Dipole moment of adsorbate

Miller A R 1946 The variation of the dipole moment of adsorbed particles with the fraction of the surface covered Proc. Camb.Phil. See. 42 292-303... 

Kds are the constants of rates of chemical reactions of oxygen adsorption and desorbtion from ZnO film and Aq are electron work function from ZnO before oxygen gets adsorbed and its variation caused by dipole moment of adsorbed complexes being formed U is the adsorption activation energy of non-electrostatic nature [ M] is the concentration of solvent molecules. Apparently we can write down the following expression for the stationary system ... 

Surface induced dipole moment of adsorbed atoms... 

The dipole moment of adsorbed atoms can be derived by measuring the change in work function as a result of the presence of the adatoms. In macroscopic measurements, the states of the deposited adatoms are unknown. They may combine into clusters of different sizes and some of them may be absorbed into lattice steps. One can also use the field emission microscope for this purpose. However, similar uncertainties exist. To achieve a well characterized surface and a known number of individual adsorbed atoms on a surface, a combined experiment with field ion microscope observations and field emission Fowler-Nordheim (F-N) plots has been most successful.198,200... 

Even though this equation is based on an overly simple model, it illustrates well the double-layer properties that govern the electrosorption valency in the absence of pet. In particular, it shows that a fractional value of / need not necessarily indicate pet. We shall return to the hard-sphere electrolyte model when we discuss dipole moments of adsorbates. 

Cm. The dipole moments of adsorbates are by convention taken positive in this chapter when the adsorbate dipole vector, is pointing to the vacuum (electronegative adsorbates, e.g., O, Cl ) and negative when Pj is pointing to the surface (electropositive adsorbates, e.g., Na ). 

The principle of PEEM is based on the different dipole moments of adsorbate complexes giving rise to modifications of the local work function. The )deld of photoelectrons emitted from a surface irradiated by ultraviolet light is thus determined by the t)rpe and concentration of adsorbed species, and the lateral... 

A number of molecules have, when they are free, a non-negligible dipole moment, for example the sulfur dioxide molecule SO2, whose dipole moment is g = 1.61 debye. The dipole component is related to the dipole moment of adsorbed molecules through ... 

Within DFT, dipole moments of adsorbates are best obtained from the change A<1> in the work function caused by the adsorbate ... 

With the CO oxidation reaction on Pt(llO) a rich variety of concentration patterns was observed by means of photoemission electron microscopy (PEEM) with typical dimensions in the xm-range [21]. This technique is based on the different dipole moments of adsorbate complexes (Oad ad) giving rise to variations of the local work function. This in turn affects the yield of photoemitted electrons which is imaged, spatially resolved, on a fluorescent screen. 

Subsequent elegant work by Lambert and coworkers61 has shown that, while under UHV conditions the electropumped Na is indistinguishable from Na adsorbed by vacuum deposition, under electrochemical reaction conditions the electrochemically supplied Na can form surface compounds (e.g. Na nitrite/nitrate during NO reduction by CO, carbonate during NO reduction by C2FI4). These compounds (nitrates, carbonates) can be effectively decomposed via positive potential application. Furthermore the large dipole moment of Na ( 5D) dominates the UWr and O behaviour of the catalyst-electrode even when such surface compounds are formed. 

P° initial dipole moment of the adsorbate in the adsorbed state Cm... 

The physical meaning of the g (ion) potential depends on the accepted model of an ionic double layer. The proposed models correspond to the Gouy-Chapman diffuse layer, with or without allowance for the Stem modification and/or the penetration of small counter-ions above the plane of the ionic heads of the adsorbed large ions. " The experimental data obtained for the adsorption of dodecyl trimethylammonium bromide and sodium dodecyl sulfate strongly support the Haydon and Taylor mode According to this model, there is a considerable space between the ionic heads and the surface boundary between, for instance, water and heptane. The presence in this space of small inorganic ions forms an additional diffuse layer that partly compensates for the diffuse layer potential between the ionic heads and the bulk solution. Thus, the Eq. (31) may be considered as a linear combination of two linear functions, one of which [A% - g (dip)] crosses the zero point of the coordinates (A% and 1/A are equal to zero), and the other has an intercept on the potential axis. This, of course, implies that the orientation of the apparent dipole moments of the long-chain ions is independent of A. 

However, this equation is valid only in the case when the dipole moment of the adsorbed XY molecule and the direction of the bond, defined by the arrangement of the... 

If the surface potential alters as a result of adsorption as JV/j/e 0, where is the dipole moment of the adsorbate and N the number of adsorbed molecules. 

In an effort to understand the mechanisms involved in formation of complex orientational structures of adsorbed molecules and to describe orientational, vibrational, and electronic excitations in systems of this kind, a new approach to solid surface theory has been developed which treats the properties of two-dimensional dipole systems.61,109,121 In adsorbed layers, dipole forces are the main contributors to lateral interactions both of dynamic dipole moments of vibrational or electronic molecular excitations and of static dipole moments (for polar molecules). In the previous chapter, we demonstrated that all the information on lateral interactions within a system is carried by the Fourier components of the dipole-dipole interaction tensors. In this chapter, we consider basic spectral parameters for two-dimensional lattice systems in which the unit cells contain several inequivalent molecules. As seen from Sec. 2.1, such structures are intrinsic in many systems of adsorbed molecules. For the Fourier components in question, the lattice-sublattice relations will be derived which enable, in particular, various parameters of orientational structures on a complex lattice to be expressed in terms of known characteristics of its Bravais sublattices. In the framework of such a treatment, the ground state of the system concerned as well as the infrared-active spectral frequencies of valence dipole vibrations will be elucidated. 

Table 4.1 Dipole moments of a few ions adsorbed from an aqueous solution at low coverage.

The same effect exists for adsorption on a metal surface from the gas phase. In this case the adsorbate-induced dipole potential changes the work function by an amount A. If nad is the number of adsorbed molecules per unit area, the component mx of the dipole moment of single adsorbed molecule can be inferred from the relation ... 

In the gas phase the dipole moment determined through Eq. (4.10) refers to an individual adsorbed particle. This is not so in the electrochemical situation. The dipole moment of an adsorbed species will tend to align neighboring solvent molecules in the opposite direction, thereby reducing the total dipole potential drop (see Fig. 4.3). Only the total change in dipole potential can be measured, and there is no way of dividing this into separate contributions from the adsorbate bond and the reorientation of the solvent. The apparent dipole potential of an ion adsorbed from a solution on a particular metal is often substantially smaller than that of the same ion adsorbed in the vacuum (see Table 4.1), since it contains a contribution from the solvent. For comparison we note that the dipole moments of alkali ions adsorbed from the vacuum are usually of the order of the order of 10 29 C m. 

Figure 4.3 Two alternative ways of viewing the charge distribution in an adsorption bond. The upper part of this figure shows the dipole moment the lower part shows a partially charged adsorbate and its image charge. The dipole moments of the surrounding solvent molecules are oriented in the direction opposite to the adsorbate dipole.

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