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

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