Ion-Exchange Properties of Cellulosic Polyanions

Abstract

Thermodynamic parameters have been determined for the ion exchange of inorganic cations with sodium ions on cellulosic polyanions which contain carboxyl, carboxymethyl, phosphate, or sulphate groups. These polyanions show similar properties to both ion-exchange resins and biological polyelectrolytes with respect to their ion-binding character istics. Thus both cellulose sulphate (—SO2-) and cellulose phosphate (=PO2 -) bind potassium ions in preference to sodium ions, whereas with oxycellulose (—COO-) and carboxymethylcellulose (—CH2COO -) the order is reversed. The order of binding for copper(II) ions when exchanged for sodium ions is carboxymethyl > carboxyl > sulphate. For binding to carboxymethyl groups, the order of binding for divalent ions is Cu2+ > Mn2+ > Ca2+, but at sulphate groups the order is Mn2+ > Ca2+ > Cu2+. This behavior is here explained on the basis of two postulates: (1) that the ions are bound in the hydrated form to -SO2- and = PO2- groups, and (2) that at -CH 3COO- and —COO- groups the effective radii of the ions are in order of their crystal radii. The cause of this difference in behavior is held to be the effective radius of the anionic group. A theory of ion-exchange selectivity is advanced which gives quantitative expression to these ideas and allows calculation of free-energy changes that are in good agreement with the experimental values.