Ion Adsorption on Modified Electrodes as Determined by Direct Force Measurements under Potentiostatic Control

Abstract

Ion adsorption is a charging process that is of central importance for many hydrophobic surfaces, such as gas bubbles, oil drops, and cell membranes. This process has been studied by various techniques and different adsorption mechanisms have been proposed so far. However, different analytical methods seem to indicate the adsorptions of different types of ions, particularly hydroxyl or hydronium ions, at hydrophobic surfaces. In this work, we studied ion adsorption on modified electrodes by direct force measurements with the colloidal probe technique. The electrodes were modified with a self-assembled monolayer (SAM), and their potential was controlled externally by means of a potentiostat. Ion adsorption onto OH- or CH3-terminated SAMs was determined as a function of pH and background electrolyte concentration. Charge at the interface was found to result primarily from the adsorption of hydroxyl and hydronium ions, whereas the influence of the background electrolyte (KCl) can be neglected. For the hydron...