Polyion extensions, interactions and precipitation

The precipitation of polyelectrolytes by the addition of multivalent counterions may be explained in these terms. When there are no multivalent ions in solution there is a strong repulsive force between polyions and the osmotic pressure is large. The solubility of polyions is a result of these repulsive forces. 

The binding of multivalent counterions decreases the repulsion and causes attraction between polyions. This attraction is the result of the fluctuation of the counterion distribution and is equivalent to a multivalent counterion bridge between polyions. 

The theory of gelation (Flory, 1953,1974) has been summarized in Section 2.2.3. This theory regards gelation as the consequence of the random crosslinking of linear polymer chains to form an infinite three-dimensional network. The phenomenon is, of course, well illustrated by examples drawn from the gelation of polycarboxylic acids by metal ions. 

The concept of ionic crosslinking is in accord with the idea that a gel must possess a coherent structure. However, although crosslinking may be essential to gel formation it does not necessarily have to be a simple ionic salt bridge. 

Michaeli (1960) opposed these views. He concluded that whatever the exact mechanism was, the binding of divalent cations caused contraction and coiling of the polyelectrolyte as was the case with adds. He disagreed with the concept of ionic crosslinking. The phenomenon of precipitation could be explained simply in terms of reduced solubility. From this he concluded that precipitation took place in an already coiled molecule and the matrix consisted of spherical macromolecules containing embedded cations. 

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