Effective interactions between oppositely charged polyelectrolytes in the presence of salt

We generalize the familiar effective DLVO (Derjaguin-Landau-Verwey-Overbeek) pair potential between charged, hard core colloidal particles to the case of solutions of oppositely charged, penetrable polyelectrolyte coils in the presence of microions, within the framework of classical Density Functional Theory. The limiting behaviour of the effective potentials is derived in the limits of weak and strong microion screening; in the latter regime the effective potentials are shown to go over to a universal Gaussian form, multiplied by the square of the microion Debye screening length. The physical implications of screening on polyelectrolyte aggregation are discussed and illustrated by preliminary Monte Carlo simulations and the results of fluid integral equations for the polyelectrolyte pair structure.