LIGHT-SCATTERING METHOD OF DETERMINING THE 2ND VIRIAL-COEFFICIENT FOR SIMPLE MOLECULES AND OLIGOMERS

A procedure of determining the molecular weight and second virial coefficient for simple molecules and oligomers from light-scattering measurements on a binary solution is presented on the basis of the well-established theory of fluctuations in multicomponent systems. It consists of determining first the total isotropic scattering from the solution and then the composition scattering by subtracting from the former the density scattering (the Einstein-Smoluchowski term), where the values at finite concentrations should be used for the density scattering and also for the refractive index and its increment appearing in the optical constant. The required value of the density scattering is obtained by multiplying its observed value for the pure solvent by the concentration correction factor that is evaluated from a proper relationship between the refractive index and density of the solution. From experimental results obtained for a solution of toluene in cyclohexane at 25.0-degrees-C as an example, the Lorentz-Lorenz equation is recommended as such a relationship. It is demonstrated that the multiple scattering theory by Bullough, which has often been used, is erroneous.