aa r X i v : . [ c ond - m a t . m t r l - s c i ] J un Vapor Pressure of Ionic Liquids
Markus Bier ∗ and S. Dietrich Max-Planck-Institut f¨ur Metallforschung, Heisenbergstr. 3, 70569 Stuttgart,Germany, and Institut f¨ur Theoretische und Angewandte Physik,Universit¨at Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany (Dated: 11 November, 2009)We argue that the extremely low vapor pressures of room temperature ionic liquids near theirtriple points are due to the combination of strong ionic characters and of low melting temperatures.
An extremely low vapor pressure (e.g., ca. 100 pPaat 298 K for [C mim][PF ] [1] compared with 3 kPa at298 K for H O [2]) is one of the extraordinary prop-erties of room temperature ionic liquids (RTILs), i.e.,molten salts with melting points below 100 ◦ C. As aconsequence, RTILs such as [C mim][PF ] at 298 K areliquids which do not evaporate significantly even underultrahigh vacuum (UHV) conditions (i.e., for a pressurerange 100 nPa . . .
100 pPa [3]), which offers the possibil-ity to use RTILs, e.g., as substitutes for volatile organicsolvents [4, 5]. Only a decade ago RTILs were still de-scribed as “non-volatile” [4], but meanwhile direct mea-surements of their vapor pressures and enthalpies of va-porization at elevated temperatures have been carriedout [6, 7]; even the distillation of RTILs [8] has beenachieved. Since non-ionic liquids (NILs, such as benzeneand water) exhibit triple point pressures p above 1 Pa(see Tab. I(a)), one might be tempted to attribute the ex-tremely low triple point pressures of RTILs exclusively totheir ionic character. However, a comparision of RTILs,which are composed of organic ions, with inorganic fusedsalts (IFSs), which are also of ionic character, reveals thatthe triple point pressures of the latter are above 1 Pa (seeTab. I(c)), such as for NILs. This rules out that the ioniccharacter is the only reason for the low triple point pres-sures of RTILs. We shall show below that it is in fact the combination of the melting point to occur below roomtemperature and of the ionic character of RTILs whichleads to the observed low triple point pressures. In otherwords, any substance with a strong ionic character ful-filling the definition of an RTIL inevitably exhibits ex-tremely low vapor pressures near its triple point.Figure 1 displays the experimental vapor pressures p sat ( T ) for liquid-vapor coexistence at temperature T forthe non-polar liquid benzene (C H , see Ref. [2]), the hy-drogen bond forming liquid water (H O, see Ref. [2]), theparadigmatic RTILs [C mim][dca], [C mim][NTf ], and[C mim][NTf ] (see Refs. [7, 9]), as well as fused cadmiumchloride (CdCl ) and sodium chloride (NaCl) as represen-tatives of IFSs (see Ref. [10]). At low temperatures theboiling curves terminate at the triple point temperature T (see Tab. I and Refs. [2, 11–13]), which is close thestandard melting temperature of the corresponding sub-stance because the melting curve is very steep. At hightemperatures the boiling curves of the NILs and the IFSs NaClCdCl [C mim][NTf ][C mim][NTf ][C mim][dca]H OC H IFSRTILNIL
