Orientation of solvent

According to Fig. 2, as M comes in contact with S,3 4 the electron distribution at the metal surface (giving the surface potential XM) will be perturbed X ) The same is the case for the surface orientation of solvent molecules (Xs + SXS). In addition, a potential drop has to be taken into account at the free surface of the liquid layer toward the air (xs). On the whole, the variation of the electron work function (if no charge separation takes place as assumed at the pzc of a polarizable electrode) will measure the extent of perturbation at the surfaces of the two phases, i.e.,... 

Figure 4. Sketch to illustrate the situation believed to exist at a metal surface upon adsorption of water from the gas phase (or at the surface of an emersed electrode). In particular, the layer thickness is so small that the orientation of solvent molecules at the external surface is strongly affected by the orientation at the internal surface.

The temperature coefficient of the potential of zero charge has often been suggested to indicate the orientation of solvent molecules at the met-al/solution interface. However, this view is based only on the response of a simple two-state model for the interfacial solvent, and on neglecting any contribution from the electronic entropy.76,77 This is in fact not the case. The temperature coefficient of 0in many instances is negative and of the... 

Almost all that is known about the crystal face specificity of double-layer parameters has been obtained from studies with metal single-crystal faces in aqueous solutions. Studies in nonaqueous solvents would be welcome to obtain a better understanding of the influence of the crystallographic structure of metal surfaces on the orientation of solvent molecules at the interface in relation to their molecular properties. 

At a phase boundary (or interface) the molecular species experience anisotropic forces, which vary with the distance from the interface. This causes a net orientation of solvent and other molecular dipoles and a net excess of ions near the phase boundary, on both sides of the solution. The term electrified interface means that there occur differences in potential, charge densities, dipole moments, and electric currents. 

More recently, electrostatic theory has been revived due to the concept of molecular electrostatic potentials. The potential of the solute molecule or ion was used successfully to discuss preferred orientations of solvent molecules or solvation sites 50—54). Electrostatic potentials can be calculated without further difficulty provided the nuclear geometry (Rk) and the electron density function q(R) or the molecular wave function W rxc, [Pg.14]

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. 

F-centre in an alkali halide crystal but with OH dipoles replacing the cations. The trap may be stabilized further to a varying extent by the orientation of solvent molecules outside this first coordination sheath. 

Orientation of solvent molecules in the neighbourhood of the ion, and their coordination to the ion, is known as ion solvation. The manner of solvation is very important. If the surface of the ion pair as a whole is surrounded by... 

Nonpolar solute in a polar solvent. In the absence of a solute dipole moment there is no significant orientation of solvent molecules around the solute molecules, and again a general red shift, depending on the solvent refractive index n, is expected. Solute quadrupole/solvent dipole interactions also have to be taken into account in this case, as shown for nonpolar aromatic solutes e.g. anthracene) [469]. 

Electrostatic interactions between ions such as G, H+, G", etc., and dipolar solvent molecules such as THF, monoglyme, etc., would likely alter the values of AS° and ACp° of the system in the same direction. More explicitly, the orientation of solvent molecules by the ions in close proximity should decrease the values of these quantities. Indeed, this is the trend that is observed in this study (cf. Tables X-XV). 

Relative Galvanl potentials cannot be made absolute because there is no way to establish the conditions under which there is no potential difference across the boundary between dissimilar phases. For lack of better knowledge, i/° is therefore usually referred to the point of zero charge which, although not thermodynamically defined, can often be established with some confidence, see below and sec. 3.8. However, the point of zero charge is not necessarily identical to the point of zero potential because even at cr = 0 the interfacial potential Jump X, caused by preferential orientation of solvent (water) dipoles and polarization of the particle is non-zero. Anticipating sec. 3.9 it is realized that x may change with [Pg.334]

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. 

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