Metal ion – ionic polysaccharide interactions: theoretical developments and experimental results

Abstract

A modification of the Counterion Condensation (CC) theory of linear polyelectrolytes has been developed to take into account the presence of specific interactions between the polymer and the counterions. The particular case in which the polymer is the Ionic Polysaccharide (IPS) K-carrageenan and the ions are monovalent alkaline ions with high atomic number (e.g. K + , Cs + ) has been considered. K-carrageenan in the presence of such ions undergoes a conformational transition eventually leading to the formation of side-by-side ordered pairs of chains, acting as junctions in a 3-D gel network. The presence of the conformational equilibrium leading to the fundamental ordered single chain (1h) and of the following one leading to chain pairing (1h.1h) complicates the description of the overall process. Nevertheless, they have been successfully accommodated together with the specific ion-polymer interactions to give a self-consistent set of thermodynamic equations. The application of the model to the experimental calorimetric data of mixing K-carrageenan with Cs + ions has been successful. The thermodynamic parameters for the main chain conformational changes, as well as those for the specific interactions, have been calculated. In addition, the approach allowed for the evaluation of the number of polymer repeating units in the cooperative units of both 1h and of 1h.1h. They are as large as 50 and 35, respectively.