Correlation of counterions with rodlike macroions as assessed by anomaloussmall-angle X-ray scattering

We consider the analysis of a rodlike synthetic polyelectrolyte in solution by anomalous small-angle X-ray scattering (ASAXS) in order to elucidate th e correlation of the counterions with the highly charged macroion. ASAXS ca n be applied to these systems because the absorption edge of typical counte rions, for example, bromine or iodine ions can be attained by synchrotron r adiation. Model calculations using the Poisson-Boltzmann cell model show th at ASAXS furnishes two terms caused by the anomalous dispersion of the coun terions. The leading terms is a cross-term between the ordinary scattering amplitude of the polyelectrolyte and the real part of the scattering length f ' of the counterions. A second term refers solely to the anomalous contr ibution of the counterions, i.e., to f ' and f " (f ": imaginary part of sc attering length). Preliminary data obtained from rodlike synthetic macroion s having iodine counterions corroborate the theoretical deductions. They de monstrate that ASAXS is capable of furnishing information that is not avail able by the ordinary SAXS experiment.