(p, Vm, T) measurements of (cyclohexane + nonane) at temperatures from 298.15 K to 328.15 K and at pressures up to 40 MPa

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Abstract

The densities and speeds of sound of (cyclohexane + nonane) were measured at four temperatures from 298.15 K to 328.15 K, and the respective values of excess volumes VmE and adiabatic compressibility were calculated. Thereafter, the densities for the last system were measured at elevated pressures (0.1 to 40) MPa at four temperatures over the range 298.15 K to 328.15 K with a high-pressure apparatus. The high-pressure density data were fitted to the Tait equation and the isothermal compressibilities were calculated with a novel procedure with the aid of this equation. The low- and high-pressure values of VmE calculated from the density data show that the deviations from ideal behaviour in the system decrease slightly as the temperature and pressure are raised. The VmE data were fitted to the fourth-order Redlich–Kister equation, with the maximum likelihood principle being applied for the determination of the adjustable parameters.

Introduction

Research activities of our laboratory comprise, among others, the systematic measurement of volumetric properties of different groups of organic compounds. Our new project is devoted to the systematic study of liquid systems modelling liquid engine fuels. As the first step, (cyclohexane + alkane) at normal pressure and temperature 298.15 K were studied [1].

To our knowledge, there exist no volumetric data for the liquid phase of (cyclohexane + nonane) at elevated pressures. Therefore, we have measured densities and calculated isothermal compressibilities and excess volumes of the system. The apparatus based on a high-pressure vibrating-tube densimeter working in a static mode [2] and designed for measuring the (p, Vm, T) behaviour of pure liquids and liquid mixtures at elevated temperatures (283 K to 333 K) and moderately high pressures to 40 MPa was used for the measurements. The measurements were carried out at the temperatures 298.15 K, 308.15 K, 318.15 K, and 328.15 K and in the pressure range 0.1 MPa to 40 MPa. To have sufficiently accurate values of densities at the reference (atmospheric) pressure for fitting the high-pressure density data to the Tait equation, the temperature dependence of densities at atmospheric pressure was measured first with vibrating-tube densimeter DSA 5000. In addition, excess volumes and adiabatic compressibilities were calculated from the data.

The densities and excess volumes of the investigated liquids and their mixtures are required, for instance, for relating excess enthalpy and excess Gibbs free energy values. From a practical point of view, the data are useful for the design of mixing, storage, and process equipment. Last but not least, the data measured reflect interactions between the molecules of the mixtures studied and can serve for testing the theories of the liquid state.

Section snippets

Materials

The nonane used in the experiments was the product from Fluka, purum, g.c. mass fraction ⩾0.99. The cyclohexane was from Sigma–Aldrich, for HPLC, g.c. mass fraction purity ⩾0.997. Both the hydrocarbons were dried and stored over 0.4 nm molecular sieves. In order to check the purity of the substances, their density and refractive index values were determined at T = 298.15 K and compared with the literature data [3], [4], [5] with the agreement being, in general, good (table 1). The contents of the

Atmospheric pressure measurements

The results of the atmospheric pressure measurements are given in table 2. The measured densities are tabulated along with excess volumes and adiabatic compressibility. The values of excess volume VmE were calculated from the mixtures densities, ρ, and the densities, ρi, and molar masses, Mi, of pure components i (i = 1, 2) using the relationVmE=[xM1+(1-x)M2]/ρ-[xM1/ρ1+(1-x)M2/ρ2],where subscript 1 refers to cyclohexane and 2 to nonane and x stands for the mole fraction of cyclohexane. The

Acknowledgements

The authors acknowledge the partial support from the Grant Agency of the Czech Republic. The work has been carried out under Grant No. 104/06/0656.

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