Abstract
This work concerns the activity coefficient determination for sodium and calcium ions in a semi-dilute solution of xanthan when they are mixed in various ratios, and their correlation with the conformational structures. From these values we show that the conformation can be related to the dissociation of the polymer carboxylic acid sites. Below an apparent dissociation coefficient of about 0.29, the conformation is ordered and above 0.55 the xanthan conformation is entirely disordered. The value of the counterion activity coefficient in pure solution of sodium or calcium xanthan salts agrees fairly well with the theoretical ones derived from Manning's and Lifson-Katchalsky's theories if we make the assumption that the Na form is a stretched coil and the Ca form a single fivefold helix.
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