Counterion and polyion dynamics in highly asymmetrical electrolyte solutions

Aqueous solutions of 1-10, 1-20, and 2-20 electrolytes with an asymmetry in size of 2:15 are studied for several volume fractions between 4% and 44%. The use of an efficient Brownian dynamics simulation method allows the inve stigation of the structural and dynamical properties of these systems. The pair interaction potential between ions is modeled by a pairwise soft-core repulsion (1/r(9)) and the Coulomb interaction. Self-diffusion coefficients of both polyions and counterions are calculated. The residence times of co unterions in the vicinity of polyions are also computed. It is shown that a crystallization of macroions in a face centered cubic phase occurs for the 1-20 and 2-20 electrolytes at the highest concentration. In the 1-10 elect rolyte at the same volume fraction, the macroions stay in a disordered stru cture. In any case the residence time of counterions around polyions is rel atively small. Indeed, the self-diffusion of counterions is slightly increa sed in the 2-20 electrolyte when the electrolyte concentration is increased , whereas it is roughly constant in the other systems. Finally, the self-di ffusion coefficients of polyions decrease strongly with regards to the infi nite dilution, especially as they are charged. (C) 2001 American Institute of Physics.