Ionization behavior, polyelectrolytes

The degree of ionization of weak polyelectrolytes depends on the degree of neutralization, a, and therefore on the pH. To monitor the ionization behavior of the weak polyelectrolytes, one can measure the pH while adding strong acid or base solution. The degree of neutralization a can be calculated from the added amount of acid or base. An important value is the pKa or pKb, which denotes the negative decadic logarithm of equilibrium constant of dissociation of acids or protonation of bases. 

In a first approach, we have studied the polyelectrolyte properties of a series of Q-P(TDAE)X,. with X = 7, 17, 22, 35 and 48. The conformational and the ionization behaviors of these compounds were investigated by potentiometry, viscometry, laser light scattering and by Optical Rotatory Dispersion (ORD) and Circular Dichroism (CD) for some optically active derivatives.2"4... 

The behavior of hydrogen ions in the gel is very similar to that in polyelectrolyte solutions and the same theoretical treatment may be applied. The pH of a polyelectrolyte solution is dependent on the intrinsic dissociation constant (p °) of the functional group—which is normally that of a monomer—on the degree of ionization (a), and on the potential... 

Addition of polymers can both stabilize and destabilize a solution. If the polymer contains ionizable units it is usually referred to as a polyelectrolyte. In this report we will focus on the effect from polyelectrolytes on the colloidal stability. In high dielectric media like water, where the monomers are ionized, the behavior of a polyelectrolyte is mainly governed by electrostatics and the connectivity of the monomers. Therefore, in theoretical studies, many important features of the polyelectrolyte behavior in water solution can be studied by a schematic description of the polyelectrolyte as a linear chain of charged monomers connected with springs. The bonding interaction between two monomers is Ub=K(r —a)2, where K is the spring constant, a is the equilibrium value and r is the distance between the two monomers (see Fig. 11). 

The properties of ionomer solutions are sensitive to not only the degree of the ionic functionality and the polymer concentration, hut perhaps even to a greater extent, the ability of the solvent to ionize the ion-pairs (64). Thus, non-ionizing solvents, usually those with relatively low dielectric constant, favor association of the ionic groups even in dilute solutions. In contrast, ionomer solutions may exhibit polyelectrolyte behavior in polar solvents due to solvation of the ion-pair that leaves the hound ions unshielded. 

Synthetic polymers are widely applied to modify the surface properties of materials, and their adsorption mechanism is very different from small ions or molecules discussed in previous sections. Moreover, special methods are applied to study polymer adsorption, thus, polymer adsorption became a separate branch of colloid chemistry. Polymers that carry ionizable groups are referred to as polyelectrolytes. Their adsorption behavior is more sensitive to surface charging than adsorption of neutral polymers. Polyelectrolytes are strong or weak electrolytes, and the dissociation degree of weak polyelectrolytes is a function of the pH. The small counterions form a diffuse layer similar to that formed around a micelle of ionic surfactant. 

Similar solution behavior was reported(9-11) for sulfonate ionomers. Rochas eit al. (9) observed a polyelectrolyte effect for acrylonitrile-methallylsulfonate copolymers in DMF. Lundberg and Phillips(10) studied the effect of solvents, with dielectric constants ranging from c 2.2 to e 46.7, on the dilute solution viscosity of the sulfonic acid and Na-salt derivatives of sul-fonated polystyrene (SPS). For highly polar solvents such as DMF and dlmethylsulfoxide (DMSO, e 46.7) they observed a polyelectrolyte effect, but for relatively non-polar solvents such as THF and dioxane (c = 2.2) no polyelectrolyte effect was observed. Like Schade and Gartner, these authors concluded that polar solvents favor ionization of the metal sulfonate group while non-polar solvents favor ion-pair interactions. 

A polyelectrolyte can be defined as a macromolecule with ionizable repeat units, which can consequently display pH-dependent behavior when dissolved in aqueous solution. 

As the term polyelectrolyte can also be extended to encompass natural biopolymers, which contain ionizable groups such as peptides, proteins, and DNA, it is perhaps not surprising that synthetic polyelectrolytes are often used as simple models for more complex biological systems. Indeed, many of the photophyscial techniques discussed in this chapter were developed by biologists and biophysicists interested in the conformational behavior and interactions of biopolymers [1]. 

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