Partial Molar and Specific Volumes of Polyelectrolytes: Comparison of Experimental and Predicted Values in Salt-free Solutions

Abstract

The partial molar volume, V̄, and the partial specific volume, ν̄, were estimated for more than 25 polyelectrolyte structures. The materials investigated by density measurements in highly diluted aqueous solutions (c < 10-2 monomol/L) included both synthetic polyelectrolytes and chemically modified natural polymers. Through a detailed analysis, related to the chemical structure and macromolecular parameters, a linear dependence between the copolymer composition and the partial volumes could be identified for diallyldimethylammonium chloride/acrylamide copolymers. Additivity could also be shown for sodium cellulose sulfate having various degrees of substitution. For a homologous series of poly(vinylbenzyltrialkylammonium chloride)s, a linear correlation between the molar mass of the monomer unit and the partial molar volume was obtained. No influence of the degree of polymerization was observed as long as the contour length exceeds the Debye length. The experimental results were used to evaluate the general applicability of the additivity schemes of Durchschlag and Zipper, as well as Gianni and Lepori, to polyelectrolytes. Agreement between experimental and calculated partial volumes strongly depends on the chemical structures. In the case of the synthetic polycations, the deviations are in a similar range for both models though somewhat smaller for the Gianni/Lepori model. The use of the Durchschlag/Zipper model yields much better agreement for the anionic biopolymers with deviations generally less than 3%. The tendency of the empirical models is correct; however, the precision may be improved by regressing of parameters from the experimental results. The new experimental data may also be useful in polyelectrolyte characterization.