Polyion–Counterion Interactions in Mixtures of Sodium Carboxymethylcellulose (aq) with Different Molar Masses

Abstract

This study explores polyion–counterion interactions in aqueous mixtures of a polyelectrolyte sodium carboxymethylcellulose (NaCMC) having two different molar masses. Previous dynamic light scattering (DLS) studies indicated an invariance of the polyion–counterion interactions with the total concentration or with the composition in mixed sodium polystyrene sulfonate (NaPSS) solutions, while conductivity studies reveal a definite dependence of these interactions in these systems. The objective of this research is to shed more light on these interactions for a different system not considered earlier namely mixtures of NaCMC with a quite low charge density compared to NaPSS, and to explore whether such effect prevails in other systems as well. Counterion-dissociation governing the polyion–counterion interactions are quantified using conductivity as the probe to unravel the nature of these interactions. Degrees of counterion-dissociation and hence the effective charges on the carboxymethylcellulose chains are found to increase with the total polyelectrolyte concentration. This study reveals a reduction in the degree of counterion-dissociation in mixtures richer in the higher molar mass sample, which becomes more pronounced in dilute solutions. Roles of the effective dielectric constant, screening of chain charges, and polyion chain-coiling in governing counterion-dissociation are critically assessed. Substantial proportion of the counterions has been found to move with the polyions, corroborating the inferences drawn on the basis of counterion-condensation. This study demonstrates a definite dependence of polyion–counterion interactions on both the total polyelectrolyte concentration and the composition of the mixtures.