Donnan Potential and Surface Potential of a Charged Membrane and Effect of Ion Binding on the Potential Profile
A model is presented for the electrical potential distribution across a charged biological membrane which is in equilibrium with an electrolyte solution. We assume that a membrane has charged surface layers of thickness d on both surfaces of the membrane, where the fixed charges are distributed at a uniform density N within the layers, and that these charged layers are permeable to electrolyte ions. It is demonstrated that this model smoothly unites two different concepts, the Donnan potential and the surface potential (or the Gouy-Chapman double layer potential). Namely, the present model leads to the Donnan potential when d >> 1/κ′ (κ′ is the Debye-Hückel parameter of the surface charge layer) and to the surface potential as d → 0, keeping the product Nd constant. It is also shown that the potential distribution depends significantly on the thickness d of the surface charge layer when d ≤ 1/κ′, as well as on the types of ion binding to the charge sites of the surface charge layer. The conventional method to estimate surface potential and surface charge density from cell electrophoretic mobility by assuming the zero thickness of the surface charge layer may result in largely underestimated values of those for the membrane having the non-zero thickness of the surface charge layer.
KeywordsSurface Potential Potential Distribution Surface Charge Density Charge Layer Core Layer
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