Light scattering from two-component solutions

In Chapter 3, the scattering function for a single macromolecule was introduced. The scattered intensity was shown to be a function of the magnitude of the scattering vector q. hi this section, the value of q is restricted to the very low range. At each concentration, there is a characteristic distance, L between solute molecules that is proportional to When the quantity qL 1, it is convenient to treat the solution as a continuum characterized by a local concentration, that fluctuates in both position and time. If the 

The results of a light-scattering experiment are usually expressed in terms of a scattering cross section called the Rayleigh ratio, R(q). It is defined as  

The partial derivative can be related to the osmotic pressure. The excess Rayleigh ratio for concentration fluctuations can then be expressed as  

To focus on the variables that change with the solution, the excess Rayleigh ratio is often written as  

In the thermodynamic limit ( - 0), measurement of the solution structure factor is a good method for obtaining the osmotic second virial coefficient. Measmements of the excess Rayleigh ratio as a function of concentration yield the molecular weight of the solute within this limit. The usual procedure is to plot the quantity Kc/AR c) against concentration  

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