A Possible Connection between HI and Experimental Quantities

In the second form on the rhs, we have added and subtracted Ay , and we have also used relation (8.20). The last form on the rhs of (8.31) employs the definition of AAtr in (8.22). 

The above relation has been read off by looking at the cyclic process in Fig. 8.3. We shall prove it in a more formal way later. Before doing this, it is appropriate to note that although Afi and are measurable quantities, Afia is not. The reason for this is that we do not have a molecule denoted by a + 6. We therefore refer to the cycle in Fig. 8.3 as an ideal cycle.  

We now develop a formal and convenient expression for From (8.23) and (8.31), we get (in the following, we assume that a and b are the same kind of solute, denoted by S) 

The most important aspect of this relation is the absence of the internal properties as well as of the direct pair potential for the two solute particles. One may therefore envisage a hypothetical system of pure solvent subjected to an external field of force originating from two fixed points and given by (7(X /Ri, Rg). In such a system, (5y4 (Ri, R2) is the work required to move the two sources of the field of force from infinite separation to the final positions Ri and Rg. This point of view will be exploited further in the next section. 

We now use a procedure similar to the one employed in Sections 3.5 and 3.6 to rewrite (8.34) in a more convenient form. 

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