Simple Systems with Interactions

Throughout this chapter, we use very simple models to introduce a number of concepts that are frequently used in the context of the theory of fluids. Examples are the pair-correlation function, direct and indirect correlations, potential of average force, nonadditivity of the triplet correlation function, and so on. All these will be introduced again in Chapter 5. However, it is easier to grasp these concepts within the simple models. This should facilitate understanding them in more complex systems. 

In actual examples the polymers are not localized. They form a dilute solution within the solvent. The consideration of M indistinguishable polymers causes only a minor difference in the PF of the system (see section 2.9.2). A more serious modification should be considered when taking into account the presence of the solvent. This will be deferred to Chapter 8. 

For most of the present chapter, we can think of the M polymers, very large compared with the ligands, as forming an ideal gas in which the translational and rotational degrees of freedom have been frozen in. This makes the polymers localized, and therefore distinguishable. The release of the polymers from their fixed positions adds the translational-rotational PF and a factor M. These factors do contribute to the PF of the system, but do not change the thermodynamics of adsorption. 

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