Orientation of solvent dipoles

Trasatti ° assumed that the value of at ct = 0 is constant (-0.31 V) and independent of the nature of the solvent. Therefore, if the contact potential difference at cr = 0 is known, the values of Sx for a given metal can be calculated. It should be noted that the idea that the potential shift due to the interaction of metal electrons with solvent is independent of the nature of the solvent is open to criticism. For example, the local solvent field can interfere with electron distribution in the metal in the vicinity of the interface. The data obtained for a mercury electrode and different solvents show that the contact potential difference is mainly determined by the orientation of solvent dipoles at the interface. The positive values of gjUdip)o are due to orientation of the solvent dipoles with their negative ends directed toward the mercury surface. 

Fig. 1. Scheme of the capture of electrons in a polar matrix, (a) Orientation of solvent dipole molecules around an electron (b) potential well for et (du and dlt the ground and the excited levels of an electron in a trap). The arrows indicate the optical transitions of the trapped electron. 

Besides the ionic double layers that may be present at phase boundaries there Is also a second type of double layer, caused by polarization of the interfacial region, l.e. a double layer not attributable to free ions. An important contribution is the preferential orientation of solvent dipoles and multipoles close to the surface. These molecules may also have induced dipoles. In the surfaces of solids the centres of positive and negative charges are, as a rule, displaced as compared with the situation in the bulk. All these charge displacements together constitute the interfacial polarization. The associated potential difference across phase boundaries is called the interfactal potential (drop) or x-potential. 

The actual situation is in fact more complicated, because solvent response about a newly formed charge distribution is characterized by more than one timescale. In particular, solvent polarization has a substantial electronic component whose characteristic timescale is fast or comparable to that of electronic transitions in the solute, and a nuclear component, here associated with the orientation of solvent dipoles, that is slow relative to that timescale. In the present introductory discussion we disregard the fast electronic component of the solvent response, but it is taken into account later. 

In the case of water, Sop is 1.776, so that go is equal to 1.57 V. In assessing this result one should remember that the total surface potential has two opposing contributions, namely, g and is also large and contains the contribution from orientation of solvent dipoles in the monolayer at the interface. 

Temperature-Dependent Orientation of Solvent Dipoles in the Inner Region of the Double Layer and the Entropy of Activation... 

A possible effect of the state of orientation of solvent dipoles in the inner region of the double layer, as a function of potential and temperature, on the temperature dependence of Tafel slopes was suggested in Ref. 2. Reacting ions or molecules, especially in the case of the h.e.r., are intimately involved in solvational interactions with the inner-region solvent dipoles, so the temperature-dependent state of the latter region could lead to effects on the Tafel slope. It would be anticipated that these effects might also be associated with the entropy of activation (Section IV. 1). 

Several theories have been proposed in the past 20 years for changing solvent polarization in the double layer with electrode potential. One of the most useful has been that of Mott and Watts-Tobin and the development of it by Bockris et The model represents the inner region of the double layer in terms of two-states of orientation of solvent dipoles, parallel and antiparallel, ti, to the electrode field across the interphase, analogous to the... 

When the free electrostatic charge in phase a turns to zero, = 0 and = X . The surface potential of a liquid phase is dictated by a certain interfacial orientation of solvent dipoles and other molecules with inherent and induced dipole moments, and also of ions and surface-active solute molecules. For solid phases, it is associated with the electronic gas, which expands beyond the lattice (and also causes the formation of a dipolar layer) other reasons are also possible. 

In further developments, with Schmickler et al. (1984), two models of the solvent layer at the metal interface were considered. These self-consistent calculations of charge-induced electron relaxation predict in one form or another the well known hump in the compact-layer capacitance and introduce a dependence of the capacitance behavior on the properties of the metal electron system, that is, of course, not indicated in previous, purely molecular treatments of the metal/electrolyte interface. In general, metal-specific behavior (apart from that associated with specific orientation of solvent dipoles due to donor-acceptor interaction with the metal) is related to the free electron density of the metal. For further details, readers are referred to the review mentioned earlier (Feldman et al., 1986). 

Tunneling transition. The A—H bond elongates to the critical r value, and then the tuimeiing transfer of a proton occurs from level to level Uf. This transition can take place if the vibrational levels of the proton in the initial and final states are equalized. This is achieved due to the thermal fluctuation of orientation of solvent dipoles. In the framework of the model, this looks like a change in the coordinates of the molecules from the 9 to 9 values. The turmeling proton transfer occurs in this state. 

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