Counterion activity of highly charged strong polyelectrolytes

Comprehensive counterion activity studies have revealed clear influence of the polyelectrolyte concentration and chain length on the polyion-counterio n interaction, neither of which is evident from the existing theoretical mo dels. The counter-ion activity coefficient (f(a)) increases below the overl ap concentration c* as calculated from Odijk's scaling approach. At dilutio ns where the Debye length (l(D)) exceeds the contour length (L), this incre ase is more pronounced. It is concluded that these changes in the ionic int eractions are primarily responsible for the strong increase of the equivale nt conductivity (Lambda) below c*. The findings are not restricted to oligo mers but apply to all chain lengths of polyelectrolytes at concentrations l ess than c*. The correlation of f(a) and Lambda with both c* and the ratio l(D)/L leads to a general description of polyelectrolyte behavior. Experime nts were performed above and below c* and over the range 0.1 less than or e qual to l(D)/L less than or equal to 10 using a series of poly(vinylbenzylt rialkylammonium) model polyelectrolytes with contour lengths over a range 7 nm less than or equal to L less than or equal to 102 nm and characterized by a charge density parameter xi = 2.85.