Background

Ionic liquids

An ionic liquid is a liquid salt which is reported to be in a liquid form at temperatures below 100°C or at room temperature due to poor alignment of ions. The crystal lattice of ionic liquid is not stable due to the presence of one delocalized charge on one ion and one organic component. The formation of ionic liquid is studied in detail but it is found that pyridinium and methylimidazolium ionic liquids are the most stable ones (Wasserscheid and Welton, 2008). With the help of ion and organic component attached with that ion, properties of viscosity, melting point and solubility of solvent are measured. For the purpose of specific synthesis in industries and laboratory level, many ionic liquids are developed in that perspective. These types of ionic liquids due to their unique properties are called “designer solvents” (Berthod et al., 2008). The mixture of 1-ethyl-3-methylimidazolium chloride ({emim} Cl) with AlCl₃ is the first ever reported room temperature ionic liquids. They are known as the ionic liquids to form equilibrium among (emim) (Al₂Cl₇), (emim) (AlCl₄) and (emim) (Al₃Cl₁₀). These transition metal catalysts with the ionic liquids are recycled by separation method with the help of extraction with water and non-polar organic solvents. In some cases these transition metal catalyst along with ionic liquids are recycled many times (Kim et al., 2001; Mathews et al., 2001; Fernandez et al., 2007).

Protic ionic liquids

It has been found many years ago that to remove dangerous gases from a stream it is necessary to absorb the gas in a liquid or solvent. The focus of the present study was to capture CO_2, for this purpose various techniques are being used world-wide. One of the methods is known as chemical scrubbing which involves the absorption of carbon dioxide in a solvent or simple single phase liquid (Kapdi et al., 2005; Couvert et al., 2006; Charron et al., 2006). Kim et al. (2001) suggested a method of CO₂ capturing that involves a reaction between the targeted stream and a solvent or a liquid. These methods are old but a relatively new method was also developed which included the use of alkanol amine absorbents. The disadvantages of the above methods are huge capital investment and driving cost of CO₂ at high temperatures. Because CO₂ is only released at high temperatures therefore the above methods were reported to be inefficient after a certain period of time. These methods are old and require careful handling of the waste product which releases CO₂ but these systems became unstable after high heating process due to boiling and vaporization of the solvent or liquid used. To eliminate these types of limitations which were encountered in many old process scientists paid attention towards a new system of absorbents that are known as ionic liquids. Ionic liquids are reported as good CO₂ absorbents through various experiments and are highly stable due to their stable thermo-physical properties for high temperature systems. In the current study, adenosine triphosphate (ATP) is also used which is mixed in ionic liquid and water mixture. ATP show great promise of CO₂ capture when dissolved in water and along with ionic liquid, ATP is reported to become more efficient as a CO₂ absorbent due to its unique thermos-physical properties (Couvert et al., 2006; Charron et al., 2006).

Adenosine triphosphate

Adenosine triphosphate (ATP) is a nucleotide and is used as a coenzyme in many small cell reactions. The transfer of chemical energy in the cells for metabolism is possible due to ATP. The process of synthesis of proteins, synthesis of membranes, movement of the cell, cellular division, transport of various solutes need energy which is transferred by ATP and it transfer energy exactly at the point where it is needed. The energy is transferred by the breakage of ATP in ADP (adenosine diphosphate) and Pi (phosphate).

The Objectives of the present study were as under:

(1) To fine out the thermo-physical properties of protic ionic/ATP/water mixtures to be used as good synthetic media.

(2) To fine out the thermo-physical properties of protic ionic/ATP/water mixtures for carbon capturing phenomena.

(3) To fine out the excess molar volume properties through REDLICH-KISTER equation to study mixture behavior at different mole fractions of ATP.

(4) To compare the two ionic liquids used for carbon capturing with the effect of Adenosine triphosphate (ATP) solubility on carbon capturing for each of the two ionic liquid s used.

