Article pubs.acs.org/jced
Liquid−Liquid Phase Equilibria of the Binary Ionic Liquid Systems [Cn MIM][NTf2] + n‑Butanol, + n‑Pentanol, + H2O Using UV Spectroscopic and Densimetric Analytical Methods Christiane Wertz,‡ Jochen K. Lehmann, and Andreas Heintz* Department of Physical Chemistry, University of Rostock, D-18059 Rostock, Dr.-Lorenz-Weg 1, Germany ABSTRACT: Two new analytical methods using UV-spectroscopy and densimetry have been applied for measuring LLE of the following systems at lower temperatures in the region of restricted mutual solubilities: [CnMIM][NTf2] (n = 2,4,6), + n-butanol, + n-pentanol, + water. The data obtained agree very well with results obtained at higher temperatures for the same systems using the synthetic laser scattering method.
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INTRODUCTION The amount of LLE data of binary systems containing ionic liquids has increased during the last years.1−19 The methods applied for these measurements are based on cloud point observations using optical techniques. In the case of systems with an upper critical solution temperature such synthetic methods are restricted to temperatures, where the mutual solubility is high enough. However, at lower temperatures the mutual solubilities become much smaller and the two phase lines on both sides of the miscibility gap have very steep slopes which causes high uncertainties for measuring cloud point temperatures. Under these conditions only analytical methods are successful, where the two phases are analyzed by taking samples from them using a suitable analytical method. We have tested and successfully applied two different methods studying ionic liquid mixtures containing imidazolium cations. The UV-spectroscopic analysis of the phase with low concentration of the IL can be applied due to the light absorption of the imidazolium cation at 212 nm. The other phase containing the solvent in a large excess of the IL can be analyzed by measuring the mixture density with a vibrating tube instrument. Both methods require the knowledge of corresponding calibration curves. The binary systems [C2MIM][NTF2] + butanol, + pentanol and [CnMIM][NTF2] (n = 2,4,6) + H2O have been studied at lower temperatures, where the mutual solubilities are small and where no data are available for these systems in the literature. A survey of the systems studied in this work and the methods used is presented in Table 1.
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Table 1. Binary Systems Studied (+ sign). The UV Method Is Designated by UV, the Densitmetric Method by DEN [C2MIM][NTf2] n-butanol n-pentanol water
UV
DEN
UV
+ + +
+ + +
+
DEN
[C6MIM][NTf2] UV
DEN
+
Erlangen-Nuernberg, Germany), C6MIM][NTf2] was obtained from Dr. M Muldoon (University Notre Dame, USA). All volatile compounds were removed under high vacuum at 313 K to 343 K. The ionic liquids have been characterized by 1H, 13C, and 19F-NMR spectroscopy, and elemental analysis. The purity of the samples applied is estimated with >98 %. The purity of the solvents was given by the manufacturer with >99.8 % for nbutanol (Merck), > 99.0 % for n-pentanol (Fluka); water content was determined by Karl Fischer-titration with
