Pressure-volume-temperature-composition properties of complex spherical molecules
Authors:   , Benson Montford Stokley 
Published: June 2024

"Important advances to the better understanding of the liquid state have been made recently as a result of studies conducted which use as a starting point the hard-sphere fluid model. The structure of a real fluid is similar to that of a hard-sphere fluid in that the distribution of the molecules in space is determined primarily by the molecules repulsive forces while the attracting forces may be thought of as a uniform potential field which holds the molecules together. When the hard-sphere repulsive term is used as a model for the real fluid repulsive forces, the success of theory in describing the real fluid properties is determined mainly by the way the attractive forces are accounted for and the molecular diameters determined. Excess thermodynamic functions for liquid mixtures are very useful in testing liquid theories for real fluids since these functions can be related to the intermolecular forces, shape, size, and polarity of the molecules which comprise the fluid mixture. Any successful theory must be able to take these factors into account. Most of the measurements of excess properties of liquid mixtures have been limited in two ways. First, most of the experimental studies have been for liquid mixtures where the disparity in the size of the molecules in the mixture has been relatively small. Second, the studies have been restricted to the low pressure region. The temperature and pressure dependence of the excess volume is particularly useful information. In practice there is no convenient method for measuring the pressure derivative of the excess Gibbs free energy other than that of measuring the excess volume. In addition the pressure variation of the excess enthalpy can be calculated once the temperature derivative of the excess volume is known. The excess volume data as a function of temperature and pressure are also useful since they can be used to provide a sensitive test of theory. While excess volume, excess Gibbs free energy, and excess enthalpy data have been measured for a number of binary systems in the low pressure region, these data are extremely scarce in the compressed liquid region. Theories which use the hard-sphere repulsive term as a starting point are extremely sensitive to molecular size differences. In addition, the methods that the various theories use to describe the attractive forces in the real fluid mixture become increasingly important in the compressed liquid region. As a result, a study in which an excess property, namely the excess volume, is experimentally measured in the compressed liquid region for a binary system where the size disparity of the two molecules is relatively large should provide a very severe test of theory. One binary liquid mixture that has received recent attention in the literature is the CCl4 + OMCTS (octamethyl-cyclotetrasiloxane) system. OMCTS is a large, globular, quasispherical molecule with a molar volume approximately 3.2 times that of CCl4. The excess volume, excess Gibbs free energy, and excess enthalpy have been measured for this system but only at negligibly small pressures. The experimental variation of the excess volume with pressure for the CCl4 + OMCTS system should provide a very good test of the theory. Both the effects of differences in molecular size and the methods used to describe the attractive forces in real fluids will be severely tested. The purpose of this investigation is then to obtain experimental excess volumes as a function of temperature, pressure, and composition for the CCl4 + OMCTS liquid system and to use the data to examine several theories of the liquid state. Four theories which use the hard-sphere repulsive term as a starting point for the description of real fluid properties will be examined in the following chapter. Two of the theories will be examined in detail and used to compare experiment and theory for the CCl4 + OMCTS system."--Introduction.