Dependence of structural forces in polyelectrolyte solutions on charge density:: A combined AFM/SAXS study

We have investigated the structural forces in aqueous, salt-free, semidilute solutions of partially sulfonated polystyrene (PSS) of various charge fractions f using colloidal probe atomic force microscopy and small-angle X-ray scattering (SAXS). Near interfaces, we find an exponentially damped oscillatory force-distance characteristic whose period d corresponds to the characteristic length of the density fluctuations in bulk. d scales with monomer concentration C as d = d(0)C(alpha), where alpha varies progressively from -0.51 for fully charged PSS to -0.31 for the lowest charge fraction investigated. This can be explained by changes in the polyelectrolyte configuration as the charge is reduced, due to the hydrophobic nature of the backbone. The decay length lambda of the force is in all cases proportional to C-1/2 and is closely matching the range of positional correlations extracted from the SAXS data. The amplitude A of the force is proportional to the effective charge (f(eff)) of the polyelectrolyte.