Brownian motion of highly charged poly(<scp>L</scp>‐lysine). Effects of salt and polyion concentration

Abstract

Abstract The Brownian motion of a single sample of high‐molecular‐weight poly( L ‐lysine) [(Lys) n , n = 955] has been studied by dynamic light scattering over a wide range of NaBr concentrations and at three different polyion concentrations. A substantial decrease in scattered intensity is associated with the transition from the ordinary phase to the low‐salt extraordinary phase. At the salt concentration where the transition takes place the relaxations are non‐exponential and appear to exhibit at all angles a rapid relaxation (τ ≅ 10 μsec) that is presumed to be a manifestation of the kinetics of the transition process. The K 2 dependence of the slow relaxation rates in the extraordinary phase has been confirmed within the experimental error. The extrapolated infinite‐dilution values of the diffusion coefficients in the ordinary phase are observed to decline precipitously below 10 −2 M salt to astonishingly small values, indicating a dramatic rise in the friction factors of the isolated polyions. An extensive discussion of these findings in relation to the theory employed here and to existing data in the literature is also given.