Liquid-Liquid Phase Behavior of Solutions of 1-Octyl- and 1-Decyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide (C 8,10 mimNTf 2 ) in n-Alkyl Alcohols † Vlad R. Vale, ‡ Bernd Rathke,* ,‡ Stefan Will, ‡ and W. Schro ¨er § Universita ¨t Bremen, Technische Thermodynamik, Badgasteiner Str. 1, 28359 Bremen, Germany, and Universita ¨t Bremen, FB2, Institut fu ¨r Anorganische und Physikalische Chemie, Leobener Str. NWII, 28359 Bremen, Germany Systematic measurements of liquid-liquid phase diagrams of binary mixtures of the ionic liquids 1-octyl- 3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C 8 mimNTf 2 ) and 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C 10 mimNTf 2 ) with n-alkyl alcohols (1-octanol, 1-nonanol, 1-decanol, 1-undecanol, 1-dodecanol, 1-tetradecanol, 1-hexadecanol, 1-octadecanol, and 1-eicosanol) were performed. The phase diagrams were obtained at atmospheric pressure in the temperature range (283 to 423) K applying the cloud-point method on a set of samples. With the assumption of Ising criticality, analysis of the shapes of the phase diagrams yielded data on the critical points and the parameters characterizing the width and the asymmetry of the coexistence curves. Introduction The melting temperatures of typical inorganic salts are above 1000 K. In contrast, certain organic salts, termed ionic liquids (ILs), 1 have melting temperatures below 373 K; some ILs become liquids at temperatures as low as 200 K. 2 The remarkably low melting points originate from the chemical structure of the ILs, which are composed of an inorganic or organic anion and a rather large, asymmetrical organic cation. Because of the presence of free charges with Coulomb interactions and long organic side chains, ILs have many special properties. 3-5 The strong Coulomb interactions cause the vapor pressure to be so low that it can hardly be measured, 6 and the boiling temperature is estimated to be above 1300 K, 7 although this is not observable because the chemical stability of ILs ceases near 600 K. 8,9 Neverthe- less, it can be said that the liquid range of up to 400 K is remarkably large 10 in comparison with nonionic solvents. Chemical modification of the ions enables variation of the solvent properties, so some ILs are miscible with water 11-13 while others are soluble in hydrocarbons. 14-17 Because of those properties, there is a high interest in applying ILs in chemical engineering, e.g., as solvents in synthesis and separation techniques. 18,19 Phase diagrams of liquid-liquid equilibria are important basic data for applications such as extractions. Although quite a few data for binary mixtures with ILs of different anions have recently been reported, 11-32 systematic studies are still required in order to provide information for assessing the relations between the phase diagrams of the solutions of ILs and the chemical structures of the components by empirical analysis and theoretical methods. This work aims to contribute to this program. Mixtures with alcohols are of particular interest because the influence of the chain length of the alcohols on the phase diagrams can be studied in detail. In principle, such an investigation can cover the whole range from water to hydrocarbons and from highly polar to nonpolar solvents. 12,14,32 ILs with the anion bis(trifluoromethylsulfonyl)imide (NTf 2 ) are presently of high interest because this anion is rather hydrophobic, making the ILs soluble in nonpolar solvents. Furthermore, it is stable with respect to hydrolysis, in contrast to ILs with the anions BF 4 - and PF 6 - , 33 whose solutions have been investigated extensively. 11-13,23-26 In this work, we have extended previous investigations 21,22,25,26,29,31 of mix- tures of ILs containing the NTf 2 anion and imidazolium cations with n-alkyl alcohols (C n OH) of short chain length to include alcohols with longer chain lengths (n ) 8 to 20) and to consider ILs with slightly longer side chains (x ) 8, 10) on the methylimidazolium (C x mim) cation. Because of the increasing tendency for the formation of liquid-crystalline domains of the methylimidazolium salts with longer side chains (x >10), the present study was limited to systems with x e 10, which show only increasing segregation but no formation of liquid crystals (e.g., see ref 34). Figure 1 gives the chemical structure of C x mimNTf 2 . In total, we report data for 17 phase diagrams. Experimental Section Materials. The n-alkyl alcohols 1-octanol (C 8 H 18 O, CAS no. 11-87-5, g 99 %), 1-nonanol (C 9 H 20 O, CAS no. 143- 08-8, g 98 %), 1-decanol (C 10 H 22 O, CAS no. 112-30-1, g † Part of the “Josef M. G. Barthel Festschrift”. * To whom correspondence should be addressed. Tel.: +49 421 218 3334. Fax: +49 421 218 7555. E-mail: rathke@uni-bremen.de. ‡ Technische Thermodynamik. § Institut fu ¨r Anorganische und Physikalische Chemie. Figure 1. Structures of the ionic liquids 1-octyl- and 1-decyl-3-methylimi- dazolium bis(trifluoromethylsulfonyl)imide, abbreviated as C 8 mimNTf 2 and C 10 mimNTf 2 , respectively. J. Chem. Eng. Data 2010, 55, 2030–2038 2030 10.1021/je900988a  2010 American Chemical Society Published on Web 03/11/2010