by CO2, high-pressure phase behavior of ILscCO2 systems, solubility of ILsn CO2 phase, and the interaction of the ILscCO2 system at molecular level are also included.

2007 Elsevier B.V. All rights reserved.

eywords: Ionic liquids; Supercritical carbon dioxide; Review

ontents

1. General description of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512. History of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1513. Basic properties of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

3.1. Solvent properties of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1533.2. Polarity of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1543.3. Toxicology of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1543.4. Air and moisture stability of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

4. Structure and synthesis of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1554.1. Anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

4.2. Cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.3. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Major applications suggested for ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1. Solvent replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. Tel.: +90 212 3596867; fax: +90 212 2872460.E-mail address: akman@boun.edu.tr (U. Akman).

896-8446/$ see front matter 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2007.05.013

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

mailto:akman@boun.edu.trdx.doi.org/10.1016/j.supflu.2007.05.013

S. Keskin et al. / J. of Supercritical Fluids 43 (2007) 150180 151

5.2. Purification of gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1575.3. Homogenous and heterogeneous catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1585.4. Biological reactions media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1595.5. Removing of metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

6. Challenges of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1607. ILs and scCO2 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

7.1. High-pressure phase behavior of ILCO2 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1637.1.1. The [bmim][PF6]CO2 system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1637.1.2. Other ILCO2 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

7.2. IL solubility in CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1677.3. ILCO2 interaction at the molecular level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1687.4. Solute recovery from ILs with scCO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1687.5. Other applications of ILscCO2 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. General description of ILs

Ionic liquids (ILs) have been accepted as a new green chemi-al revolution which excited both the academia and the chemicalndustries. This new chemical group can reduce the use ofazardous and polluting organic solvents due to their uniqueharacteristics as well as taking part in various new syntheses.he terms room temperature ionic liquid (RTIL), nonaqueous

onic liquid, molten salt, liquid organic salt and fused salt havell been used to describe these salts in the liquid phase [1]. ILsre known as salts that are liquid at room temperature in con-rast to high-temperature molten salts. They have a unique arrayf physico-chemical properties which make them suitable inumerous applications in which conventional organic solventsre not sufficiently effective or not applicable. Short [2] pointedut in 1980, that there were only a few patent applications forLs, in 2000, the number of patent applications increased to 100,nd finally by 2004, there were more than 800. This is a clearndication of the high affinity of the academia and industry tohe ILs.

. History of ILs

ILs have been known for a long time, but their extensive uses solvents in chemical processes for synthesis and catalysis hasecently become significant. Welton [1] reported that ILs areot new, and some of the ILs such as [EtNH3][NO3] was firstescribed in 1914 [3]. The earliest IL in the literature was createdntentionally in 1970s for nuclear warheads batteries [4]. During940s, aluminum chloride-based molten salts were utilized forlectroplating at temperatures of hundreds of degrees Celsius.n the early 1970s, Wilkes tried to develop better batteries foruclear warheads and space probes which required molten saltso operate [4]. These molten salts were hot enough to damagehe nearby materials. Therefore, the chemists searched for salts

hich remain liquid at lower temperatures and eventually they

dentified one which is liquid at room temperature. Wilkes andis colleagues continued to improve their ILs for use as batterylectrolytes and then a small community of researchers began

scnr

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

o make ILs and test their properties [5,6]. In the late 1990s, ILsecame one of the most promising chemicals as solvents.

The first ILs, such as organo-aluminate ILs, have lim-ted range of applications because they were unstableo air and water. Furthermore, these ILs were not inertowards various organic compounds [7]. After the firsteports on the synthesis and applications of air stableLs such as 1-n-butyl-3-methlyimidazolium tetrafluoroborate[bmim][BF4]) and 1-n-butyl-3-methlyimidazolium hexafluo-ophosphate ([bmim][PF6]), the number of air and water stableLs has started to increase rapidly [7]. Recently, researchers haveiscovered that ILs are more than just green solvents and theyave found several applications such as replacing them witholatile organic solvents, making new materials, conducting heatffectively, supporting enzyme-catalyzed reactions, hosting aariety of catalysts, purification of gases, homogenous and het-rogeneous catalysis, biological reactions media and removal ofetal ions [4].Some of the basic physical properties of ILs such as density

nd viscosity are still being evaluated by the researchers sincehe study of the IL is a relatively young field [8]. The numberf research on ILs and their specific applications is increasingapidly in the literature. For example, the cation 1-n-ethyl-3-ethylimidazolium has been the most widely studied until 2001,

nd nowadays, 1-3-dialkyl imidazolium salts are the most popu-arly used and investigated class of ILs. For the future of ILs, theim of research is the commercialization of ILs in order to usehem as solvents, reagents, catalysts and materials in large-scalehemical applications.

. Basic properties of ILs

ILs are made of positively and negatively charged ions,hereas water and organic solvents, such as toluene andichloromethane, are made of molecules. The structure of ILs is

imilar to the table salt such as sodium chloride which containsrystals made of positive sodium ions and negative chlorine ions,ot molecules. While, salts do not melt below 800 C, most of ILsemain liquid at room temperature. The melting points of sodium

1 ritical Fluids 43 (2007) 150180

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52 S. Keskin et al. / J. of Superc

hloride and lithium chloride are known as 801 and 614 C,espectively. Since these conventional molten salts exhibit highelting points, their use as solvents in applications is severely

imited. However, RTILs are liquid generally up to 200 C. ILsave a wide liquidus ranges. The adopted upper melting tem-erature limit for the classification as IL is known as 100 Cnd higher melting ion systems are generally referred as moltenalts.

Researchers explained that ILs remain liquid at room tem-erature due to the reason that their ions do not pack well9]. Combination of bulky and asymmetrical cations and evenlyhaped anions form a regular structure namely a liquid phase.he low melting points of ILs are a result of the chemical com-osition. The combination of larger asymmetric organic cationnd smaller inorganic counterparts lower the lattice energy andence the melting point of the resulting ionic medium. In someases, even the anions are relatively large and play a role in low-ring the melting point [10]. Most widely used ILs and theirtructures are given in Table 1.

As solvents, ILs posses several advantages over conventionalrganic solvents, which make them environmentally compatible1,4,8,1015]:

ILs have the ability to dissolve many different organic, inor-ganic and organometallic materials.ILs are highly polar.ILs consist of loosely coordinating bulky ions.ILs do not evaporate since they have very low vapor pressures.ILs are thermally stable, approximately up to 300 C.Most of ILs have a liquid window of up to 200 C whichenables wide kinetic control.ILs have high thermal conductivity and a large electrochem-ical window.ILs are immiscible with many organic solvents.ILs are nonaqueous polar alternatives for phase transfer pro-cesses.The solvent properties of ILs can be tuned for a specificapplication by varying the anion cation combinations.

Generally, the above statements are valid for the most com-only used ILs. However, one should note that there are many

Ls containing different anions and cations and their propertiesover a vast range. Therefore, the above statements should note generalized for all existing ILs and for those designed in theuture.

ILs exhibit the ability to dissolve a wide variety of materialsncluding salts, fats, proteins, amino acids, surfactants, sugarsnd polysaccharides. ILs have very powerful solvent propertiesuch that they can dissolve a wide range of organic molecules,ncluding crude oil, inks, plastics, and even DNA [9].

Two important groups of ILs are those based on imidazoliumnd pyridinium cations with PF6 and BF4 anions [13,14].igs. 1 and 2 illustrate the imidazolium and pyridinium deriva-

ives of ILs and their possible anions which are extensivelynvestigated in literature.

ILs tend not to give off vapors in contrast to traditionalrganic solvents such as benzene, acetone, and toluene. The

Fig. 1. Imidazolium derivatives of ILs (www.sigmaaldrich.com).

apor pressures of the ILs are extremely low and are considereds negligible. For example, Kabo et al. [16] gave the vapor pres-ure of [bmim][PF6] at 298.15 K as 1011 Pa. ILs are introduceds green solvents because unlike the volatile organic compoundsVOCs) they replace, many of these compounds have negligibleapor pressure, they are not explosive and it may be feasibleo recycle and repeatedly reuse them. It is more convenient toork with ILs in the laboratory since the non-evaporating prop-

rties of ILs eliminate the hazardous exposure and air pollutionroblems.

ILs are also known as designer solvents since they give thepportunity to tune their specific properties for a particular need.he researchers can design a specific IL by choosing negativelyharged small anions and positively charged large cations, andhese specific ILs may be utilized to dissolve a certain chemicalr to extract a certain material from a solution. The fine-tuningf the structure provides tailor-designed properties to satisfy thepecific application requirements. The physical and chemicalroperties of ILs are varied by changing the alkyl chain lengthn the cation and the anion. For example, Huddleston et al. [17]oncluded that density of ILs increases with a decrease in thelkyl chain length on the cation and an increase in the moleculareight of the anion.Although ILs are studied by a great number of research

roups, there are still many questions that scientist are notble to answer. For example, one of the basic rules of chem-stry like dissolves like is seem to be broken by some ILs:onpolar benzene is up to 50% soluble (by volume) in polar

etrachloroaluminate-based ILs [9]. Therefore, studies on whyLs are able to dissolve uncharged covalent molecules are con-inuing.

