Excess thermodynamic functions derived from densities and surface tensions of (p- or o-xylene + ethylene glycol dimethyl ether) between the temperatures (298.15 and 308.15) K

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Abstract

Densities (ρ) for binary systems of (p-xylene or o-xylene + ethylene glycol dimethyl ether) were measured over the full mole fraction range at the temperatures of (298.15, 303.15 and 308.15) K along with the densities of the pure components. The excess molar volumes (VE) calculated from the density data show that the deviations from ideal behaviour in the two binary systems are negative, and they become more negative with the temperature increasing. Surface tensions (σ) of these binary systems were determined at the same temperatures (298.15, 303.15 and 308.15) K by the pendant drop method. The surface tension deviations (δσ) for p-xylene system are negative over the whole composition range, and become less negative with the temperature increasing, but for the o-xylene system, δσ are negative at high o-xylene concentration, and change to positive with the o-xylene concentration decreasing. The VE and δσ were fitted to the Redlich–Kister polynomial equation. Surface tensions were also used to estimate surface entropy (Sσ) and surface enthalpy (Hσ).

Introduction

The surface tension and density of liquid mixtures are important thermodynamic properties in the mass and heat transfer at the interface such as occurs with liquid–liquid extraction, gas absorption, distillation, and crystallization. They are also useful for understanding and interpreting the nature of interactions between unlike molecules in the mixtures. For the purpose of separation of mixed xylene and trimethylbenzene, our group has developed a high performance separation method named “urging rectification” [1], which involves adding some special solvent (named “urging solvent”) to the rectification system. The urging solvent passes through the mixtures and condenses in the condenser, aiding light component to leave the mixtures quickly. It is vital for selecting an urging solvent to assess excess functions and to understand the interactions between molecules in the mixtures. In previous papers, densities and surface tensions were measured and the derived excess functions were calculated for the following systems [2], [3], [4], [5], [6]: (trimethylbenzene + propyl acetate or butyl acetate), {trimethylbenzene + ethylene glycol ether (ethylene glycol monomethyl ether or ethylene glycol dimethyl ether)}, (trimethylbenzene + dimethyl carbonate or diethyl carbonate), (xylene or ethylbenzene + propyl acetate), {xylene + alkanone (acetone or 2-butanone)}. Herein, density and surface tension for binary systems of (p-xylene or o-xylene + ethylene glycol dimethyl ether) from T = (298.15 to 308.15) K were determined. The excess molar volume (VE) and surface tension deviations (δσ) were also derived. The surface tension values have been further used to estimate the surface entropy (Sσ) and surface enthalpy (Hσ). It is worth noting that the surface tension deviations (δσ) for the (o-xylene + ethylene glycol dimethyl ether) system are negative in the rich o-xylene region, but become positive in the poor o-xylene region.

Section snippets

Materials

Ethylene glycol dimethyl ether (ACROS ORGANICS), p-xylene (AR, Guangzhou Chem., China), and o-xylene (AR, Guangzhou Chem., China) were obtained commercially and further purified by distillation. All the chemicals were stored over molecular sieve before use. The mass fraction purity of the substances, analyzed by a PE auto system XL gas chromatograph, is as follows: ethylene glycol dimethyl ether (>0.993), p-xylene (>0.995), o-xylene (>0.996). The density and surface tension of the pure

Results and calculations

Excess molar volumes were determined from the following equation [8], [9]:VE=(x1M1+x2M2)/ρ-(x1M1/ρ1+x2M2/ρ2),where xi is the mole fraction of component i, Mi is the molar mass of component i, ρ and ρi are the densities of the mixture and component i, respectively. Experimental density (ρ) and excess molar volume (VE) for the two binary mixtures (p-xylene + ethylene glycol dimethyl ether, and o-xylene + ethylene glycol dimethyl ether) from T = (298.15 to 308.15) K are listed in table 2. The results

Discussion

Figure 1 shows the excess molar volume VE against composition for the binary system of {ethylene glycol dimethyl ether (EGDE, x) + p-xylene (1  x)} at T = (298.15, 303.15, and 308.15) K. The VE values for this system are negative over the entire range of composition. The minimum values of VE occur close to the equimolar composition, and VE becomes more negative at higher temperature.

Figure 2 shows the plot of excess molar volume VE against composition for the binary system of {EGDE (x) + o-xylene (1  x

Summary

We measured the liquid densities and surface tension for the binary systems (p-xylene + ethylene glycol dimethyl ether, or o-xylene + ethylene glycol dimethyl ether). The excess molar volume VE and surface tension deviation δσ were calculated from these experimental results. A Redlich–Kister type equation was used for fitting each set of VE and δσ. The surface tension was also used to estimate surface entropy (Sσ) and surface enthalpy (Hσ). These measurements and calculations may be useful to

Acknowledgments

The authors acknowledge the assistance of Q.-P. Dong for obtaining some experimental data, and the financial support of National Natural Science Foundation of China (50572125), and Guangdong Natural Science Foundation (7003708).

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