Forces between surfaces immersed in polyelectrolyte solutions

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.