Thermophysical properties of pure and water-saturated tetradecyltrihexylphosphonium-based ionic liquids Catarina M.S.S. Neves, Pedro J. Carvalho, Mara G. Freire, João A.P. Coutinho ⇑ Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal article info Article history: Received 17 December 2010 Received in revised form 13 January 2011 Accepted 27 January 2011 Keywords: Solubility Density Viscosity Ionic liquids Water Tetradecyltrihexylphosphonium Bromide Chloride Bis(trifluoromethylsulfonyl)imide Decanoate Methanesulfonate Dicyanamide Bis(2,4,4-trimethylpentyl)phosphinate abstract In this work, the solubility of water in several tetradecyltrihexylphosphonium-based ionic liquids at 298.15 K, and densities and viscosities of both pure and water-saturated ionic liquids in a broad temper- ature range were measured. The selected ionic liquids comprise the common tetradecyltrihexylphospho- nium cation combined with the following anions: bromide, chloride, bis(trifluoromethylsulfonyl)imide, decanoate, methanesulfonate, dicyanamide and bis(2,4,4-trimethylpentyl)phosphinate. The isobaric thermal expansion coefficients for pure and water-saturated ionic liquids were determined based on the density dependence with temperature. Taking into account that the excess molar volumes of the cur- rent hydrophobic water-saturated ionic liquids are negligible, the solubility of water was additionally estimated from the gathered density data and compared with the experimental solubilities obtained. Moreover, the experimental densities were compared with those predicted by the Gardas and Coutinho model while viscosity data were correlated using the Vogel–Tammann–Fulcher method. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Ionic liquids (ILs) are widely recognized as promising new sol- vents for industrial applications. Several processes and products have been developed and are commercially available nowadays [1]. Although most studies in literature address imidazolium-based ILs, the number of industrial applications regarding phosphonium- based ILs is facing a progressive increase. Cytec produces these ILs in bulk quantities; Central Glass Co. Ltd., from Japan, developed pharmaceutical intermediates using phosphonium-based ILs [1]; Texaco used a ‘‘ruthenium melt catalyst’’ dispersed in phospho- nium- or quaternary ammonium-based ILs to convert syngas into acetic acid, esters, alcohols and glycol [2]; Rhone-Poulenc used tet- rabutylphosphonium chloride to stabilize zero-valent palladium catalysts for carbonylation [2]; and Eastman Chemical Company, one of the oldest examples of the industrial applications of ionic liquids, used a Lewis basic phosphonium iodide IL along with a Lewis acid catalyst for the isomerization of 3,4-epoxybut-1-ene to 2,5-dihydrofuran [1]. The adequate design, optimization and operation of industrial processes involving phosphonium-based ILs require the knowledge of their thermophysical properties, in particular their viscosities and densities. Most of the studies concerning the measurement of thermophysical properties of ILs have been focused on imidazo- lium- and pyridinium-based ILs [3–9]. However, compared with those nitrogen-based ILs, phosphonium-based salts are less toxic, thermally more stable, readily available in bulk quantities and less expensive [10–12]. Furthermore, unlike the majority of common ILs, most of them are less dense than water [4,13] and more stable towards nucleophilic and basic conditions due to the absence of acidic protons [10]. These characteristics offer greater practicality and scope and have proved to be valuable for specific applications, for instance, in the purification of biomolecules with aqueous two- phase systems [14,15], in the separation of ethanol-water mixtures [16], in the extraction of metals from aqueous phases [17–19], in removing diamondoids from gas condensates and natural gas [20] and in the CO 2 capture and gas separation processes [21–23]. One of the major characteristics inherent to ILs is their high hygroscopicity that is a direct result of their ionic nature. Thus, any large scale application involving ILs must have this issue into account. Most of ILs’ thermophysical properties are sensitive to 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.01.016 ⇑ Corresponding author. Tel.: +351 234 370200; fax: +351 234 370084. E-mail address: jcoutinho@ua.pt (J.A.P. Coutinho). J. Chem. Thermodynamics 43 (2011) 948–957 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct