Contact charge density

C. Cohen-Addad et al., Nature of S—O interaction in short X-S—O contacts Charge density experimental studies and theoretical interpretation. J. Chem. Soc. Perkin Trans. 2, 191-196 (1984)... 

Assume is -25 mV for a certain silica surface in contact with O.OOlAf aqueous NaCl at 25°C. Calculate, assuming simple Gouy-Chapman theory (a) at 200 A from the surface, (b) the concentrations of Na and of Cr ions 10 A from the surface, and (c) the surface charge density in electronic charges per unit area. 

At positive surface charge density, the CP density exhibits a large maximum at a very short distance from the electrode. The position of this maximum is closer to the electrode than that of the first water layer, thus giving a,clear indication of contact adsorption of this anion. The Na" ... 

At negative surface charge density, the Na" density exhibits a large maximum at around z = 4 A. This position is very similar to the one at vanishing and positive surface charge densities. Obviously, with the models used in this study, Na does not contact-adsorb. The C density profile at a — —9.9/iCcm is similar to the one at cr = 0. 

To investigate the charge dependence of the capacitance we have to calculate the dependence of the profile contact values on the charge density. 

The Stern model (1924) may be regarded as a synthesis of the Helmholz model of a layer of ions in contact with the electrode (Fig. 20.2) and the Gouy-Chapman diffuse model (Fig. 20.10), and it follows that the net charge density on the solution side of the interphase is now given by... 

Clearly, the optimal injection condition is for electrons and holes to be in balance, but this does not necessarily guarantee that b- I. For example, when both contacts are injection limited and the injected charge densities arc small, the prob-... 

The fact that there is a potential difference between points close to the surfaces of two conductors in contact implies that the excess charge densities on their exposed snrfaces are different. This also implies that when two condnctors come in contact, there will be a redistribution of free electrons not only at the actnal inner contact snr-face (which gives rise to the Galvani potential) bnt also at their exposed surfaces. 

Work in this area has been conducted in many laboratories since the early 1980s. The electrodes to be used in such a double-layer capacitor should be ideally polarizable (i.e., all charges supplied should be expended), exclusively for the change of charge density in the double layer [not for any electrochemical (faradaic) reactions]. Ideal polarizability can be found in certain metal electrodes in contact with elelctrolyte solutions free of substances that could become involved in electrochemical reactions, and extends over a certain interval of electrode potentials. Beyond these limits ideal polarizability is lost, owing to the onset of reactions involving the solvent or other solution components. 

In the analysis of molecular capacitors, the diffuse layer and elastic capacitors, we have always assumed that the electrode charge density a could be controlled. Under such conditions it is generally possible for C to become negative while the system remains stable. For example, contraction of the gap z in an elastic capacitor proceeds smoothly with cr growing until the plates come in contact, while C becomes negative for z < 2/3. At the same time, as shown in Section II for an EC connected to a battery, the EC collapses after z 0.6 is reached. How can these seemingly contradictory results be reconciled And how can cr-control be related to reality Is C < 0 observable These questions are addressed in this section. 

When a membrane based on a derivative of azobis(benzo-15-crown-5) in contact with a solution of a primary cation is exposed to visible light, we assume that the iono-phore within the membrane phase is exclusively in the trans isomer and forms a 1 1 ionophore (I)-cation (M+) complex with a stability constant, trans. According to Eq. (10), the corresponding charge density at the membrane side of the interface, o is > can be expressed as... 

Figure 2.1 (a) A schematic representation of the apparatus employed in an electrocapillarity experiment, (b) A schematic representation of the mercury /electrolyte interface in an electro-capillarity experiment. The height of the mercury column, of mass m and density p. is h, the radius of the capillary is r, and the contact angle between the mercury and the capillary wall is 0. (c) A simplified schematic representation of the potential distribution across the metal/ electrolyte interface and across the platinum/electrolyte interface of an NHE reference electrode, (d) A plot of the surface tension of a mercury drop electrode in contact with I M HCI as a function of potential. The surface charge density, pM, on the mercury at any potential can be obtained as the slope of the curve at that potential. After Modern Electrochemistry, J O M. 

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