A model for ion-exchange behaviour of polyampholytic resins: Using polystyrene polyampholytic latex

Abstract

A polystyrene based polyampholytic (zwitterionic) latex was synthesised with an isoelectric point (IEP) of 6.9 and particle size of about 110nm. Ion-exchange (IEX) ability of this latex with sodium chloride and ammonium bicarbonate (AB) was studied and it was found that Na+, NH4+, Cl− and HCO3− ions exchange with the surface groups following the law of mass action. The latex typically had an adsorption capacity of about 0.11mmol/m2, for both NaCl and NH4HCO3 electrolyte solutions. Surface adsorption constants, i.e. KNH2 and KCOOH, were assumed to be similar to solution values, that is: 1×10−9M and 1×10−6M, respectively. Precise ionization ability of the active groups is specific for the type of polymer used to form the latex particles. Calculated surface charge densities, obtained from zeta potential studies, shows that the best pH range for IEX studies with this polyampholytic latex was in the range 6.8–8.0. The average affinity of HCO3− ions was found to be higher than Cl− during the IEX process. This latex was used to demonstrate an ion adsorption model which could be applied to the IEX properties of polyampholytic resins containing the same weak acid (WA) and weak base (WB) groups.