Boundary-activated charge-separation

As shown in Fig. 2. the charge separation is the result of the occurrence of an oriented layer for ionophore/ analyte ion complexes at the organic/aqueous interface in the organic side of the interface, leaving their hydrophilic counterions in the aqueous side at the interface. The extent of this charge separation is dependent on the activity (concentration) of respective analyte ions and is measurable as changes in the membrane boundary potentials. 

But the activity of C+ in the membrane (dAm) is very nearly constant for the following reason The high concentration of LC+ in the membrane is in equilibrium with free L and a small concentration of free C+ in the membrane. The hydrophobic anion R is poorly soluble in water and therefore cannot leave the membrane. Very little C+ can diffuse out of the membrane because each C+ that enters the aqueous phase leaves behind one R in the membrane. (This separation of charge is the source of the potential difference at the phase boundary.) As soon as a tiny fraction of C diffuses from the membrane into solution, further diffusion is prevented by excess positive charge in the solution near the membrane. 

Depending on its nature, the layer promotes the separation of molecules by (1) physical sorption of solutes from solution onto the surface-active groups of the layer particles (adsorption) (Scott and Kucera, 1979), (2) dissolving of solutes into a stationary liquid held on the layer (partition), (3) attraction of ions to sites of opposite charge on the layer (ion exchange), or (4) retention or rejection of solutes on the basis of molecular size and/or shape (size-exclusion or gel-permeation TLC). The boundary between adsorption and partition is quite obscure because both can involve the same types of physical forces, that is, permanent and... 

Electrodes are devices with which one can detect the movement and separation of charges occurring at phase boundaries, as well as induce and vary such processes by means of a forced current flow. An electrode can be a piece of some sufficiently inert conductor (such as Pt, Ag, Cu, etc.). If such an electrode is immersed in an electrolyte solution which contains ions of the electrode material, a potential difference will develop between electrode and solution which depends on the activity of this particular metal ion in solution. This is referred to as an electrode of the first kind. 

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