Structure of poly (sodium 4-styrenesulfonate) (PSS) in electrolyte solutions: Theoretical modeling and measurements

In this work, the structure of poly (sodium 4-styrenesulfonate) (PSS) molecules in electrolyte solutions obtained from theoretical simulations was compared with experimental data derived from dynamic light scattering (PCS), electrophoretic and dynamic viscosity measurements. Simulations and experiments were carried out for polymer having molecular weight of 15.8 kD and for various ionic strength of the supporting electrolyte (NaCl). It was predicted from molecular dynamic simulations that for the entire range of electrolyte concentration studied (I = 10−3 to 0.15 M) the molecule behaved as a flexible rod. Its effective length Lef varied from 12.5 to 8.5 nm, which corresponds to 0.79–0.56 of the contour length Lext = 16 nm predicted for fully extended polymer chain. Thus, for electrolyte concentration of 0.15 M, a significant folding of the molecule was predicted, whose shape resembled a semi circle (torus). These predictions were compared with PCS measurements of the diffusion coefficient of the molecule, which allowed one to calculate its hydrodynamic radius RH. It was found that RH varied between 3.1 for I = 5 × 10−3 M and 4 nm for I = 0.15 M. These RH values were in a good agreement with theoretical predictions stemming from Brenner's theory, approximating the true particle shape by prolate spheroids, bent to various forms. Using these RH values and electrophoretic mobility data derived from microelectrophoresis, the average number of uncompensated (free) charges on the PSS molecule and the effective ionization degree were calculated. The number of free charges was determined to be 14–16 (decreasing slightly with ionic strength), which gives the ionization degree of 18–20%, which was comparable with theoretical predictions. Additional shape information was derived from the dynamic viscosity measurements of dilute PSS solutions using a capillary viscometer. The intrinsic viscosity derived from these measurements varied between 28.3 and 8 for the ionic strength 10−3 to 0.15 M. It was shown, after introducing the correction for hydration, that the experimental results were accounted well by the Brenner's viscosity theory for slender particle suspensions. The effective lengths derived from viscosity measurements using this theory was comparable with values predicted from the molecular dynamic simulations.