Adsorption of the Neutral Macromonomeric Surfactant Tween-80 at the Mercury/Electrolyte Solution Interface as a Function of Electrode Potential and Time

Abstract

The adsorption of the neutral macromonomeric surfactant Tween-80 from electrolyte solutions on a polarized mercury electrode was studied by means of differential capacitance measurements of the electrode double layer. Its variation with potential and time can provide qualitative information on the state of the Tween-80 adsorbate. The time evolution of the phenomenon is followed by sampling the capacitive current at different time periods after stepping the electrode potential at given values, and the steady-state differential capacitance (C) vs electrode potential (E) curve is obtained from long-duration data. Features associated with surface aggregation processes, such as capacitance plateaus, deformed peaks, and increase in capacitance with increasing time or surfactant concentration, start to appear even at concentrations below the bulk critical micelle concentration (cmc). The type of 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 concentration gives rise to a rather continuous change of the corresponding C vs E characteristics. Two distinct capacitance plateaus are observed for the higher concentrations studied and are attributed to two-dimensional surface micelles of different monomer-unit orientations, but (unlike the charged micelles studied previously) these surface micelles do not collapse at extreme potentials into condensed polylayers. The very slow attainment of a steady state even at high concentrations is indicative of a surfactant surface concentration and orientation (and, hence, state too) which are dependent on the slow three-dimensional interaction of the first adsorption layer with outer adsorbed layers.