Slow diffusion coefficient, polyelectrolyte

The slow diffusion coefficient is measurable only at high enough polyelectrolyte concentrations. The value of c at which the slow mode appears is higher if Cj is higher. When the ratio X = c/cg is about 1, the onset of the slow mode and the crossover between the smaller Df for A, < 1 and higher Df for A > 1 occur. Dj depends [33] on c strongly. 

We have identified three diffusion coefficients. These are the self-translational diffusion coefficient D, cooperative diffusion coefficient Dc, and the coupled diffussion coefficient fly. fl is the cooperative diffusion coefficient in the absence of any electrostatic coupling between polyelectrolyte and other ions in the system, fly is the cooperative diffusion coefficient accounting for the coupling between various ions. For neutral polymers, fly and Dc are identical. Furthermore, we identify fly as the fast diffusion coefficient as measured in dynamic light scattering experiments. The fourth diffusion coefficient is the slow diffusion coefficient fl discussed in the Introduction. A satisfactory theory of flj is not yet available. 

X 10 cmr/s. Thus it appears that colloidal crystal formation bears further investigation as a source of extraordinarily slow diffusion coefficients in low salt polyelectrolyte solutions. 

The hght scattering data [24—34] of polyelectrolyte solutions are characteristic of two diffusion coefficients Df (fast) and Dj (slow). D/ for a solution with a given Cj increases with monomer concentration c and above a certain concentration c, Df is independent [31, 33, 34] of both c and M. 

The presence of a slow diffusion seems to be characteristic for most polyelectrolyte solutions. The slow mode was first detected by Schurr et al. [218] for poly(L-lysine) upon variation of salt concentration. A sudden drop of the diffusion coefficient was observed as the salt concentration was decreased below some critical value. Since then, this transition is called ordinary-extraordinary transition , the fast diffusion being referred to as ordinary and the slow diffusion as extraordinary . Drifford and Dalbiez [219] later gave an empirical expression which describes the relation between this critical salt concentration... 

Influence of Added Salt on the Slow Mode. As with NaCl and CaCl2, the system NaPSS/LaCl3 presents a pseudo splitting phenomenon between the two modes at a critical salt concentration [32], The amplitude of the slow mode becomes very low and undetectable. Only the fast component of the autocorrelation function is present. These results are analogous to many observations made on a lot of polyelectrolyte solutions and recall the pseudo-transition from extraordinary phase to ordinary phase [31,32,34,37,64]. At last, in the upper one-phase at Cs 0.5 M (D-point on Figure 15), a large scattered intensity is observed with only one relaxation time. The value of the effective coefficient diffusion is about 10 7 cm2/s. 

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