| Abstract: | A simple theory for the relaxation of concentration fluctuations in polyelectrolyte solutions is presented, and particular results for the high-salt and no-salt limiting cases are discussed. Autocorrelation functions for the fluctuating intensity of scattered light from dilute aqueous solutions of poly(L -lysine HBr) (PLL-HBr) with and without added salt have been observed over a wide range of pH. The observed autocorrelation functions are in general very satisfactorily represented by single exponentials except at high pH (>10.5), where considerable aggregation is manifested. Solutions of PLL HBr without added salt exhibit extraordinary behavior, evident at low pH, involving a species with a very slowly decaying autocorrelation function. Though this species is readily annealed to a more ordinary individual free-molecule form by cycling the pH to 9.5 or higher and back, the resulting molecules are found to require unusually long times to reach internal configuration equilibrium under low pH conditions. Solutions of PLL-HBr in 0.2M NaBr and 0.1M NaCl are apparently free of similar extraordinary effects and show the normal isothermal helix–coil transition accompanied by a 15–30% rise in the diffusion coefficient to a maximum at pH 10.5, which is interpreted in terms of a change in molecular dimension of an interrupted helix. The predicted K2 dependence of the reciprocal relaxation time and enhancement of the apparent diffusion coefficient of the polyelectrolyte in the absence of salt is confirmed. |
