Quantitative characterization of the concentration-dependent interaction between molecules of Dextran 70 in aqueous solution: Measurement and analysis in the context of thermodynamic and compressible sphere models

The static light scattering and sedimentation equilibrium of solutions of Dextran 70 were measured as functions of concentration up to 100 g/L in pH 7.4 phosphate-buffered saline at temperatures between 5 and 37 °C. The concentration dependence of scattering intensity and the apparent molar mass obtained from sedimentation equilibrium were found to be nearly independent of temperature over this range to within the uncertainty of measurement. Global analysis of the concentration dependence of both properties yielded a reliable estimate of the concentration-dependent thermodynamic activity coefficient, a quantitative measure of the free energy of self-interaction. The self-interaction between Dextran molecules is compared with that of a globular protein (BSA) and a highly crosslinked polymer of similar molar mass (Ficoll 70). The observed concentration dependence of the free energy of Dextran self-interaction may be quantitatively accounted for by a semi-empirical model in which the polymer molecule is represented by a compressible sphere.