Dynamics Of Polyelectrolyte In Salt-free Aqueous Solutions

Dynamics of polyelectrolytes in solution is important for us to understand fundamental issues in biology and their applications in a number of technological and industrial fields. To date, the charging property makes polyelectrolytes so complex that there are many questions still open. In this thesis, we have investigated polyelectrolytes and polyzwitterion aqueous solutions with and without the addition of salt by using a combination of analytical ultracentrifugation (AUC) via sedimentation velocity (SV) and laser light scattering (LLS). We also have clarified the nature of the fast and slow mode, and revealed the reason of complex dynamics of polyelectrolytes. The main content and results of thesis as follows:(1) We have investigated the dynamics of xanthan aqueous solutions with and without added NaCl by using analytical ultracentrifugation (AUC) via sedimentation velocity (SV) and laser light scattering (LLS). The fast and slow modes are observed in salt-free and low-salt xanthan solutions by dynamic light scattering (DLS). The apparent diffusion coefficient (Ds,app(q))and scattering ratio (KC/Rs(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 (as) 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 dynamic inhomogeneity 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 aqueous solutions.(2) We have investigated the sedimentation and diffusion of xanthan and poly(N-methyl 4-vinyl pyridine iodide) (P4VPI) in salt-free dilute solutions as a function of polyelectrolyte concentration (Cp) by using analytical ultracentrifugation (AUC) via sedimentation velocity (SV). Our study reveals two concentration regimes distinguished in either polyanion (xanthan) or polycation (P4VPI) dilute aqueous solution. When Cp is below the Debye concentration (Cd) at which the chain separation (d) is close to the Debye length (lD), the interchain electrostatic repulsion is negligible, and the apparent diffusion coefficient (D), reciprocal apparent sedimentation coefficient (1/s) or reciprocal apparent molecular weight (1/Mw) is linearly related to Cp. In the range Cp>Cd, d<lD, the interchain electrostatic repulsion is present, the dynamics of polyelectrolytes becomes complex. The real diffusion coefficient (Do), sedimentation coefficient (s0) and molecular weight (Mw,0) of the single polyelectrolyte chain in salt-free dilute solution can be obtained by extrapolating the concentration to zero. The present study reveals the complex dynamics of polyelectrolytes in salt-free dilute solutions arises from the interchain electrostatic repulsion.(3) Dynamics of polyzwitterions remains largely unclear. We have prepared zwitterionic poly[1-(3-sulphopropyl) betaine-2-vinylpyridinium] (PSB) and investigated its dynamics in solution as a function of added salt (NaCl) concentration (Cs) by using sedimentation velocity (SV) in analytical ultracentrifligation (AUC) and dynamic laser light scattering (DLS). A slow and a fast mode can be observed by DLS in a salt-free and low-salt solution, where the latter exhibits a maximum intensity at Cs-103 M. SV measurements demonstrate that the fast mode corresponds to the diffusion of individual chains and the slow mode arises from the dynamic inhomogeneity due to interchain electrostatic repulsion. As Cs increases, the sedimentation coefficient exhibits a maximum at Cs-0.1 M whereas the diffusion coefficient has a minimum at Cs-10-3 M and a maximum at Cs-0.1 M. Namely, PSB shows a complex dynamics in a salt-free and low-salt solution and anti-polyelectrolyte behavior in a high-salt solution. Our studies reveal that the dynamics of polyzwitterion is mediated by the short range intrachain attraction and long range interchain electrostatic repulsion, which are determined by effective charges on the chains.