Stanislaw L. Randzio

n-hexane as a model for compressed simple liquids

Isobaric thermal expansivities, αp, ofn-hexane have been measured by pressure-controlled scanning calorimetry from just above the saturation vapor pressure to 40 MPa at temperatures from 303 to 453 K and to 300 MPa at 503 K. These new data are combined with literature data to obtain a correlation equation for αp valid from 240 to 503 K at pressures up to 700 MPa. Correlation equations are developed for the saturated vapor pressure, specific volume, and isobaric heat capacity of liquid n-hexane from 240 to 503 K. Calculated volumes, isobaric and isochoric specific heat capacities. isothermal compressibilities, and thermal coefficients of pressure are presented for the entire range of pressure and temperature. The pressure-temperature behavior of these quantities is discussed as a model behavior for simple liquids without strong intermolecular interactions.

An isothermal scanning calorimeter controlled by linear pressure variations from 0.1 to 400 MPa. Calibration and comparison with the piezothermal technique

An isothermal scanning calorimeter controlled by linear pressure variations is described for the pressure range 0.1–400 MPa at temperatures from 303 to 573 K. The rate of pressure variations can be as low as 0.002 MPa/s over the whole pressure range. The functioning of the instrument was tested by measuring the coefficient of thermal expansivity of liquid n‐hexane, with calibration performed with gaseous nitrogen and by measuring the coefficient of thermal expansivity of benzene with calibration performed with liquid n‐hexane. The results are compared with literature results obtained by the piezothermal technique and with results obtained in the present instrument by the use of a pressure stepwise scanning mode. An example of an investigation of an isothermal solid‐to‐liquid transition in benzene is also given.

An attempt to explain thermal properties of liquids at high pressures

Abstract The behavior of the heat capacity and the thermal expansion of liquids at high pressures is explained on the basis of a molecular interpretation of thermal expansion and its dependence on temperature. According to this interpretation an application of pressure to a liquid markedly changes its intermolecular potential.

Thermophysical properties of 1-hexanol over the temperature range from 303 K to 503 K and at pressures from the saturation line to 400 MPa

Abstract Isobaric thermal expansivities, α P (T,p), of 1-hexanol have been measured in a pressure-controlled scanning calorimeter from just above the saturation vapour pressure to 400 Mpa at temperatures from 302.6 K to 503.15 K. The specific volume isotherm, v(T R ,p), at T R =302.6 K has been derived from measurements of isothermal compressibilities up to 400 MPa and from the specific density at atmospheric pressure. Specific volumes, isothermal compressibilities, thermal pressure coefficients, and isobaric and isochoric heat capacities for the whole pressure and temperature range are derived from these data and from literature data on the saturation vapour pressures and on the isobaric heat capacities at atmospheric or saturation vapour pressure.

Scanning calorimeters controlled by an independent thermodynamic variable: Definitions and some metrological problems

Abstract A novel definition of scanning calorimetry is introduced. Thermodynamic formulae are derived to prove the accuracy of the new definition which is based on the calorimetric measurement of the power of the process of linear variation of an independent thermodynamic variable. A detailed analysis is given of the dynamic precision of the diathermic and compensation methods of power determination. To achieve this, dynamic errors in the time domain and dynamic errors in the parameter space are used. The main problems arising in obtaining the linear variation of independent thermodynamic variables are discussed. Problems of dynamic precision related to the organisation of a scanning calorimeter experiment controlled by an independent thermodynamic variable are discussed for the case of first-order phase transitions.

Supercritical transitiometry of polymers.

Employing supercritical fluids (SCFs) during polymers processing allows the unusual properties of SCFs to be exploited for making polymer products that cannot be obtained by other means. A new supercritical transitiometer has been constructed to permit study of the interactions of SCFs with polymers during processing under well-defined conditions of temperature and pressure. The supercritical transitiometer allows pressure to be exerted by either a supercritical fluid or a neutral medium and enables simultaneous determination of four basic parameters of a transition, i.e., p, T, Δ(tr)H and Δ(tr)V. This permits determination of the SCF effect on modification of the polymer structure at a given pressure and temperature and defines conditions to allow reproducible preparation of new polymer structures. Study of a semicrystalline polyethylene by this method has defined conditions for preparation of new microfoamed phases with good mechanical properties. The low densities and microporous structures of the new materials may make them useful for applications in medicine, pharmacy, or the food industry, for example.

Isobaric thermal expansivities of binary mixtures ofn-Hexane with 1-hexanol at pressures from 0.1 to 350 MPa and at temperatures from 303 to 503 K

Isobaric thermal expansivities, αp(p, T), of seven binary mixtures ofn-hexane with l-hexanol (0.0553, 0.1088, 0.2737, 0.2983, 0.4962, 0.6036, and 0.7455 mol fraction of l-hexanol) have been measured with a pressure-controlled scanning calorimeter over the pressure range from just above the saturation pressures to 350 MPa and at temperatures from 302.6 to 503.1 K. The low-temperature isotherms of αp for particular mixtures observed with respect to the unique crossing point ofn-hexane isotherms reveal an association effect which is reduced when the temperature increases. The high-temperature isotherms of αp are very similar to the isotherms of puren-hexane, especially for lower mole fractions ofn-hexanol. No known equation of state can reproduce these properties.

Pressure effects on the thermodynamic properties of (n-hexane + 1-hexanol) binary mixtures

Abstract On the basis of pressure-scanning measurements of isobaric thermal expansivities, αp(p,T), of n-hexane, 1-hexanol and their binary mixtures at 0.1088, 0.2735, 0.4965, 0.6034, and 0.7453 hexanol molar fractions, and of isotherms of molar volumes at reference temperatures, the pressure derivatives of excess thermodynamic functions have been derived over the pressure range up to 400 MPa and at temperatures from 303 to 503 K. The obtained results presented in the form of isobars-isopleths clearly demonstrate the shifting, from positive to negative, of the pressure effects on thermodynamic functions when composition, temperature or pressure varies. The observed phenomena are discussed in terms of association equilibria in this system.

The equation of state for molecules with shifted Lennard-Jones pair potentials

Abstract Compressibility factors and internal energies have been calculated for fluids with softly repulsive pair potentials by means of the Maxwell distribution method. The pair potentials used are Lennard-Jones potentials, truncated at the minimum and with the minimum shifted towards zero energy. The exponent of attraction has been varied between 4 and 8, the exponent of repulsion between 8 and 40. An empirical equation of state has been developed which permits the calculation of thermodynamic properties for all truncated Lennard-Jones potentials. When this equation of state is substituted for the hard-sphere term in simple equations of state of the var der Waals type, a better representation of caloric data is obtained, especially over the high density region.

Excess enthalpy in the methanol-water system at 278.15, 298.15 and 323.15 K under pressures of 0.1, 20 and 39 MPa. II: Experimental results and their analytical presentation

Abstract Results are presented of flow-calorimetric measurements of excess enthalpy in the methanol-water system at temperatures 278.15, 298.15 and 323.15 K under pressures of 0.1, 20 and 39 MPa. The experimental results are correlated with the use of Redlich-Kister, NRTL and UNIQUAC formulas.