Liquid phase behavior of hexafluorophosphate ionic liquids with polyhydric alcohols
Highlights
► The longer alkyl chain in imidazolium cation of investigated ionic liquid, the worse miscibility with 1,2-ethanediol. ► Miscibility of [omim][PF6] with (1,ω) diols much worse than with (1,2) diols. ► Miscibility of butanediol isomers with [omim][PF6] is better for the solvent with low dielectric constant.
Introduction
Over the last decade room temperature ionic liquids (RTILs) have emerged as a new class of solvents for practical applications due to their unique combinations of low volatility, chemical stability, high conductivity, wide electrochemical window, ability to dissolve organic and inorganic solids and gases and tunable solvent properties [1]. Moreover, ionic liquids exhibiting low toxicity and low bioaccumulation would be elected as environmentally suitable green solvents. Hence, they are highly attractive as alternatives to hazardous volatile organic compounds. It is then obvious that besides the academic interest, the potential applications stimulate the broad investigations of ionic liquids and their solutions [2], [3]. Among others the extensive study of the miscibility behavior has been reported by many researchers. The existence of polar and nonpolar domains in ionic liquids solutions is responsible for the complex nature of intermolecular interactions [4]. Because of such comprehensive properties between solvent and solute, abundant phase behavior was observed [5].
Over the last years, much scientific attention has been devoted to liquid–liquid equilibria of ionic liquid with hydrocarbons or alcohol systems [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. However, to the best of our knowledge the detailed study on miscibility of RTILs with polyhydric alcohols is still rather rare [23], [24], [25], [26], [27]. Recently we initiated the systematic study on liquid–liquid equilibria involving ionic liquids and polyhydroxy alcohols [25]. Continuing our work on miscibility of ionic liquids with this group of alcohols, especially diols, we decided to determine the influence of diol's structure on critical solution temperature. Diols are very interesting compounds whose molecules can participate in the formation of the network structure. Owing to the presence of two hydroxy groups in the molecule, diols are capable of forming intermolecular and intramolecular hydrogen bonds, which give rise to a diversity of structures existing in the liquid. 1,2- and 1,ω-alkane diols are amphiphiles with distinct hydrophilic and hydrophobic parts in their molecules. Therefore, they often behave as non-ionic surfactants in their aqueous solutions. The series of [Cnmim][PF6] (based on the 1-n-alkyl-3-methylimidazolium cation coupled with the hexafluorophosphate anion, [PF6]−) have been chosen which is among the most popular and commonly investigated group of ionic liquids and therefore is suitable for comparison purposes.
Section snippets
Materials and methods
The list of ionic liquids purchased from Aldrich and used in this study is as follows: 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6] (>97%) (Aldrich), 1-hexyl-3-methylimidazolium hexafluorophosphate, [hmim][PF6] (>97%) (Fluka) and 1-methyl-3-octylimidazolium hexafluorophosphate, [omim][PF6] (>95%) (Aldrich). All ILs were carefully purified to reduce the water content and volatile compounds. The procedure of purifying all ionic liquids was the same. The samples were kept under
Results and discussion
The liquid–liquid phase equilibrium for the 16 binary systems was determined. We examined 3 hexafluorophosphate ionic liquids with 10 different polyhydroxy alcohols. The systems are as follows:
([bmim][PF6])-1,2-ethanediol, 1,2-hexanediol,
([hmim][PF6])-1,2-ethanediol, 1,2-propanediol, 1,2-butanediol and 1,2-hexanediol,
([omim][PF6])-1,2-ethanediol, 1,2-hexanediol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, 1,2-propanediol, 1,3-propanediol, 1,2-pentanediol, 1,5-pentanediol.
T–x
Conclusions
Following the liquid–liquid equilibria measurements in the systems consisting of ionic liquids ([omim][PF6], [hmim][PF6] and [bmim][PF6]) and polyhydric alcohols (1,2-ethanediol, 1,2-propanediol, 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol) the appropriate phase diagrams have been constructed. The results obtained clearly show how the structure of both ionic liquids and polyhydric alcohols affects the phase
Acknowledgement
This work was partially supported by the Ministry of Science and Higher Education under the Grant No. N N204 030336.
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