The molar surface Gibbs energy and its application to the binary mixtures of N-butylpyridinium dicyanamide [C4py][DCA] with alcohols

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Highlights

  • The density and surface tension for binary mixtures of [C4py][DCA]+alcohols were measured over the whole composition.

  • We improved Li’s model of surface tension so that a new function– molar surface Gibbs energy, gs, was obtained.

  • According to new function gs, a new Eӧtvӧs equation was obtained and applied to binary mixtures and discussed.

  • A new method for predicting the surface tension, γ, of the mixture is proposed.

Abstract

The density and surface tension for the binary mixtures of ionic liquid (IL) [C4py][DCA] with ethanol, 1-propanol and 2-propanol were measured across the entire range of mole fraction (x1 = 0.0000–1.0000) at T = (288.15–318.15) K. The average molar volume and the thermal expansion coefficient of the binary mixtures were calculated from the experimental density. The excess molar volumes, VE, of the binary mixtures were also calculated and well fitted by Redlich−Kister equation. The obtained VE are negative values over the whole composition range. The values of VE are related to the alcohol chain length and temperature. In order to predict the surface tension of the mixtures, the new function – the molar surface Gibbs energy of mixtures was obtained from Li’s model improved by us in this work. So, the molar surface Gibbs energy, gs, and its excess function, gsE, of mixtures for [C4py][DCA] with ethanol, 1-propanol and 2-propanol were calculated. The excess molar surface Gibbs energy, gsE, was well fitted by Redlich−Kister equation. In terms of the Redlich−Kister’s parameters, a new method for predicting the surface tension, γ, of the mixtures is proposed and the predicted values, γ (pre.), are in good agreement with the corresponding experimental ones, γ(exp.). On a basis of molar surface Gibbs energy of mixtures, a new Eӧtvӧs equation was obtained and applied to binary mixtures of [C4py][DCA] with alcohols. The absolute value of the slope of the new Eötvös equation is the mole surface entropy, s, and the intercept is the molar surface enthalpy, h, which is temperature-independent.

Introduction

Ionic liquids are molten salts with melting points below 373 K and are composed of organic cations and inorganic or organic anions. Owing to their unique properties, such as non-volatility, thermal stability and non-flammability they have been successfully applied in some physical and chemical fields [1], [2]. In addition, pyridinium-based ionic liquids can capture large amounts of SO2 by simple physical absorption [3]. Some of them have been used as a extractant [4] and novel catalysts [5], [6] for the synthesis in chemical reactions. ILs containing N(CN)2 anion have been widely used in electrochemical preparation [7], [8] and as absorbents to capture CO2 [9], [10] and electrolytes for dye-sensitized solar cells [11]. It is also believed that mixtures of ILs with molecular solvents may have better use than those of the pure compounds in many technological processes [12], [13]. Thus the fundamental physico-chemical properties of IL-molecular solvent mixtures are of great valuable from both practical and theoretical point of view. Therefore, in recent years, binary mixtures of ionic liquid with other solvents have attracted more and more attention from industry and academic community because of the high viscosity of ILs [14]. Among these investigations, there are a number of workers who have studied the physical properties of binary systems of imidazolium-based ILs with water or alcohols [15], [16]. However, for binary systems of pyridinium-based ILs with alcohols, these properties are scarce. Therefore, as our continuous work [17], [18], [19], the aim of this work is as follows: (1) The densities and surface tensions for binary mixtures of [C4py][DCA] with ethanol, 1-propanol and 2-propanol were measured across the entire range of mole fraction (x1 = 0.0000–1.0000) at T = (288.15–318.15) K interval 5 K. (2) The volumetric properties such as average molar volume and excess molar volume for binary mixtures are discussed. Excess molar volumes were fitted by Redlich−Kister equation. (3) We improved Li’s model of surface tension so that a new function– molar surface Gibbs energy, gs, was obtained and the values of gs and its excess function, gsE, of mixtures {[C4py][DCA]+alcohol} were calculated. (4) The values of gsE were fitted by Redlich–Kister equation and in terms of Redlich–Kister parameters, a new method of predicting surface tension of mixtures {[C4py][DCA]+alcohol} was introduced. (5) According to new function gs, a new Eӧtvӧs equation was obtained and applied to binary mixtures of [C4py][DCA] and discussed.

Section snippets

Chemicals

Pyridine, 1-bromobutane, silver nitrate, ethyl acetate, acetonitrile and sodium dicyanamide were purchased from Sinopharm Chemical Reagent Co., Ltd. The sources and purities of all chemicals are detailed listed in Table S1 in Supporting Information. [C4py][DCA] was prepared by us according to literature [17], [18]. [C4py][DCA] was heated at about 318 K under vacuum (<10 Pa) for a few hours to remove water before used. The mass fraction of water in the pure IL [C4py][DCA] was 425 ppm determined

The average molar volume for binary mixtures of [C4py][DCA] with alcohols

According to the theory of electrolyte solution [21], when the alcohol and ionic liquid form a mixture, two kinds of effects may occur simultaneously. Firstly, the organic solvent in the mixture provides an environment with a dielectric constant D and reduces the electrostatic interaction between the positive and negative ions. Secondly, due to the solvation of polar alcohol molecules, free solvation ions are produced. However, since the alcohols have much lower dielectric constant and much

Conclusions

The density and surface tension for binary mixtures of [C4py][DCA] with ethanol, 1-propanol and 2-propanol were measured over the whole composition range at T = (288.15–318.15) K. The excess molar volumes, VE, were calculated and well fitted by a Redlich−Kister equation. The minimum excess volume of the mixture occurs at about x1 = 0.3, so that this indicates that alcohols strongly influence intermolecular interactions of the mixtures. In order to predict the surface tension of the mixtures,

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21673107) and (21373005).

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