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.