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  < 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  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  2.0 and remains stable regardless of bulk concentration increase, reflecting formation of a rather uniform and homogenous layer of perpendicularly oriented molecules of linoleic acid. The observed changes of fractal dimension are result of restructuring of the adsorbed layer and 2 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.

Hydrodynamic influence on the fractal morphology of the linoleic acid adsorbed layer at the mercury/electrolyte interface

“…Recently, we have established that the adsorbed layer of LA has a fractal structure [14]. That makes it a suitable testbed for our study because any hydrodynamical 6 influence on the adsorption process and the structure of the adsorbed layer should be reflected in a change of its fractal properties.…”

“…To determine the fractal dimension of the adsorbed layer, we rely on the recently introduced method of capacitive current measurement in a system with size-scaling of hanging mercury drop electrode [14]. This method relies on relation (1) where the "feature" used is the capacitive current measured at a selected constant parameters (potential, AC amplitude and frequency) and the "scale" is the surface area of the hanging mercury drop electrode represented through corresponding drop radius, r. This is facilitated by the fact that the extent of adsorption of organic substances on the electrode surface is reflected in a change of differential capacitance of the electrode/solution interface, consequently in a change of measured capacitive current:…”

“…The study of adsorption kinetics was 3 also extended to kinetics of diffusion limited adsorption on not smooth but fractal electrode surfaces [7]. In general, hitherto more work has been directed towards studying adsorption on fractal surfaces [8][9][10][11] than fractality of the adsorbed layer itself or its changes during the adsorption process [12][13][14]. On the other hand, considerable theoretical [15][16][17] and experimental [18][19][20] efforts have been made to understand and to explain the fractal structures that result from processes of aggregation in two and three dimensions.…”

“…It has been demonstrated that in the case of the linoleic acid (LA) adsorption at the mercury electrode, subtle changes in the adsorption process resulting in structural changes of the adsorbed layer are reflected in, and can be observed through, the change in the corresponding fractal dimension [14]. Moreover, recently it was shown that geometrical features, such as fractality also reflect itself in basic material properties such as permittivity, polarization and specific 4 capacity [25].…”

Impact of Fractal Geometry on Permittivity and Related Quantities

“…Although, most of the adsorption studies deal with the fractality of the surface [25][26][27][28] there is also evidence of a fractality of the adsorbed layer itself. 16,29,30 The approach used in study of fractal structures is based on the possibility to describe quantitatively complex objects that are statistically scale-invariant, physical realizations of mathematical fractals that appear the same on all length scales. This property manifests itself as a power-law-scaling ratio that characterizes one or more features of an object or a process carried out near or at the object:…”