Molecular weight determination from light scattering and refraction in solutions. A new and coherent theoretical equation

The classical theoretical equation allowing for the determination of the solute's molecular weight (M(2)) from light scattering (LS) and refractive measurements in diluted solutions is reexamined. Serious theoretical and practical defects in its derivation and application are pointed out, especially about the refraction increment dn/dc. A new theoretical equation is deduced from a more consistent application of the basic molecular orientation formulae to binary liquid mixtures. More adequate additive molecular properties, the specific mean polarizability <(alpha)over bar>/M and the specific square of the mean polarizability <(alpha)over bar>)(2)/M instead of <(alpha)over bar> and (<(alpha)over bar>(2), are considered with respect to refraction and LS, respectively. Inadequate approximations are avoided and a corrected LS intensity expression is used. The new equation is tested upon several typical mixtures of small molecules from measurements either performed in this work or collected in the literature (for LS perpendicular to the incident beam) to ensure objectivity. Examples of measurements in macromolecule solutions from other authors are also analysed. The new equation is suitable for practical applications, because it is based upon easy relative measurements only, and allows for a reliable determination of the M(2) values in macromolecules. The reasons for which the classic equation leads to realistic values, in spite of its theoretical inconsistency, are also explained.