NOTE A Perturbed Electric Double Layer Near a Soft Polar Interface dipoles can exert influence on the electric potential profile in the system (11-13). The hydrophilic zone thickness is usually characterized by a pa-A perturbation analysis is applied to a nonequilibrium double rameter L p that describes the extent of water and ion penetration into this electric layer (NDEL) near a membrane with extended soft permeloose region of the membrane. L p values reported for lipid membranes (14, able interfaces (polar zones) under steady-state ion flux conditions. 15, 18-20) range from L p Γ
3-8 A Λto 12-14 A Λ, for erythrocytes (17) L p The membrane has been modeled as composed of three layers: an Γ
35-55 A Λ, and for some cells (21) L p Γ
40-70 A Λ. inner hydrophobic layer and two polar zones with fixed charges The studies concerned with membranes with soft interfaces (7-12) have and dipoles. Ion flux is described according to Nernst-Planck and dealt mainly with the influence of surface charge spatial distribution on the Poisson equations. A first order approximation of the nonequilibelectrophoretic behavior of cells, and the interaction between lipid bilayers. rium electric potential has been obtained, using the electric current The influence of polar zone features on the electric potential profile in the as the small parameter. The influence of the ionic flux, the surface system under nonzero electric current has not been analyzed yet. dipole density, and the thickness of the polar zone on the NDEL near the membrane has been analyzed. The electric potential and 2. THEORY hence the ion concentration profiles in the NDEL change considerably with surface dipole density. A maximum in the electric poten-Let us consider a planar, charged membrane separating two aqueous tial, which is determined by ion flux and polar zone parameters, solutions of a binary electrolyte. The membrane consists of two hydrophilic is predicted in certain cases. The effects shown can help in the layers flanking a central hydrophobic core. Let the origin of the X-axis, normal to the membrane surface, be on the right boundary of the hydropho-interpretation of permselectivity measurements. α§ 1997 Academic bic layer (see Fig. 1). We are interested in the structure of the NDEL, and Press to fix ideas we shall focus on the polar zone and the ionic solution on the