Until recently, ILs have been considered to be scarce but its now known that many salts form liquids at or close to roomemperature. There are literally billions of different structureshat may form an IL. The composition and the specific prop-rties of these liquids depend on the type of cation and anionn the IL structure. By combining various kinds of cation andnion structures, it is estimated that 1018 ILs can be designed

Fig. 2. Pyridinium derivatives of ILs (www.sigmaaldrich.com).

http://www.sigmaaldrich.com/http://www.sigmaaldrich.com/

S. Keskin et al. / J. of Supercritical Fluids 43 (2007) 150180 153

Table 1Most widely used ILs, their structures and short names

Ionic liquid Structure Short name

1-Butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4]

1-Butyl-3-methylimidazolium triflate [bmim][TfO]

1-Butyl-3-methylimidazolium methide [bmim][methide]

1-Butyl-3-methylimidazolium dicyanamide [bmim][DCA]

1-Butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6]

1-Butyl-3-methylimidazolium nitrate [bmim][NO3]

1-Butyl-3-methylimidazolium bis(trifluoromethylsulfony1) imide [bmim][Tf2N]

l-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide; R = C6H17 [hmim][Tf2N]

l-Octyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide; R = C8H17 [omim][Tf2N]

2,3-Dimethyl-1-hexylimidazolium bis(trifluoromethylsulfonyl) imide [hmmim][Tf2N]

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eprinted with permission from [121]. Copyright 2004 American Chemical So

.1. Solvent properties of ILs

Both the chemical industry and academia search for alter-ative solvents to meet the cleaner technology requirements

sioa

ince the most widely used solvents are volatile and damag-ng. ILs are good solvents for a wide range of substances;rganic, inorganic, organometallic compounds, bio-moleculesnd metal ions. They are usually composed of poorly coordinat-

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54 S. Keskin et al. / J. of Superc

ng ions which makes them highly polar but non-coordinatingolvents. ILs are immiscible with most of the organic sol-ents, thus they provide a nonaqueous, polar alternative forwo-phase systems [19]. Furthermore, ILs which are not mis-ible with water can be used as immiscible polar phasesith water. Although all other conventional solvents evap-rate to the atmosphere, ILs do not evaporate and theironvolatility gives an opportunity to utilize them in high-acuum systems. The negligible volatility is the basic propertyhich characterizes them as green solvents. Considering poten-

ial as solvents, ILs can easily replace other conventionalrganic solvents which are used in large quantities in chem-cals processing industries to eliminate major environmentalroblems.

Many chemical reactions are carried out in conventional sol-ents. Upon the completion of reaction, chemical products muste taken out of the solvent. There are several techniques toecover a product from a solvent: For example, water-solubleompounds may be extracted with water; distillation may besed for chemicals with high vapor pressures. On the otherand, for the chemicals with low vapor pressures, distillationust be performed at low pressures, which may not be eco-

omical. In addition to this, there are some chemicals that canecompose as a result of heating, such as pharmaceuticals.herefore, ILs seem to be potentially good solvents for manyhemical reactions in the cases where distillation is not practi-al, or water insoluble or thermally sensitive products are theomponents of a chemical reaction. Although, ILs are not con-idered to be distilled due to their low volatility, Earle et al.20] showed that many ionic liquids, especially bistriflamideLs, can be distilled at 200300 C and low pressure withoutecomposition. It was once more understood that there is aong way for total investigation of the properties of ILs. Theuthors suggested that the possibility of IL distillation intro-uced a new method for IL purification, and also new applicationreas (such as isolation of highly soluble products by high-emperature crystallization) could emerge. But, distillation stillannot be applied when heat-labile products are encounteredn ILs.

In most chemical applications, extraction is used for sep-ration since it is an energy efficient technique. Generally,xtraction consists of two immiscible phases such as an organichase and an aqueous phase. Many organic solvents usedn extractions are known with their flammable and toxicalroperties. In order to improve the safety and environmen-al friendliness of this conventional technique, ILs may besed as ideal substitutes due to their stability, nonvolatility anddjustable miscibility and polarity [15].

The solvent properties of ILs are mainly determined by thebility of the salt to act as a hydrogen bond donor and/orcceptor and the degree of localization of the charges on thenions [1,21]. Charge distribution on the anions, H-bondingbility, polarity, dispersive interactions are the main factors

hat influence the physical properties of ILs [22]. For exam-le, imidazolium-based ILs are highly ordered hydrogen-bondedolvents and they have strong effects on chemical reactions androcesses.

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l Fluids 43 (2007) 150180

.2. Polarity of ILs

Polarity of chemicals is commonly used to classify the sol-ents. The terms used as polar, nonpolar and apolar are generallyelated to the values of dielectric constants, dipole moments,olarizabilities. If a solvents has the ability to dissolve and sta-ilize dipolar or charged solutes, it is defined as a polar solvent.nder this simple definition, ILs are highly polar solvents, but

t is not completely true to make such strict conclusions sincehere ILs can be designed in a vast range.

The existences of polar and nonpolar domains, believed toe associated with the unique amphiphilic solvent proper-ies of ILs, are found in the structures of PF6 and BF4alts [23]. Since polarity is the simplest indicator of solventtrength, researchers compared polarities of ILs and conven-ional solvents: Carmichael and Seddon [24] showed that-alkyl-3-methylimidazolium ILs with anions [PF6], [BF4],(CF3SO2)2N], and [NO3] are in the same polarity region as-aminoethanol and lower than alcohols such as methanol,thanol and butanol. Aki et al. [25] indicated that [bmim][PF6],C8mim][PF6], [bmim][NO3] and [N-bupy][BF4] are more polarhan acetonitrile and less polar than methanol and these ILs arexpected to be at least partially miscible with water. ILs basedn [PF6] anion is preferred as solvents in most extraction appli-ation to form biphasic systems due to their immiscibility withater.

.3. Toxicology of ILs

The green character of ILs has been usually related with theiregligible vapor pressure; however their toxicology data haveeen very limited until now. Several authors [2629] alreadyentioned this lack of toxicological data in the literature [30].lthough ILs will not evaporate and thus will not cause air pollu-

ion, it does not mean that they will not harm the environment ifhey enter. Most of ILs are water soluble and they may enter thequatic environment by accidental spills or effluents. The mostommonly used ILs [bmim][PF6] and [bmim][BF4] are knowno decompose in the presence of water and as a result hydroflu-ric and phosphoric acids are formed [31]. Therefore, bothoxicity and ecotoxicity information which provide metabolismnd degradability of ILs are also required to label them as greenolvents or investigate their environmental impact.

The ecotoxicological studies performed to understand theffects of different ILs on enzymatic activities, cells and microor-anisms are utilized to obtain LC50 levels (lethal concentration).ecreasing LC50 values indicate higher toxicities according to

he toxicity classes of Hodge and Sterner scale (1956) [32]. Thiscale indicates that the LC50 value (in terms of mg/L) of 10r less shows that the chemical is extremely toxic, LC50 valueetween 10 and 100 shows that chemical is highly toxic, LC50alue between 100 and 1000 shows that chemical is slightlyoxic, and finally LC50 value between 1000 and 10,000 means

hat chemical is practically nontoxic.

The impact of ILs on aquatic ecosystems is highly importantince some of ILs have a high solubility in water. Maginn [33]rovided the LC50 levels for two imidazolium-based ILs with

S. Keskin et al. / J. of Supercritica

Table 2LC50 values for certain solvent [33]

Compound LC50 (mg/L)

[bmim][PF6] 250300[bmim][BF4] 225275Acetone 30,642Dichloromethane 310Toluene 60313Benzene 203Chlorobenzene 586PAC

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aphnia magna, common fresh water crustaceans. Due to theeason that D. magna are filter feeders at the base of the aquaticood chain, their responses to ILs are essential to understand howhese new solvents may impact an environmental ecosystem. AsLs, 1-n-butyl 3-methylimidazolium cation with PF6 and BF6nions are used and the results are tabulated in Table 2: Thesewo ILs are as toxic to Daphnia as benzene and even far moreoxic than acetone, but much less toxic than ammonia, chlorine,henol, etc. Wells and Coombe [34] also provided the resultsf freshwater ecotoxicity tests of some common ILs with imi-azolium, ammonium, phosphonium and pyridinium cations onnvertebrate D. magna and the green alga Pseudokirchneriellaubcapitata (formerly known as Selenastrumcapricornutum).he results were reported using medium effective concentra-

ion (EC50) values. The toxicity values of the most toxic ILere four orders of magnitude more than the least toxic IL.here was a relation between the order of toxicity and alkylide chain length of the cation. For alkyl methylimidazoliumLs with C4 side chain constituents showed moderate toxicity,hereas the C12, C16, and C18 species were very highly toxic

o both organisms under investigation. Pyridinium, phospho-ium, and ammonium species with C4 side chain constituentsad also only moderate toxicity, whereas C6 and longer sidehains showed significant increases in toxicity. It was shownhat the least toxic ionic liquids ecotoxicity were comparable toydrocarbons such as toluene and xylene. The most toxic ioniciquids are many orders of magnitude more acutely ecotoxichan organic solvents such as methanol, tert-butyl methyl ether,cetonitrile, and dichloromethane. The authors also emphasizedhat simple acute ecotoxicity measurements did not enough toully characterize the full impact of a solvent released to thenvironment but were only part of the environmental impactssessment.