1 Materials and Methods

1.1 Adenosine triphosphate

Adenosine triphosphate (ATP) is a nucleoside which is also named as nucleoside triphosphate. The use of ATP in cells is as a coenzyme and in general mentioned as the “molecular unit of currency” of intracellular energy transfer. It is made by the processes of photophosphorylation, aerobic respiration and fermentation. As ATP is the source of energy within the cell and behaves as shuttle to transfer energy to the parts of the cell where it is needed, it is also responsible for carbon capturing by transferring energy to suitable carbon dioxide capturing solvents and activating their cellular potential to capture carbon dioxide. Its structure is given as follows:

1.2 Density meter

The density meter used was Rudolph Research analytical density meter DDM 2911 which was first calibrated according to air and distilled water densities for the measurement of densities.

1.3 Standard molar volume

The standard molar volume (V_m) is the volume of one mole of a substance occupied by that mole of a substance at a given temperature and pressure. The formula used for the calculation of standard molar volume is given below: 

The SI units of standard molar volume are m³/mol. While cm³/mol units are used for liquids and solids and dm³/mol units are used for gases. In the above equation (M) is the molar mass of the targeted compound and (ρ) is the density of the compound under study. For N number of components the equation become complex and shows the summation of the single components involved and is known as excess volume. It is very rational type of excess volume equation. It is written as

Then the molecular volume was obtained through the following formula:

Where (Vm) is the molar volume calculated above and (Na) is the Avogadro’s constant.

1.4 Mole fraction

As discussed earlier, we weighed every single vial used in the experiment without water, with water, with ionic liquid and water, with ionic liquid water and ATP, with pure ionic liquid only and with ATP + water mixture. The reason for these calculations was to calculate the mole fraction of water, ATP and ionic liquid in the whole mixture. Those mole fractions were later used in excess volume calculations. Therefore, the formula used for mole fraction calculation was:

Where (N) is the number of moles, (i) is the name of the component, (m) is the mass of component in grams and (M) is the molar mass of the chemical component used.

Where (X) is the mole fraction, (i) is the name of the component, (Ni) is the number of moles of component under study and (Nt) is the total number of moles of the mixture. Further approaches applied to calculate the mole fractions of three components of a ternary mixture are:

Both calculation methods were employed to find the accuracy of the data obtained from the analysis of data by mass and the most accurate data was selected.

1.5 Thermal expansion coefficient

The coefficient of thermal expansion was found by the density values at various temperatures ranging from 25°C to 75°C with 5°C temperature interval. The formula used for thermal expansion of liquids was:

Where A₁ and A₀ are the density fitting parameters derived from the graphs between density and temperature and T is the temperature at which densities are measured. By finding coefficient of thermal expansion we can easily find out that how much a liquid covered the space in the vile by expansion at high temperatures and how much is its expansion potential at high temperatures. That will help us in explaining a significant effect of carbon capturing at high temperatures in context of the mixture used in the current study.

1.6 Standard molar entropy

The standard molar entropy is the entropy of 1 mole of mixture or a compound. It is actually the amount of disorder in a mixture or a compound when the heat energy contact of the compound or mixture is increased. Here we found the entropy by using the Glasser equation:

In this equation (V) is the molar volume. The outcome is interpreted in results and discussions. As a close system always moves towards an increase in entropy therefore an increase in entropy was studied in this portion.

1.7 Lattice potential energy

The lattice potential energy is the energy of the formation of a crystalline solid with the help of infinite and separated ions. It is the amount of energy that is not possible to calculate experimentally but can be calculated theoretically by the help of various empirical relations. The value of lattice potential energy is always positive and is a very specific amount of energy. Born-Haber cycle helps us to calculate lattice potential or crystal energy thermodynamically. In our study, we calculated the lattice potential energy values by the help of Glasser equation given as follows:

In this equation (ρ) is the density and (M) is the molar mass of the component or mixture. It is seen after the calculations that the lattice potential energy is inversely correlated with the volume of the mixture. It means that on increasing volume the energy between the crystal lattices of the mixture is decreasing due to splitting of the crystal ions.

