Insight into dynamics of polyelectrolyte chains in salt-free solutions by laser light scattering and analytical ultracentrifugation

Abstract

We have investigated the dynamics of xanthan aqueous solutions with and without added salt (NaCl) by using laser light scattering (LLS) and analytical ultracentrifugation (AUC) via sedimentation velocity (SV). The fast and slow modes are observed in salt-free and low-salt xanthan solutions by dynamic light scattering (DLS). The scattering ratio (KC/Rs(q)) and apparent diffusion coefficient (Ds,app(q)) of the slow mode is linearly related to scattering vector (q2), indicating that it is related to the diffusion of scattering objects. The intensity contribution (αs) of the slow mode is independent of scattering angles, indicating that the slow mode is not related to some scattering objects larger than the LLS observation length. However, the slow mode disappears in SV experiments, indicating that it arises from the temporal aggregates due to long range electrostatic interactions between chains, which can be destroyed in centrifugal field. The diffusion coefficient measured by SV is close to that of the fast mode in DLS measurements, indicating that it is the coupling diffusion of macroions and counterions. The present studies also demonstrate that the chain stiffness does not change the characteristics of the dynamics of polyelectrolyte in solutions.