Orientational transition and complexation of DNA with anionic membranes: Weak and intermediate electrostatic coupling

Electric dipole image forces in three-layer systems: The classical electrostatic model

General exact analytical expressions have been derived for the image force energy W_i(Z, φ) of a point dipole in a classical three-layer system composed of dispersionless media with arbitrary constant dielectric permittivities ε_i. Here, i = 1-3 is the layer number, and Z and φ are the dipole coordinate and orientation angle, respectively. It was found that the long-range asymptotics W_i(Z→∞,φ) in both covers (i = 1, 3) are reached unexpectedly far from the interlayer (i = 2). Another specific feature of the solution consists in that the interference of the fields created by polarization charges emerging at both interfaces leads to the appearance of a constant contribution inside the interlayer with a non-standard dependence on the dipole orientation angle φ. It was shown that by changing the dielectric constants of the structure components, one can realize two peculiar regimes of the W_i(Z, φ) behavior in the covers; namely, there arises either a potential barrier preventing adsorption or a well far from the interface, both being of a totally electrostatic origin, i.e., without involving the Pauli exchange repulsion, which is taken into account in the conventional theories of physical adsorption. The results obtained provide a fresh insight into the physics of adsorption in physical electronics, chemical physics, and electrochemistry.

Bistable colloidal orientation in polar liquid near a charged wall

We examine the translation and rotation of an uncharged spheroidal colloid in polar solvents (water) near a charged flat surface. We solve the nonlinear Poisson-Boltzmann equation outside of the colloid in two dimensions for all tilt angles θ with respect to the surface normal. The colloid's size is assumed to be comparable to the Debye's length and hence field gradients are essential. The Maxwell stress tensor, including the ideal gas pressure of ions, is integrated over the colloid's surface to give the total force and torque on the colloid. From the torque we calculate the effective angular potential U_eff(θ). The classical behavior where the colloid tends to align in the direction perpendicular to the surface (parallel to the field, θ=0) is retrieved at large colloid-surface distances or small surface potentials. We find a surprising transition whereby at small separations or large potentials the colloid aligns parallel to the surface (θ=90°). Moreover, this colloid orientation is amplified at a finite value of the aspect ratio. This transition may have important consequences to flow of colloidal suspensions or as a tool to switch layering of such suspensions near a surface.