Polyelectrolyte ionic strength

The conformations adopted by polyelectrolytes under different conditions in aqueous solution have been the subject of much study. It is known, for example, that at low charge densities or at high ionic strengths polyelectrolytes have more or less randomly coiled conformations. As neutralization proceeds, with concomitant increase in charge density, so the polyelectrolyte chain uncoils due to electrostatic repulsion. Eventually at full neutralization such molecules have conformations that are essentially rod-like (Kitano et al., 1980). This rod-like conformation for poly(acrylic acid) neutralized with sodium hydroxide in aqueous solution is not due to an increase in stiffness of the polymer, but to an increase in the so-called excluded volume, i.e. that region around an individual polymer molecule that cannot be entered by another molecule. The excluded volume itself increases due to an increase in electrostatic charge density (Kitano et al., 1980). 

Ermi BD, Amis EJ. Domain structures in low ionic strength polyelectrolyte solutions. Macromolecules 1998 31 7378-7384. 

The viscosity of sodium algiaate solutioas is slightly depressed by the additioa of moaovaleat salts. As is frequeatly the case with polyelectrolytes, the polymer ia solutioa coatracts as the ionic strength of the solution is increased. The maximum viscosity effect is obtained at about 0.1 N salt concentration. 

In most cases, the swelling of polyelectrolyte hydrogels depends only on ionic strength of the solution but not on the size and nature of the ions [101]. Therefore, the ionic suppression curves similar to those of Fig. 2 and 3 are to some extent universal and allow to predict quantitatively the swelling of hydrogels for practically any ionic situation. 

For polyelectrolytes the second virial coefficient is very sensitive to ionic strength. Preston and Wik [28] have shown a tenfold increase in B—from 50 mlmolg" to 500 ml mol g —upon decreasing the ionic strength from 0.2 down to 0.01 moll h... 

It was found earlier by experiment and theory that the viscosity intrinsic of polyelectrolyte solutions is nearly linear with the reciprocal square root of the ionic strength over a certain range, such as... 

The increasing dilution of flexible polyelectrolytes at low ionic strength, the reduced viscosity may increase first, reach a maximum, and then decrease. Since a similar behavior can also be observed even for solutions of polyelectrolyte lattices at low salt concentration, the primary electroviscous effect was thought as a possible explanation for the maximum, as opposed to conformation change. 

Note that when the concentration of added salt is very low, Debye length needs to be modified by including the charge contribution of the dissociating counterions from the polyelectrolytes. Because the equilibrium interaction is used, their theory predicts that the intrinsic viscosity is independent of ion species at constant ionic strength. At very high ionic strength, the intrachain electrostatic interaction is nearly screened out, and the chains behave as neutral polymers. Aside from the tertiary effect, the intrinsic viscosity will indeed be affected by the ionic cloud distortion and thus cannot be accurately predicted by their theory. 

The effects of ion valence and polyelectrolyte charge density showed that at very low ionic strength found that when the counterion valence of added salt changes from monovalent (NaCl) to divalent (MgS04), the reduced viscosity decreases by a factor of about 4.5. If La(N03)3 is used, the reduced viscosity will be further decreased although not drastically. As for polyelectrolyte charge density, the intrinsic viscosity was found to increase with it because of an enhanced intrachain electrostatic repulsion (Antonietti et al. 1997). 

One of the main characteristic of polyelectrolyte is the pK of the - COOH function as usually with polyelectrolyte only the intrinsic pK (pKo) extrapolated to zero charge characterizes the polymer [41] one gets 3.30 which is in same range as other carboxylic polymers the apparent values of pK (pKa) depends on the charge distribution, on the polymer concentration, on the ionic strength of the solution and on the nature of the counterions. 

Satoh, M., Komiyama, J. lijima, T. (1984). Counterion condensation in polyelectrolyte solutions a theoretical prediction of the dependences on the ionic strength and degree of polymerization. Macromolecules, 18, 1195-2000. 

Garcia, R., Porcar, I., Campos, A., Soria, V. and Figueruelo, J. E., Solution properties of polyelectrolytes. X. Influence of ionic strength on the electrostatic secondary effects in aqueous size-exclusion chromatography, /. Chromatogr. A, 662, 61, 1994. 

Water-soluble polymers in general, and especially polyelectrolytes, are often difficult due to their specific and long range electrostatic interactions, which complicate all analytical techniques that rely on single particle properties that are usually realized by high dilution. In most cases the ionic strength of the solution must be increased by the addition of salt in order to screen electrostatic forces. Ideally, SEC separation is predominantly governed by entropic interactions,... 

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