Effect of Confinement on the Interaction between Two Like-Charged Rods

Monte Carlo simulations were employed to study two charged rods confined between two unlike charged plates, all immersed in a model electrolyte. Recently, it was shown that two rods immersed in a multivalent counterion solution may show attraction [Phys. Rev. Lett. 78, 2477 (1997)]. Here we show for a monovalent electrolyte that rod-rod attraction and repulsion can switch sign depending on confinement and ionic size. We also propose a simple self-assembling mechanism which may be helpful to understand the DNA-lipid bilayers complexation.

Figure 1

Scheme of the system showing regions $r_1$, $r_2$, and $r_3$.

Figure 2

(a) Cation, ○, and anion, ●, densities for region $r_1$. (b) Normalized charge densities for regions $r_1$ (●), $r_2$ (○), and $r_3$ (×). (c) Normalized charge density of region $\mid z\mid \le \tau /2$.

Figure 3

Negative of ${\rho }_{\mathrm{el}}(x,z)$ for $h/a=2.5$ (a), 1.5 (b), 1.0 (c), and 0 (d). Blue indicates a larger concentration of anions than cations and red the opposite. Darker tones indicate larger absolute values of ${\rho }_{\mathrm{el}}(x,z)$. The inset of each figure schematizes the charge distribution.

Figure 4

(a) Forces on rods per unit of rod area (pressure) as a function of $h$ for rod-rod contact. Crosses were obtained when no plates were included in the simulation box. (b) Forces on plates per unit of plate area (pressure) as a function of $h$. (c) Pressure on rods as a function of $t$ for different $h$.

Figure 5

Force acting on rods as a function of the ionic size.