There were also some studies on investigation of toxicity ofLs performed on animals such as the nematode model organ-sm (Caenorhabditis elegans) [35], freshwater pulmonate snailsPhysa acuta) [36], Fischer 344 rats [37] and zebra fish (Danioerio) [38].

One of the most important points that must be taken intoccount during the toxicological study of the ILs is to pay atten-

ion to the purity of the IL studied. Therefore, different authors35,39] attached significant importance to proper analyzing tech-iques [30]. ILs are introduced under the concept of greenhemistry in all research papers due to their nonvolatile nature.

cotd

l Fluids 43 (2007) 150180 155

he environmental persistence [34,40] of commonly used ILs,long with their possible toxicity should be taken into account.here are still very few results about the (eco)toxicological effectf ILs and they can be evaluated more satisfactorily as greenolvents or not after more data on the subject will be provided.he possible toxic and non-biodegradable nature of the existing

Ls also led to the development of new types of nontoxic andiodegradable ILs [4046].

.4. Air and moisture stability of ILs

The stability of ILs is crucial for optimum performance. Manyf ILs are both air and moisture stable, some are even hydropho-ic. On the other hand, most imidazolium and ammonium saltsre hydrophilic and if they are used in open vessels, hydrationill certainly occur. The hydrophobicity of an IL increases with

ncreasing length of the alkyl chain [25]. Despite their widepread usage, ILs containing PF6 and BF4 have been reportedo decompose in the presence of water, giving off HF. Wasser-cheid et al. [47] pointed out that ILs containing halogen anionsenerally show poor stability in water, and also give off toxic andorrosive species such as HF or HCl. Therefore, they suggest these of halogen-free and relatively hydrolysis-stable anions suchs octylsulfate-compounds.

The degree to which this hydration is a problem dependsn the application. For instance, small amounts of highly reac-ive species which are used as catalysts may be deactivated byven very small amounts of water. For this kind of application,Ls must be handled under an inert atmosphere. Moreover, theolutes used may be sensitive for air or moisture, thus an inerttmosphere is required for the ILsolute systems.

The interaction between water and ILs and their degree ofydroscopic character are strongly dependent on anions. Themount of absorbed water is highest in the BF4 and lowest inF6 [48]. However, Tf2N is much more stable in the pres-nce of water as well as having the advantage of an increasedydrophobic character.

ILs immiscible with water tend to absorb water from thetmosphere. The infra-red (IR) studies of Cammarata et al. [31]emonstrated that the water molecules absorbed from the airre mostly present in the free state, bonded via H-bonding withhe PF6 and BF4 anions. The presence of water may haveramatic effect on IL reactivity. Since water is present in all ILs,hey are usually utilized after a moderate drying process.

The new ILs synthesized are more stable than the oldalogenoaluminate systems. Certain ILs incorporating 1-3-ialkyl imidazolium cations are generally more resistant thanraditional solvents under harsh process conditions, such as thoseccurring in oxidation, photolysis and radiation processes [10].

. Structure and synthesis of ILs

There are a great number of different cation and anion

ombinations to synthesize IL. Different types of ILs give anpportunity to modify the physical and chemical properties ofhe IL. The most widely used cations are imidazolium, pyri-inium, phosphonium and ammonium. The properties of ILs

156 S. Keskin et al. / J. of Supercritica

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ig. 3. Most commonly used cation structures and possible anion types [50].

re determined by mutual fit of cation and anion, size, geom-try, and charge distribution. Among the similar class of salts,mall changes in types of ions influence the physico-chemicalroperties. The overall properties of ILs result from the com-osite properties of the cations and anions and include thosehat are superacidic, basic, hydrophilic, water miscible, watermmiscible and hydrophobic. Usually, the anion controls theater miscibility, but the cation also has an influence on theydrophobicity or hydrogen bonding ability [49]. The structuresf most commonly used cations and some possible anion typesre tabulated in Fig. 3 [50].

.1. Anions

The properties of ILs are determined by the anion type. Thentroduction of different anions results in an increasing num-er of alternative ILs with various properties. There are twoifferent types of IL anions: ILs containing fluorous anionsuch as PF6, BF4, CF3SO3, (CF3SO3)2N and ILs withon-fluorous anions such as AlCl4.

In designing ILs, fluorous anions are usually used. Theost popular anions consist of chloride, nitrate, acetate, hex-

fluorophosphate and tetrafluoroborate [13]. The most widelynvestigated ILs are the ones with anions PF6 and BF4. Espe-ially PF6 is the most prominent anion used in IL research.ince the anion chemistry has a large effect on the propertiesf IL, although the cations are the same, there are significantifferences between ILs with different anions. For example ILith 1-n-butyl-3-methylimidazolium cation and PF6 anion is

mmiscible with water, whereas IL with same cation and BF4nion is water soluble. This example represents the designerolvent property of ILs: different ion pairs determine physicalnd chemical properties of the liquid. By changing the anion theydrophobicity, viscosity, density and solvation of the IL systemay be changed [8].Although PF6 and BF4 are the two anion types that are

tilized in most of IL applications, they have an important dis-dvantage: these two anions may decompose when heated in theresence of water and liberate HF. After the researchers realizedhe production of HF in the presence of water, the bonding style

f anion was altered and fluorous anions inert to hydrolysis weresed. The fluorine of the anion is bonded to carbon and CF bondecomes inert to hydrolysis. In this way, ILs such as CF3SO3nd (CF3SO3)2N are produced [50].

5

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l Fluids 43 (2007) 150180

Fluorinated anions tend to be expensive and in response toost and safety concerns new ILs with non-fluorous ions haveeen introduced. In the synthesis of these ILs, anions are derivedrom inexpensive bulk chemicals. Alkylsulfate anions are theost popular non-fluorous anions due to their nontoxic and

iodegradable structures. The first commercially available IL forhich toxicology data are available contains alkylsulfate anion

methosulfate) [50].

.2. Cations

The cation of IL is generally a bulk organic structure withow symmetry. Most ILs are based on ammonium, sulfonium,hosphonium, imidazolium, pyridinium, picolinium, pyrroli-inium, thiazolium, oxazolium and pyrazolium cations. Theesearch mainly focuses on RTILs composed of asymmet-ic N,N-dialkylimidazolium cations associated with a varietyf anions. 1-n-butyl-3-methylimidazolium and 1-n-ethyl-3-ethylimidazolium are the most investigated structures of this

lass.Chiappe and Pieraccini [18] indicated that the melting points

f the most ILs are uncertain since ILs undergo considerableupercooling. Therefore, by examining the properties of a seriesf imidazolium cation based ILs, it has been concluded thats the size and asymmetry of the cation increases, the meltingoint decreases. Further, an increase in the branching on thelkyl chain increases the melting point. The melting point of ILss essential because it represents the lower limit of the liquiditynd with thermal stability it defines the interval of temperaturesithin which it is possible to use ILs as solvents [18].

.3. Synthesis

There are three basic methods to synthesize ILs: metathe-is reactions, acidbase neutralization, direct combination [1].

any alkylammonium halides are commercially available; theyan also be prepared simply by the metathesis reaction of theppropriate halogenoalkane and amine. Pyridinium and imida-olium halides are also synthesized by metathesis reaction. Onhe other hand, monoalkyllammonium nitrate salts are best pre-ared by the neutralization of aqueous solutions of the amineith nitric acid. After neutralization reactions, ILs are processednder vacuum to remove the excess water [1]. Tetraalkylammo-ium sulfonates are also prepared by mixing sufonic acid andetraalkylammonium hydroxide [51]. In order to obtain pure IL,roducts are dissolved in an organic solvent such as acetoni-rile and treated with activated carbon, and the organic solvents removed under vacuum. The final method for the synthesisf ILs is the direct combination of halide salt with a metalalide. Halogenoaluminate and chlorocuprate ILs are preparedy this method. The synthesis methods of ILs have been givenn numerous articles [5256].

