Chain conformation, polyelectrolyte-counterion interactions

The situadon is even more interesting in the case of polyelectrolyte chains. In this case, chain conformations are dirertly coupled with the intrachain electrostatic interactions between charges that are controlled by the amount of the condensed counterions. ° ° 5-72,86,88-93 jjj addition to counterion... 

These experiments show the effect of the counterion on the structure of the charges stored on the polymer chain. On the other hand tiie electrostatic interaction between charges, as for polyelectrolyte, must change the chain conformation. Then, one can ask the influence of the chain conformation in die neutral state on the stracture of the charges and the way the chains are sensitive to the different kinds of charges. 

Let us consider the conformation of polyelectrolyte macromolecules immersed in an infinite quantity of solvent. The counterions having high translational entropy are distributed over the whole volume of the solution their concentration in the vicinity of the macromolecules is extremely low, and their influence on the molecular conformation can be omitted completely. The conformation of the polyelectrolyte macromolecule is determined by rather strong unscreened repulsive interactions between charged groups attached to the chain. Due to this repulsion, the macromolecule assumes a strongly stretched conformation in the sense that its end-to-end distance R is a linear function of the degree of its polymerization m [14-18]. 

It is obvious that both intra-molecular and inter-polymer phenomena in polyelectrolyte solutions are dominated by coulomb forces. Repulsive interactions would be expected to diminish aggregation. At the same time, extended chain dimensions and the long-range character of electrostatic effects can promote forms of ordering unique to polyelectrolytes. Hydrodynamic, spectroscopic and thermodynamic methods have all been brought to bear on the coupled problems of conformation, counterion distribution and inter-polymer ordering in polyion solutions. These approaches are well represented in Part III by the works on Paoletti, Berry and Jamieson. 

The presence of charge influences both inter- and intramolecular interactions. The charged polyelectrolyte molecules are surrounded by a diffuse distribution of counterions (cf. Chapter 9). The molecules repel each other by electrical double layer overlap, so that a polyelectrolyte solution may be colloidally stable even when the solvent quality is poor. Intramolecular electrostatic repulsion causes a more stretched conformation of the chain. This can be accounted for by an electrostatic contribution to the persistence length L ... 

Many macromolecules in aqueous solution are polyelectrolytes. The remarkable changes in the conformation of linear polyelectrolytes as a function of concentration, ionic strength, and pH are discussed. The various theories of chain expansion are reviewed. The thermodynamic properties of polyelectrolyte solutions reveal dramatic behavior. The large increase in the reduced osmotic pressure, jr/c, as the solution is diluted is explained in terms of the entropy of the counterions. The strong dependence of the conformation of the chains with solution conditions also leads to large changes in the viscosity. The viscosity is also explained in terms of the coil size and the interactions of the chains. 

---