Concentration and molecular weight regime diagram of salt-free polyelectrolyte solutions as studied by light scattering

Abstract

Static and dynamic light-scattering measurements were carried out on salt-free aqueous sodium poly(styrene sulfonate) solutions. The concentration dependence of the solution behavior was investigated in the range c=0.01– 45.6 g/L for three samples with Mw=5000, 100 000, and 1 200 000. A critical concentration ccr≂0.5 g/L was experimentally established. Above ccr, two diffusion coefficients Df (fast) and Ds (slow) were measured. Df, corresponding to the coupled diffusion of polyions and counterions, is independent of concentration. Ds, corresponding to the presence of multichain domains in solution, increases upon dilution. The angular dependence of the scattering intensity as it reflects the size of domains becomes less pronounced upon dilution and, at c=ccr, the scattering intensity is independent of angle. The concentration dependence of the excess scattering intensity is stronger than linear (the reduced intensity I/c increases with concentration). Below ccr, Df decreases upon dilution and finally merges with Ds. The scattering intensity is first independent of angle, then upon dilution a broad maximum appears. The concentration dependence of the excess scattering intensity is weaker than linear (the reduced intensity I/c decreases with concentration). These experimental data indicate that, for c<ccr, repulsive Coulombic interactions screened by the presence of counterions dominate. For c>ccr, the solution structure and dynamics are predominantly influenced by coupled polyion-counterion dynamics and the presence of attractive forces giving rise to domains. The critical concentration ccr is independent of molecular weight. A concentration molecular weight regime diagram is constructed combining the present data with previous results on the molecular weight dependence and with literature data on very dilute solutions.

Ordinary–extraordinary transition in dynamics of solutions of charged macromolecules

“…Because D c of a macromolecule is orders of magnitude smaller than the monomer diffusion coefficient (if the monomer were to be alone and not a part of the chain), the fast diffusion coefficient is predicted as D f ' αD 0 , independent of N and c. [16] As the effective degree of ionization is about 0.25-0.3 due to counterion adsorption, and taking D 0 as that of a metallic ion, the fast diffusion coefficient is about 5 × 10 −6 cm 2 =s as seen in Fig. 1 As a typical example, UðrÞ ∼ − 10k B T if the separation distance between the dipoles and the dipole length are 0.25 nm, comparable to the monomer separation distance along the chain for saltfree water at room temperature.…”

“…The diffusion coefficients D f and D s , collected from literature originating from many laboratories worldwide (1,11,15,16), are given in Fig. 1 as functions of polymer concentration c and degree of polymerization N. In addition to the abovementioned puzzles, several features characterize the ubiquitous and intriguing simultaneous occurrence of the fast and slow modes.…”

Interactions between cationic starch and anionic surfactants

“…When the added electrolyte concentration is lower t han the equivalent polyelectrolyte concentration, the intensity correlation functions of polyelectrolyte solutions are no longer of the unimodal type but display a bimodal behaviour, with two characteristic decay rates termed fast and slow. Both a slow and a fast decay rate are found for rod-like and flexible polyelectrolyte molecules such as DNA (Wang et al, 1991) and NaPSS (Sedlak et al, 1992). It has been demonstrated for quaternised poly(2-vinylpyridine) (Förster et al, 1990) that for polyelectrolyte solutions without added salt the intensity correlation functions are unimodal at very low polyelectrolyte concentrations and bimodal at the higher ones.…”

Inter- and intrachain attractions in solutions of flexible polyelectrolytes at nonzero concentration

“…Equally unclear is how the essential results of the theory can be generalized to nonzero concentrations instead of just at infinite dilution. Recent experiments [4][5][6], theories [7][8][9][10] and simulations [11,12] have attempted to address some of these issues, but no consistent picture has yet emerge.…”