Electrophoretic light scattering studies of poly(L-lysine) in the ordinary and extraordinary phase. Effects of salt, molecular weight, and polyion concentration

Abstract

Electrophoretic light scattering (ELS) is employed to determine the electrophoretic mobilities μE and apparent diffusion coefficients DELS obtained from ELS linewidths for three samples of poly(L-lysine)HBr (PLL) of degree of polymerization n=406, n=946, and n=2273 as a function of salt (Cs) and polyion (Cp) concentration. Dynamic light scattering (DLS) is used to determine apparent diffusion coefficients Dapp of these same PLL samples in the absence of the applied field. The polyelectrolyte system exhibits an ordinary to extraordinary phase transition at low salt concentrations that is manifested by a more than 50-fold decrease in Dapp as well as a twofold decrease in scattered intensity. In the extraordinary phase, the DELS values are typically very small, though still somewhat (frequently twofold) larger than Dapp. When the salt concentration is raised to enter the ordinary phase DELS remains small, less than one tenth of Dapp, in the low-salt end of the ordinary phase, but increases with increasing Cs to become comparable to, though still about twofold smaller than, Dapp in 0.1 M NaBr. The fluctuations in polyion density responsible for the observed ELS signal are evidently relaxed very slowly under low-salt conditions, regardless of which phase prevails. In the ordinary phase μE increases rapidly with decreasing Cs, but in the extraordinary phase μE either remains nearly constant for (n=946), or actually decreases (for n=2273), as Cs decreases further to 0. The present μE data disagree with a previous empirical prediction of Schurrr and rule out the particular interpretation of the diffusion virial coefficient upon which that was based. The theory of Manning predicts a value of the tracer mobility μp that is quite close to the value of μE observed for (1ys)406 at Cs=0.1 M, but predicts much too large an increase in μp with decreasing Cs, and fails to predict the significant molecular weight dependence of μE observed in the intermediate Cs range near 0.02 M. It is pointed out that incorporation of electrolyte friction into Manning’s theory could improve the agreement with experiment.