SODIUM-ION ACTIVITY AND ELECTRICAL-CONDUCTIVITY OF POLY(MALEIC ACID) AND POLY(ISOBUTYLENE-ALT-MALEIC ACID) IN AQUEOUS SALT-FREE SOLUTION

The sodium ion activity and electrical conductivity of poly(maleic acid) (PMA) and an alternating copolymer of isobutylene with maleic acid (PIM) in aqueous salf-free solution were measured at 25-degrees-C as a function of the degree of dissociation (alpha) and polymer concentration (C(p)). These results were compared with those of poly(acrylic acid) (PAA) to investigate the effect of charge density and distribution of charged groups in a polyelectrolyte chain on the counterion binding. The activity coefficients of sodium ions of PMA and PIM increased above alpha = 0.6 at low C(p), whereas those of PAA decreased gradually with alpha at all C(p)'s. Correspondingly, the electrical conductances of PMA and PIM increased steeply above alpha = 0.5 at low C(p), whereas those of PAA increased slightly above alpha = 0.5 at all C(p)'s. The values of the activity coefficient of Na+ in PMA under salt-free conditions deviated from those predicted by the theories of Manning and Katchalsky-Lifson. The same tendency was observed in PIM for higher alpha-regions. These deviations became remarkable with increasing alpha or decreasing C(p). These results suggest that the bound sodium ions are fairly labile for alpha > 0.5 in dilute, salt-free solutions of PMA and PIM. This means that, in these cases, condensed counterions may be more or less replaced by dissociated protons in the neighborhood of a polyion, corresponding to the polyion phase.