Pure and Applied Chemistry

Abstract: The Poisson­Boltzmann (PB) equation is used to investigate effective colloid-interface interactions and the phase behavior of charge-stabilized colloidal suspensions. When a colloidal particle, immersed in an electrolyte, approaches an interface, which may be neutral (such as air) or charged (like electrodes, glass, etc.), image charge effects plus the deformation of the colloidal ion atmosphere by the interface lead to an effective interaction which can be attractive or repulsive, depending on the surface charge density and the dielectric constants of the interface and the electrolyte. Two cases are considered: i) a spherical particle near a like-charged interface, and ii) a rod-like particle in the vicinity of an oppositely charged interface. The latter serves as a model for the adsorption of (anionic) DNA on a cationic membrane, and it is shown that the effective attraction, induced by the release of counterions on approach of the DNA to the membrane, makes up an essential contribution to the total DNA-membrane effective interaction. To understand the phase behavior of charge-stabilized colloidal suspensions, we study a PB cell model of a bulk suspension and investigate how the PB equation can best be linearized. It is found that the previously predicted gas­liquid phase coexistence results when the PB equation is linearized about the Donnan potential. No indication of such a spinodal instability could, however, be found, when the free energy is evaluated using the numerical solution of the full PB equation. This suggests that the predicted gas­liquid phase coexistence is an artifact of the linearization.