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Activity coefficient for ionic liquids

Activity coefficients give information concerning the interaction between solute and IL. This parameter describes also the degree of non-ideality for a species i in a mixture. The large dataset of activity coefficients at infinite dilution published in the literature may be used to present a general behaviour of solutes with ionic liquids [6, 7].
The values of activity coefficients at infinite dilution for n-alkanes increase with an increase in carbon number. In most ionic liquids, the high values observed with n-alkanes indicate their low solubility in ionic liquids. values of n-alkanes are higher than the values obtained with cyclohexane, alkenes, alkynes and aromatics. Introduction of a double or triple bond in the n-alkanes decreases the values.
Cyclization of the alkane skeleton reduces the value of in comparison to that of the corresponding linear alkanes. Aromatics with their-delocalized electrons have smaller values, presumably because of the interaction with the cation species. Using computer simulation, Lynden-Bell et al. [18] showed that the cations are found to interact predominantly with the ring of the benzene while the anions interact with the ring hydrogens to a first approximation.
In the series of chloromethanes, it is usually observed that values strongly increase from dichloromethane to tetrachloromethane. This behavior observed with all types of ionic liquids indicates that polar compounds have better solubility in the ILs when attractive interaction between polar molecules and the charged ions of the solvent is possible. The values for the alcohols are relatively small (ranging between 1.2 and 4.6). The lone pair of electrons on the oxygen atom could interact with the ionic liquid cation, and the acidic proton is attracted by oxygen atoms in the cation. values of branched alkanol skeleton are smaller than values of the corresponding linear alcohol. values of n-alkanols increase with increasing chain length. values of ethers and amine are higher in comparison with those of the alcohols. For most solutes, their solubility increase when the alkyl chain length grafted on the ionic liquid increase. The behavior of solutes with ionic liquids is also strongly affected by the nature of the chain grafted on the ionic liquids. For example, grafting a polar chain on the cation of dicyanamide based ionic liquid increases strongly the interactions. Replacing the 1-ethyl-3-methylimidazolium cation by 1-(3-cyanopropyl)-3-methylimidazolium in dicyanamide based ionic liquids, the activity coefficients values of n-hexane are divided by two (241 to 111).
The alkoxymethyl-group grafted on the imidazolium cation makes the ionic liquid more polar and with possible anti-microbial activities [19]. Domanska and Marciniak [20] studied the interaction between organic compounds and 1-hexyloxymethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)-imide and 1,3-dihexyloxymethyl-imidazolium bis(trifluoromethylsulfonyl)-imide. The authors found that ILs with two alkoxymethyl groups in the cation reveals stronger interactions with solutes, e.g. additional interaction of the IL with n-alkanes, alkenes and alkynes (i.e. Van der Waals interaction between alkane chains of the solute and the cation), and also stronger interaction with aromatic hydrocarbons, thiophene and alcohols (hydrogen bonding, n–, or– interactions).
Revelli et al. [21] measured activity coefficients at infinite dilution of organic compounds in the ionic liquid trihexyl(tetradecyl) phosphonium bis(trifluoromethylsulfonyl)imide. As observed with imidazolium-based ionic liquids, cations with a long alkyl chain tend to increase the solubility of most organic compounds in IL. The activity coefficients of 39 organic compounds in this IL are below unity apart from the alkanes and alcohols indicating a strong affinity of the solutes for the ionic liquid. The introduction of a cyanoalkyl chain dramatically decreases the solubility of a polar compounds in ILs. Aromatics, alkenes and alkynes have lower interactions with cyanoalkylimidazolium based ILs than dialkylimidazolium. There is a lack of information concerning piperidinium based ionic liquids. In their recent study, Domanska and Paduszynski, [22] demonstrate that 1-propyl-1-methylpiperidinium bis(trifluoromethyl)sulfonyl)imide behaves like the other measured ionic liquids based on different cations.

Miscibility with water

Solubility of water in ionic liquids mainly depends on their structure and especially on the nature of the anion. At room temperature, ILs based on [PF6]¯ , [Tf2N]¯ and [CF3SO2)2N]¯ anions are insoluble in water (Figure I.2). Most of the ionic liquids based on anions such as ethanoate, trifluoroacetate, nitrate, acetate, halides are fully miscible with water. Nevertheless, the water solubility in [BF4]¯ and [CF3SO3]¯ based ILs depends on the alkyl chain length grafted on the cation. For instance, [EMIM][BF4] and [BMIM][BF4] are fully water miscible. However, [CnMIM][BF4] ionic liquids with alkyl chain length higher than four (n > 4) are immiscible with water. Hence, the anion has a key role on water miscibility and the cation a secondary effect. It is well known that ionic liquids based on [PF6]¯ and [Tf2N]¯ are hygroscopic. However, this is not in agreement with the literature results which shows that water saturation of hydrophobic ionic liquids such as [C4-8MIM][BF4] and [C6-10MIM][PF6] is a function of both alkyl chain and the anion. Chellapan [7] stated that [PF6]¯ based ionic liquids dissolve less water in comparison to the [BF4]¯ based ionic liquids and the water intake decreases as a the chain length of the alkyl group increases. Seddon et al. [24] reported that the so-called hydrophobic ionic liquids are in fact hygroscopic. They investigated the extent of hygroscopicity of [OMIM][Cl], [OMIM][NO3], [BMIM][BF4], and [BMIM][PF6]. Although the fully water-soluble chloride and nitrate ionic liquids absorb much more water than the corresponding hexafluorophosphate, an uptake of 1% w/w (0.16 mole fraction of water) over 3 hour is significant.