1.8 Excess volume

The values of densities, molar mass and calculated mole fractions of three components of a ternary mixture were used to find out the excess molar volume of a ternary mixture. The formula used was:

In the above equations (X1, X2 and X3) are the mole fractions of component 1, component 2, and component 3. (ρ) is the density of the mixture and (ρ1, ρ2 and ρ3) are the densities of component 1, component 2 and component 3. The above equation was used to fit the data with the Redlich-Kister equation. The excess volume formula for the binary mixture was used to find the excess molar volume of the ternary mixture. Therefore for binary mixture the excess molar volume formula is given below:

After calculating excess molar volume for binary mixture of ATP/Ionic liquid/Water, we will use the following formula for excess molar volume of ternary mixture and then we will use those calculated excess molar volume and correlate with the Redlich-Kister equation for curve fitting. From that curve fitting we will find Redlich-Kister parameters. These parameters will give us the value of calculated excess molar volume for ternary mixture. The variance was calculated with the following formula:

Where P is the number of parameters and N_exp is the number of experimental data points.

2 Results and Discussion

2.1 Coefficient of thermal expansion

For the ionic liquid 1-hydrogen-3methylimidazolium acetate [(HMIM) C₂H₃O₂] the data of the coefficient of thermal expansion was measured with the help of density measurements over a diverse temperature range from 25°C to 75°C. The results showed an increase in coefficient of thermal expansion on increasing temperature. For the density measured for 25 mg/ml to 45 mg/ml of ATP concentration, for only 45 mg/ml ATP concentration in water and for pure acetate based ionic liquid from 25°C to 75°C (Table A.1, Table A.2, Table A.3, Table A.4, Table A.5, Table B, Table C), the thermal expansion data showed an increase with respect to an increase in temperature. Nelyubina, et al., (2016) also reported the findings of similar nature in its tabular data over coefficient of thermal expansion as that of ours. This indicates that mixture under consideration shows expansion over increasing temperature.

Molar volume

For the ionic liquid 1-hydrogen-3methylimidazolium acetate [(HMIM) C₂H₃O₂] the data of the molar volume was measured by the help of density measurements over a diverse temperature range from 25°C to 75°C. For the density measured for 25 mg/ml to 45 mg/ml of ATP concentration, for only 45 mg/ml ATP concentration in water and for pure acetate based ionic liquid from 25°C to 75°C (Table A.1, Table A.2, Table A.3, Table A.4, Table A.5, Table B, Table C) the molar volume data showed an increase with respect to an increase in temperature. Murray, et al., (2013) reported same work as of ours over molar volume but the pattern of the data was different. This showed that the interactions between the molecules of the experimental mixture were expanding due to temperature effect.

Molecular volume

For the ionic liquid 1-hydrogen-3methylimidazolium acetate [(HMIM) C₂H₃O₂] the data of the molecular volume was measured by the help of density measurements over a diverse temperature range from 25°C to 75°C. For the density measured for 25 mg/ml to 45 mg/ml of ATP concentration, for only 45 mg/ml ATP concentration in water and for pure acetate based ionic liquid from 25°C to 75°C (Table A.1, Table A.2, Table A.3, Table A.4, Table A.5, Table B, Table C), the molecular volume data showed an increase with respect to an increase in temperature. Gonfa, et al., (2015) concluded findings over molecular volume similar to our work.

Standard molar entropy

For the ionic liquid 1-hydrogen-3methylimidazolium acetate [(HMIM) C₂H₃O₂] the data of the standard molar entropy was measured by the help of density measurements over a diverse temperature range from 25°C to 75°C. For the density measured for 25 mg/ml to 45 mg/ml of ATP concentration, for only 45mg/ml ATP concentration in water and for pure acetate based ionic liquid from 25°C to 75°C (Table A.1, Table A.2, Table A.3, Table A.4, Table A.5, Table B, Table C), the molar entropy data showed an increase with respect to an increase in temperature. This showed that the entropy of the system is increasing on increasing temperature for the Table A.1, Table A.2, Table A.3, Table A.4, Table A.5, Table B, Table C. Kurnia, et al., (2011) highlighted findings over standard molar entropy in its paper similar to our work.

Lattice potential energy

For the ionic liquid 1-hydrogen-3methylimidazolium acetate [(HMIM) C₂H₃O₂] the data of the lattice potential energy was measured by the help of density measurements over a diverse temperature range from 25°C to 75°C. For the density measured for 25 mg/ml to 45 mg/ml of ATP concentration, for only 45 mg/ml ATP concentration in water and for pure acetate based ionic liquid from 25°C to 75°C (Table A.1, Table A.2, Table A.3, Table A.4, Table A.5, Table B, Table C), the lattice potential energy data showed a decrease with respect to an increase in temperature. Kermanioryani, et al., (2016) made findings for lattice potential energy that was similar to our work. This showed that the potential energy of the crystal lattice was decreased and hence makes the crystal lattice weak. That weak crystal lattice results in an increase of volume due to expansion in crystal structure.

