Raman spectroscopic studies on the interaction between divalent counterion and polyion

The nature of the interaction between polyacrylate ion and several divalent cations, such as Cu2+, Mn2+, Zn2+, Ba2+ and Mg2+, was investigated using Raman spectroscopy. A specific Raman band characteristic of a carboxyl group is shifted upon addition of Cu2+, Zn2+ and Mn2+ to partially neutralized poly(acrylic acid). On the other hand, no frequency shift of the specific Raman band is observed on addition of Mg2+ and Ba2+, though the intensity of the specific Raman band decreases with concentration of MgCl2. It is concluded from these Raman data that the interaction between polyacrylate ion and Cu2+, Zn2+ or Mn2+ includes a specific interaction with bond formation, whereas in the case of Mg2+ and Ba2+, the electrostatic interaction is dominant.