Polyelectrolytes, coiling

Significantly, counterion activity coefficients generally are not strongly affected by polyelectrolyte concentration. In contrast, simple salts exhibit a monotonic increase in activity upon dilution. This effect may be attributed to the fact that the charge density in the vicinity of the polyelectrolyte coil is insensitive to dilution, while dilution of a simple salt solution results in uniform separation of all of the charges and hence weaker ionic interactions. 

Only if an inert electrolyte like NaCl is added, the strong electrostatic interactions are increasingly screened and the highly expanded polyelectrolyte coils start to shrink. Eventually the unperturbed dimensions are approached at high enough concentrations of inert salt [18-20]. In such cases, phase separation occurs if this inert salt level is surpassed [21-23]. The latter phenomena has been denoted as salting out of polyelectrolytes, or alternatively, as H-type precipitation, because the concentration of inert salt required to cause precipitation of the polyelectrolyte is high and independent of the polymer concentration [23]. 

Direct Determination of Unperturbed Polyelectrolyte Coil Dimensions... 

Another approach taken by Flory is to consider the polyelectrolyte coil xmder large ionic strength conditions as a phase subject to the conditions of the Dorman equilibrium. The coil will expand to reduce the Dorman potential, but the chain will resist due to the decreasing entropy. The coil is characterized by an effective volume, V, and the free salt concentration is c°. The condition of equilibrium is given by Morawetz as ... 

As the previous section illustrated, different theories provide different values of R, making a theoretical assessment of c problematic. Substituting measured value of M and R into equation 21 offers the best way to determine c. Although flexible polyelectrolyte coils overlap above c, these coils are not necessarily entangled. As demonstrated by Boris and Colby (96) through measurements of the c dependence of viscosity, the polyelectrolyte concentration Ce required for onset of entanglement may be substantially higher than c. ... 

The obvious effect of changing the salt concentration in a solution of a flexible polyelectrolyte is that the latter will expand in low salt concentration, as a result of the mutual repulsion of the charges along the chain, and contract in high salt concentrations, as discussed briefly in Chapter 2. In any particular solution the size of the polyelectrolyte coil will be such that the energy... 

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