Toxicity and human health

Due to their low vapor pressure, the risk of air pollution by ionic liquids is minimal and so, they have low impact on the environment and human health. It should be noticed that an ideal green solvent should also be non-toxic and not persist in the environment. Even if ionic liquids can be used in large-scale in industrial applications, their entry to the aquatic environment through accidental spills or as effluents is the most possible pathway for their contributing to environmental hazards. Consequently, aqueous toxicology investigations are the most important topic of interest concerning ionic liquids environmental safety. However, their toxicology data have been very limited until now. Several authors [29-31] already mentioned this lack of toxicological data in the literature. The eco-toxicological studies performed to understand the effects of different ILs on enzymatic activities, cells and microorganisms mainly use LC50 levels (lethal concentration). Decreasing LC50 values indicate higher toxicities according to the toxicity classes of Hodge and Sterner scale [32]. This scale indicates that the LC50 value (in terms of mg/l) of 10 or less shows that the chemical is extremely toxic, LC50 value between 10 and 100 shows that chemical is highly toxic, LC50 value between 100 and 1000 shows that chemical is slightly toxic, and finally LC50 value between 1000 and 10,000 means that chemical is practically nontoxic.
Early studies suggested that quaternary ammonium and pyridinium ILs have significant toxic effects on a variety of bacteria and fungi [7]. Bernot et al. [33] studied the toxicity of imidazolium and pyridinium ionic liquids using fresh water snails. The toxicity was tested by the LC50 method. They observed that the alkyl chain length of the cation has a great impact on the toxicity of the ionic liquids. The ionic liquids with a longer carbon chain (C8) were found to be more toxic than the alkyl chains with C4 and C6. Maginn [34] presented the LC50 levels for two imidazolium-based ionic liquids with Daphnia magna. Daphnia are common fresh water crustaceans (a large group of Phylum Arthropoda). The LC50 values obtained for [BMIM]+ cations with [PF6]¯ and [BF4]¯ anions illustrated that these two ionic liquids are about as toxic to Daphnia as benzene and are far more toxic than acetone (Table I.3).
Mikkola et al. [35] investigated the toxicity of some promising amidinium, imidazolium, and phosphonium based ILs toward two different cell lines, human corneal epithelial cells and Escherichia coli bacterial cells. They found that the toxicity of the phosphonium ILs was highly dependent on the longest linear chain of the IL as mentioned above. They referred this to the increasing hydrophobicity, with the long-chain phosphonium ILs which leads to toxicity. While the shorter-chain versions were significantly less toxic or not toxic at all. Amidinium and imidazolium ILs showed no significant effect on the cells, within the concentration range used.

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Absorption heat pumps (AHP)

Recently, absorption heat pump (AHP) has attracted considerable attention due to its ability to use sustainable energy systems giving a high primary energy efficiency and low environmental impact [42]. The driving heat source for AHP is various, such as hydrocarbon fuels, solar energy, geothermal energy, district heating network or waste heat. Heated by the high-grade driving source supplied to the generator the AHP can extract low-grade heat in the evaporator side from low temperature sources, such as the ambient air, underground soil, surface water or waste heat. Then, medium temperature hot water is produced in the condenser and absorber. Because the heat supplied to the generator and evaporator is converted into the supplied hot water, the heat quantity is increased [49].
The operating sequence of the basic AHP is explained as follows: refrigerant vapor is produced in the evaporator which is heated by a low-medium grade heat source. The refrigerant vapor is absorbed in the strong solution that enters the absorber and the resulting weak solution returns back to generator. In the generator some refrigerant vapor is separated from the weak solution to be sent to the condenser and therefore the strong solution from the generator is returned back to the absorber. The vaporized refrigerant is condensed in the condenser then it is pumped to a higher pressure level as it enters the evaporator. The waste heat given to the evaporator causes its vaporization. Once again, the absorber absorbs the refrigerant vapor at a higher temperature. Hence, the absorption heat pump has the ability of raising the temperature of the solution above the temperature of the waste heat source [37, 39].

Absorption heat transformers (AHT)

A different category of absorption cycle is known as “absorption heat transformer” or “reverse absorption heat pump”. AHT operates in a cycle that is the reverse of the AHP. The basic AHT, shown schematically in Figure I.10, has similar components as a single-effect absorption cycle. The difference is the inversion of pressure levels between generator/condenser and evaporator/absorber. AHT also uses heat from an intermediate temperature reservoir as the driving heat (generally an industrial waste heat) (Figure I.8). The system rejects heat out at a low temperature level (normally to the surroundings). The useful output is obtained at the highest temperature level. The use of AHTs allow any waste heat to be upgraded to a higher temperature level without any other heat input except some work required to circulation pumps.

