Excess molar volumes and excess molar enthalpies of binary and ternary mixtures of 1-butanol, a tertiary amine (tri-n-butylamine or tri-n-octylamine) and n-hexane: Experimental results and ERAS-model calculations

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Abstract

Experimental data at T = 298.15 K and ambient pressure of excess molar volumes VmE and excess molar enthalpies HmE are reported for two ternary and the corresponding binary mixtures of {1-butanol, tri-n-butylamine (TBA), or tri-n-octylamine (TOA), and n-hexane}. A vibrating-tube densitometer was used to determine VmE. HmE was measured using a quasi-isothermal flow calorimeter. The experimental results are used to test the applicability of the ERAS model for describing both thermodynamic excess properties of ternary mixtures containing an alkanol, a tertiary amine and an n-alkane. Qualitative agreement of calculated values with experimental results was achieved using only system-specific parameters determined from binary mixtures. The results are explained in terms of the self-association between alkanol molecules, cross-association between the alkanol and the tertiary amines and structural effects.

Introduction

Excess properties of mixtures provide information about the molecular interactions between the various components and can be used for the development of molecular models describing the thermodynamic behavior of mixtures. Experimental results for multi-component mixtures are rather scarce, because determination of all these properties is time consuming and expensive especially if the data have to be known at various state conditions and compositions. Therefore, the properties of multi-component mixtures are often estimated from the corresponding data of the constituting binary mixtures but the reliability of such an estimation has to be tested experimentally. Tertiary amines are important organic bases. Their solubility depends on the length of the alkyl chain. The lower members of a homologous series are highly soluble in polar and non-polar solvents, while the higher members are soluble only in non-polar solvents. Some long-chain tertiary amines are considered as the effective extractant for carboxylic acid. Liquid tertiary amines are weakly polar, non-associated, strong proton acceptors. It is well known, that mixtures containing associating components like alkanols and/or amines are highly non-ideal systems. Due to the formation of hydrogen bonds between the different species large negative heat as well as volumetric effects are observed upon mixing. Non-associating molecules like n-alkanes act as inert components when mixed with associating components. As the excess properties of ternary mixtures are reflecting differences in molecular size, shape, and interaction of three components it is of interest and significance to use ternary data for testing models for the prediction and correlation of the excess properties of multi-component systems. The extended real associated solution (ERAS) model developed by Heintz [1] has been successfully applied for correlating excess properties of (binary alcohol + amine, alkane + amine, and alcohol + alkane) mixtures [2], [3], [4], [5]. Moreover, the extent of capability of the ERAS-model for predicting excess properties of ternary systems has been investigated based on the ternary mixture (1-propanol + triethylamine + n-heptane) [6].

In a previous article [7], the excess properties for ternary and constituting binary mixtures of {1-alkanol (ethanol or 1-butanol), triethylamine and n-hexane} were presented. Continuing these systematic investigations in this paper experimental results of VmEandHmE are reported for the ternary and corresponding binary mixtures {1-butanol + tertiary amine (tri-n-butylamine (TBA), or tri-n-octylamine (TOA)) + n-hexane} at T = 298.15 K. The objective of the present investigations examine the influence of the shape and the size of the tertiary amine molecules on the specific interactions in ternary mixtures. The experimental data are used to test the applicability of the ERAS model for describing and predicting both excess properties of ternary mixtures containing a 1-alkanol, a tertiary amine and an n-alkane using only binary parameters.

Section snippets

Materials

The substances were obtained from standard laboratory sources and were used without further purification. The GC-purities were >0.998 for 1-butanol (Merck), >0.995 for n-hexane (Fluka), >0.995 for tri-n-butylamine (Fluka) and >0.993 for tri-n-octylamine (Fluka). HPLC gradient grade water (J.T. Baker) has been used for the calibration of the densitometer. The n-hexane was carefully dried over molecular sieves and all substances were degassed by applying reduced pressure for a couple of minutes

Experimental results and discussion

The excess molar volumes VmE and excess molar enthalpies HmE at T = 298.15 K of the binary mixtures are listed in TABLE 4, TABLE 5, respectively. The dependence on mole fraction is shown in FIGURE 1, FIGURE 2, FIGURE 3, FIGURE 4, together with the results of the model calculations and the comparison with the literature data available [11], [13], [14], [22]. The values of VmE are negative for all binary systems investigated. The values of HmE are small and negative (exothermic) for the systems of

Modeling and discussion

The ERAS model [1] combines the linear chain association model with Flory’s equation of state [25]. It is rather successful in describing, VmE,HmE,Cp,mEandGmE simultaneously for binary [2], [3] as well as for multi-component systems [26], [27], [28]. In those papers, the relevant model equations are given. The pure component parameters within the framework of this model which usually used to be adjusted to pure component experimental data are available from the literature sources [11], [15],

Conclusion

Excess molar volumes and excess molar enthalpies at T = 298.15 K have been reported for ternary and their constituting binary systems containing 1-butanol, a tertiary amine (tri-n-butylamine or tri-n-octylamine) and n-hexane. The absolute VmE values for the mixture of (TBA + n-hexane) are considerably lower than the corresponding ones for the (TOA + n-hexane) system. HmE values of these two binary mixtures are negative and show similar dependence on the mole fraction of the tertiary amine. For the

Acknowledgements

M.K. is grateful for having received a scholarship from the Landes-Graduierten-Förderung, Baden-Württemberg, Germany during the course of this work. E.N.R. is grateful to the Deutsche Forschungsgemeinschaft for granting a scholarship (436 RUS 17/96/05).

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    Permanent address: Department of Chemical Thermodynamics and Kinetics, Faculty of Chemistry, St. Petersburg State University, Universitetsky pr. 2, Stary Petergof, St. Petersburg, 198904, Russia.

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