Sedimentation equilibrium of polydisperse nonideal solutes: a comparison of three approximations.

In the evaluation of sedimentation equilibrium data for polydisperse nonideal solutions, extrapolation procedures are required for the determination of the true solute weight-average molecular weight and of the light-scattering second virial coefficient. In an attempt to decide just which concentration-dependent parameter should be used in making these extrapolations, Van Holde and Williams, J. Polym. Sci., 11, 243 (1953), and Fujita, J. Phys. Chem., 63, 1326 (1959), and ibid., 73, 1759(1969), have derived relations between apparent and true weight-average molecular weights which, starting from approximate forms of the same differential equations, appear to give different working expressions. The present analysis of these results will demonstrate new and perhaps unexpected relations between them.Further, these approximations are discussed in terms of certain of the available experimental data, the purpose being to clarify the conditions under which the behavior of a polydisperse nonideal solution in the ultracentrifuge may be treated as if a nonideal two-component system were involved.