J. Phys. Chem. B 2007, 111, 4109-4115
4109
Solubility of Phosphonium Ionic Liquid in Alcohols, Benzene, and Alkylbenzenes Urszula Doman´ ska*,† and Lidia M. Casa´ s‡,§ Physical Chemistry DiVision, Faculty of Chemistry, Warsaw UniVersity of Technology, Noakowskiego 3, 00-664 Warsaw, Poland, and Chemical Engineering Department, Faculty of Sciences, UniVersity of Vigo, Lagoas Marcosende s/n, 36200 Vigo, Spain ReceiVed: January 13, 2007; In Final Form: February 20, 2007
The (solid + liquid) phase equilibria and (liquid + liquid) phase equilibria of binary mixtures containing quaternary phosphonium salt-tetrabutylphosphonium methanesulfonate and alcohols or alkylbenzenes were investigated. The systems {[(CH3CH2CH2CH2)4P][CH3SO3] + 1-butanol, or 1-hexanol, 1-octanol, 1-decanol, or 1-dodecanol} and {[(CH3CH2CH2CH2)4P][CH3SO3] + benzene, or toluene, ethylbenzene, or propylbenzene} have been measured by a dynamic method at a wide range of temperatures from 220 to 386 K. Solid-liquid equilibria with immiscibility in the liquid phase were detected with the aromatic hydrocarbons ethylbenzene and propylbenzene. The basic thermodynamic properties of pure ionic liquidsthe melting point, enthalpy of fusion, enthalpy of solid-solid-phase transition, and glass transitionshave been determined by differential scanning calorimetry. The experimental data of systems with alcohols were correlated by means of the UNIQUAC ASM and NRTL1 equations and of systems with alkylbenzenes with Wilson and NRTL equations utilizing parameters derived from the (solid + liquid) equilibrium. The root-mean-square deviations of the solubility temperatures for all calculated data are dependent upon the particular system and the particular equation used.
Introduction An understanding of solid-liquid equilibria is of paramount importance for the design of separation processes, especially antisolvent crystallization. There is a pressing need to develop better solvents for separation, especially in the cases of nonideal complexing mixtures. There are numerous publications reporting the suitability of imidazolium ionic liquids (ILs) as entrainers for extractive distillation and as separation solvents for liquidliquid extraction.1-6 With an increasing awareness of how the structure of an IL affects its physical properties, the advantages of ILs as compared to volatile organic solvents become evident. Until now, we have focused on the determination of phase equilibria, (solid + liquid) phase equilibria (SLE) and (liquid + liquid) phase equilibria (LLE), of ionic liquids based on the imidazolium cation with popular organic solvents.7-11 The main disadvantage of imidazolium-based ionic liquids is their relatively high cost for bulk applications, whereas very well-known quaternary ammonium salts seem to be much cheaper. The ammonium ionic liquids presented in our previous work were tetraalkylammonium bromides with a hydroxyl group as a side chain.12-15 To the best of our knowledge, the phase equilibria of phosphonium ionic liquids were not measured. The results can be compared to the generally known ammonium ILs, which could be used as a phase-transfer catalyst or as the additive to new processes for large-scale electroplating, or as substanceenhancing lubricating properties of motor oils, or as substances exhibiting antimicrobial and antifungal activity and antielectrostatic effects.15 More complex substances with functional groups or polar anions (here, [CH3SO3]-) are capable of having additional * To whom correspondence should be addressed. E-mail: ula@ ch.pw.edu.pl. † Warsaw University of Technology. ‡ University of Vigo. § On leave at Warsaw University of Technology.
interactions with polar solvents. Given their structure and diversity of functionality, they are capable of most types of interactions (i.e., dispersive, π-π, n-π, hydrogen bonding, dipolar, ionic/charge-charge, van der Waals forces). In every solution, there can be a number of different (in terms of type and strength) and often simultaneous solute-solvent interactions.16-19 The ILs can be totally miscible or immiscible with alcohols or alkylbenzenes that depend on the structure of cation, or anion. Complete understanding of the phase behavior of ILs with organic solvents is an important issue. The majority of studies have focused upon the solid-liquid or liquid-liquid phase behavior of ionic liquids with alcohols and the subsequent determination of the upper critical solution temperatures (UCST).10-16,19-24 Many factors that control the phase behavior of ionic liquids with other liquids, especially with alcohols, have been discussed in recently published papers focusing on pyridinium ILs, as opposed to imidazolium ILs,20 or on ammonium ILs, as opposed to imidazolium ILs.14 The impact of different alcohol and IL characteristics, including alcohol chain length, cation (alkyl or alkoxy) chain length, and cations with different substituent groups on the nitrogen in ammonium ILs, have been discussed.7,10,12-16,19-24 In every system measured, the LLE or, for the crystalline solutes, the SLE were observed as a simple eutectic binary mixture with immiscibility in the liquid phase. All systems exhibit UCSTs, with low solubility of the IL in the alcohol and high solubility of the alcohol in the IL. Increasing the alkyl chain length on the alcohol causes an increase in the UCST of the system. By contrast, increasing the alkyl chain length on the cation (imidazolium salts) results in a decrease in the UCST of the system or in an increase in the solubility (ammonium salts). The solubility of imidazolium or ammonium IL in alkylbenzenes showed UCST behavior with very low solubility of an aromatic solvent in the IL. An increase in the alkyl chain length
10.1021/jp070293j CCC: $37.00 © 2007 American Chemical Society Published on Web 03/29/2007
4110 J. Phys. Chem. B, Vol. 111, No. 16, 2007