T / K p s a t ( T ) / P a − FIG. 1: Experimental vapor pressures p sat ( T ) at liquid-vaporcoexistence of non-ionic liquids (NILs), room temperatureionic liquids (RTILs), and inorganic fused salts (IFSs) as afunction of temperature T for the non-polar liquid benzene(C H , see Ref. [2]), the hydrogen bond forming liquid water(H O, see Ref. [2]), the paradigmatic RTILs [C mim][dca],[C mim][NTf ], and [C mim][NTf ] (see Refs. [7, 9]), as wellas fused cadmium chloride (CdCl ) and sodium chloride(NaCl) as examples of IFSs (see Ref. [10]). At low temper-atures all curves terminate at the corresponding triple pointtemperature T (see Tab. I), which is close to the standardmelting temperature of that substance. At high temperturesthe boiling curves for the RTILs terminate at the decomposi-tion temperature T d , whereas the boiling curves of the otherliquids end at their critical points (see Tab. I). Room tem-perature T = 298 K and ambient pressure p = 10 Pa areindicated. terminate at their critical temperatures T c (see Tabs. I(a)and (c) and Refs. [2, 14]), whereas RTILs decompose ata substance specific decomposition temperature T d (seeTab. I(b) and Refs. [4, 11, 15]). As it is apparent fromFig. 1, RTILs do not boil at ambient pressure p = 10 Pabecause boiling is preempted by decomposition; conse-quently Tab. I(b) displays only extrapolated standardboiling temperatures T extr b for RTILs.In order to understand the position of the boilingcurves of RTILs in Fig. 1, we note that with respect tothe strength of the particle-particle interaction, RTILslie in between NILs, which interact via relatively weakdispersion forces and possibly hydrogen bonds, and IFSs, (a) NIL T / K p / Pa T b / K T c / K Refs.C H . . . O 273 . . . . T / K p / Pa T d / K T extr b / K Refs.[C mim][dca] 267 1 . × −
695 719 [9, 11][C mim][NTf ] 271 8 . × −
712 906 [7, 13, 15][C mim][NTf ] 264 7 . × −
698 857 [7, 13, 15](c) IFS T / K p / Pa T b / K T c / K Refs.CdCl
837 214 1233 ? [10, 12]NaCl 1074 46 1738 > T and p denote the temperature and the pressure, respectively, at thetriple point, T c is the critical temperature, and T d denotesthe temperature for the onset of decomposition of an RTIL[11, 15]. T b denotes the standard boiling temperature at am-bient pressure p = 10 Pa for NILs and IFSs, whereas thestandard boiling temperatures T extr b for RTILs are estimatedby extrapolation [19] because boiling of RTILs is preemptedby decomposition. which interact predominantly via strong Coulomb forces.Due to the larger size of the RTIL ions and a possible de-localization of the charge their interaction is, however,weaker than that of IFS ions. Hence, ignoring for thetime being the decomposition of RTILs at T d , the mo-lar enthalpies of vaporization ∆ vap H ( p ) > p are expected to be ordered as ∆ vap H NIL ( p ) < ∆ vap H RTIL ( p ) < ∆ vap H IFS ( p ). On the other hand, inthe spirit of Trouton’s rule [16], the molar entropies ofvaporization ∆ vap S ( p ) at pressure p are expected to de-pend only weakly on the kind of substance, because theirvalues are dominated by the translational and rotationaldegrees of freedom whereas vibrational and electronicmodes and the structural arrangements contribute onlyas small corrections [17]. Data for organic and inorganicliquids tabulated in Refs. [12, 18] suggest a Trouton-likerule ∆ vap S ( p ) ≈ (95 ±
15) J / mol at ambient pressure p = 10 Pa. According to ∆ vap H ( p ) = T b ( p )∆ vap S ( p )[16] with T b ( p ) denoting the boiling temperature at pres-sure p one expects the relation T NIL b ( p ) < T RTIL b ( p ) IFS3 , see Tab. I) induced by a large differ-ence in standard melting temperatures. The mechanismfor leading to the low standard melting temperatures ofRTILs has been explained in terms of a frustrated crys-tallization due to asymmetric ion shapes, charge delocal-ization, packing inefficiency, and conformational degen-eracy [20–23]. Hence the extremely low vapor pressues ofRTILs near their triple points can be understood on verygeneral grounds based on both a strong ionic