Swelling Equilibria of Hydrogels with Sulfonate Groups in Water and in Aqueous Salt Solutions

Abstract

Swelling equilibria for five sulfonate gels were carried out in water and in aqueous solutions of NaCl, KCl, CaCl 2 , Na 2 SO 4 , K 2 SO 4 , and CaSO 4 with concentrations ranging from 10 -6 to 1 M at 25 o C. These samples were the same DS series of gels used in our previous paper, which were copolymerized using the ionizable monomer 2-(acrylamido)-2-methylpropanesulfonic acid (AMPS), the comonomer N,N-dimethylacrylamide (DMMA), and the cross-linker N,N'-methylenebis(acrylamide) (BIS). The mole fraction f of AMPS in the gel network ranged from 0.0899 to 0.6076. Before swelling the proton in the gel was exchanged by neutralization to the same cation as that in the salt solution to be equilibrated. The swelling capacity increases with increased charge density in the gels and with decreasing salt concentration. NaCl or KCl solutions more concentrated than 1 M were needed to screen the charge interaction in the DS50 (f=0.4318) and DS70 (f=0.6076) samples. The swelling ratios of DS50 and DS70 gels swollen in pure water and in the salt solutions were found to depend on the counterion species in the increasing sequence as Ca 2+ , Na + , and K + , which cannot be interpreted with the concept of ionic strength. These phenomena seem to be the results of counterion condensation