Application of the vibrating plate (VP) technique to measuring electric surface potential, ΔV, of solutions; the flow cell for simultaneous measurement of the ΔV and the surface pressure, Π

This paper describes the measuring procedure which enables overcoming the base difficulty in application of the vibrating plate (VP) technique to measuring electric surface potential (ΔV) of free solution surfaces, namely loosing the zero level set for the reference sample (water) while replacing it with the solution measured. (The ΔV measurement is performed by comparing two measurements in series, i.e., for the pure interface and that containing a surface film.) Main improvement in the measuring procedure is achieved by (i) applying a flow for exchanging the reference sample with the measured solution and (ii) elimination of an error driven by dependency of the measuring signal on the air gap width. The air gap width is fixed by contact of the bottom edge of a platinum Wilhelmy plate with the solution surface which the plate is simultaneously used for measurement of the surface pressure (Π). The results received with the new methodology for n-decanoic acid, n-decyl-trimethylammonium bromide (C10TABr) and sodium n-dodecyl sulphate (SLS) are discussed.

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► Application of the vibrating plate (VP) technique to free solution surfaces met with difficulties, so far.
► A methodology for using the VP technique to free solution surfaces is proposed.
► The ΔV vs. concentration isotherms of a cationic and of an anionic surfactant are compared and discussed together with the corresponding surface pressure vs. concentration isotherms.
► Sources of errors in the ΔV measurement and their dependency on the air gap width are discussed.