Use of anomalous small-angle X-ray scattering for the investigation of highly charged colloids

We present a study of the radial structure of a spherical polyelectrolyte brushes by anomalous small-angle X-ray scattering. The spherical polyelectrolyte brushes consist of a solid poly(styrene) core of ca 100 nm diameter onto which long linear chains of poly(acrylic acid) (PAA) are densely grafted. A sufficiently high pH, these polyelectrolyte chains are fully charged. Rubidium ions are used as counterions because their adsorption edge (15 199.6 eV) can be conventiently reached by synchrotron radiation. By performing small-angle X-ray scattering studies at different energies of the incident radiation, the contribution of the Rb⁺ counterions can be analyzed separately. The scattering contribution of the counterions can be derived and compared to the scattering intensity of the entire particle. The distributions of the macro-ion and of the counterions are shown to be very similiar. This shows that the counterions must be confined within the brush.