Ionic Liquids IIIA: Fundamentals, Progress, Challenges, and

Downloaded by PENNSYLVANIA STATE UNIV on May 31, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch013

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C h a p t e r 13 Ciencias del Campus de Ourense, Aslagoas 32004, Ourense, Spain Instituto de Tecnologia Química e Biológica, ITQB-2, Universidade Nova de Lisboa, Av. da Republica, Apartado 127, 2780-901, Oeiras, Portugal *Corresponding author: [email protected] *Corresponding author: [email protected]

A study of the behavior of the response functions of the [C mim][BF ] + water ionic binary solution near its liquid— liquid critical point at atmospheric pressure is presented. Phase equilibrium temperatures, which allow to obtain the critical coordinates of this system, are determined. Measurements of the isobaric heat capacity per unit volume in the critical region indicate Ising-like behavior. The slope of the critical line, (dT/dp) , is estimated by means of Prigogine and Defay's equation using experimentally determined excess volu mes and excess enthalpies as a function of temperature. (dT/dp) is found to be near zero. The consequences of this fact for the global critical behavior of second-order volumetric derivatives are discussed. 4

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© 2005 American Chemical Society

175

In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

176


Introduction Liquid-liquid phase transitions in ionic solutions are divided into two main groups depending on the value of the dielectric constant ε of the non-ionic solvent (1,2). According to the restrictive primitive model, when ε is high (solvophobic systems), Coulombic forces are weak and phase transitions are driven by specific interactions (hydrogen bonds, hydrophobic interactions, etc.). On the other hand, Coulombic forces are responsible for phase separation in low dielectric constant media (Coulombic systems). In both cases, interactions are effectively short-ranged: (i) long-ranged Coulombic interactions are screened by low ε and (ιϊ) solvophobic interactions are always short-ranged. Within the frame of the crossover theory (3), these facts mean that Ising-like behavior is expected close to the critical point, whereas, as for other complex systems, non-universal crossover behavior will be observed in the critical domain. Although no general explanation is presently accepted, it is now believed that ionic solutions exhibit crossover from Ising to some type of meanfield tricritical behavior (2,4), which is not yet well understood. Chemical instability and other experimental factors make the interpretation of the existing data a difficult task. These issues are believed to play an important role in the aforementioned incomplete understanding of ionic solutions' criticality. Further knowledge of this subject requires much more reliable data than that currently available. In this regard, the recent appearance of room temperature ionic liquids (RTIU) is of interest since these liquids are generally very stable and are easily controlled for purity. In addition, mixtures of RTILs with water and alcohols show liquid-liquid phase separation at experimentally accessible conditions. Historically, the most commonly studied family of RTILs is that containing l-alkyl-3-methylimidazolium cations [C„mim] combined with the anions, [BF ~] (tetrafluoroborate), or [PF

Experimental

Chemicals [C mim][BF ] (purity > 98 %) was purchased from Solvent Innovation, and Milli-Q water was used. Prior to the measurements, vacuum was applied to the ionic liquid at 60 °C for two days owing to its hygroscopic character and possible contamination with traces of volatile compounds originated at the synthesis procedure. Then, it was placed into a dry box. Before each run, [C mim][BF ] was maintained under vacuum for two hours in order to eliminate extra small traces of water. 4

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Equipment and Techniques Liquid-liquid phase separation temperatures were obtained by means of cloudpoint determinations, where a He-Ne laser light shines through a homemade Pyrex glass cell. The whole device as well as the experimental procedure can be found elsewhere (9). Cloud-point temperatures were determined to an uncertainty of about ± 0.01 K .


178 Heat capacities per unit volume were determined by means of a Micro DSC Π differential scanning calorimeter from Setaram. This apparatus as well as the experimental procedure (scanning method) were described elsewhere (10). Isobaric runs at a scanning rate β = 0.01 K'min" were performed. The device's ability to obtain reliable data in the critical region was established in a previous work (11).


