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Two rods confined by positive plates: Effective forces and charge distribution profiles

Monte Carlo simulations were employed to study two negative rods confined between two parallel plates. The system is immersed in a 1-1 restricted primitive model electrolyte. Ion distributions and forces per unit of area (pressures) on rods and plates are accessed by sampling the NVT ensemble. Pressures are analyzed by means of their corresponding electric and contact (depletion) contributions. This was done for several charge distributions on plates, plates charge densities, and plate-plate surface separation distances. We found an enhancement of the inherent repulsive rod-rod effective force when uncharged plates are the confining species. On the contrary, a strong decrease of the rod-rod effective repulsion was obtained for positively charged plates. Moreover, attraction was also found for plates having a charge equal to that of fully charged bilipid bilayers. These results agree with DNA-phospholipid complexation experiments. On the other hand, for a model having the plates charges fixed on a grid, very long range rod-rod sin-like effective forces were obtained. As explained in the text, they are a consequence of the rod-plate double layer coupling.