A NEW APPROXIMATION TO SEDIMENTATION EQUILIBRIUM EQUATION FOR POLYDISPERSE NONIDEAL SOLUTIONS

A new approximation to the sedimentation equilibrium equation for polydisperse nonideal solutions is proposed, and it is shown that this leads to the statement Mapp-1 = Mw-1 + (B*/2)(cm + cb) + . . ., with B* = B(1 + Δ) Here Mapp is the apparent weight-average molecular weight calculated from the concentrations (cm and cb) at the meniscus and bottom of the solution column in the centrifuge cell, Mw is the weight-average molecular weight of the solute, B is a nonideality parameter essentially equal to the light-scattering second virial coefficient of the solution, and Δ is a small correction which primarily depends on the polydispersity of the solute and the experimental conditions chosen. This result indicates that the plot of Mapp-1 against the average concentration, (cm + cb)/2, may be applied in certain situations for the analysis of polydisperse non-ideal solutions.