. Major applications suggested for ILs

The research areas on ILs are growing very rapidly and theotential application areas of ILs are numerous. The unique

S. Keskin et al. / J. of Supercritica

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Fig. 4. Major application areas of ILs.

hemical and physical properties of ILs bring about severalpplication areas including reaction and synthesis media. Thepplication areas of ILs can be expressed as solvents for organic,rganometallic synthesis and catalysis; electrolytes in electro-hemistry, in fuel and solar cells; lubricants; stationary phasesor chromatography; matrices for mass spectrometry; supportsor the immobilization of enzymes; in separation technologies;s liquid crystals; templates for synthesis nano-materials andaterials for tissue preservation; in preparation of polymergel

atalytic membranes; in generation of high conductivity materi-ls [7]. Fig. 4 represents the major applications suggested for ILsnd these essential applications are discussed in detail below.

.1. Solvent replacement

A majority of common solvents have potential health hazardslthough they are extensively utilized. For example, approxi-ately half of 189 hazardous air pollutants regulated by Cleanir Act Amendment of U.S. (1990) are VOCs including solvents

uch as dichloromethane [57]. The VOCs are the workhorses ofndustrial chemistry in the pharmaceutical and petrochemicalreas. The use of VOCs by these industries can be assessedsing the Sheldon E-factor. This factor is responsible to mea-ure process by-products as a proportion of production on theass basis. Researchers investigated how VOC use is distributed

cross the chemical industry and found that the value of E-factors between 25 and 100 for pharmaceuticals industries with a pro-uction of 10 to 103 t/year although oil refining industries withproduction of 106 to 108 t/year have an E-factor of 0.1 [9]

adapted from [58]). These values suggest that pharmaceuticalsndustries use inefficient and dirty processes although on smallercale as compared to the oil refining industries. The oil and bulkhemicals industries which are commonly regarded as dirty arepparently remarkably waste conscious when the E-factors arenalyzed [9].

As the introduction of cleaner technologies has becomemajor concern throughout both industry and academia, the

earch for the alternative solvents has become a high priority.herefore, environmentally friendly ILs can easily replace theazardous VOCs in large scale to reduce E-factors.

ILs are able to dissolve a variety of solutes. They can be usednstead of traditional solvents in liquidliquid extractions whereydrophobic molecules such as simple benzene derivatives willartition to the IL phase. Huddleston et al. [17] showed that

bmim][PF6] could be used to extract aromatic compounds fromater. Fadeev and Meagher [59] demonstrated that two imida-

olium ILs with PF6 anion could be used for the extractionf butanol from aqueous fermentation broths. Selvan et al. [60]

saCi

l Fluids 43 (2007) 150180 157

sed ILs for the extraction of aromatics from aromatic/alkaneixtures, whereas Letcher et al. [61] used ILs for the extrac-

ion of alcohols from alcohol/alkane mixtures. Moreover, binaryemperaturecomposition curves of ILs with alcohols, alkanes,romatics and water; ternary temperaturecomposition curves ofLs with alcohols and water; solubilities of some organics andater in ILs are all investigated by various groups to completelyenefit from the solvent properties of ILs [6264].

Arce et al. [65] studied essential oil terpenless by extractionsing organic solvents or ILs. Citrus essential oil is simulateds a mixture of limonene and linalool and 2-butene-1,4-diolnd ethylene glycol are used as solvents. They choose 1-thyl-3-methylimidazolium methanesulfonate as the IL andiquidliquid equilibria data for the ternary systems are reported.rce et al. [65] concluded that IL presents the highest selectivityut close to the other organic solvents and they reported that theesults for solute distribution ratio depend on the concentrationange of extraction.

.2. Purification of gases

Reliable information on the solubility of gases in ILs iseeded for the design and operation of any possible processesnvolving IL. Processes using ILs to purify gas streams wereeveloped after solubilities of various gases in ILs were reportedy some researchers [6668]. These experimental studies showhat some gases, especially CO2 is highly soluble in ILs. The sim-lations performed explain that the anion of the IL is responsibleor high gas solubility. With this property ILs, can be replaceds solvents in reactions involving gaseous species.

Anthony et al. [69] investigated solubility of nine differentases up to 13 bar: carbon dioxide, ethylene, ethane, methane,rgon, oxygen, carbon monoxide, hydrogen, and nitrogen inbmim][PF6]. These gases were chosen for several reasons: CO2olubility is important due to the possibility of using scCO2 toxtract solutes from ILs; ethylene, hydrogen, carbon monoxide,nd oxygen are reactants in several types of reactions studiedn IL such as hydroformylations, hydrogenations, and oxida-ions. Due to the nonvolatile nature of IL, the gas solubilitiesn IL were measured using a gravimetric technique, usuallyith a microbalance. The study of Anthony et al. [69] showed

hat CO2 has the highest solubility and strongest interactionsith [bmim][PF6], followed by ethylene and ethane. Argon andxygen had very low solubilities and immeasurably weak inter-ctions. Fig. 5 demonstrates the solubility of various gases (CO2,2H4, C2H6, CH4, Ar, O2) in [bmim][PF6] at 25 C and at differ-nt pressures. Except for CO2, all gases remained in the Henrysaw regime up to 13 bar. However, CO2 showed some non-inearity, indicating some degree of saturation. Henrys constantsf these gases in various organic solvents and in [bmim][PF6]ere compared and the results showed that the gases that are

ess soluble in the IL are less soluble in the other solvents asell. However, CO2 is more soluble in the IL than in the other

olvents. The relatively high solubility of CO2 was explaineds a result of its large quadrapole moment. The solubility ofO2 in [bmim][PF6] at different temperatures is demonstrated

n Fig. 6.

158 S. Keskin et al. / J. of Supercritica

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ig. 5. Solubility of various gases in [bmim][PF6] at 25 C. (Reprinted withermission from [69]. Copyright 2002 American Chemical Society)

Camper et al. [66] measured the solubility of CO2 and2H4 in [bmim][PF6], [emim][Tf2N], [emim][CF3SO3] (ethyl-ethylimidazolium trifluoromethanesulfone), [emim][dca]

ethylmethylimidazolium dicyanamide) and [thtdp][Cl] (tri-exyltetradecylphosphonium chloride) to demonstrate thathe regular solution theory can be used to model the gasolubilities in RTILs at low pressures and studied the effectsn pressure and the temperature on the solubility of gases inTILs. The previous works; Blanchard et al. [70] and Anthonyt al. [71] related that the solubility of the gases in ILs tohe intermolecular interactions between the anion of the ILnd the gas. On the other hand, Camper et al. [66] indicatedhat at low pressures, the solubility of CO2 and C2H4 may bexplained using the regular solution theory without consideringhe intermolecular interactions between the anion of the IL andhe gas. At higher pressures, regular solution theory is limitednd Camper et al. [66] attributed this limitation to the dominantntropic effects.

Recently, the results of solubility of hydrogen in [bmim][PF6]

as presented for temperatures from 313 to 373 K and pres-

ures up to 9 MPa. The results demonstrated that the solubilityf hydrogen in [bmim][PF6] is low and increases slightly with

ig. 6. Solubility of CO2 in [bmim][PF6] at different temperatures. (Reprintedith permission from [69]. Copyright 2002 American Chemical Society)

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l Fluids 43 (2007) 150180

emperature [72]. Since ILs can dissolve certain gaseous species,hey may be used in conventional gas absorption applications.he nonvolatility of ILs prevent any cross contamination of theas stream by the solvent during the process. Moreover, regener-tion of the solvent may be performed easily by a simple flash oristillation to remove the gas from the solvent without any crossontamination. The other advantages of ILs as separating agentsre no solvent loss and no air pollution. Currently, researchersre interested in examining the potential of ILs for the separa-ion of CO2 from flue gases emitted from fossilfuel combustionperations [73]. ILs may also be utilized as supported liquidembranes. In conventional membranes, gas dissolves in liquid

ut then the liquid in which the gas dissolved evaporates ren-ering the membrane useless [50]. Due to the nonvolatility ofLs, they can be immobilized on a support and used in supportediquid membranes.

ILs are also used for storage and delivery of hazardous spe-ialty gases such as phosphine (PH3), arsine (AsH3) and stibineSbH3). GASGUARD Sub-Atmospheric Systems supply theajor ion implant gases: AsH3, boron trifluoride (BF3), enrich

oron trifluoride (11BF3) and PH3 sub-atmospherically [74]. Theystem is combined with gas supply technologies for the deliv-ry of the gases when needed. In the complexed gas technology,he desired gases (BF3 and PH3) are chemically bond to ILs sub-tmospherically, then pulling the vacuum on the ILgas complexrovides the mechanism to evolve high purity gas, similar toesorbing a gas from active carbon.

