MONTE-CARLO SIMULATIONS OF A SINGLE POLYELECTROLYTE IN SOLUTION - ACTIVITY-COEFFICIENTS OF THE SIMPLE IONS AND APPLICATION TO VISCOSITY MEASUREMENTS

Monte Carlo simulations of linear polyelectrolytes together with expli cit ions have been performed in a spherical cell model to study confor mational changes and activity coefficients in relation to the isoionic dilution method used in viscosity measurements. The results show that it is possible to define an effective ionic strength that will keep t he average chain conformation constant on isoionic dilution and that t his ionic strength can be predicted from the activity of the counterio ns, as has been suggested experimentally. Activity coefficients have b een calculated from the simulations and compared with theoretical esti mates based on various applications of the Debye-Huckel approximation, including Manning theory and an expression for a rigid rod with discr ete charges. Manning theory generally gives poor agreement with the si mulations, while the rigid-rod expression, which includes an ion-ion t erm, is able to predict the mean activity coefficient at not too high charge densities. Assuming that the co-ions are completely inert, the rigid-rod expression also leads to a reasonable approximation for the counterion activity. The simulation results have been used as input fo r two theoretical expressions for the reduced viscosity. The first, wh ich is only based on the average chain conformation, does not reproduc e the qualitative features of experimental curves. Our chains, with on ly 80 monomers, do not display large conformational changes upon dilut ion with salt solutions of varying ionic strength. In contrast, the se cond viscosity expression, which takes intermolecular electrostatic in teractions into account, gives a correct qualitative behavior.