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Excess volumes were obtained by vibrating-tube densitometry using an Anton-Paar 512P cell (12), whereas Calvet calorimetry was employed to determine the excess enthalpies (7). The latter were obtained at a high single temperature, 333.15 K. This measuring temperature -usually, h is determined at 298.15 K - was selected owing to the large viscosity of [C mim][BF4] at room temperature, a fact which produced a very long mixing process with a nearly vanishing calorimetric signal. The uncertainties in v and h are estimated to be ± 0.003 cm -mor and 1 %, respectively. The procedure for obtaining h as a function of temperature involves the determination of excess molar isobaric heat capacities c (7). They were determined combining Cp/V with density E

4

E

3

E

!

E

E

measurements. An uncertainty about ± 0.15 J mor -K" in Cp using a e

!

1

1

scanning rate of 0.25 K-min" was attained.

Results and Discussion

Coexistence Curve Table I shows the phase separation temperatures at atmospheric pressure as a function of the ionic liquid weight fraction, w. The critical composition is located at the low-ionic liquid mole fraction region resulting in a highly unsymmetrical curve. This is partially due to the high difference between the molar volumes of the two liquids. As can be seen in Fig. 1, a nearly symmetrical curve is obtained if w is chosen as the composition variable. Data were fitted to a scaling function: |H>-U>

1-J

ιΐ/ιΛ

(D


179 where F , w , A, and b are fitting parameters, the values of which are listed in Table I. The critical coordinates (w ,F ) for this system are 0.489 (equivalently, x = 0.070) and 277.59 K, respectively. In this particular case of the phase diagram, and due to the lack of more data points, the parameters reported in Table I should be considered as mere fitting parameters with no special significance in terms of critical phenomena. c

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Heat Capacity per unit Volume Figure 2 shows isobaric heat capacities per unit volume, Cp/K, for a critical mixture as a function of temperature. The λ-curve observed is a clear indication of Ising-like behavior in the immediate proximity of the critical point, confirming the expectations already mentioned in the Introduction. Data in the homogenous region were fitted to the following expression which contains the power-law and regular terms: 280

w Figure 1. Liquid-liquid phase equilibrium as a T-w curve. The two-phase region is located inside the envelope.


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Table L Liquid-liquid phase separation temperatures, Γ, at atmospheric pressure for H>[C mim][BF ] + ( 1 - M>)water mixtures and parameters of Eq.(l).


4

4

w

τ/κ

0.1992

τ/κ

w

266.11

0.4568

277.71

0.2415

271.94

0.5217

277.46

0.2819

275.20

0.5840

277.34

0.3502 0.3854

277.23 277.65

0.7531

269.96

w

b

Λ

0.218

0.0822

277.59

0.489

C / K = B+E p

c

I

C

T.

A*(T-T '

J «I +

C

r,

Γ

(2)

,

+

where α and Λ are the critical exponent and critical amplitude of Cp/K, respectively. Β and £ are coefficients that characterize the regular part. The fitting strategy consisted of fixing a to its universal value (0.110) allowing the remaining ones (T included) to vary. The results thus obtained as well as the standard deviation of the fit can be found in Table II. The value of A* (0.017 J'K~ -cm~ ) lies within the typical range for molecular liquids. This is a consequence of the short-range nature of the interactions responsible for phase separation - in the present case, solvophobic interactions owing to the high ε of water. Unfortunately, the quality of the data do not permit us to obtain information about crossover behavior. c

l

3

Slope of the Critical Line. Prigogine and Defay's equation has proven to be an useful tool for predicting the value of the slope of the critical line (dT/dp) (7,13). In its simplified, restrictive version it is expressed as: c

(3)



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274

276

278 Γ(Κ)

280

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Figure 2. Isobaric heat capacities per unit volume Cp/Vfor a critical mixture.

