Structure function in the grand canonical formalism

Let us consider the simple case where the solute consists of homogeneous (but different) polymers (see Chapter 7, Section 2.2.3). In this case, the monomer concentration is given by [See (9.2.14)] 

The partial structure function (q) concerning species si and 38 is defined by [see (9.2.16)] 

For reasons expounded at the beginning of this chapter, it is convenient to express these mean values in the grand canonical formalism. Thus, for instance, the quantity C C j (r) Ca (r) is just one of these functions p 38, / ) defined 

We observe that in the limit of large volumes, Hjm can be expressed in terms of Qa / Indeed, using the definition (9.3.9) of Qa, we find 

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