375 J. Electroanal. Chem, 324 (1992) 375-385 Elsevier Sequoia S.A., Lausanne JEC 01878 Three-dimensional phase-sensitive ac voltammetry for the study of adsorption processes An application to the adsorption of sodium dodecyl sulphate at the mercury-electrolytic solution interface N. Papadopoulos l , S. Sotiropoulos and P. Nikitas Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54006 (Greece) (Received 4 June 1991; in revised form 14 October 1991) Abstract Three-dimensional phase-sensitive ac voltammetry is proposed as an alternative approach to the study of adsorption from aqueous solutions on mercury with respect to both applied potential and time. The entire curve of differential capacitance C versus applied potential E for various time periods is constructed from isochronous C-,? data sampled during many ac phase-sensitive chronoamperometric experiments. The method is used to study the adsorption of sodium dodecyl sulphate from a premicellar solution on a mercury electrode. It is found that the development of the adsorbed film is characterized by strong kinetic effects. These effects are attributed to (a) short time diffusion-controlled processes, (b) phase changes leading to the formation of a compact layer at intermediate polarizations and (cl long time processes due to micellar formation on both sides of the compact layer. The above processes can be distinguished by means of capacitance transients, which are also presented and discussed. INTRODUCTION The formation of compact layers and, in general, any process leading to a phase change on the polarized mercury electrode has usually been studied by measuring the differential capacitance C on a hanging mercury drop electrode (HMDE) or a dropping mercury electrode (DME) with relatively high drop times. In studies of the equilibrium state of the adsorption layer the differential capacitance is mea- * To whom correspondence should be addressed. 0022-0728/92/$05.00 0 1992 - Elsevier Sequoia S.A. All rights reserved