Counterion condensation, ion pairing and scattering properties of carboxymethyl cellulose with mono- and di-valent ions

We study the scattering and conductometric properties of a semiflexible polyelectrolyte, carboxymethyl cellulose (CMC), with monovalent and divalent counterions in aqueous media without added salts. The scattering patterns for the magnesium salts of CMC display a broad shoulder instead of the scattering peak observed for the monovalent salts. This suggests weaker electrostatic repulsion between chains and a consequent loss of local order. The result is consistent with conductivity measurements, which reveal that the effective charge of the backbone for MgCMC is approximately half that of NaCMC. The decrease in charge density agrees with Oosawa–Manning condensation, which expects the charge density to be inversely proportional to the counterion valence. Alkali metal counterions show large differences in ion-pair formation but only a weak effect in counterion condensation. We suggest that paired ions are a subset of condensed ions. A review of different methods to evaluate counterion condensation, including potentiometry, osmometry and viscosity-based methods is presented. Qualitative agreement between these methods is found and possible reasons for the discrepancies are discussed.