Hydration of the polyion

The structure of this water can be affected by ion binding. If the counterions are tightly bound at the sites of individual charged groups, the 

As a increases, the average distance between ionized groups decreases so that these neighbouring groups begin to have an effect. When a exceeds 0-3, individual water spheres begin to overlap and eventually coalesce into a cylindrical form. With further increases in a, a second outer cylindrical sheath of water appears in which water molecules are oriented by the cooperative effect of two or more carboxyl groups. 

The explanation for this volume increase is as follows. For a cylindrical model of uniform charge density the electric field around the cylinder is 

The formation of a stable hydrogen-bonded ring structure as in poly(itaconic acid) and in poly(maleic acid) has also been shown to affect hydration states (Muto, Komatsu Nakagawa, 1973 Muto, 1974). 

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