Dynamics of single polyelectrolyte chains in salt-free dilute solutions investigated by analytical ultracentrifugation

Abstract

The dynamics of polyelectrolytes in salt-free solution is an unsolved problem. We have investigated the sedimentation and diffusion of xanthan and poly(N-methyl 4-vinyl pyridine iodide) (P4VPI) in salt-free dilute solutions by analytical ultracentrifugation (AUC) using sedimentation velocity (SV) as a function of polyelectrolyte concentration (C_p). Our study reveals two concentration regimes distinguished in either polyanion (xanthan) or polycation (P4VPI) dilute aqueous solution. When C_p is below the Debye concentration (C_d) at which the chain separation (d) is close to the debye length (l_D), the interchain electrostatic repulsion is negligible, and the reciprocal apparent sedimentation coefficient (1/s), apparent diffusion coefficient (D) or reciprocal apparent molecular weight (1/M_w) is linearly related to C_p. In the range C_p > C_d with d < l_D, the interchain electrostatic repulsion is present, and the dynamics of polyelectrolytes becomes complex. The real sedimentation coefficient (s₀), the diffusion coefficient (D₀) and the molecular weight (M_w,0) of the single polyelectrolyte chain in salt-free dilute solution can be obtained by extrapolating the concentration to zero. The present study reveals that the complex dynamics of polyelectrolytes in salt-free dilute solutions arises due to the interchain electrostatic repulsion.