.3. Homogenous and heterogeneous catalysis

One of the most important targets of modern chemistry iso combine the advantages of both homogenous and heteroge-eous catalysis [75]. Greater selectivity is generally observedn homogenous catalysis compared to its heterogeneous coun-erparts, but separation of the catalyst from the product streamr from the extract stream causes a problem [8]. ILs offer thedvantages of both homogenous and heterogeneous catalystsith their two main characteristics: A selected IL may be immis-

ible with the reactants and products, but on the other hand the ILay also dissolve the catalysts. ILs combine the advantages of a

olid for immobilizing the catalyst, and the advantages of a liq-id for allowing the catalyst to move freely [76]. Brennecke andaginn [8] indicated that the ionic nature of the IL also gives an

pportunity to control reaction chemistry, either by participatingn the reaction or stabilizing the highly polar or ionic transitiontates.

ILs have an active role in chemical reactions and catalysis.ome of the examples where ILs are utilized are: reactions ofromatic rings; clean polymerization [77]; Friedel Crafts alky-ation [78]; reduction of aromatic rings [79]; carbonylation [80];alogenation [81]; oxidation [82]; nitration [83]; sulfonation84]; solvents for transition metal catalysis; immobilization ofharged cationic transition metal catalysis in IL phase without

eed for special ligands [85]; in situ catalysis directly in ILather than aqueous catalysis followed by extraction of productsrom solution: this process eliminates washing steps, minimizesosses of catalysis and enhances purity of the products [86].

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S. Keskin et al. / J. of Superc

any applications of ILs in catalytic reactions can be found inarious articles in the literature [1,12,85,8789].

Holbrey and Seddon [90] described many of the catalyticrocesses which use low temperature ILs as reaction media andndicated that the classical transition metal catalyzed hydro-enation, hydroformylation, isomerization, dimerization andoupling reactions can be performed in IL solvents. In theireview, Holbrey and Seddon [90] concluded that ILs may be useds effective solvents and catalysts for clean chemical reactionsnstead of the volatile organic solvents.

Brennecke and Maginn [8], concluded that ILs have beensed successfully for hydrogenations, hydroformylations, iso-erizations, dimerizations, alkylations, Diels-Alder reactions

nd Heck and Suzuki coupling reactions, and in generalesearchers have concluded that the reaction rates and selec-ivities are as good or better in ILs than in conventionalrganic solvents. The catalytic hydrogenation of cyclohex-ne using rhodium-based homogenous catalysts [91] andydrogenation of olefins using ruthenium and cobalt-basedomogenous catalyst [92] in various ILs are studied and theesults indicated that there is a certain increase in the reac-ion rates and selectivity compared to the other normal liquidolvents.

Lagrost et al. [12] used immidazolium and ammonium-ased ILs ([emim][NTf2], [bmim][NTf2], [bmim][PF6],(C8H17)3NCH3][NTf2]) as reaction media for different typesf electrochemical reactions and investigated the oxidationf organic molecules (anthracene, naphthalene, durene, 1,4-ithiafulvene and veratrole) in ILs. Their results suggest thathe nature of investigated mechanisms is almost unchanged inLs as compared with the conventional organic media althoughhe structure of molecular solvents and ILs are expected toe quite different. Lagrost et al. [12] also concluded that theiffusion coefficient of the organic compounds are about 100imes smaller than those in conventional media as expectedrom the lower viscosity of RTILs versus organic solvents. Theositive results of this study demonstrated that ILs can be useds a new media for organic electrochemistry [12].

.4. Biological reactions media

ILs are used in biological reactions such as the synthesisf pharmaceuticals due to the stability of enzymes in ILs, andn separation processes such as the extraction of amino acids15]. IL biphasic systems are used to separate many biologi-ally important molecules such as carbohydrates, organic acidsncluding lactic acid [93,94]. Carbohydrates are renewable andnexpensive sources of energy and raw material for the chem-cal industry. The underivatized carbohydrates are not solublen most of the conventional solvents although they are solu-le in water. Their insolubility in most solvents prevents theransformation of carbohydrates. Therefore, the ability of ILso dissolve carbohydrates enables transformation possibilities

15,93,95,96].

Lau et al. [95] studied the alcoholysis, ammoniolysis, anderhydrolysis reactions by Candida antarctica lipase cataly-is using the [bmim[[PF6] and [bmim[[BF4] as reaction media.

rant

l Fluids 43 (2007) 150180 159

eaction rates were generally comparable with, or better than,hose observed in organic media. Park and Kazlauskas et al. [96]tudied the acetylation of 1-phenylethanol catalyzed by lipaserom Pseudomonas cepacia (PCL) in several ILs and the reac-ion was as fast and as enantioselective in ILs as in toluene. Theylso investigated the acetylation of glucose catalyzed by lipase

from C. antarctica (CALB) and found that the transforma-ion was more regioselective in ionic liquids because glucoses up to one hundred times more soluble in ionic liquids. Liut al. [93] stated that carbohydrates are only sparingly solublen common organic solvents as well as in weakly coordinat-ng ionic liquids, such as [bmim][BF4]. They found that ILs thatontain the dicyanamide anion could dissolve approx. 200 g L1f glucose, sucrose, lactose and cyclodextrin and the esterifica-ion of sucrose with dodecanoic acid in [bmim][dca] could beerformed with CALB.

Swatloski et al. [97] showed that ILs can also be used ason-derivatizing solvents for cellulose, the most abundant biore-ewable material. Cellulose, which is insoluble in water andn most of the common organic solvents, has many derivitizedroducts in many applications of the fiber, paper, and polymerndustries. ILs incorporating anions which are strong hydrogenond acceptors are most effective solvents for cellulose, whereasLs containing non-coordinating anions including PF6 andF4 are not effective. Furthermore, Przybysz et al. [98] exam-

ned the influence of ILs on a cellulose product, paper and foundhat the wettability of paper is improved, whereas the strengthecreased as a result of weakening of cellulose hydrogenonds.

Finally, Pfruender et al. [99] tested the water immiscible ILsamely; [bmim][PF6], [bmim][Tf2N] and [oma][Tf2N] (methyl-rioctylammonium bistrifluoromethanesulfonylimide) for theiriocompatibility towards Escherichia coli and Saccharomyceserevisiae. The results of this study showed that these watermmiscible ILs do not damage microbial cells and thereforene can utilize these water immiscible ILs as substrate reser-oirs and in situ product extracting agents for biphasic wholeell biocatalytic processes. Generally, toxic organic solventsave been used as substrate reservoirs and with this study its shown that water immiscible ILs may be used as biocompati-le solvents for microbial biotransformations. The experimentalesults demonstrated that there is an increase of chemical yieldrom 50 M s1 L1) compared to thequeous system [99]. Although ILs are known by their highlyiscous characteristics, good mass transfer rates were obtainedn their study.

.5. Removing of metal ions

Dai et al. [100] studied the effects of ILs (with PF6 andf2N anions) on improving the ability of crown ethers to

emove metal ions from aqueous solutions. Strontium nitrate,fission product for which there is no available extraction tech-ique for its removal from radioactive waste sites, was used inhis study.

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60 S. Keskin et al. / J. of Superc

Visser et al. [57] designed and synthesized several ILs toemove cadmium and mercury from contaminated water. Theydrophobic ILs come into contact with contaminated water andhey snatch the metal ions out of water. Task-specific ionic liq-ids (TSIL) concept is introduced in order to synthesize ILs withesired properties to extract metal ions. Visser et al. [57] pro-uced TSIL cations by appending different functional groupsnamely thiother, urea and thiourea) to imidazolium cations.hese IL cations can be considered as a new IL class, or aovel class of IL extractants. Synthesized TSIL cations wereombined with PF6 anion and used alone or in a mixture withbmim][PF6]. The results of the study gave significant distribu-ion ratios for mercury and cadmium in liquidliquid separationsnd minimized the reliance on traditional organic solvents forhis process. Davis [101] gave a detailed analysis and relatednformation on TSILs.

In traditional solvent extraction technologies, adding extrac-ants that reside quantitatively in the extracting phase increaseshe metal ion partitioning to the more hydrophobic phase. Thedded extractant molecules dehydrate the metal ions and pro-ide a more hydrophobic environment enabling their transporto the extracting phase [50].

In TSILs, attaching a metal ion coordinating group directlyo the imidazolium cation makes the extractant an integral partf the hydrophobic phase and in this way the chance for ILoss to the aqueous phase is reduced. Therefore, TSILs act ashe hydrophobic solvent and the extractant at the same time50]. However, the cost of TSIL is generally high. In order toliminate this drawback, TSILs may be added to the mixturesf less expensive ILs. Furthermore, Davis [101] and Zhao et al.15] stated in their recent reviews that TSILs are not limited toxtraction processes; they can be also used as versatile solventsn organic catalysts, solid phase synthesis and even in productionf liquid Teflon.

