Elsevier

Fluid Phase Equilibria

Volume 344, 25 April 2013, Pages 125-138
Fluid Phase Equilibria

Liquid phase PVTx properties of binary mixtures of (water + ethylene glycol) in the range from 278.15 to 323.15 K and from 0.1 to 100 MPa. I. Experimental results, partial and excess thermodynamics properties

https://doi.org/10.1016/j.fluid.2013.01.025Get rights and content

Abstract

The coefficient of compressibility, k = ΔV/Vo, of {water (1) + ethylene glycol (2)} binary mixture was measured at pressures from 0.1 to 100 MPa and temperatures from 278.15 to 323.15 K over the whole concentration range. At all state parameters the following characteristics were calculated: excess molar volumes of the mixture, VmE; changes of excess molar Gibbs energy, ΔPoPGmE; changes of excess molar entropy, ΔPoPSmE; changes of excess molar enthalpy, ΔPoPHmE; mixing enthalpy, HmE, of water with ethylene glycol at high pressure, and partial molar volumes of the components. It was shown that the coefficients of compressibility, k, sharply decreased over the concentration interval x2 = 0 –0.2, where x2 was ethylene glycol mole fraction, and further changed insignificantly. It was revealed that excess molar volumes were negative at all temperatures studied and went down with pressure increasing. The dependence of partial molar volumes of the mixture components on ethylene glycol mole fraction passes extreme at all isobars. Limiting partial molar volumes of water and ethylene glycol decrease with pressure growth. The mixture compression leads to its ordering due to new hydrogen bonds formation.

Introduction

Characteristics of volume change of liquid mixture on composition, temperature, and pressure are important source of information to reveal the peculiarities of its components interactions. During the liquid mixture formation the changes of molecule interactions occur, and difference in the components packing becomes apparent. When there is developed hydrogen bond network in, at least, one of the solvents then the mixture properties change in a special way. Characteristic features of the solvents with spatial H-bond network are relatively large free volume, small values of coefficients of isothermal compressibility and of volume thermal expansion, high viscosity, etc. Both water and ethylene glycol refer to such solvents [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Though these substances differ by many micro- and macroproperties, ethylene glycol can be considered as interesting water analog. The values of cohesion energy density and intrinsic pressure of ethylene glycol indicate that it is associated solvent [12], [13], and free energy of transfer of inert solutes from water to EG is vey small [14]. Moreover the surfactant micellization has been shown to occur in EG [15].

Water–ethylene glycol (ethanediol, EG) system is permanent subject of fundamental investigations and simulation of intermolecular interactions in it [4], [6], [7], [16], [17], [18], [19], [20], [21], [22], [23], as EG is the most simple compounds among diols. Also EG is widely applied in many branches of industry due to its important properties. Ethylene glycol decreases the water freezing point and stabilizes proteins in solutions. The mixtures based on it are irreplaceable as heat carriers or refrigerants in many power-producing systems, in addition EG is used extensively as hydraulic fluid. Thermophysical properties of ethylene glycol aqueous mixtures are significant for industrial equipment calculation.

In connection with all above-stated, the investigation of the mixture of both these solvents over the wide range of state parameters is of special interest. The present work is a continuation of our investigations of volume properties as function of composition, temperature, and pressure in binary mixtures with different nature of interactions.

Volume properties of {water (1) + ethylene glycol (2)} system were investigated by us earlier at atmospheric pressure [24]. For better understanding of the processes in the system, its compressibility on composition, pressure, and temperature was measured. Experimentally measured compressibility coefficients, and calculated excess and partial molar thermodynamic parameters are performed in the article. Whereas pure ethylene glycol compressibility was measured for several times [25], [26], [27], [28], [29], [30], water–EG mixture compressibility was earlier studied by three investigators only. So Hamann and Smith [31] have measured the compressibility at one temperature 303.15 K and one pressure 101.3 MPa over the whole concentration range (13 compositions); Nakagawa et al. [32] have determined the compressibility at 298.15 K and 101.3 MPa within the entire composition range (34 mixtures); Miyamoto et al. [33] have measured the system compressibility at 298.15 K over full composition range (11 mixtures) and at four pressures (50, 100, 150, 200) MPa. Furthermore by direct measurements with dilatometer, Götze and Schnaider [34] have determined excess molar volume of the equimolar mixture (x = 0.5) at 273.15, 298.15, 323.15, 348.15 K and 10, 50, 100, 150, 200, 250 MPa, Hynčica et al. [35] have measured the density of {water (1) + ethylene glycol (2)} mixture at low EG concentrations over the temperature range from 298.21 to 573.14 K (from 11 to 13 points) and at pressures from 0.64 to 30.09 MPa (3 points).

As one can see from this review, the results of earlier investigations of volume properties at high pressures, unfortunately, are rather scattered and can not be used for calculation of all thermodynamic coefficients. In Fig. 1 the comparison of literature [32], [33] and our experimental data on compressibility coefficients is performed. For this comparison k values obtained in the present work were fitted by polynomial of six degree for concentrations matching.

As follows from Fig. 1, the data obtained by us are above the ones from [33]; the upper deviation, 100%(kthisworkklit)/kthiswork, is observed at 100 and 50 MPa for compositions about x2 = 0.1 and 0.5, accordingly.

Section snippets

Chemicals

In the work the solvents of the highest available purity were used. Their description is given in Table 1. Ethylene glycol used was purified by double distillation according to [36], [37] and was kept under vacuum. The water content in EG was determined by K. Fisher method and did not exceed 0.02 wt.% (or 6 × 10−5 mole fraction) after distillation. Double distilled water with conductivity <2 S cm−1 were used for all mixtures preparation. All mixtures were prepared by mass from degasified solvents

Results

The compressibility coefficients k were determined as following:k=vovvo=ρρoρwhere vo, ρo and v, ρ are specific volumes and densities of {water (1) + ethylene glycol (2)} mixture, accordingly, at atmospheric pressure (po = 0.101 MPa) and at pressure in question p. Measured compressibility coefficients of the mixture are presented in Table 1.

Excess molar volume VmE was determined by equation:VmE=Vmx1V1ox2V2owhere Vm is the mixture molar volume; V1o,x1, and V2o,x2 are molar volumes of pure

Discussion

In one of our previous works [24] we have discussed the volume properties of {water (1) + ethylene glycol (2)} mixture and their temperature dependences at atmospheric pressure. In the present work changes of volume properties of the mixture on external pressure are considered.

The presence of developed spatial hydrogen-bond network both in water and ethylene glycol [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] predetermines properties of these solvents mixture. In the mixture the H-bond

Conclusion

Volume properties of {water (1) + ethylene glycol (2)} mixture corroborate hydrophilic nature of ethylene glycol, and formation and breaking of hydrogen bonds take place in the mixture.

Extremes on concentration dependences of partial molar volumes of the mixture components display hydrophobic properties of ethylene glycol. Origin of these extremes is connected with breakdown of water structure consisting in displacement of “free” H2O molecules, located in cavities of own water structure, by

Acknowledgment

This work was supported by the Russian Foundation for Basic Research (project 12–03-97525-r_centre_a).

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