A microscopic theory is presented for the behavior of polyelectrolyte solutions between surfaces. The theory predicts oscillatory forces between hydrophobic surfaces immersed in dilute salt-free polyelectrolyte solutions. The period of the oscillation, p, scales with concentration, ρ, as p∼ρ−1/3 and p∼ρ−1/2, in the dilute and semidilute solutions, respectively. With a small amount of excess salt, the oscillatory forces are replaced by a short-ranged, predominantly attractive force. Further addition of salt causes the force to become purely repulsive. These predictions can, in principle, be tested experimentally.

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It has been suggested (K. S. Schweizer, private communication) that this integral equation approach is not accurate for the depletion interactions in neutral polymers, and therefore the predictions with high concentrations of added salt might not be reliable.
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