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