Fractal and Voltammetric Study of Linoleic Acid Adsorption at the Mercury/Electrolyte Solution Interface

Abstract

A new method, size scaling of the hanging mercury drop electrode, is introduced and used to determine the fractal properties of the adsorbed layer and its dependence on fractional electrode coverage. Simultaneously, the adsorption process of linoleic acid at a mercury electrode surface has been studied by means of phase-sensitive ac voltammetry in 0.5 M NaCl electrolyte solution, pH 8.3, simulating seawater conditions. Three concentration domains that exhibit different fractal behavior characterize the adsorption process. In the first domain, corresponding to the fractional electrode coverage theta < 0.5, the fractal dimension of the adsorbed layer D 2. Further increase in surface concentration (electrode coverage) is followed by a swift change in fractal dimension, reaching the peak value D approximate to 2.4. This indicates that, at fractional electrode coverage of 0.7-0.8, depending on adsorption conditions, a second-order phase change in the adsorbed layer of linoleic acid takes place. In the third domain, the fractal dimension rapidly decreases to D approximate to 2.0 and remains stable regardless of bulk concentration increase, reflecting formation of a rather uniform and homogeneous layer of perpendicularly oriented molecules of linoleic acid. The observed changes of fractal dimension are a result of restructuring of the adsorbed layer and a change in the orientation of adsorbed molecules due to molecular interactions. The influence of electrode polarity on the adsorption process of linoleic acid and fractal properties of the adsorbed layer is discussed. Comparison with the apparent isotherm, obtained in a classical voltammetric way, shows that the analysis of fractal properties of the adsorbed layer provides a useful tool for studies of the adsorption process and a sensitive method for detection of phase changes.