Study of Oleate Adsorption at the Mercury/Electrolyte Solution Interface as a Function of Electrode Potential and Time

Abstract

The adsorption of sodium oleate from electrolyte solutions on a polarized mercury electrode can be studied by monitoring the differential capacitance of the electrode double layer. Its variation with potential and time can provide qualitative information on the state of the oleate adsorbate. A new experimental methodology, three-dimensional phase-sensitive ac voltammetry, is used to provide safer information on the equilibrium transition characteristics and also to follow the time evolution of the phenomenon. The steady-state differential capacitance vs electrode potential curve is interpreted by means of recently developed thermodynamic criteria for phase transitions of surfactants at charged interfaces. At concentrations below the bulk critical micelle concentration monolayer adsorption takes place at moderate and negative polarizations, whereas there is some evidence of multilayer adsorption at more positive potentials. At higher concentrations, multilayer adsorption occurs throughout the potential range studied. In both cases, the formation of a close-packed structure is likely to be restricted to the first layer and the phase transitions of the latter to be realized via aggregates formed by its interaction with the outer layers. In addition, a close-packed polylayer is also observed at extreme positive potentials for high bulk oleate concentrations. Analysis of the capacitance transients based on simple diffusion, adsorption, or Avrami formalisms cannot describe successfully the time dependence of oleate adsorption on mercury, indicating that the latter is under mixed control or/and a complex process.

On the Adsorption Kinetics of Octanoic Acid at the Mercury/Electrolyte Interface

“…The two-dimensional analogues of surface hemimicelles are propeller-like surface accommodations discussed in ref for the adsorption of n -alkanoic acids at the air/water interface. For the system 5 × 10 -4 M sodium oleate + 0.1 M Na 2 SO 4 , Sotiropoulos et al also found an increase of the capacity with time after an initially falling capacity transient in the potential region −0.1 to −0.16 V when the potential was stepped from −1.8 V. The increase started from about 3 μF cm -2 and attained a plateau value of about 15 μF cm -2 . Avrami plots, corresponding to the phase transitions, gave a mean slope of 2.66.…”

“…In recent times, the formation of surface hemimicelles from adsorption layers at the interfaces metal/electrolyte − and air/water − has been intensively discussed. This problem is of fundamental interest, because mostly two-dimensional phase transitions in adsorption layers were found but not transitions from monolayers to three-dimensional microstructures.…”

“…The long-term capacitance transients can reveal relatively small changes of the double-layer structure, and only recently has an increase of this capacity with time been observed, the reason of which is not yet quite clear. Therefore, in the present paper octanoic acid was included in these investigations.…”

Adsorption and Interaction of Bovine Serum Albumin and Pluronic P103 Triblock Copolymer on a Gold Electrode: Double-Layer Capacitance Measurements

“…Surface plasmon resonance (SPR) and ellipsometry are techniques that have been used to characterize protein adsorption on metallic surfaces and its interactions with the medium. , However, electrochemical techniques are adequate and highly sensitive for successfully evaluating not only the protein adsorption–desorption process on electrodes but also its possible denaturation at the surface. Techniques such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), along with electrochemical quartz crystal micro- and nanobalance and double-layer capacitance measurements, have been applied to the study of adsorption–desorption processes of proteins and emulsions on metallic electrodes. − According to the results, where bovine serum albumin (BSA) and human serum albumin have been the most studied proteins, the electrochemical techniques adequately demonstrate the adsorption–desorption process as a function of protein concentration, substrate charge, pH, and polarization potentials, allowing studies in the presence and absence of charge transfer. The versatility of the electrochemical techniques and the information acquired in the study of protein adsorption has increased their application, and thus, they are currently preferred over optical techniques for many researchers …”

“…It should be noted that electrochemical studies concerning these interactions are scarcer and the most common imply the adsorption of the triblock copolymers or the proteins in an isolated manner. − ,, The aim of this work is to study the triblock copolymer–protein–electrode interactions from the fundamental point of view of double-layer capacitance as calculated from EIS measurements and to obtain information about the behavior and apparent size of the interface formed by the adsorbed species on a gold disc electrode at different temperatures. The information obtained from this study is important in sensor and bio-surface design based on the detection of protein liberation or a reduced protein adsorption process in micelles.…”