Effect of counterion species on the dynamics of polystyrenesulfonate in aqueous solution as studied by dynamic light scattering

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Abstract
The diffusion behavior of macroions in aqueous solutions especially at low salt concentrations has been interesting and also mysterious problem since Schurr et al. found slow mode in 1978. By the systematic investigations on the structure and dynamics of charged particles and macromolecules in solution, it has been suggested that some novel factor(s) should be taken into account for understanding of interesting phenomena in these systems. In this study, the dynamic character, i.e., diffusion behavior of polystyrenesulfonate (PSS) in aqueous solution has been studied by dynamic light scattering (DLS) as a function of salt concentration in various salt forms. Both fast and slow modes were clearly detected. With increasing salt concentration, the fast mode became dominant and its decay rate became slower. On the other hand, the slow mode was not so sensitive to salt concentration but its contribution markedly decreased with increasing salt concentration and then became almost negligible at higher salt concentrations. The fast mode of LiPSS, NaPSS, and CsPSS was in the order of LiPSS≦NaPSS<CsPSS, although the difference was small, and was consistent with the order of diffusion coefficients of counterions. The fast mode of HPSS was much faster than that of other PSS salts. The same tendency was also observed for HPES and NaPES, salts of polyethylenesulfonate. The slow mode also depended on the counterion species, which may predict the important role of counterion dynamics in electric interaction between macroions.