The distribution and dynamics of small ions in simulations of ordered polyelectrolyte solutions

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Abstract
The diffusion limit brownian dynamics simulation method has been used to investigate some properties of the counterions in infinite arrays of hexagonally packed cylindrical polyelectrolytes. The level of description is that of the primitive model, and the hard cylinders representing the macroions have been given characteristics appropriate for B-DNA molecules. The simulations were carried out in a semi-infinite model system of the ordered macroion phase. The minimum image convention was used in all directions, and the long-range electrostatic interactions were described using the Ewald summation method. The results are compared with simulations in the corresponding cell model representations of the systems, and with the predictions of mean field theory. Some aspects of the static distribution of mono- and divalent counterions in ordered systems of different polyelectrolyte concentrations are presented. The dynamic behaviour of the counterions is characterized in terms of average residence times, and macroscopic diffusion coefficients for the motion in the infinite ordered array of macroions are calculated. The interactions of the nuclear quadrupole moments of the counterions with the electric field gradients of the systems are discussed, and estimates of the quadrupolar splitting for 23Na+ counterions in systems of hexagonally packed B-DNA are given.