Potential dependence of the capacitance of ionizable bilayers

The effect of the applied potential across a biomimetic model membrane, ΔΦ, on the extent of dissociation of surface bound ionizable groups has been examined theoretically using a formalism that shares common features with that recently introduced by White and coworkers (Langmuir 9 (1993) 1) to account for the occurrence of peaks in the cyclic voltammetry of self-assembled monolayers rigidly attached to an electrode surface, bearing the same type of ionizable groups facing the electrolyte. Numerical solutions of the governing coupled, highly non-linear system of equations yielded for reasonable membrane parameters, and under conditions of physiological relevance, linear changes in the extent of ionization as a function of ΔΦ, about ΔΦ = 0, of ca. 0.07 units/V. Also considered in this work was a more general situation in which the contributions to the total interfacial capacity, CT, due to the diffuse double layers, CjS, and to the fixed charges Cj(fj) on each of the sides of the membrane-like assembly denoted by 'j', were comparable in magnitude to the potential independent capacity of the intervening hydrophobic layer, i.e. low-electrolyte concentration. In such case, plots of CT versus ΔΦ displayed a global minimum at ΔΦ = 0 and two local maxima at values of ΔΦ slightly negative and positive to the maxima observed in the corresponding Cj(fj) versus ΔΦ plots. Implications of the results obtained to excitable bilayer membranes are briefly discussed.