Thermodynamics of a model for flexible polyelectrolytes in the binding mean spherical approximation

We discuss our previous solution of the mean spherical approximation (MSA) for flexible polymers made of arbitrarily charged spheres of arbitrary diam eter. Closed form analytical results for some particular cases are presente d, in particular thermodynamic excess functions and an equation for the MSA screening parameter Gamma(T). It is found that the ideal chain approximati on does not satisfy the exact Debye-Huckel limiting law since the polymer t erm appears with an effective charge squared that is 8.5 instead of 9 for a trimer, 14 instead of 16 for the tetramer, and 7n-16 instead of n(2) for v ery large n. We propose a new chain approximation that takes three-body har d core exclusion into account. We show that this triplet exclusion chain ap proximation satisfies explicitly the Debye-Huckel limiting law for all leng ths n, and also for all charge combinations. (C) 2000 American Institute of Physics. [S0021-9606(00)50415-2].