Surface Potential AV of Lipid Monolayers

Any liquid surface, especially aqueous solutions, will exhibit asymmetric dipole or ions distribution at surface as compared to the bulk phase. If SDS is present in the bulk solution, then we will expect that the surface will be covered with SD ions. This would impart a negative surface charge (as is also found from experiments). 

Due to the surface adsorption of SDS to water not only surface tension changes (reduces) but SDS also imparts negative surface potential (due to SD at the surface). Similarly, solutions of cationic detergent, such as CTAB, give rise to a positively charged potential of water surface. 

On the other hand, nonionic surfactants do not change the surface charge of water solutions. 

Of course, the surface molecules of methane (in liquid state) obviously will exhibit symmetry in comparison to water molecule. This characteristic can also be associated to the force field resulting from induced dipoles of the adsorbed molecules or spread lipid films (Adamson and Cast, 1997 Birdi, 1989). The surface potential arises from the fact that the lipid molecule orients with polar part toward the aqueous phase. This gives rise to a change in dipole at the surface. There would thus be a change in snrface potential when a monolayer is present, as compared to the clean surface. The surface potential, AV, is thus 

The magnitude of AV is measured most conveniently by placing an air electrode (a radiation emitter e.g., Po ° [alpha-emitter]) near the surface (ca. mm in air) connected to a very high impedance electrometer. This is required, since the resistance in air is very high, but it is appreciably reduced by the radiation electrode. 

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