Self-excitation in a porous membrane doped with sorbitan monooleate (Span-80) induced by an Na+/K+ concentration gradient.

The electrical potential across a fine-pore membrane doped with sorbitan monooleate (Span-80) imposed between aqueous solutions of NaCl and KCl was studied. It was found that this system showed rhythmic and sustained oscillations of electrical potential between the two aqueous solutions. These oscillations were attributed to the change of permeability of Na+ and K+ across the membrane, which originated from the phase transition of Span-80 molecules within the fine pores. Impedance measurement across the membrane also suggested a change in permeability. It was found that this membrane exhibited the property of differential negative resistance. In relation to this, it was shown that Na+ and K+ have different effects on the aggregation of Span-80 molecules. The mechanism of oscillation is discussed in relation to the ability of Span-80 molecules to behave as a dynamic channel through the membrane. This oscillatory phenomenon is interesting because in biological nervous membranes a difference between the concentrations of Na+ and K+ across the membranes is essential for excitability.