Excess volume and density

For the ionic liquid 1-hydrogen-3methylimidazolium acetate [(HMIM) C₂H₃O₂] the excess volume data was calculated by the help of new density data and Redlich-Kister equation. The density in Table G showed an increase due to increase in ATP mole fraction in the mixture. The graph showed the result discussed above for density from 25°C to 70°C. On the other hand the excess volume also showed an increase with the increase of ATP mole fraction in the mixture. The phenomenon is reported in graphs given after each excess volume table from Table G, Table G.1, Table G.2, Table G.3, Table G.4, Table G.5, Table G.6, Table G.7, Table G.8, Table G.9 and Table G.10. The calculated values of excess volume were reported to be positive for the entire mole fraction range. Kermanpour and Sharifi, (2014) reported similar excess molar volume findings as that of ours but the property trend was negative.

2.2 Hydrogen Sulphate-based IL

Coefficient of thermal expansion

For the ionic liquid 1-hydrogen-3methylimidazolium Hydrogen sulphate [(HMIM) HSO₄] the data of the coefficient of thermal expansion was measured by the help of density measurements over a diverse temperature range from 25°C to 75°C. The results showed an increase in coefficient of thermal expansion on increasing temperature. For the density measured for 25 mg/ml to 45 mg/ml of ATP concentration, for only 45 mg/ml ATP concentration in water and for pure acetate based ionic liquid from 25°C to 75°C (Table D.1, Table D.2, Table D.3, Table D.4, Table D.5, Table E and Table F), the thermal expansion data showed an increase with respect to an increase in temperature. This indicates that mixture under consideration shows expansion over increasing temperature. Coquelet, et al., (2009) showed similar findings over coefficient of thermal expansion as that of ours.

Molar volume

For the ionic liquid 1-hydrogen-3methylimidazolium hydrogen sulphate [(HMIM) HSO₄] the data of the molar volume was measured by the help of density measurements over a diverse temperature range from 25°C to 75°C. For the density measured for 25 mg/ml to 45 mg/ml of ATP concentration, for only 45 mg/ml ATP concentration in water and for pure acetate based ionic liquid from 25°C to 75°C (Table D.1, Table D.2, Table D.3, Table D.4, Table D.5, Table E and Table F), the molar volume data showed an increase with respect to an increase in temperature. This showed that the interactions between the molecules of the experimental mixture were expanding due to temperature effect. Machanová, et al., (2012) reported molar volume properties similar to our results.

Molecular volume

For the ionic liquid 1-hydrogen-3methylimidazolium hydrogen sulphate [(HMIM) HSO₄] the data of the molecular volume was measured by the help of density measurements over a diverse temperature range from 25°C to 75°C. For the density measured for 25 mg/ml to 45 mg/ml of ATP concentration, for only 45 mg/ml ATP concentration in water and for pure acetate based ionic liquid from 25°C to 75°C (Table D.1, Table D.2, Table D.3, Table D.4, Table D.5, Table E and Table F), the molecular volume data showed an increase with respect to an increase in temperature. Gonfa, et al., (2015) concluded results over molecular volume similar to our findings.

Standard molar entropy

For the ionic liquid 1-hydrogen-3methylimidazolium hydrogen sulphate [(HMIM) HSO₄] the data of the standard molar entropy was measured by the help of density measurements over a diverse temperature range from 25°C to 75°C. For the density measured for 25 mg/ml to 45 mg/ml of ATP concentration, for only 45 mg/ml ATP concentration in water and for pure acetate based ionic liquid from 25°C to 75°C (Table D.1, Table D.2, Table D.3, Table D.4, Table D.5, Table E and Table F), the molar entropy data showed an increase with respect to an increase in temperature. This showed that the entropy of the system is increasing on increasing temperature for the Table D.1, Table D.2, Table D.3, Table D.4, Table D.5, Table E and Table F. Zaitsau, et al., (2016) declared results for standard molar entropy similar to our results.

