Connected, charged surfaces

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Much of soft matter deals with fluid surfaces - direct measures of surface potential are sometimes possible and always preferred.

Mercury electrodes

Any film that can be assembled on a metal surface can have its potential (or capacitance) measured directly. The dropping mercury electrode allows for fresh surface to be generated to avoid contamination by oxidized or reduce species.

The dropping mercury electrode (for polarography).
An example showing the simultaneous determination of Zn , Cd, Pb and Cu using standard addition.

Monomolecular films

With high enough molecular weight hydrophobe, a surfactant forms a monomolecular film.
The conductivity and capacitance can be measured directly.(J.W. McBain; C.R. Peaker, Proc. Roy. Soc. A, 125, 394-401, 1929; Martin, A.S.; Sambles, J.R. Molecular Rectifier, PRL, 70, 218, 1993.)
McBain 1929

Interesting information is gotten by (a) varying the surface concentration, (b) varying the ionic concentration and composition in the liquid, (c) adding other materials to the surface film and (d) adding other materials to the bulk liquid.

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