Previous chapters established many relations between various thermodynamic properties of a fluid. We have seen, for example, that the heat capacity of a fluid is always positive. We have also shown that the thermal, mechanical, and chemical equations of state must be mutually consistent (i.e. they must satisfy the Gibbs–Duhem equation). In the previous chapter, we showed how statistical mechanics can be used to derive fundamental thermodynamic relations from simple models of molecular interactions. In this chapter we discuss in greater detail important molecular interactions and their relative magnitudes. Specific mathematical expressions taken from physical chemistry are used to estimate when interactions are important and how they might influence thermodynamic quantities. Molecular interactions ultimately determine the behavior of materials and fluids. Hence they have received considerable attention and entire texts have been devoted to their study. Interested readers are referred to [68, 80, 99] for a more complete and thorough discussion.
The equations of state and the transport properties of gases, liquids, and solids are intimately related to the forces between the molecules. The methods of statistical mechanics provide a connection between these forces and measurable thermodynamic properties. An introduction to statistical mechanics was presented in the previous chapter. Here we merely discuss the nature and the origin of intermolecular forces. In the following chapter we will illustrate how everything comes together to generate thermodynamic property predictions from intermolecular interactions. Finally, we note that calculation of intermolecular forces requires knowledge of several fundamental properties of molecules.