character and low melting temperatures; the conclusions are inde-pendent of substance specific properties which explainswhy this phenomenon is a common feature of RTILs.In summary, we have shown that near its triple pointthe vapor pressure of a room temperature ionic liquid ofstrong ionic character is very small, because it dependsexponentially on the ratio of a large enthalpy of vapor-ization — which is almost as large as that of inorganicsalts — and a small thermal energy near the triple point,which is as small as that of non-ionic liquids. Accordingto p sat ( T ) ∼ exp( − ∆ vap H/ ( RT )), where the prefactoris approximately independent of the kind of substance,an increase of ∆ vap H , reflecting the ionic character ofroom temperature ionic liquids relative to non-ionic liq-uids, leads to a downshift of p sat ( T ). For room tempera-ture ionic liquids these low vapor pressures are physicallyaccessible due to their low triple points, induced by theirlow melting temperature — which is part of the definitionof room temperature ionic liquids (see Fig. 1). The evenstronger ionic character of inorganic fused salts wouldin principle lead to even lower vapor pressures; however,these cannot be reached for their liquid state becausethey are preempted by a significantly higher freezing andthus triple point temperature (see Fig. 1). ∗ Electronic address: [email protected] [1] Y. U. Paulechka, G. J. Kabo, A. V. Blokhin, O. A. Vy-drov, J. W. Magee, and M. Frenkel, J. Chem. Eng. Data , 457 (2003).[2] E. W. Lemmon, M. O. McLinden, and D. G.Friend, Thermophysical Properties of Fluid Systems , inNIST Chemistry WebBook, NIST Standard ReferenceDatabase Number 69, edited by P. J. Linstrom and W.G. Mallard, http://webbook.nist.gov [3] P. A. Redhead, J. Vac. Sci. Technol. A , S12 (2003).[4] P. Wasserscheid and W. Keim, Angew. Chem. Int. Ed. , 3773 (2000).[5] R. Ludwig and U. Kragl, Angew. Chem. Int. Ed. , 6582(2007).[6] Y. U. Paulechka, Dz. H. Zaitsau, G. J. Kabo, and A. A.Stechan, Thermochim. Acta , 158 (2005).[7] Dz. Zaitsau, G. J. Kabo, A. A. Stechan, Y. U. Paulechka,A. Tschersich, S. P. Verevkin, and A. Heintz, J. Phys.Chem. A , 7303 (2006).[8] M. J. Earle, J. M. S. S. Esperan¸ca, M. A. Gilea, J. N. C.Lopes, L. P. N. Rebelo, J. W. Magee, K. R. Seddon, andJ. A. Widegren, Nature , 831 (2005).[9] V. N. Emel’yanenko, S. P. Verevkin, and A. Heintz, J.Am. Chem. Soc. , 3930 (2007).[10] J. L. Barton and H. Bloom, J. Phys. Chem. , 1413(1956).[11] C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. V. K. Aki, and J. F. Brennecke, J. Chem. Eng. Data ,954 (2004).[12] D. R. Lide (Ed.), CRC Handbook of Chemistry andPhysics (CRC Press, Boca Raton, 1998).[13] Y. U. Paulechka, A. V. Blokhin, G. J. Kabo, and A. A.Stechan, J. Chem. Thermodyn. , 866 (2007).[14] P. J. McGonigal, J. Phys. Chem. , 1931 (1963).[15] H. Tokuda, K. Hayamizu, K. Ishii, M. A. Bin Hasan Su-san, and M. Watanabe, J. Phys. Chem. B , 6103(2005).[16] P. W. Atkins, Physical chemistry , 6th Ed. (Oxford Uni-versity Press, Oxford, 1998).[17] P. S. Vincett, J. Phys. Chem. , 2797 (1978).[18] G. J. Janz, Molten Salts Handbook (Academic Press, NewYork, 1967).[19] L. P. N. Rebelo, J. N. C. Lopes, J. M. S. S. Esperan¸ca,and E. Filipe, J. Phys. Chem. B , 6040 (2005).[20] K. R. Seddon, J. Chem. Tech. Biotechnol. , 351 (1997).[21] A. S. Larsen, J. D. Holbrey, F. S. Tham, and C. A. Reed,J. Am. Chem. Soc. , 7264 (2000).[22] J. D. Holbrey, W. M. Reichert, M. Nieuwenhuyzen, S.Johnston, K. R. Seddon, and R. D. Rogers, Chem. Com-mun. , 1636.[23] N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev.37