The extraction of radioactive metals (lanthanides andctinides) has particular industrial significance among IL extrac-ion of metal ions for the handling of nuclear materials [15]. Theehaviors of uranium species in various ILs were investigatedn early studies [102105]. Recently, researchers have focusedn the fundamental understanding of ILs in nuclear chemistryuch as radiochemical stability of ILs [106].

Recently, Nakashima et al. [107] examined the feasibilityf extracting of rare earth metals into ILs from aqueous solu-ions and stripping of metal ions from ILs into an aqueous phasey complexing agents. They successfully accomplishing toecycle the extracting IL phase. In this study, octyl(phenyl)-N,N-iisobutylcarbamoylmethyl phosphine oxide (CMPO) dissolvedn [bmim][PF6] showed an extremely high extraction abilitynd selectivity of metal ions as compared to in an ordinaryiluent, n-dodecane. The results of this study indicate thatLs are a promising medium for actinide and fission producteparation.

In the literature, various studies were performed to extract

etal ions using ILs [108113]. Different metal ions including

lkali, alkaline earth metals, heavy metals and radioactive metalsre researched by using different ILs. Generally, the side chainf the IL on the cation is varied and the effect of structure of the

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l Fluids 43 (2007) 150180

L on the extraction efficiency of the metal ions is investigated.he side chain of the cation influences the hydrophobic characterf the IL and thus the partition coefficient of the metal ions isffected.

Visser et al. [108] extracted Na+, Cs+ using [Cnmim][PF6]n = 4, 6, 8); Chun et al. [109] investigated extraction ofther alkali metals such as Li+, K+, Rb+ using [Cnmim][PF6]n = 49); Luo et al. [112,113] studied the extraction of Na+,

+, Cs+ ions using [Cnmim][Tf2N] (n = 2, 4, 6, 8). Not onlyhe alkali metals but also extraction of alkaline earth metalsere studied by various groups: Visser et al. [108] removed Sr2+

sing [Cnmim][PF6] (n = 4,6,8); Bartsch et al. [110] studied theemoval of Mg2+, Ca2+, Sr2+, Ba2+; Luo et al. [112,113] utilizedCnmim][Tf2N] (n = 2, 4, 6, 8) to extract Sr2+. The extraction ofeavy and radioactive metals such as Cu2+, Ag+, Pb2+, Zn2+,d2+, Hg2+ were studied by using [Cnmim][PF6] (n = 49) andSILs [110,111,114].

. Challenges of ILs

The unique properties of ILs and the ability to designheir properties by choice of anion, cation and substituentsreate many more processing options, alternative to the onesith conventional solvents. However, high cost, lack of phys-

cal property and toxicity data restrict the advantageous usef ILs as process chemicals and processing aids at theresent.

The challenges in the use of ILs must be also addressed asell as their advantages. The major challenge is the cost. Ailogram of IL costed about 30,000-fold greater than a commonrganic solvent such as acetone. Renner [9] reported that this costould be reduced to approximately 1000-fold greater dependingn the composition of IL and the scale of production. Wagnernd Uerdingen [115] anticipated that the price of cation systemsased on imidazole will be in the range of D 50100 kg1, ifarger quantities of ILs are produced. The price can be loweredven below D 25 kg1 if ILs are prepared with cheaper cationources on a ton scale. Another estimation was done by Wasser-heid and Haumann [116]. They expected that for bulk ioniciquids choosing proper (relatively cheap) cations and anionsead to prices approximately D 30 l1 for production rates ofulti-ton. Moreover, scientists emphasized that the price of the

Ls may look still discouraging however, the essential factors the price to performance ratio. If the performance of an ILs extremely higher than that of the material (solvent) it aimso replace, less amounts of the IL may be needed for a givenpecific job [2], thus totally or partially overcoming the priceisadvantage.

The second problem is associated with the manufacturingethod of ILs. In manufacturing ILs environmental issues also

eed to be tackled since some VOCs are used to manufactureLs. Recently, some advanced methods have been developedn the solventless syntheses of ILs. For example, 1-alkyl-

-methylimidazolium halides have been synthesized in openontainers in a microwave oven without any VOCs by Varma andamboodiri at the Environmental Protection Agency of U.S.,001 [9].

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Researchers need to find alternative ways to recycle ILs due tohe reason that many processes for cleaning up ILs involve wash-ng with water or VOCs which creates another waste stream. Thisroblem has been solved by adopting supercritical extractionechnologies to recover the dissolved organic compounds fromLs or using membrane separation processes. However, there arether solid matrixes which adsorb some part of the ILs. Thus, aecond rinse would be required and this would create an aque-us waste stream that contains ILs. Despite this disadvantagehere may be some cleaning applications where ILs would bettractive [8].

Incomplete physico-chemical data are another challenge forhe application of ILs. At the present most available data areocused on bulk physical properties such as viscosity, densitynd phase transitions. Relatively little is known about the micro-copic physical properties of ILs. After these properties arenvestigated properly and the influence of ILs on chemical reac-ion rates is found, new ILs with precisely tailored propertiesan be synthesized.

It is extremely important to obtain reliable thermophysicalata and transport properties of ILs in order to make themvailable for many applications and to design IL-based pro-esses efficiently. Harris et al. [117] measured the viscositiesf two members of one of the most commonly studied ILroups, that are based on imidazolium cations; [omim][PF6]nd [omim][BF4] between 0 and 80 C and at pressures to76 MPa ([omim][PF6]) and 224 MPa ([omim][BF4]) with aalling body viscometer and densities between 0 and 90 Ct atmospheric pressure. The bulk physical properties of mostidely used ILs at wider temperature and pressure ranges are

ssential.Another barrier to the large-scale application of ILs arises

rom their high viscosities. The viscosities of ILs are higherhan most organic solvents and water, usually similar to viscos-ty of oils. This high viscosity may be responsible to produce

reduction in the rate of many organic reactions and even aeduction in the diffusion rates of species. Also, handling of ILsith high viscosities is difficult however; increasing tempera-

ure, changing anioncation combinations may yield ILs withower viscosities. To overcome mass transfer limitations in gas-L systems resulting from high viscosity reactions using ILs maye run at high pressures and in efficient gasliquid contactingquipment.

In chemical processing, pharmaceuticals, fine chemicals,etroleum refining, metal refining, polymer processing, pulp andaper, and textiles where a nonvolatile liquid with a wide liq-idus range could work better, ILs are the best choice however,he challenges of turning ILs into useful and environmentallyenign fluids must be overcome.

. ILs and scCO2 systems

Green chemistry, also known as sustainable chemistry,

escribes the search for reducing or even eliminating the usef substances in the production of chemical products and reac-ions which are hazardous to human health and environment.he goal of green chemistry is to create a cleaner and more

Ioit

l Fluids 43 (2007) 150180 161

ustainable chemistry and it has received more and more atten-ion in recent years. Green chemistry searches for alternative,nvironmentally friendly reaction media as compared to the tra-itional organic solvents and at the same time aims at increasedeaction rates, lower reaction temperatures as well higherelectivities.

The ideal situation for a safe and green chemical processs using no solvent, however most of the chemical processesepend on solvents. Some of these solvents are soluble in waternd therefore they must be stripped from water before it leaveshe process not only for ecological but also for economic reasons.olvents must be recovered for recycle and reuse for an econom-

cally viable process. Water, perfluorinated hydrocarbons andupercritical fluids (SCFs) are alternative solvents which maye used in green chemistry. Among these, the most promisinglements of green chemistry are ILs and scCO2.

The low volatility of ILs is the key property that makes themreen solvents. However, this advantage also causes a problemor product separation and recovery [15]. Several techniques forolute recovery from ILs exist: volatile products can be extractedrom IL by distillation or simply by evaporation. However, non-olatile or thermo-sensitive products cannot be separated fromLs with these methods. ILs exhibiting immiscibility with wateran be extracted with water to separate water-soluble solutesrom IL into the aqueous phase; but this method is not suitableor hydrophilic ILs [17]. Of course, organic solvents such as hex-ne and toluene may be effective to recover the products fromL but this approach obviously compromises the ultimate goalf green technologies [15]. Furthermore, the cross contami-ation between the phases presents another problem. Finally,nother green solvent is discovered which solves all the prob-ems and recovers various kind of solutes from ILs without crossontamination: supercritical fluids (SCFs).

SCFs are compounds which are above their critical temper-ture and pressure and they can be manipulated from gas likeo liquid like densities due to their unusual properties near theritical point. They are commercially viable solvents in severalpplications such as dry cleaning and polymer impregnation.cCO2 is the most widely used SCF as a result of nontoxic andon-flammable characteristics. scCO2 has low critical tempera-ure and pressure and it is not expensive.