Doman´ska and Casa´s
TABLE 1: Investigated Compound: Structure, Name, and Abbreviation of Name structure
name
abbreviation
tetrabutylphosphonium methanesulfonate [(CH3CH2CH2CH2)4P][CH3SO3]
of an n-alkane at a benzene ring resulted in an increase in the UCST.8,9,11,15 Recently published by us, SLE and LLE of (benzyl)dimethylalkylammonium nitrate demonstrated the influence of the structure of the IL on its phase behavior.25 Although the main interest in ILs is still focused on organic chemistry synthesis or electrochemistry, various aspects of physical chemistry in ionic liquids are being discussed now in the literature. The wide overview of the physicochemical aspects of air- and water-stable ionic liquids was recently presented by Endres and Al Abedin.26 The other review, more about thermodynamic and thermophysical properties, such as phase equilibria VLE, LLE, and SLE; activity coefficients; heats of mixing; gas solubility data; densities; and transport properties such as viscosity, electric conductivity, and mutual diffusion coefficients, speed of sound, and other properties, were presented by Heintz27 and Hu and Xu.28 In our previous work, the LLE or SLE of binary systems comprising 1-hexyloxymethyl-3-methylimidazolium bis(trifluoromethylsylfonyl)imide with an alcohol (1-butanol, 1-hexanol, or 1-octanol), a ketone (3-pentanone, or cyclopentanone), or water were measured, as were binary systems comprising 1-hexyloxymethyl-3-methylimidazolium tetrafluoroborate with an alcohol (methanol, ethanol, 1-butanol, 1-hexanol, or 1octanol), a ketone (3-pentanone, or cyclopentanone), or water.11 In addition, the solubility of two dialkoxyimidazolium salts, 1,3dihexyloxymethylimidazolium bis(trifluoromethylsylfonyl)imide in an alcohol (1-butanol, 1-hexanol, 1-octanol, or 1-decanol), in a hydrocarbon (benzene, hexane, or cyclohexane), and in water along with the systems 1,3-dihexyloxymethylimidazolium tetrafluoroborate, in an alcohol (1-hexanol, 1-octanol, or 1-decanol) and in water, have been previously determined in our laboratory.11 SLE of quaternary ammonium ILs in alcohols, hydrocarbons, and water have previously been measured, as well.12-15,25 The experimental SLE phase diagrams investigated for four quaternary ammonium salts, ethyl(2-hydroxyethyl)dimethylammonium bromide, C2Br; (2-hydroxyethyl)dimethylpropylammonium bromide, C3Br; butyl(2-hydroxyethyl)dimethylammonium bromide, C4Br; and hexyl(2-hydroxyethyl)dimethylammonium bromide, C6Br; in water and alcohols, have been published in the literature.12,13 Other anions, including [BF4]-, [PF6]-, [(CN)2N ]-, and [NTf2]-, have also been investigated.14,15 This paper follows the discussion of room-temperature ionic liquids and is a continuation of our systematic study of the impact of different factors on the phase behavior of ILs. This work includes physicochemical properties and the phase equilibria measurements of the quaternary phosphonium salt tetrabutylphosphonium methanesulfonate [(CH3CH2CH2CH2)4P][CH3SO3], (B4PCH3SO3), with 1-butanol, 1-hexanol, 1-octanol, 1-decanol, or 1-dodecanol and with benzene, toluene, ethylbenzene, propylbenzene. The chemical name and the abbreviation of the substance under study are presented in Table 1. Characteristics investigated here include the effect of the alkyl
B4PCH3SO3
TABLE 2: Thermophysical Constants of Pure Salt (DSC Data) and Some Solvents compound
Tfus,1 K
∆fusH1 kJ mol-1
Ttr,1 K
∆trH1 kJ mol-1
B4PCH3SO3 1-octanolb 1-decanolb 1-dodecanolb benzenec
335.35 258.35 280.15 296.95 278.61
11.105 23.70 28.79 38.42 9.834
307.09; 227.7 (g)a
3.19
a (g), glass transition. ∆Cp at the glass transition is 126 J mol-1 K-1. b Ref 29. c Ref 30.
chain length of the alcohol (solvent) and the effect of the alkyl chain length at the benzene ring (solvent). For a better understanding of the IL behavior and with a view to application in chemical engineering or development of thermodynamic models, reliable experimental data are required. A basic IL can be expected to interact with solvents with both accepting and donating sites. On the other hand, polar solvents such as alcohols are very well-known to form a hydrogenbonded net with both high enthalpies and constants of association. In this work, higher interaction may be expected between the anion of the phosphonium salt and the polar solvent as an alcohol. The better solubility of an IL in a chosen solvent means possible hydrogen bonding between the IL and the solvent. The determination of the IL-solvent interaction of this IL via the solubility measurements, especially with alcohols and alkylbenzenes, has been performed. The characterization and purity of the compounds were obtained by the water content (Fischer method) and differential scanning microcalorimetry (DSC). The melting point, the glass transition temperature, enthalpy of fusion, and enthalpy of solid-solid-phase transition were determined by DSC. Experimental Procedures and Results Materials. Investigated tetrabutylphosphonium methanesulfonate (CAS 98342-59-7) was obtained from Flucka, BuchsGG, Switzerland. The sample’s purity was g98 mass %, and it was used without any purification. All solvents were delivered from Sigma-Aldrich Chemie GmbH, Stenheim, Germany. Before direct use, they were fractionally distilled over different drying reagents to the mass fraction purity g99.8 mass %. The solvents were stored over freshly activated molecular sieves of type 4 Å (Union Carbide). The brief characterization of IL and some solvents is presented in Table 2. All compounds were checked by GLC analysis, and no significant impurities were found. Water Content. Water content was analyzed by the Karl Fischer titration technique (method TitroLine KF). Samples of all compounds were dissolved in methanol and titrated with steps of 2.5 µL. Analysis using the Karl Fischer technique showed that the water content in the solvents was
