Ultracentrifugal extrapolated molecular weights of polyelectrolytes

The extrapolated ultracentrifugal molecular weight for a polyelectrolyte in a polar solvent is shown to be equal to M ext = M P —Z P M B where M P is the true molecular weight of the polyelectrolyte, Z P is its electrostatic charge, and M B is the molecular weight of a low molecular weight electrolyte. This relationship is shown to be true for a polyelectrolyte in the absence or presence of added salt. Hence, the relationship M ext = M P /(1 + Z P ) is never obtained for a polyelectrolyte for equilibrium ultracentrifugation in a polar solvent.