On the “filterable aggregates and other particles” interpretation of the extraordinary regime of polyelectrolytes

Quasi‐elastic light scattering studies on some polyelectrolyte systems exhibit a somewhat “bizarre” behavior in the profile of the apparent diffusion coefficient Dapp as a function of the salt concentration Cs. As Cs is decreased, Dapp first increases in accordance with polyelectrolyte theories, and then undergoes a precipitous drop in value by over an order of magnitude at a well‐defined critical value Cs = C . This “transition” from Cs > C (ordinary) to Cs < C (extraordinary) is referred to as the “ordinary‐extraordinary” (o‐e) transition. Ghosh, Peitzsch, and Reed [(1992) Biopolymers, Vol. 32, pp. 1105–1122] proposed a “filterable aggregate” (FA) and “other particle” interpretation for the o‐e transition and its reversibility in regard to ionic strength changes. The present communication examines in detail the FA model as applied to the o‐e transition. It is shown that the FA model fails to account of the established characteristics of the o‐e transition.