Static Light Scattering from Mixtures of Polyelectrolytes in Low Ionic Strength Solutions

Static light scattering behavior is explored for salt-free and low ionic strength solutions of pure, highly charged polyelectrolytes and polyelectrolyte mixtures at very low concentration (∼10 -5 g/mL). The pure polyanions (xanthan and proteoglycan subunits) show pronounced, high-contrast angular scattering peaks in the scattering function I ( q ), whereas hyaluronate shows none. When I ( q ) is divided by the experimentally extrapolated form factor P ( q ) to obtain the structure function S ( q ) using I ( q ) ∼ P ( q ) S ( q ), a high-contrast peak remains in S ( q ). This can be interpreted in terms of a quasi-periodic, screened (i.e. damped) pair correlation function g ( r ), which suggests that the peak is measuring an average interparticle separation and not any type of electrostatic scaling length. The particle/particle correlations are thought to be dynamic in the sense that no crystallization or local static ordering takes place. Adding neutral polymer (dextran) causes marked changes in I ( q ) which can be rationalized in terms of an absence of correlation between xanthan and the polymer. Adding polyanions (hyaluronate or polystyrenesulfonate) to xanthan produces a completely different change in I ( q ), which suggest that the xanthan molecules are being locally concentrated.