The advantages of using SCFs as extraction medium includeow cost, nontoxic nature, recoverability and ease of separationrom the products. SCFs have been adapted for product recoveryrom ILs and supercritical fluid extraction (SCFE) is shown toe a viable technique with the additional benefits of environ-ental sustainability and pure product recovery [118]. Among

he SCFs, an inexpensive and readily available one, scCO2 hasecome a partner of IL and two environmentally benign sol-ents are utilized together in several applications. The volatilend nonpolar scCO2 forms different two-phase systems withonvolatile and polar ILs. The product recovery process withhese systems is based on the principle that scCO2 is soluble in

Ls, but ILs are not soluble in scCO2 [70]. Since most of therganic compounds are soluble in scCO2, with the high solubil-ty of scCO2 in ILs, these products are transferred from the ILo the supercritical phase.

162 S. Keskin et al. / J. of Supercritical Fluids 43 (2007) 150180

Table 3ILgas systems phase behaviors

System Temperature Pressure Findings Reference

[bmim][PF6]CO2 40, 50, and 60 C Up to 93 bar As pressure increases, solubility of CO2 in the IL increases [70][C8-mim][PF6]CO2 Solubility of CO2 in IL-rich phase decreases with temperature[C8-mim][BF4]CO2 CO2 solubility depends on the nature of the anion and cation[bmim][NO3]CO2 The solubility of CO2 in IL-rich phase is highest for ILs with fluorinated anions[emim][EtSO4]CO2 The general trend of the phase behavior is almost identical for all ILs[N-bupy][BF4]CO2

[bmim][PF6]CO2 10, 25, and 50 C Up to 13 bar Water and carbon dioxide exhibited the strongest interactions and the [69][bmim][PF6]C2H4 Highest solubilities in [bmim][PF6], followed by ethylene, ethane, and methane[bmim][PF6]C2H6 Argon and oxygen both had very low solubilities and essentially no

interactions with the IL[bmim][PF6]CH4[bmim][PF6]Ar[bmim][PF6]O2[bmim][PF6]CO[bmim][PF6]N2[bmim][PF6]H2

[bmim][PF6]CO2 20, 40, 60, 80,100, and 120 C

Up to 9.7 MPa Total pressure increases linearly with increasing amount of CO2 in IL [119]

[bmim][BF4]CO2 3070 C Atmospheric pressure CO2 is found to be one order of magnitude more soluble in the IL than O2 [131][bmim][BF4]O2 The solubility of CO2 in the IL decreases with temperature

The solubility of O2 in the IL slightly increases with temperatureSimilar results are obtained in [bmim][PF6]

[bmim][PF6]CO2 25, 40, and 60 C Up to 150 bar Solubility of CO2 in ten different IL is reported [121][bmim][BF4]CO2 The solubility of CO2 is strongly dependent on the choice of the anion[bmim][TfO]CO2 Increasing the alkyl chain length, increases the solubility of CO2 in IL[bmim][NO3]CO2 All of the ILs expand a relatively small amount when CO2 is added[bmim][methide]CO2[bmim][DCA]CO2[bmim][Tf2N]CO2[hmim][Tf2N]CO2[omim][Tf2N]CO2[hmmim][Tf2N]CO2

[emim][PF6]CHF3 36.1594.35 C 1.651.6 MPa The solubility of supercritical CHF3 in [emim][PF6] is very high [142]At low CHF3 concentrations (mole fraction

S. Keskin et al. / J. of Supercritical Fluids 43 (2007) 150180 163

Table 3 (Continued )

System Temperature Pressure Findings Reference

[bmim][PF6]CO2 4090 C Up to 97 MPa CO2 has good solubilities in these ILs at lower pressures [13][emim][PF6]CO2 There is a linear relationship between the alkyl chain length and solubility

of CO2[hmim][PF6]CO2

[hmim][BF4]CO2 2095 C 0.54100 MPa CO2 was found to be more soluble in [hmim][PF6] than in [hmim][BF4] [133][hmim][PF6]CO2

[bmim][BF4]CO2 5.3295.07 C 0.58767.62 MPa CO2 has a high solubility in [bmim][BF4] at lower pressures, but the solubilitydecreases dramatically at higher pressures

[132]

The phase behavior of the system [bmim][BF4]CO2 shows similarities withthe phase behavior of the system [hmim][BF4]CO2CO2 is more soluble in [hmim][BF4] than in [bmim][BF4]

[omim][BF4]CO2 29.8589.95 C 0.1100 MPa High solubilities of CO2 for low CO2 mole fractions (mole fraction < 0.6) areattained at relatively low pressure, whereas for mole fraction >0.6, the pressureneeded for completely dissolving the CO2 increases drastically

[139]

Increase in temperature slightly decreases the solubility of CO2The CO2 solubility increases in the IL with increasing chain length of the alkylgroup

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.1. High-pressure phase behavior of ILCO2 systems

Preliminary works have shown that scCO2 extraction is aiable method for solute recovery from an IL. However, thenowledge of phase behavior of ILCO2 systems is an essentialspect of this methodology. scCO2 dissolution in the IL phase isot only necessary for contact with the solute but it also reduceshe viscosity of the IL and therefore enhancing the mass transferrocess.

Early studies of ILCO2 phase behavior indicated that theseystems are very unusual biphasic systems. No measurablemount of [bmim][PF6] was soluble in the CO2-rich phase,lthough a large amount of CO2 dissolved in the IL-rich phase,educing the viscosity of IL [70]. Blanchard and Brennecke118] concluded that the system remained as two distinct phasesven under pressures up to 400 bar. Therefore, high-pressurehase behavior of [bmim][PF6]CO2 is totally different fromhat of any ordinary organic liquidCO2 systems. This differenthase behavior is the key phenomena which makes extractionf solutes from IL with CO2 attractive.

Table 3 summarizes the phase behavior studies performedor ILgas systems, demonstrates the type of IL and gases usedn these studies, the experimental conditions, the basic findingsnd the related references.

.1.1. The [bmim][PF6]CO2 systemThe phase behaviors of ILscCO2 systems are studied very

xtensively in the literature for a better understanding of the pro-esses involving both IL and scCO2. Since [bmim][PF6] is theost widely studied IL in the literature, many researchers stud-

ed the high-pressure phase behavior of the [bmim][PF6]CO2ystem [13,69,70,119122].

Blanchard et al. [70] measured the high-pressureaporliquid phase behavior of [bmim][PF6]CO2 systemy using two different apparatus sets: a static high-pressurehase equilibrium apparatus and a dynamic flow apparatus. In

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urther study to investigate the effect of the anion on the

2.when the cation is fixed as [omim]

he static high-pressure vaporliquid equilibrium apparatus, alass cell was loaded with a known amount of IL sample andnown amounts of CO2 were metered into the cell while theample within was vigorously stirred to ensure equilibrium.

ith the assumption of pure CO2 vapor phase, the compositionf the IL-rich phase was calculated by knowing the amount ofO2 added to the cell. At the end of the equilibration period,O2 was completely removed from IL phase upon depressur-

zation. The same group also used a dynamic apparatus, i.e. aigh-pressure extractor to determine the solubility of the IL inhe CO2 phase. The detailed description of these apparatus setsnd experimental procedures can be found in the literature [70].ifferent experimental set-ups were used by other researchers:

schematic diagram of a general ILscCO2 experimentalpparatus, which was used to measure the solubility of CO2n IL ([bmim][PF6]) is given by Kim et al. [123]. Shiflett andokozeki [124] measured the gas solubility and diffusivityf CO2 in [bmim][PF6] using a gravimetric microbalance forhich the details of the experimental set-up is given in the

elated reference.The solubility of CO2 in [bmim][PF6] was determined at 40,

0 and 60 C and pressures up to 93 bar [70]. As the pressurencreases, the solubility of CO2 in the IL-rich phase increasesramatically and the solubility value reaches a mole fraction of.72 at 40 C and 93 bar. A general rule suggests that an increasen temperature results with a decrease in the solubility of gasesn liquids. As expected, the solubility of CO2 in [bmim][PF6]ich phase decreases with temperature. However, they noticedhat the temperature dependence of the solubility is quite smalln this temperature and pressure range. Another crucial points the effect of large degree of CO2 solubility on the viscosityf IL. The viscosity of IL decreases when a certain amount of

O2 is dissolved in IL and this effect can easily be observedy the reduced drag on the stirring magnet when a static high-ressure vaporliquid equilibrium set-up is used. This reductionn viscosity of the liquid facilitates the solution process.

164 S. Keskin et al. / J. of Supercritical Fluids 43 (2007) 150180

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Kamps et al. [119] presented the solubility of CO2 inbmim][PF6] for temperatures 293393 K in 20 K intervals andressures up to about 9.7 MPa. The total pressure is plotted ver-us the stoichiometric molality of the gas (number of moleser kilogram of the IL) in Fig. 7. The total pressure increasesinearly with increasing amount of the gas in IL. The solubilityata represented by Kamps et al. [119] differ from the previouslyeported solubility data of Blanchard et al. [70]. The comparisonf experimental data of two studies is given in Fig. 8.