Lattice potential energy

For the ionic liquid 1-hydrogen-3methylimidazolium hydrogen sulphate [(HMIM) HSO₄] the data of the lattice potential energy was measured by the help of density measurements over a diverse temperature range from 25°C to 75°C. For the density measured for 25 mg/ml to 45 mg/ml of ATP concentration, for only 45 mg/ml ATP concentration in water and for pure acetate based ionic liquid from 25°C to 75°C (Table D.1, Table D.2, Table D.3, Table D.4, Table D.5, Table E and Table F), the lattice potential energy data showed a decrease with respect to an increase in temperature. This showed that the potential energy of the crystal lattice was decreased and hence makes the crystal lattice weak. That weak crystal lattice results in an increase of volume due to expansion in crystal structure. Chhotaray, et al., (2014) reported data over lattice potential energy similar to our nature of study and data analysis.

Excess volume and density

For the ionic liquid 1-hydrogen-3methylimidazolium hydrogen sulphate [(HMIM) HSO₄] the excess volume data was calculated by the help of new density data and Redlich-Kister equation. The density in Table G showed an increase due to increase in ATP mole fraction in the mixture. The graph showed the result discussed above for density from 25°C to 70°C. On the other hand the excess volume also showed an increase with the increase of ATP mole fraction in the mixture. The phenomenon is reported in graphs given after each excess volume table from Table H, Table H.1, Table H.2, Table H.3, Table H.4, Table H.5, Table H.6, Table H.7, Table H.8, Table H.9 and Table H.10. The calculated values of excess volume were reported to be positive for the entire mole fraction range. Wang, et al., (2003) reported similar nature of data over excess molar volume and found the deviations to be positive as that mentioned in our work.

3 Conclusions

It was concluded that the nature of results found over thermophysical properties and excess volume data of 1-hydrogen-3methylimidazolium acetate [(HMIM) C₂H₃O₂] and its mixture with ATP and water for different concentrations of ATP and 1-hydrogen-3methylimidazolium hydrogen sulphate [(HMIM) HSO₄] and its mixture with different concentrations of ATP in water are similar to each other but from the data analyzed, the ionic liquid 1-hydrogen-3methylimidazolium acetate [(HMIM) C₂H₃O₂] will show promising results for carbon capturing at maximum solubility level of ATP in water and ionic liquid mixture than the ionic liquid, 1-hydrogen-3methylimidazolium acetate [(HMIM)C₂H₃O₂]. From the data measured it is also added in the results after the careful examination of the properties that for maximum solubility of ATP in water, we have increased level of carbon capture. Therefore, 1-hydrogen-3methylimidazolium hydrogen sulphate [(HMIM) HSO₄] with maximum solubility in water is suitable for carbon capturing more than 1-hydrogen-3methylimidazolium acetate [(HMIM) C₂H₃O₂]. It was also found that excess volume values remained positive for a variety of mole fractions throughout the calculations. Densities and lattice potential energies were found to decrease with increasing temperature. Whereas, the molecular volume, molar volume, standard molar entropy and coefficient of thermal expansion were increased due to increase the temperature.

Authors’ contributions

Author II conducted research under the supervision of JAA and NM. The author II wrote up the initial draft of manuscript. The final corrections were made by JAA. All authors read and approved the final manuscript.

Acknowledgments

The authors of manuscript are very pleased by Institute of Chemical Engineering and Technology, University of the Punjab Lahore, Pakistan for providing funding to conduct research. The authors are thankful to Dr. Qurban Ali from Centre of Excellence in Molecular Biology, University of the Punjab Lahore, Pakistan for providing help in writing, submitting and publishing of manuscript.

List of Abbreviations

[αpX10^-3 (1/K)] Coefficient of thermal expansion, (V) Molecular Volume, S*10^2 (Molar entropy), Uop*10^3 (Lattice potential energy), Vm*10^2 (Molar Volume), (ρ) Density, Vcc (Specific Volume), V12 (Excess volume of binary mixture 1 and 2), V13 (Excess volume of binary mixture 1 and 3), V23 (Excess volume of binary mixture 2 and 3), VE (Experimental excess volume), VEcal (Calculated excess volume), Ecart (Standard deviation), Ecart^2 (Square of standard deviation), σ (variance).

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