Anomalous Small Angle X-ray Scattering Study of Counterion Distribution around Macroion and Biopolyelectrolytes*

The counterion condensed around marcoion is important for their solution behavior. While other techniques can hardly achieve, anomalous small angle X-ray scattering (ASAXS) can deterimine numbers and spatial distribution of counterions around the macroion. {Mo₁₃₂}, a negetively charged sphere of ~3 nm diameter, was used as a model of macroions to be investigated by ASAXS. The result shows: 1. Extra Rb⁺ can neutralize most of negative charges and induce coagulation of {Mo₁₃₂}. 2. Extra Sr²⁺ can effectively screen the electrostatic interaction between {Mo₁₃₂} but cannot induce coagulation. Contrary to DLVO or Debye-Huckel theory, Sr²⁺ only loosely associate with macroion. While the Coulomb force dominate the interactions between macroions and counterions in most cases, the energy penalty of break the hydration shell of the divalent cation (Sr²⁺) upon counterion-macroion association is important, which caused this abnormal counterion association behavior. Counterion distribution around several other soft and rigid charged polysaccharides were also studied.