There are several [bmim][PF6]CO2 high-pressureaporliquid equilibrium data sets available in the litera-ure. However, these sets differ from each other considerablyn the values they report for similar conditions. The reasonf differing solubility data reported may be due to the smallmounts of water dissolved in the IL sample used. For example,lanchard et al. [125] presented a solubility data of CO2

n [bmim][PF6] which is different than the data reported byhe same group in 2001. In the first study, this group usedbmim][PF6] which was saturated with water at 22 C, contain-ng 2.3 wt.% water. In the second study, they used [bmim][PF6]

ig. 8. Comparison of experimental data of Blanchard et al. [70] (, , and )nd Kamps et al. [119] ( and ) for [bmim][PF6]CO2 system. (Reprintedith permission from [119]. Copyright 2003 American Chemical Society)

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hich was dried to approximately 0.15 wt.% water. Drying ofL has a significant effect on the phase behavior with CO2.hus, Blanchard et al. [70] showed that the solubility of CO2

n ILs was decreased in the presence of water. In Fig. 9,bmim][PF6]CO2 liquid phase compositions are given forried and wet IL samples.

In order to observe the effect of water impurity in IL, phaseehaviors of two IL samples (dry and wet) with CO2 were com-ared. The effect of water impurity in IL is significant at 57 bar.or dried IL sample, the mole fraction of CO2 is 0.54, whereasor the wet (water saturated) IL sample it is only 0.13. The effectf water in IL may be explained by CO2-phobic nature of water.ven at high pressures, mutual solubilities of water and CO2 areery low [126]. Another point is the formation of carbonic acidrom the reaction of CO2 with water that can result in a reductionf the aqueous phase pH to as little as 2.80 [127].

Rubero and Baldelli [128] investigated gasliquid interfacef imidazolium ILs using surface-sensitive vibrational spec-roscopy sum frequency generation. The results indicated thathen the IL is dry, the cation is oriented with the imida-

olium ring parallel to the surface plane for both hydrophilicnd hydrophobic ILs. But the cation reorients itself with respecto the surface for the hydrophobic liquid when water is added,hile the orientation in the hydrophilic liquid is unaffected.After the influence of water is noticed, researchers working

n ILCO2 solubility and equilibrium have started to dry andegas all ILs under vacuum at room temperature for severalays prior to use. After ILs are dried and degassed, the waterontents are estimated by the Karl Fischer analysis before solu-ility data are taken. Measurements show that the most widelytudied IL; [bmim][PF6] absorbs a couple wt.% water when lefto the atmosphere. The estimated water content of [bmim][PF6]

fter drying was approximately 0.15 wt.% water as measured byarl Fischer analysis [70].A number of high and low-pressure [bmim][PF6]CO2 solu-

ility studies have appeared in the literature. Although consistent

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esults have been established for low-pressure solubility data ofO2 in [bmim][PF6], there are large discrepancies among high-ressure solubility data of several researchers [13,70,119121].hese large solubility differences in the literature are mostrobably due to the differences in the purities of the ILssed.

Since the behaviors of ILCO2 systems are different fromther organic liquidCO2 systems, full phase diagrams ofLCO2 systems are investigated. Blanchard and co-workers125] found two-phase immiscibility regions with three cloudoint measurements of 1.31, 4.92 and 7.15 mole% IL mix-ures with the balance being CO2. Although, these experimentsere conducted with water-saturated ILs, qualitatively a similarehavior is expected with dried samples.

Blanchard et al. [70] gave a qualitative phase behavior ofbmim][PF6]CO2 system over a wide pressure range. Theyoticed that the phase behavior where a large miscibility gapxists even at extremely high pressures. Blanchard et al. [70]eported and referred that as a complementary work, McHughnd co-workers studied [bmim][PF6]CO2 phase behavior atigher pressures up to 3100 bar and found two distinct phasest all conditions. The existence of large immiscibility gap event very high pressures is not expected for organic liquidCO2ystems and the existence of two distinct phases is explainedy the following discussion: At high-pressures density of pureO2 phase increases but since the liquid phase does not expand,

he two phases will never become identical and a mixture criti-al point will never be reached. Therefore, the ILCO2 systememains as two phases even at very high pressures, although theO2 solubility is quite high, the mixtures never become a singlehase [70].

Anthony et al. [69] reported the solubilities and Henrysonstants of different gases (carbon dioxide, ethylene, ethane,ethane, argon, oxygen, carbon monoxide, hydrogen, and nitro-

en) in [bmim][PF6] and showed that CO2 has the highestolubility and strong interaction with [bmim][PF6]. The solu-ility data of CO2 in [bmim][PF6] is in good agreement withhe published results of Blanchard et al. [70] although differentechniques were used in these studies. Furthermore, Baltus et al.68] reported that Henrys constants for Kamps et al. [119] datare in reasonable agreement with those obtained by Anthony etl. [69].

Aki et al. [121] studied the high-pressure phase behavior ofO2 in imidazolium-based ILs and compared the phase behav-

or of the system [bmim][PF6]CO2 with the other solubilityata present in the literature. The solubility results of CO2 inbmim][PF6] at 25 C measured by Aki et al. [121] agreedemarkably well with the solubility results of Anthony et al.69] at low pressures and with the solubility results of Kamps etl. [119]. Aki et al. [121] investigated the solubility of CO2 inbmim][PF6] at 40 C and compared the results with the previ-us studies of Kamps et al. [119], Blanchard et al. [70] and Liu etl. [120]. As expected the agreement between the data points is

ood at low pressures but the discrepancy is obvious at high pres-ures. Aki et al. [121] explained that in their previous work [70],hey were not aware of the various impurities and degradationroducts that were present in the samples they used. There-

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l Fluids 43 (2007) 150180 165

ore, they attributed the difference between their study [121]nd the previous study of the same group [70] to the purity ofhe IL.

At 40 C and at all pressures, there is a very good agree-ent within the solubility values reported by Aki et al. [121]

nd Liu et al. [120]. However, this statement is not correct forhe reported solubility data of Aki et al. [121] and Kamps et al.119]. The results of two studies agree at low pressures, but atigh pressures, there is a significant difference: At about 43 bar,he solubility of CO2 in [bmim][PF6] was measured as 0.43mole fraction) by Aki et al. [121], however, at the same pointamps et al. [119] reported the solubility of CO2 as 0.38. By con-

idering the studies mentioned above, it may thus be concludedhat the solubility of CO2 in one of the most widely studied IL,bmim][PF6], varies among different groups in the literature ands not a good agreement especially at higher pressures.

The solubility of CO2 in [bmim][PF6] was experimen-ally studied at 298.15 K and up to 1.0 MPa by Kim et al.123]. A group contribution form of a non-random latticefluidodel (GC-NLF) was applied to predict solubility of CO2 in

bmim][PF6]. They used the solubility data of Kamps et al. [119]or a wider pressure range for the group parameter determina-ion. Comparisons of calculated solubility data with Kamps etl. data [119] for [bmim][PF6] demonstrated that the methodpplied is fairly accurate except for regions close to the criticalonditions of CO2. Kim et al. [123] also compared the calcu-ated values of solubility of CO2 in different ILs ([emim][PF6],bmim][PF6], and [C6mim][PF6]) with the experimental sol-bility data reported by other groups for the same ILs. Thegreements are generally good up to 10 MPa pressure, however,urther comparisons for higher pressure shows some degree ofiscrepancy between the calculated solubility data of Kim et al.123] and the experimental solubility data of Shariati and Peters129,130].

Finally, Shariati and Peters [13] studied the comparison ofhe phase behavior of [bmim][PF6]scCO2 system with thether studies present in the literature. The solubility of CO2n [bmim][PF6] was determined by measuring the bubble pointressure of the binary system at different temperatures for sev-ral isopleths and pressures up to 97 MPa. The solubility dataf CO2 in [bmim][PF6] at 323.15 K was compared with that oflanchard et al. [70] and Anthony et al. [71] and the results ofhariati and Peters [13] were in a good agreement with thosef Anthony et al. [71]. The solubility data taken at 333.15 Kas compared with that of Blanchard et al. [70], Kamps et

l. [119], Liu et al. [120]. Although the experimental meth-ds were completely different, there is also a good agreementetween the results of Shariati and Peters [13] and Kamps etl. [119] at a temperature of 333.15 K. The solubility data oflanchard et al. [70] and Liu et al. [120] show greater devi-tions from the data of Shariati and Peters [13] especially atigher pressures. Shariati and Peters [13] reported the existencef the three-phase equilibrium liquidliquidvapor (LLV). As

he other studies demonstrated, this study also indicated thatO2 has a high solubility in [bmim][PF6] and there is a linear

elationship between the alkyl chain length and the solubilityf CO2.

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