Working fluids containing {water + ILs} for absorption cycles

Water can be considered as a green refrigerant, nontoxic, having high latent heat and excellent thermal characteristics. If {H2O + ILs} are selected as the working fluids, the ILs selected should be very hygroscopic and water stable.
It is well established that many ILs are completely miscible with water, especially imidazolium-based ILs. Nevertheless, there is still a lack of thermodynamic data for IL and water systems.

{H2O + Ionic liquids} binary systems in literature

Recently, many research groups have investigated {H2O + IL} as alternatives binary systems in absorption cycle [49]. Numerous articles present thermodynamic studies of binary systems containing water and IL. Annex I lists the most studied ILs in the literature and the thermo physical properties available. Alkylsulfate and alkylphosphaste based ILs are well known and their performance as working fluids {H2O + IL} were evaluated in different absorption cycle [46, 49, 54- 57].
Among others, the binary systems {H2O + [DMIM][DMP]} [46] was extensively studied by different research groups. Kato and Gmehling [58] have measured the vapor-liquid equilibria (VLE) of the {H2O + [DMIM][DMP]} system with a computer-driven static apparatus. He et al. [57] measured the thermodynamic properties including VLE, density, viscosity, heat capacity and excess enthalpy of this system. Wang et al. [59] measured VLE for the same binary system using a quasi-static ebulliometric method, while Dong et al. [46] simulated the performance of absorption refrigeration cycle based on the measured data of VLE and heat capacity of the same binary mixture. The simulation results show that the cycle performance of {H2O + [DMIM][DMP]} is close to that of conventional working pair {H2O + LiBr}. Nevertheless, using the {H2O + [DMIM][DMP]} system increases the operating temperature range and stops crystallization and corrosion caused by {H2O + LiBr}.
Zhang and Hu [55]; Gong et al. [60]; Ren et al. [54]; and Wang et al. [61] have studied the binary system {H2O + 1-Ethyl-3-methylimidazolium dimethyl phosphate [EMIM][DMP]} using different techniques. Zhang and Hu [55] simulated the performance of the working pairs {H2O + [EMIM][DMP]} in an absorption refrigeration cycle. The results of the simulation show that the coefficient of performance (COP) of {H2O + [EMIM][DMP]} is lower than {H2O + LiBr} but still higher than 0.7, while the generation temperature was lower than that for {H2O + LiBr}. Ren and his coworkers [54] stated that the binary system {H2O + [EMIM][DMP]}can meet the basic requirements for working pairs and has potential to be new working pair for absorption heat pump or absorption refrigeration.

Table of contents :

I.1. Introduction
I.2. Ionic Liquids
I.3. History and progress of ionic liquids
I.4. Classes of ionic liquids
I.5. Physico-chemical properties of Ionic liquids
I.5.1. vapor pressure
I.5.2. Activity coefficient for ionic liquids
I.5.3. Density
I.5.4. Viscosity
I.5.5. Surface tension
I.5.6. Miscibility with water
I.5.7. Melting point
I.5.8. Thermal stability
I.5.9. Flammability and corrosion
I.5.10. Toxicity and human health
I.6. Potential Applications of Ionic Liquids
I.7. General information about Absorption cycles
I.8. Classification of Absorption cycles
I.8.1. Absorption refrigerators (Chillers) (AC)
I.8.2. Absorption heat pumps (AHP)
I.8.3. Absorption heat transformers (AHT)
I.9. Working fluids containing {water + ILs} for absorption cycles
I.9.1. {H2O + Ionic liquids} binary systems in literature
I.10. Thermodynamic properties of {H2O + IL}
I.10.1. Thermodynamic model for the representation of binary system {H2O + IL}
I.10.2. Experimental thermodynamic data of {H2O + IL}
I.10.2.1. Vapor liquid equilibrium (VLE)
I.10.2.2. Heat capacity
I.10.2.3. Excess Enthalpy (HE)
I.10.2.4. Density
I.10.2.5. Viscosity
I.10.2.6. Thermal decomposition
II.1. Experimental section
II.1.1. Materials
II.1.2. Vapor Liquid Equilibrium (VLE)
II.1.2.1. Isobaric VLE apparatus
II.1.2.2. Isothermal VLE measurement
II.1.3. Heat capacity (Cp)
II.1.4. Density (ρ)
II.1.5. Excess enthalpy (HE)
II.2. Results and discussion
II.2.1. Vapor Liquid Equilibrium (VLE)
II.2.2. Heat capacity (Cp)
II.2.3. Density (ρ)
II.2.3.1. Excess molar volume VE
II.2.4. Excess molar Enthalpy (HE)
III.1. Simulation of the AHT cycle performance
III.2. COP definition for an Absorption refrigeration cycle
III.3.Absorption heat transformer


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