Partial molar and specific volumes of polyelectrolytes: Comparison of experimental and predicted values in salt-free solutions

The partial molar volume, (V) over bar and the partial specific volume, <(n u)over bar>, were estimated for more than 25 polyelectrolyte structures. Th e materials investigated by density measurements in highly diluted aqueous solutions (c < 10(-2) monomol/L) included both synthetic polyelectrolytes a nd chemically modified natural polymers. Through a detailed analysis, relat ed to the chemical structure and macromolecular parameters, a linear depend ence between the copolymer composition and the partial volumes could be ide ntified for diallyldimethylammonium chloride/acrylamide copolymers. Additiv ity could also be shown for sodium cellulose sulfate having various degrees of substitution. For a homologous series of poly(vinylbenzyltrialkylammoni um chloride)s, a linear correlation between the molar mass of the monomer u nit and the partial molar volume was obtained. No influence of the degree o f polymerization was observed as long as the contour length exceeds the Deb ye length. The experimental results were used to evaluate the general appli cability of the additivity schemes of Durchschlag and Zipper, as well as Gi anni and Lepori, to polyelectrolytes. Agreement between experimental and ca lculated partial volumes strongly depends on the chemical structures. In th e case of the synthetic polycations, the deviations are in a similar range for both models though somewhat smaller for the Gianni/Lepori model. The us e of the Durchschlag/Zipper model yields much better agreement for the anio nic biopolymers with deviations generally less than 3%. The tendency of the empirical models is correct; however, the precision may be improved by reg ressing of parameters from the experimental results. The new experimental d ata may also be useful in polyelectrolyte characterization.