Table II. Parameters of Eq. (2) and standard deviation a. Β

a

4.565 -8.562 0.017 0.110 277.60 0.05


182 Table ΠΙ. Coefficients of Eq. (4) and standard deviations ) and h (w) curves at 298.15 Κ (die nearest working temperature to die critical one) are given, illustrates the fulfillment of the expected constraints. E

E

E

E

E

E

Figure 4 shows both v (7) and A (7) for a near-critical composition. A quadratic temperature dependence, which is much more marked for v , is found in both cases. The quantity Tv /h was obtained at each temperature and, then, E

E

E


183 extrapolated to Τ - T& where Prigogine and Defay's equation has its highest validity. The results are reported in Table V. Taking into account that (dT/dp) typically ranges from 2 to 20 mKbar , the obtained result, 0.4 mKbar" , represents a quasi-null value. This can graphically be understood by observing the nearly vanishing v value at the critical point (see Fig. 4). c

1

1


E

0.5

2500

0.4

2000

0.3

C

1500

0.2

1000 j-

0.1

500

1

0.0 0.0

1.0

0.5 W

0.0

0.5

1.0

W

E

Figure 3. Excess volumes v and excess enthalpies h* for w{C mim][BF ] + (l-w)water at 298.15 Κ and 1 bar. 4

E

4

E

Figure 4. Excess volumes v and excess enthalpies h for a critical mixture as a function of T. (—) Second-order polynomial fit.


184

E

Table IV. Excess molar volumes, v , and excess molar enthalpies, Λ , for H>[C mim][BF l + (1 - *>)water * a function of temperature, T. s

4

4

V (cm mol ') e


w

3

Γ= 298.15 Κ Γ= 303.15 Κ Τ= 323.15 Κ 0.2112 0.3344 0.4563 0.5609 0.6594 0.7459 0.8441 0.9123 0.9502 0.9817 0.9924

0.045 0.081 0.132 0.181 0.255 0.338 0.453 0.480 0.437 0.251 0.137

0.078 0.147 0.233 0.309 0.409 0.510 0.626 0.628 0.543 0.259 0.109

0.059 0.100 0.160 0.214 0.296 0.382 0.498 0.521 0.468 0.255 0.110 E

0.078 0.156 0.255 0.344 0.451 0.561 0.678 0.671 0.573 0.273 0.120

1

h (J-mor )

w

Τ-298.15 Κ T= 303.15 Κ Γ= 323.15 Κ 0.1540 0.2871 0.4390 0.4906 0.7263 0.8622 0.9020 0.9420 0.9772

Τ= 333.15 Κ

216 459 797 941 1706 2160 2134 1677 1015

225 476 822 968 1743 2187 2155 1682 1008

271 549 924 1079 1895 2295 2237 1702 983

Γ= 333.15 Κ 296 590 979 1139 1974 2350 2277 1711 973


185 Ε

Ε


Table V. Experimental Γν /Λ for a eritieal mixture as a function of Γ and at atmospheric pressure. 1

Γ(Κ)

rvWCmK-bar" )

333.15 323.15 303.15 298.15 277.59

8.2 7.6 5.5 4.6 0.4*

•Extrapolated value at T=

Griffiths and Wheeler's geometrical picture of critical phenomena (8) predicts very small critical anomalies (experimentally undetectable) in the isobaric thermal expansivity Op and the isothermal compressibility ATT when (dT/dp) approaches zero. Unfortunately, the κ critical anomaly cannot be experimentally detected even in the case of a large (dT/dp) . Furthermore, density measurements as a function of temperature would also show a very small (almost undetectable) critical anomaly for Op. In future work, it would be desirable to determine directly the measured value of (dT/dp) as well as check the predicted value from Prigogine and Defay's equation using experimental (not extrapolated) v (w ,r ) and A (w ,r ) values. Current research is being performed in our laboratories to reach this goal. c

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E

E

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Acknowledgements Research at the Universidad de Vigo laboratory was supported by #BFM200309295 and #PGIDIT-03-DPI-38301-PR. Authors are indebted to the Secretaria de Estado de Politica Cientifica y Tecnologica (Ministerio de Ciencia y Tecnologia de Espafïa) and to the Direccion Xeral de I + D (Xunta de Galicia). Work at ITQB was financially supported by Fundaçâo para a Ciência e Tecnologia, Portugal, under contracts Nos. POCTI/EQU/34955 and POCTI/EQU/35437. V.N.-V. and J.M.S.S.E. are grateful to Fundaçâo para a Ciência e Tecnologia for doctoral fellowships.


