The osmotic coefficient of spherical polyelectrolyte brushes in aqueous salt-free solution

A study on the osmotic coefficient of polyelectrolyte brushes in aqueous solution is reported. The systems studied here consist of a solid polystyrene core with chemically grafted linear poly(sodium styrenesulfonate) chains. The osmotic pressure of the counterions of these spherical polyelectrolyte brushes in aqueous solution without added salt has been investigated in the dilute regime (0.05-0.50 gl-1). The osmotic coefficient, φ, as a function of particle concentration was deduced from the measured osmotic pressure. The range of φ is a few percent, which demonstrates that nearly all the counterions are confined within the charged corona of the spherical polyelectrolyte brushes. It is also observed that the greater the contour length of the grafted polyelectrolyte chains, the lower is the number of osmotically active counterions in these spherical polyelectrolyte brushes. A comparison with φ measured for free polyelectrolyte chains demonstrates that φ is much lower in the case of the polyelectrolyte brushes. This demonstrates directly the trapping of counterions in an osmotic brush predicted by theory.