Light scattering of micellar and polyelectrolyte solutions: influence of added salt

Abstract

Static and dynamic light-scattering experiments have been carried out in order to study two polyelectrolyte systems: System A consists of globular and rigid macroparticles such as micelles and system B contains flexible and anisotropic polymer chains. The dependence of the scattered intensity and mutual diffusion coefficient on concentration of polyelectrolyte-salt solutions is analyzed in terms of theoretical models (pair repulsion potential and Debye-Huckel approximation). Good fits are obtained at low salt concentration for tetradecyltrimethylammonium bromide series, sodium dodecyl sulfate, and tetradecyltrialkylammonium bromide micelle solutions with the virial development and the noninstantaneous diffusivity of small ions. The ionization degree is independent of salt concentration, but it is very sensitive to the size of ionic head groups. The static and dynamic properties of flexible polyelectrolyte-salt solutions have been reported (system B). From the q dependence of scattered intensity, it is deduced that chains have a “coil” conformation in the presence of excess of salt. The ionic strength dependence of the effective diffusion coefficient of sodium polystyrene sulfonate (NaPSS) is investigated as a function of NaCl concentration. The results indicate a splitting in the relaxation times which depends on the ratio C p C s where C p and C s are the polyion and added salt concentrations, respectively. At last, we present some comments on the different properties of these two charged macromolecule solutions (A and B). The role of counterions is discussed. In particular the counterion effects seriously modify the dynamics of polyions in low salt concentration.