Ionic strength, polyelectrolyte solutions

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

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). 

There is a range of parameters other than polyelectrolyte charge density that has an important influence on the generated surface interactions, for instance, counterion valency and ionic strength of solution [121-123], the order of addition of polyelectrolyte and salt [124], polyelectrolyte concentration [125], presence of surfactants [31, 119, 126], and finally, the chemical structure of the polyelectrolyte itself [127]. A rich literature is available on these topics (see Ref. [115] and references therein). 

The effect of simple salt on the surfactant binding confirms the electrostatic nature of the surfactant ion-polyion interactions (step 1, Scheme 1). Any increase in the ionic strength of solution shifts the onset of binding toward higher free surfactant concentrations (compare isotherms in water, 0.01 M, and 0.1 M NaCl, Figure 6) and decreases the amount of bound surfactant. These observations can be related to the screening influence of the simple salt, which acts to diminish the electrostatic interactions between surfactant cations and polyanions. This is also well documented in the literature for a variety of polyelectrolyte-surfactant pairs [26-29],... 

In the context of our further discussion on the enthalpy of binding, it has to be stressed again that the total surfactant concentration, cs, at which appreciable binding of CP+ and DP+ to PSS and PA, respectively, starts, is approximately the same (around 1 X 10 5 mol dnT3, cf. Figures 5 and 7), irrespective of the nature of the polyelectrolyte, surfactant chain length, or ionic strength of solutions. 

In terms of total surfactant concentration, appreciable binding of dodecyl-and cetylpyridinium cation starts at the same total concentration, at about 1 X 10 5 mol dm 3, irrespective of the ionic strength of solutions. Above this concentration the heat effects that accompany the complexation of surfactant with the polyelectrolyte increase sharply, and their value can be partly attributed to the micellization of detergent. The viscosity and apparent molar volume measurements reveal that above cs 1 X 10 5 mol dm-3 extensive coiling of the chain around surfactant micelles takes place. In addition to this, the unusual behavior of molar conductivity in polyelectrolyte-surfactant solutions shows that an appreciable amount of small counterions may... 

It is also known that the pH and ionic strength of solutions of polyelectrolytes can have a marked influence on their retentivities. The more a polyelectrolyte is charged in solution, and the lower the ionic strength of the medium, the larger the effective size of the polyelectrolyte for a given molecular weight. 

The literature dealing with the polyelectrolyte behavior of nylons in fluori-nated alcohols is somewhat controversial. In solutions of i lon 6 and 6,6 in 2,2,3,3-tetrafluoropropanol, some authors observed the polyel rtrolyte efifi which they suppressed with trifluoroacetate lithium chloride or water . Other authors ex dained the polyelectrolyte effect in 2,2,3,3-tetrafluoropropanol solutions by the presence of traces of water while assuming that salt suppresses this effect not by raising the ionic strength of solution, but by binding traces of water. The polyelectrolyte effect was not oteerved in 2,2,3,3-tetrafluoropropanol solutions of nylon 2,2,2-trifluoroethanol solutions of nylon 6 , octafluoro-propanol solutions of nylon 6,6 ) and 1,1,1,3,3,3-hexafluoro-2-propanol solutions ofnytonl2 >. 

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. 

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. 

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,... 

The ionic strength of the solution also significantly influences polyelectrolyte adsorption. In general, the higher the ionic strength of the medium, the less extended and the more coiled the polymer conformation becomes (due to preferential interaction with counter ions in solution rather than with other segments of the polymer chain). The coiled polymer becomes more accessible to the internal porous structure and adsorption is increased. However, for the same reason, it is less influential on the surface charge. 

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