Spatial correlation of spherical polyelectrolyte brushes in salt-free solution as observed by small-angle X-ray scattering

We report on the observation of spatial correlation of linear polyelectrolyte chains attached to latex particles. The particles are dispersed in water and consist of a solid poly(styrene) core and a shell of poly(acrylic acid) (PAA). At low pH the PAA chains are virtually uncharged, but full ionization can be reached for pH > 10. Small-angle X-ray scattering (SAXS) reveals an additional peak in the region of intermediate scattering angles when the chains are fully charged whereas no signal is seen in the uncharged state (low pH). Also, the peak is only seen if the polyelectrolyte chains attached to different particles overlap sufficiently. The position of the maximum scales with the square root of the particle concentration. The peak vanished when the ionic strength is raised by adding salt to the suspension. We assigned this signal to the weak maximum of the scattering intensity seen by small-angle scattering experiments in solutions of free linear polyelectrolytes ("polyelectrolyte peak"). The present investigation demonstrates that the same spatial correlation may occur when the polyelectrolyte chains are attached to the surface of colloidal particles.