Contrast shown by a homogeneous mixture of chains in solution

2 Contrast shown by a homogeneous mixture of chains in solution 

First let us consider a pure ideal liquid, without density fluctuations 

the structure function H(q) is identically zero. In this case, the cross-section of the isotopic mixture (of molecules with the same molecular volume) reduces in expression (7.2.30) only to the contrast term. 

Let us generalize this result to the case of a homogeneous mixture of several species si si =0, 1). Species si = 0, for instance, is the solvent  

The constraint (7.2.31) enables us to express one of the volume fractions, for instance cp0(r) in terms of the others 

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