Available values of the surface tension of 81 common fluids were fitted by using the same model presently used in the REFPROP program V9.0 by NIST. A set of data was built for every fluid by including mainly those values given in the DIPPR and DETHERM databases. For some fluids, other available sources of data were added in order to obtain adequate sets. For every fluid, we checked the accuracy of the REFPROP program and we made a new fit in order to improve the performance of the correlation. We found good general agreement between the REFPROP predictions and the data for 44 fluids, and therefore only very slight improvements are made for them. For the other 37 fluids, the REFPROP correlation can be more clearly improved. In particular, our new correlation is significantly more accurate for ammonia, deuterium, ethanol, and neon, because the version of REFPROP used gives values in clear disagreement with other data sources.
For theoretical and chemical engineering applications, accurate and, if possible, simple models of molecular interactions are needed. We have recently proposed a new procedure for determining Lennard-Jones interaction parameters for fluids, forcing agreement between the values of the pressure obtained from empirical equations of state and those obtained from computer simulations. In this work we obtain new intermolecular Lennard-Jones parameters for non-polar molecules, taking into account their deviation from the spherical shape by means of an acentric factor. Our procedure could help to connect the microscopic and macroscopic worlds and it will be progressively implemented in order to obtain a better representation of other substances and mixtures of chemical interest.
Some points about the search for analytical expressions for the equation of state of the hard-disk fluid are discussed in the light of the most recent advances in the field. New and accurate equations of state for this fluid are proposed.
Fast ionic conductors are important to study because of their use in the construction of technologically useful devices such as electrochemical cells, oxygen monitors, and the high-temperature fuel cell.Oxygen-ion conductors form a major subgroup of these materials, and, in particular, stabilized zirconia is one of the more important solid electrolytes. However, the ionic conductivity of this material is still only rather poorly understood. The aim of the present work is to describe, by means of a method of local fits (LF's) to Arrhenius's law, the experimental values of the ionic conductivity of Zr02 -12 mol % Y203 single crystals in the temperature range from 200 C to 1600'C. This method yields two sets of data: the preexponential factor, ALF, and the activation enthalpy, hHLF . The lnALF versus 1 b,S(T)Ik plot [where b,S(T) is the entropy change in the process] is a very good test of the accuracy of the LF method. The AHLF values are fitted by a least-squares procedure to an empirical temperaturedependence function with four adjustable parameters. In order to interpret these results and to understand the physical meaning of the fitted parameters, a microscopic model is proposed that allows us to deduce a theoretical function of temperature for the activation enthalpy similar to the empirical function. Then, from this function, we determine the association (0.57 eV) and migration (0.73 eV) enthalpies for oxygen vacancies, and analyze the temperature variation of the free energy (hG) and entropy (AS), as well as the degree of dissociation of the vacancies in the conduction process for this material. A noteworthy result is that, for the range of temperature studied here, the extrinsic dissociated regime (where it is assumed that all oxygen vacancies are free) is never reached. Finally, taking into account the contribution of the jumps up to the second-next-nearest anionic neighbors, we obtain the value of 1.31 X 10' Hz for the attempt frequency of the oxygen vacancies.
Accurate correlations are given for the temperature dependence of the surface tension of 37 fluids included in NIST's REFPROP program. The surface tension of these fluids was not included in the most recent version of this program, V9.12. A set of data was constructed for each of these fluids by including principally the values given in the DIPPR, DETHERM, and TDE databases and Wohlfarth and Wohlfarth's book. For some fluids, other available sources of data were added in order to obtain adequate sets. The correlation expression used is the same as used for other fluids in REFPROP. For 31 fluids it was enough to use two adjustable coefficients, whereas four coefficients were needed for the other 6 fluids. For 19 fluids, the mean average percentage deviation was below 1%, and for another 16 it ranged from 1% to 2%.
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