Polymolecularity and Polydispersity in Molecular Weight Determinations

The influence of polymolecularity on molecular weight determinations is reviewed. Several molecular weight methods give so-called mixed averages, i.e. averages which are based on more than one moment of the molecular weight distribution or on powers of the moment. Molecular weight averages can be subdivided into simple, exponential resolved, order resolved and power resolved averages. Mixed averages depend on the shape of the solute and its interaction with the solvent. Simple averages of molecular weight can be calculated from combinations of averages of different properties with the help of the H-theorem. A related problem is that of the so-called polymolecularity corrections. New experiments show a definite influence of the type and the width of the molecular weight distribution on the second osmotic virial coefficients which is not predicted by present theories. Methods are given to distinguish between open, closed, molecule-based and segment-based associations. The influence of the polymolecularity of unimers on the polydispersity of the resulting multimers is discussed for several types of association. In general, a sharpening effect is observed. Equations are given which may help detect polydispersity in open and closed association. Debye's treatment of micellar solutions cannot be used to determine premicellar equilibria.