Scattering by a homogeneous solute

The solution consists of only one solvent and one solute. In this case, it is not necessary to use any index for the solute and we write 

we write the structure function as the sum of intramolecular and intermolecular contributions [see (7.2.25)] 

Let M = Nm be the molecular mass of the solute molecules. Now, the result (7.2.58) reads 

The appearance of Avogadro s number in this expression results from the definition (7.2.57) of the constant K. This can be justified as follows. In ordinary space, the correlation function g(r) — 1 differs practically from zero only in a range of the same order as the size R of the molecules, let us say for instance 10 nm. Therefore, the Fourier transform //n(0) is a volume of the order of 103 nm3, but the product //n(0) is a macroscopic volume ( 104 cm3) which can be more directly measured, for instance, by looking at the osmotic pressure. 

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Scattering by a homogeneous solute in the zero-angle limit The solute is homogeneous if all the monomers are the same. 

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