Polyelectrolyte solutions, diffusion measurement

Therefore we expect Df, identified as the fast diffusion coefficient measured in dynamic light-scattering experiments, in infinitely dilute polyelectrolyte solutions to be very high at low salt concentrations and to decrease to self-diffusion coefficient D KRg 1) as the salt concentration is increased. The above result for KRg 1 limit is analogous to the Nernst-Hartley equation reported in Ref. 33. The theory described here accounts for stmctural correlations inside poly electrolyte chains. 

Although the theory of polyelectrolyte dynamics reviewed here provides approximate crossover formulas for the experimentally measured diffusion coefficients, electrophoretic mobility, and viscosity, the validity of the formulas remains to be established. In spite of the success of one unifying conceptual framework to provide valid asymptotic results, in qualitative agreement with experimental facts, it is desirable to establish quantitative validity. This requires (a) gathering of experimental data on well-characterized polyelectrolyte solutions and (b) obtaining the relationships between the various transport coefficients. Such data are not currently available, and experiments of this type are out of fashion. In addition to these experimental challenges, there are many theoretical issues that need further elaboration. A few of these are the following ... 

Strong evidence of ionic association was found by Stilbs and Lindman 69) in their PGSE study of aqueous polyelectrolyte solutions, polyacrylic acid and poly-methacrylic acid, neutralized by tetramethylammonium hydroxide, with or without sodium counterions. While polymer diffusion could not be detected since its T2 was too short, TMAOH and water diffusion was measured as function of degree of neutralization a, or Na+ content. A pronounced minimum of D(TMAOH) near a = 1 was interpreted in terms of a two-site model, leading to the determination that at a = 1, approximately half of the counterions are bound in both systems. Fourier transform techniques permitted the simultaneous measurement of diffusion of water and TMAOH. 

K. E. Bremmell and D. E. Dunstan. Probe diffusion measurements of polystyrene latex particles in polyelectrolyte solutions of varying ionic strength. Macromolecules, 35 (2002), 1994-1999. 

D. E. Dunstan and J. Stokes. Diffusing probe measurements in polystyrene latex particles in polyelectrolyte solutions deviations from Stokes-Einstein behavior. Macromolecules, 33 (2000), 193-198. 

Here, is a measure of the average mesh size of the entangled network. Understanding of these molecular parameters, for example, the concentration dependence of the diffusion coefficient is well developed for neutral polymer solutions (25) and polyelectrolytes with a large amount of simple salts. However, for salt-free polyelectrolytes, reliable data are lacking and understanding is rather poor. 

When the fluorescence from a DPA-labeled PSS sample was quenched with T1+, deviation from Stem-Volmer behavior was observed [146]. A measure of success was achieved in description of the data, however, through use of the Hindered Access Model [1] (see Equation 2.13). This complex behavior was considered to reflect the fact that quenching can occur via Tl+ ions which are condensed onto the polyelectrolyte in addition to deactivation by those ions which diffuse through solution. The data were also consistent with an extended PSS conformation in the presence of low concentrations of salt, which is in agreement with other fluorescence data [87,145,150] and light scattering and viscosity measurements [136-139]. 

Since ionic polysaccharides are stiffer than most synthetic polyelectrolytes, they were thought to be appropriate experimental models to test theories based on cylindrical symmetry. Systematic determinations of co interion and coion diffusion coefficioits and activity coefficients have been made in the presence of ionic polysaccharides these measurements pertain to the interaction of small ions in the ionic atmosphere with the polyelectrolyte. Here, resiilts are presented for estimating the fraction of ions dissociated from the ionic polysaccharide in simple salts solutions. 

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