Adsorption of the neutral macromonomeric surfactant Tween-80 at the mercury/electrolyte solution interface as a function of electrode potential and time

The adsorption of the neutral macromonomeric surfactant Tween-80 from elect rolyte solutions on a polarized mercury electrode was studied by means of d ifferential capacitance measurements of the electrode double layer, its var iation with potential and time can provide qualitative information on the s tate of the Tween-80 adsorbate. The time evolution of the phenomenon is fol lowed by sampling the capacitive current at different time periods after st epping the electrode potential at given values, and the steady-state differ ential capacitance (C) vs electrode potential (E) curve is obtained from lo ng-duration data. Features associated with surface aggregation processes, s uch as capacitance plateaus, deformed peaks, and increase in capacitance wi th increasing time or surfactant concentration, start to appear even at con centrations below the bulk critical micelle concentration (cmc). The type o f surface aggregates (surface micelles) formed depends on surface coverage and orientation which vary with time and surfactant bulk activity up to the cmc value. This continuous change of the surface state with bulk concentra tion gives rise to a rather continuous change of the corresponding C vs E c haracteristics. Two distinct capacitance plateaus are observed for the high er concentrations studied and are attributed to two-dimensional surface mic elles of different monomer-unit orientations, but (unlike the charged micel les studied previously) these surface micelles do not collapse at extreme p otentials into condensed polylayers. The very slow attainment of a steady s tate even at high concentrations is indicative of a surfactant surface conc entration and orientation (and, hence, state too) which are dependent on th e slow three-dimensional interaction of the first adsorption layer with out er adsorbed layers.