Dipole adsorption induced changes

The attraction forces which act most frequently in physical adsorption are the nonpolar van der Waals forces. Since London (23) described the close connection between their nature and the cause of optical dispersion, they may also be called dispersion forces. The main contribution to the nonpolar van der Waals forces arises from the interaction of continually changing inducing dipoles and induced dipoles. The interaction energy of a pair of atoms due to this contribution is inversely proportional to the sixth power of the distance ... 

Under usual conditions, molecules such as hydrogen, deuterium, nitrogen and oxygen are IR-inactive, as with respect to their normal mode there does not occur a change in the dipole moment. However, upon adsorption, for instance at the walls of zeolitic channels and cavities, the symmetry of those molecules is disturbed and a dipole is induced by the local electric field. As a first example. Fig. 31 displays the spectra of D2, Nj and O2 sorbed into zeolite Na4Ca4-A [587] at different pressures and (low) temperatures. Calculated geometries of the sor-bate/sorbent complexes are displayed in subsequent figures (vide infra). 

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 ... 

As a result of ionic contact adsorption that induces an interfacial dipole on metal electrodes, the potential, Mb, across the compact layer is altered. Figure 5-28 shows the change of the potential, ( = E — ), observed as a ftmction... 

It is now believed, however, that this intrinsic or a priori surface heterogeneity is not the most general cause of the effect under consideration and that much of the observed decrease in the heat of adsorption with increasing surface -coverage arises from effects associated with the adsorption process itself (Boudart, 85). Among these are repulsions between the adsorbed molecules and changes in the work function of the metal. The latter effect, which is due to the dipoles associated with the adsorbed species, is particularly important and has been termed induced heterogeneity (Boudart, 85) or the work function effect (de Boer, 69, US). 

It would be interesting to explore the proton-transfer (i.e., pH) dependence of the above electron transfer process. In the already mentioned study by Hsieh and Teng [11] (see Equations 5.1 through 5.3, Section 5.1), presumably Reaction 5.2, >CxO + H = >CxO//II —described as specific adsorption basically induced by ion-dipole attraction —was proposed to be responsible for the observed excess specific double layer capacitance due to the local changes of electronic charge density the invoked changes were not discussed in any detail, however. 

The adsorption of the ions of the electrolyte changes the charge of the surface and generates simultaneously surface ion pairs (dipoles). Let us first assume that only the cation is adsorbed. The electric field induced by a surface dipole on the first layer ofwater molecules is given by6... 

Figure 4.10. Adsorption of transition metal atoms at the Al(lOO) surface change of induced dipole moment p. with adsorption height z. Nonrelativistic results for Cu, Ag, Ni, and Pd. Relativistic (straight lines) and nonrelativistic (dashed lines) results for Au and Pd.

As shown in Fig. 7.26, when the sensor is exposed to vapor, individual molecules can diffuse into the semiconductor thin film and be adsorbed mostly at the grain boundaries [13], If the adsorbed analytes have large dipole moment, such as H2O ( 2 debye) and DMMP ( 3 debye), the adsorption of those analyte molecules at the grain boundaries close to or at the semiconductor-dielectric interface can locally perturb the electrical profile around the conduction channel, and hence change the trap density in the active layer. We can interpret the trapping effects by a simple electrostatic model discussed briefly in Sect. 7.2. The electric field induced by a dipole with dipole moment of p (magnitude qL in Fig. 7.4) is ... 

In general terms, physical adsorption, or physisorption, refers to weak bonding of molecules to surfaces through the interactions of induced or permanent dipoles and/or quadrupoles, whereas chemisorption describes adsorption where transfer of chemical charge between adsorbate and surface takes place. Physisorption is characteristically observed at low temperatures, is not an activated process and is completely reversible. Chemisorption, by contrast, involves the formation of bonds, persists to elevated temperatures and can lead to chemical changes. For the adsorption of molecules on microporous solids, important physisorption interactions include the uptake of simple non-polar molecules such as dinitrogen and dioxygen on cationic forms of zeolites whereas the adsorption of molecules onto acid sites is the most important type of chemisorption, because of its importance in catalysis. 

---