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References


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Equations of State of Fluids and Fluid Mixtures (Chapter 17).; Sengers, J. V.; Kayser, R. F.; Peters, C. J.; White Jr, H. J. Eds.; Amsterdam; Elsevier, 2000. 2. Gutkowski, K.; Anisimov, Μ. Α.; Sengers, J. V. J. Chem. Phys. 2001, 114, 3133. 3. Wagner, M ; Stanga, O.; Schröer, W. Phys. Chem. Chem. Phys. 2003, 5, 3943. 4. (a) Albright, P. C.; Chen, Ζ. Y . ; Sengers, J. V. Phys. Rev. Β (Rapid Comm.) 1987, 36, 877. (b) Anisimov, Μ. Α.; Povodyrev, Α. Α.; Kulikov, V. D.; Sengers, J. V. Phys. Rev. Lett. 1995, 75, 3146. (c) Melnichenko, Y. B.; Anisimov, Μ. Α.; Povodyrev, Α. Α.; Wignall, G. D.; Sengers, J. V.; Van Hook, W. A. Phys. Rev. Lett. 1997, 79, 5266. 5. (a) Najdanovic-Visak, V.; Esperanca, J. M . S. S.; Rebelo, L. P. N.; Nunes da Ponte, M . ; Guedes, H. J. R.; Seddon, K. R.; Szydlowski, J. Phys. Chem. Chem. Phys. 2002, 4, 1701; (b) Visser, A . E.; Swatloski, R. P.; Reichert, W. M . ; Griffin, S. T.; Rogers, R. D. Ind. Eng. Chem. Res. 2000, 39, 3596; (c) Swatloski, R. P.; Holbrey, J.D.; Rogers, R. D. Green Chem. 2003, 5, 361. 6. (a) Najdanovic-Visak, V.; Esperanca, J. M . S. S.; Rebelo, L. P. N . ; Nunes da Ponte, M . ; Guedes, H. J. R.; Seddon, K. R.; de Sousa, H. C.; Szydlowski, J. J. Phys. Chem. Β 2003, 107, 12797, and references therein, (b) Najdanovic-Visak, V.; Serbanovic, Α.; Esperanca, J. M . S. S.; Guedes, H. J. R.; Rebelo, L. P. N.; Nunes da Ponte, M . Chem. Phys. Chem. 2003, 4, 520. (c) Rebelo, L. P. N.; Najdanovic-Visak, V.; Gomes de Azevedo, R.; Nunes da Ponte, M . ; Guedes, H. J. R.; Visak, Z. P.; de Sousa, H. C.; Szydlowski, J.; Canongia Lopes, J. N . ; Cordeiro, T. C.; this same book, and references therein. 7. Rebelo, L. P. N.; Najdanovic-Visak, V.; Visak, Z.P.; Nunes da Ponte, M . ; Troncoso, J.; Cerdeiriña, C.A.; Romaní, L. Phys. Chem. Chem. Phys. 2002, 4, 2251. 8. Griffiths, R. B.; Wheeler, J. C.; Phys. Rev. A 1970, 2, 1047. 9. de Sousa, H. C.; Rebelo, L. P. N . J. Chem. Thermodyn. 2000, 32, 355. 10. Cerdeiriña, C. Α.; Miguez, J. Α.; Carballo, E.; Tovar, C. Α.; de la Puente, E.; Romani, L. Thermochim. Acta 2000, 347, 37. 11. Cerdeiriña, C. Α.; Troncoso, J.; Carballo, E.; Romani, L. Phys. Rev. E 2002, 66, 031507. 12. Gomes de Azevedo, R.; Szydlowski, J.; Pires, P.F.; Esperança. J.M.S.S.; Guedes, H.J.R.; Rebelo, L.P.N. J. Chem. Thermodyn. 2004, 36, 211. 13. Myers, D. B.; Smith, R. Α.; Katz, J.; Scott, R. L.; J. Phys. Chem. 1966, 70, 3341.


Ionic Liquids IIIA: Fundamentals, Progress, Challenges, and

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