Conformational changes in polyions

The conformation of macro- or polyions has been defined and discussed briefly in Section 4.1.1. The conformation of a polyion is determined by a balance between contractile forces, which depend on conformation free energy, and extension forces, which arise from electrical free energy. The extent of conformational change is determined by several factors. Changes are facilitated by the degree of flexibility of the polyion, and conformational change is greatest at low concentration of polyions. 

Conformation depends on the degree of ionization and concentration of the polyion, the type and concentration of the counterion and the interaction between counterion and polyion. Extension is favoured by low concentrations of counterion and polyion. Conformational change is also affected by the extent of the charge on the polyion. As the charge on a polyion increases, the chain uncoils and expands under the influence of repulsive forces. Thus, the neutralization of a polyacid is accompanied by 

Ionization of the carboxyl groups is accompanied by binding of the cations. But if counterions are site-bound the charge on the carboxyl groups is neutralized and chain contraction results. A special case is that of the polyacid which adopts a contracted form because the close association of hydrogen ions with carboxyl groups results in a neutral chain. 

Extensive forces arise from the electrical interaction between coimterions and polyions. There are two repulsive forces which act to extend a polyion. One results from coulombic repulsion between the charged groups on the polyion and the other from osmotic pressure of the counterions within, which seek to increase the space in which they can move. 

The coulombic force is proportional to the square of the effective charge on the polyion, i.e. n], (The effective charge is equivalent to the number of free counterions, ,.) When the charge along the polyion, Q, is small the extensive forces involved are those of purely coulombic repulsion. 

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