Collapse of a charged polymer gel induced by mixing 'unfavorable' counterions

Swelling behavior of alkali metal poly(acrylate) gels in aqueous ethanol was investigated in the mixed counterion systems. In the presence of Na+ and K+ counterions, both of which are favorable for the gel collapse in the relevant solvent system, no specific deswelling was observed. In the presence of Li+ and Cs+, both of which are relatively unfavorable for the gel collapse, however, the gel markedly deswelled in an ethanol composition that is lower than the critical compositions for the respective single counterion systems. This unexpected behavior seems to result from a subtle energy balance between the ion-pair formation (counterion binding) and dipolar aggregation of the formed ion-pairs. (C) 2001 Elsevier Science Ltd. All rights reserved.