Monolayer double-layer capacitance

Forming a monolayer involves displacing specifically adsorbed ions and solvent molecules from the interface, which changes the double-layer capacitance from that... 

In a SAM-modified electrode, the SAM functions as a dielectric, reducing the permeability of the monolayer by electrolyte and decreasing the charging current [23, 30, 48]. However, a thinner film [30, 49], one with a higher dielectric constant (e.g., containing unsaturated carbons [50]), or one which is more permeable to electrolyte (i.e., a more defective SAM), exhibits increased Cji because the resulting electrical double layer is better organized [23, 30, 48[. The double-layer capacitance is, thus, a convenient check of monolayer quality under the conditions of electrochemical electron-transfer measurements. 

J. F. Hicks A. C. Templeton S. Chen K M. Shehan R. Jasti R. W. Murray J. Debord T. G. Schaaff R. L. Whetten, The Monolayer Thickness Dependence of Quantized Double-Layer Capacitances of Monolayer-Protected Gold Clusters. Anal. Chem. 1999, 73, 3703-3711. 

Murray and co-workers have also demonstrated that the variation in Au-core sizes leads to a transition from metal-like double layer capacitive charging for larger sized particles to redox-like charging for smaller particles ranging between 1.1 nm and 1.9 nm diameter [105] (Figure 20.8). Gold particles stabilized with short chain alkanethiolate monolayers have been used in this study. The capacitance of the clusters is calculated using the concentric sphere capacitance model ... 

Figure 14.3.18 Equivalent circuit for an electroactive monolayer. Rfi = solution resistance, = double-layer capacitance, = charge-transfer resistance, and Cads capacitance of the adsorbed layer.

Fig. 2.18 An equivalent circuit representing an electrode/solution interface. The electrode surface is covered by a monolayer of a redox-active species. e ac potential across the faradaic unit of equivalent circuit, Ca double-layer capacitance, Rs -uncompensated solution resistance, Zf impedance representing solely the electron transfer reaction process of the monolayer, )> ac current due to the faradaic process, Z, total impedance of the whole system, ks. heterogeneous electron transfer rate constant of the monolayer of electroactive species, R charge transfer resistance, Q capacitance associated with the redox reaction of the adsorbed species.

Assuming that the porphyrin monolayer does not affect the dielectric properties of the liquid/liquid junction, the differential double layer capacitance can be simply obtained from... 

For several decades cyclic voltammetry (CV) has been the technique of choice in investigating charge-transfer kinetic processes. Simultaneous determination of both EIS and CV parameters is very important, as exclusively CV-based analysis often misses important electrochemical aspects of the system, such as the double-layer capacitance, details of the double-layer structure, monolayer adsorption through functional dependence of on electro-... 

In this section, selected studies are presented in which self-assembled monolayers have been used to address topics such as transition-state structures and sequential electron transfer. These studies were selected because they address fundamental mechanistic processes. SAMs have also been used to investigate such basic electrochemical phenomena as the potential profile near an electrode [134, 135], interfacial capacitance [136], the influence of redox [134] or polarizable [137] moieties on double-layer structure and the behavior of ultramicroelectrodes approaching molecular size [138]. These important topics are beyond the scope of this chapter, and the interested reader is directed to the literature for more information. 

Te and Cu monolayers on gold, as well as Ag and Bi monolayers on platinum were obtained by cathodic underpotential deposition and investigated in situ by the potentiodynamic electrochemical impedance spectroseopy (PDEIS). PDEIS gives the graphical representation of the real and imaginary interfacial impedance dependencies on ac frequency and electrode potential in real-time in the potential scan. The built-in analyzer of the virtual spectrometer decomposes the total electrochemical response into the responses of the constituents of the equivalent electric circuits (EEC). Dependencies of EEC parameters on potential, especially the variation of capacitance and pseudocapacitance of the double layer, appeared to be very sensitive indicators of the interfacial dynamics. 

Interestingly, double-layer electrostatic forces induced inclusion of inorganic cations and anions into CyD monolayers leading to an increase in the interfacial capacitance. Anion binding constants were obtained from the dependence of capacitance current on anion concentration. The charge density in the plane of nitrate inclusion is a function of total surface concentration of CyD and the nitrate occupancy. The electrostatic contribution to the potential was determined and used for the calculation of corrected binding constant for nitrates (0.11 M for f -... 

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