Solvent dipole, orientation

The experimental data have been interpreted in terms of a three-state model309 for the solvent at the interface. The three states correspond to solvent dipoles oriented up, down, and flat. The model has been found to reproduce the experiments at negative charges and around but not at... 

According to Bockris and Habib, the potential difference at the metal/solution interface at pzc is a result of the contribution of two components the surface potential (electron overlap) of the metal go and solvent dipoles oriented at the electrode surface, go- The value of go cannot be experimentally measured because the absolute value of the electrode potential is not known. However, the value of go can be estimated from the relation... 

To explain the nature of the hump, the ion-solvent molecule competition modeP can be applied. According to this model, the solvent dipoles oriented with their positive ends toward the electrode are displaced by the anions (for example PFs) that enter the inner layer at the point of maximum double-layer capacity (Cmax)- The value of Cmax is similar in different solvents (Table 5) if the activity of the PFg ions is the same and the adsorption of these anions can be neglected. Thus it is the PF anions rather than solvent molecules that are responsible for the formation of the... 

Because of the short timescale for the optical transition, solvent dipole orientations in the initially formed excited state are the same as in the ground state and there is no entropic change. For a self-exchange reaction, the contribution to AG is A0/4 as noted above. 

In addition to Trouton s rule, some other parameters for measuring the structuredness of solvents have been recommended, for example a solvent dipole orientation correlation parameter [175, 200], the solvent s heat capacity density [175, 200], and a so-called Ap parameter derived from the solvent s enthalpy of vapourization minus EPD/ EPA and van der Waals interactions [201], According to these parameters, solvents can be classified as highly structured e.g. water, formamide), weakly structured e.g. DMSO, DMF), and practically non-structured e.g. -hexane and other hydrocarbons) [200, 201]. 

Surface potentials arise from electronic polarisation and (in a polar solvent) dipole orientation of the solvent molecules at the free surface of the solution. 

Conway et al examined this question in terms of the BDM (Bockris-Devanathan-Muller) two-state model of solvent dipole orientation. The dipole polarization, expressed as a surface potential contribution, g ip, is given by... 

These results mean that the solvent environment of a discharging ion on the inner side of the outer Helmholtz plane can be quite substantially modified toward a state of orientational saturation depending on qM and T. The entropy of activation of a process involving change of charge number is closely associated with changing state of solvation in formation of the transition state. Hence it can be seen, at least qualitatively, how an influence of potential on solvent dipole orientation in the inner layer could be transmitted as a potential or field effect on the entropy of activation. This... 

Figure 11. (a),(b), and (c) The surface dipole moment, as in Figure 10, as a function of temperature for three values of the interaction parameter Uc/kT. Note that x can increase with T when U 0 (Fig. 11c). (From Conway and Dhar, Croat. Chem. Acta 45 (1973) 109 based on BDM model (Ref, 60) of solvent dipole orientation in the inner region.)...

Ten years ago, femtosecond IR spectroscopy of an excess electron in pure water showed the existence of an ultrashort-hved prehydrated state (61). This IR nonequihbrium electronic configuration is built up in less than 120 fs in H2O and represents a direct precursor of the hydrated electron ground state (equation 6). In the infrared (0.99 eV), the monoexponential relaxation of the signal toward an s-hke ground state of the hydrated electron (240 20 fs) has been analyzed in the framework of a two-state model (61, 65). With a similar model, an indirect estimate of the infrared electron relaxation in the red spectral region gives a deactivation rate of 2 X 10 s (62, 66). The very fast appearance of the infrared electron (efn) is comparable to any nuclear motion, solvent dipole orientation, or thermal motion of water molecules. The relaxation of... 

Figure 1. Model for proton transfer at electrode with elements of surrounding solvent dipoles > orientation (1) of proton source.

Scanning ion conductance microscopy was applied to investigate the interface of two immiscible electrolyte solutions (ITIES). Two opposing views describe interfacial structure one opinion is that solvent dipoles orient to form an ion-free compact layer contained in a molecularly sharp interface. The sharp interface is proposed to separate two back-to-back double layers. The opposite view suggests the interface is composed of a mixed solvent layer that ions of both phases can penetrate. To adequately examine the interface, a technique with high-resolution and small probe are desired. An SECM study performed by Bard and coworkers of a water/nitrobenzene... 

Treatments of solvent dipole orientation as a function of electrode surface charge, q, seek to evaluate a reciprocal capacitance contribution arising from the dependence of the surface-dipole layer potential difference, X on q. At electrode interfaces, AX is a function of potential and hence q then the dependence of AX on q can be written as a differential capacitance, 1/C > due to dipole orientation. It combines in a series relationship with other capacitance contributions of the overall double-layer due to free charge accumulation. Therefore, only if is sufficiently small will its effect on the overall measured capacitance be important. Hence its correct evaluation is a critical matter in modern theories of the double-layer which properly take into account the solvent layer. For example, the two-state Watts-Tobin model... 

It should be mentioned that a 3-state model of solvent dipole orientation at electrodes, with configurations t or 4-4, was considered in detail by Fawcett (1978) and gives a better account of inner layer capacitance behavior and organic molecule absorption than the Watts-Tobin 2-state model. This model was shown to be the favored one, energetically, by Parsons, with predominantly "flat (-> or <-) orientations of H2O dipoles at the potential of zero charge of metal surfaces. 

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