Effect of temperature and composition on the density, viscosity, surface tension, and thermodynamic properties of binary mixtures of N-octylisoquinolinium bis{(trifluoromethyl)sulfonyl}imide with alcohols

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Abstract

Density and viscosity were determined for the binary mixtures containing the ionic liquid N-octylisoquinolinium bis{(trifluoromethyl)sulfonyl}imide ([C8iQuin][NTf2]) and 1-alcohol (1-butanol, 1-hexanol, and 2-phenylethanol) at five temperatures (298.15, 308.15, 318.15, 328.15, and 338.15) K and ambient pressure. The density and viscosity correlations for these systems were tested by an empirical second-order polynomial and by the Vogel–Fucher–Tammann equation. Excess molar volumes were described by the Redlich–Kister polynomial expansion. The density and viscosity variations with compositions were described by polynomials. Viscosity deviations were calculated and correlated by the Redlich–Kister polynomial expansions. The surface tensions of pure ionic liquid and binary mixtures of [C8iQuin][NTf2] with 1-hexanol were measured at atmospheric pressure at three temperatures (298.15, 308.15, and 318.15) K. The surface tension deviations were calculated and correlated by the Redlich–Kister polynomial expansion. The surface thermodynamic functions such as surface entropy and enthalpy were derived from the temperature dependence of the surface tension values. The critical temperature, parachor, and speed of sound for pure ionic liquid were described. A qualitative analysis on these quantities in terms of molecular interactions is reported. The obtained results indicate that ionic liquid interactions with alcohols are strong dependent on the special trend of packing effects and hydrogen bonding of this ionic liquid with hydroxylic solvents. As previously observed, an increase by a 1-alcohol carbon chain length leads to lower interactions on mixing.

Highlights

► Density and viscosity were examined for [C8iQuin][NTf2] and 3 1-alcohols. ► The surface tensions of [C8iQuin][NTf2] was measured with 1-hexanol. ► The correlation with polynomials, Redlich–Kister and V–F–T equations were presented. ► The surface thermodynamic functions were derived. ► The critical temperature, parachor, and speed of sound of IL were described.

Introduction

Ionic liquids (ILs) are a new class of solvents, which show excellent solubility with organic and inorganic compounds and they have potential to be used as replacement solvents for extraction processes, bioproduction, electrochemistry, as herbicydes, as energy storage materials and many other [1], [2], [3], [4], [5], [6], [7]. The specific properties of ILs, i.e. high selectivity in the separation processes, makes ILs interesting for new technologies. Recently, we synthesized and measured the activity coefficients at infinite dilution for different solutes in the N-octylisoquinolinium bis{(trifluoromethyl)sulfonyl}imide, [C8iQuin][NTf2] [8]. From the results, we concluded that the selectivity S12 for the separation of aliphatic hydrocarbons from aromatic hydrocarbons, or sulphur organic compounds, or for the separation of azeotropic mixtures was on the average level. However, this new liquid showed definite interesting selectivity of separation of 2-phenylethanol from water in ternary (liquid + liquid) equilibrium [9]. We proposed to use [C8iQuin][NTf2] in the process of bio production of 2-phenylethanol for the in situ product removal. The material 2-phenylethanol is an important flavour with rose-like aroma. It can be used in cosmetics, food flavour compositions, for soft drinks, candy, and cookies. In chemical processes the product purification is a major problem. For mentioned applications natural 2-phenylethanol is preferred [10], [11]. Different entrainers were used in bioconversion by the yeast from l-phenylalanine including ionic liquids [12]. It was interesting to include in our investigations also the volumetric properties, viscosity, and surface tension, which are shown in the present research.

Recently, we measured phase equilibrium, densities and heat capacities of the quinolinium-based (N-butylquinolinium bis{(trifluoromethyl)sulfonyl}imide, [BQuin][NTf2], N-hexylquinolinium bis{(trifluoromethyl)sulfonyl}imide, [HQuin][NTf2]) and isoquinolinium-based ILs, as N-butylisoquinolinium bis{(trifluoromethyl)sulfonyl}imide, [BiQuin][NTf2]) and N-hexylisoquinolinium bis{(trifluoromethyl)sulfonyl}imide, [HiQuin][NTf2]) in binary systems [9], [13], [14], [15], [16].

The solubility of quinolinium-based, or isoquinolinium based ILs in alcohols depend on the carbon chain length of an alcohol: the ILs [BQuin][NTf2] and [HQuin][NTf2] show eutectic mixtures with complete miscibility in the liquid phase for low chain length alcohols and eutectic mixtures with immiscibility gap in the liquid phase for 1-hexanol and longer chain alcohols [13], [14]. The [BiQuin][NTf2] and [HiQuin][NTf2] show complete miscibility only with 1-butanol [15], [16].

The quinolinium-based ionic liquids are considered promising substances in terms of possible industrial applications as entrainers in the desulfurization of oils and in the 1-hexene/n-hexane separation problems [17], [18].

The aim of the present work is to study the density, viscosity, and surface tension of pure [C8iQuin][NTf2] and its mixtures with alcohols in a systematic manner. The data of new isoquinolinium-based IL ([C8iQuin][NTf2]) in binary systems with an 1-alcohol (1-butanol, 1-hexanol, and 2-phenylethanol) were obtained at five different temperatures ranging from 298.15 K to 338.15 K and ambient pressure. The surface tension of pure ionic liquid and binary mixtures of [C8iQuin][NTf2] with 1-hexanol have been measured at atmospheric pressure at three temperatures (298.15, 308.15, and 318.15) K. The data obtained are analysed to determine the effect of temperature on the fundamental physico-chemical and thermodynamic properties.

Section snippets

Chemicals and materials

The ionic liquid N-octylisoquinolinium bis{(trifluomethyl)sulfonyl}imide, [C8iQuin][NTf2] used in this work was synthesized in our laboratory from N-octylisoquinolinium bromide [C8iQuin][Br]. Synthesis and purity was described in our previous work [8].

The IL was further purified by subjecting the liquid to a very low pressure of about 5 · 10−3 Pa at a temperature about 280 K for approximately 5 h. This procedure removed any volatile chemicals and water from the ionic liquid.

The alcohols used in

Results and discussion

The chemical structure of the investigated N-octylisoquinolinium bis{(trifluoromethyl)sulfonyl)}imide is:

Conclusions

The new physico-chemical properties of the isoquinolinium IL, [C8iQuin][NTf2] were developed as densities and viscosities of the pure ionic liquid, and of mixtures with three alcohols, 1-butanol, 1-hexanol, and 2-phenylethanol. The negative deviations in mole fraction from 0 to 1 were observed for excess molar volumes only in the system with 2-phenylethanol. From the density–temperature dependence, the volume expansivity and the excess volume expansivity were described for the IL. The surface

Acknowledgements

Author M. Zawadzki wish to thank the support of the European Union in the framework of European Social Fund through the Warsaw University of Technology Development Programme. Funding for this research was provided by the Warsaw University of Technology.

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