The Partial Charge Transfer

The extent of pet from a chemisorbed molecule to an electrode is measured by the pet coefficient X. Unfortunately, the estimate of X is not straightforward, since it relies on more or less accurate modelistic assumptions. Thus, for instance, screening of the charge of the adsorbed ion by the inhomogeneous electron gas and by the solvent molecules produces an effect analogous to that ofpet.47 9 

The most common procedure for estimating X consists in measuring the thermodynamically significant electrosorption valency l and in evaluating its extrathermodynamic contributions k vkw and g of Eqs. (26) and (27) in order to extract the X value. A more direct extrathermodynamic procedure that relies exclusively on the Gouy-Chapman theory can be applied to the important class of self-assembled thiol monolayers anchored to a metal surface. 

Extrathermodynamic Estimate of the Partial Charge Transfer Coefficient from the Electrosorpion Valency 

The -vk contribution to / in Eq. (26) from the desorbing water molecules can be roughly estimated by assuming that the potential j varies linearly with the distance x in the compact layer enclosed between the electrode surface plane x = 0 and the outer Helmholtz plane x = d. In the presence of a strong excess of a nonspecifically adsorbed supporting electrolyte or upon correction for the potential difference across the diffuse layer, the electric potential in the bulk 

If the water dipoles are preferentially directed towards the solution, L is positive and the same is true for the —vtcw contribution to /. In fact, in this case the water dipoles make a negative contribution to the potential difference ( t u - fa) across the compact layer consequently, upon their removal from the contact with the electrode by the adsorbing species S, a flow of electrons to the metal surface along the external circuit is required to keep the applied potential E constant. This implies a negative contribution to ok and a consequent positive contribution to in view of its definition. Naturally, if the water dipoles are preferentially directed towards the metal surface, they make a negative contribution to l. By analogous arguments, the contribution to I from the dipoles of the adsorbed species S is written as  

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E mass spectrum. Processes of the partial charge-transfer type ... 

Stricktly speaking the partial charge transfer parameter Xj which appears in the modified electrochemical Langmuir isotherm (6.36) or (6.41) is not a constant but may vary with 0j or IT. This because, in view of Eq. (6.31) Xj is given by ... 

Effect of partial electron transfer parameter Figure 6.23 depicts the effect of the value of the partial charge transfer parameter A,d for fixed XA(= 0.15) on the rate enhancement ratio p(=r/r0) for the four main types of promotional behaviour, i.e., electrophobic, electrophilic, volcano and inverted volcano. The main feature of the Figure is that it confirms in general the global mle... 

As noted before, the partial charge transfer is not well defined. Nevertheless, let us suppose that we can treat Sz+X like a normal species. Its electrochemical potential is then ... 

Schultze and Koppitz [3] assume that g is typically of the order of 0.1 [see Eq. (18.12)]. Using this value, estimate the partial charge-transfer coefficient of the ions in Table 18.1. 

The Partial Charge Transfer Coefficient of Lorenz and Salie... 

The consequences of the partial charge transfer character in the SO2 adduct may affect in an interesting fashion the binding of further molecules in the cluster. We are pursuing the explorations of these problems, as well as that of the CO2 polarization and intend to utilize the experience acquired in this study for investigating clusters around more abundant atmospheric ions, particularly NOb . 

The basis of the model is shown in Figure 32. Each adsorbate is modeled as a dipole characterized by the distance, X, between the positive and negative charge q and by the partial charge-transfer parameter Xj dehned from the equilibrium ... 

In a polar medium, the partial charge transfer depends on two additional contributions due to the ion-solvent and metal-solvent interactions. Following the theory of electrochemical adsorption by Schmickler [232], the adsorbate energy level in solution, e is given by... 

The partial charge on the ion in vacuum, qi decreases substantially at small distances from the surface (Fig. 37). Ion hydration stabilizes the ionic charge and consequently the partial charge transfer from the ion to the surface is reduced. Substantial charge transfer in solution occurs only at distances smaller than 4.5 A from the surface. At larger distances, qh is almost equal to -1. At 4.5 A, however, the... 

Due to ionic hydration, partial charge transfer from the iodide ion to the metal is reduced in solution relative to that in vacuum. The partial charge transfer becomes important only at small ion-surface distances. 

Moreover, the very definition of this important characteristics is ambiguous in this case because of the crystallographic inhomogeneity of the solid surfaces (except for perfect single-crystal faces), a time-variation of their properties and the adsorption of the solution components with the partial charge transfer, see [13,... 

The experimental verification of the partial charge transfer and determination of the partial charge of ions adsorbed on the electrode surface is connected with a general controversy about the pure thermodynamic interpretation of the partial charge and attempts to interpret measurements by models allowing an approximate view on the charge distribution in the double layer. 

The response behavior of the sensor could effectively be influenced by the choice of the posttreatment process. The presence of noncontinuous rising responses for some sensor types shows that there is not one single response mechanism. This is consistent with the results obtained with more conventional, electrodeposited chemoresistors [107b,109]. Possible interactions are interparticle interaction, swelling [107b,107c], and the modification of the particle resistance because of the partial charge transfer [110]. 

The formation of charge transfer complex is a common chemical notion when this type of interaction involves individual molecules. However, it occurs also between solid phase and individual molecules. For example the partial charge transfer causes the chemisorption in which the adsorbing molecule strongly interacts with the surface of the solid phase. 

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