Adsorption-induced dipole moment

In addition to Eqn. 5-47, the adsorption-induced dipole moment has been derived based on molecular dynamics [Schmickler, 1993] as shown in Eqn. 5—49 ... [Pg.155]

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. [Pg.294]

The effect of cationic polyacrylamide adsorption on the stability of aqueous cellulose suspension has been electro-optically studied by Khlebtsov et al. [19,20], The variations in transmitted light modulation in low-frequency (3-45 Hz) square-pulsed a.c. fields have been found to correlate well with the adsorption isotherm. The influence of sodium carboxymethylcellulose adsorption on the electro-optical behavior of negatively charged Si02 (ani-sometric aggregates, consisting of monodisperse spherical particles) has also been studied, and an increase of the particle induced dipole moment and its time of relaxation has been found [21,22],... [Pg.306]

Figure 4 shows the most important feature of the electro-optical effect in a suspension, stabilized by polyelectrolyte adsorption. This is the appearance of an additional LF effect near the range of particle rotation (102-104 Hz) that resembles the LF effect in free polyelectrolyte solutions. The analogy is so impressive that we supposed the same origin of the LF effect in both systems. Polarization of tightly bound counterions with lower mobility in comparison to that of the free ions is proposed to explain the LF effect in stabilized suspensions [12-18], since it cannot be found in suspensions of bare oxide particles. The overlap (full or partial) of the frequency intervals of particle hydrodynamic relaxation (RF effect) and the relaxation of this additional LF effect means that the tightly bound counterions have close mobility to that of the whole colloid-polymer complex. A saturable ionic induced dipole moment probably arises due to the polarization of these coun-... [Pg.314]

One of the characteristics of chemisorption is that it permits the formation of different types of bonds between a given adsorbed species and the same adsorbent. Thus, an atom can be attached to an ionic crystal by a weak covalent bond, a strong covalent bond, or an ionic bond. The first is characterized by a localized electron and an induced dipole moment that may be larger by several orders of magnitude than the moment due to physical adsorption. When bonding is augmented by a free electron from the crystal lattice, the adsorbed atom (in the case of monovalent electropositive atom) is held by a strong covalent bond. On the other hand, localization of a hole near a weakly adsorbed atom leads to the formation of an ionic bond. Thus the same atom can represent an acceptor or a donor at the same time. [Pg.807]

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.

The adsorption of PAHs was a combination of two factors, van de Waals force and a thermodynamic gradient determined by the hydrophobicity driving them out of the solution. Adsorption was dominated by van der Waals attractive forces between instantaneous and induced dipole moments of molecules (Gauthier et al., 1987). Dipole moments depended upon the polarizability and could be estimated by the sum of bond moments in the molecule. An increase in the numbers of C=C double bonds should contribute to molecular polarizability and thereby increase the van der Waals attraction strength. As a result, heavy molecular weight PAHs with more C=C double bonds should be adsorbed more efficiently to sediment particles. [Pg.417]

If a surface is polar, its resulting electric field will induce a dipole moment in a molecule with no permanent dipole and, through this polarization, increase the extent of adsorption. Similarly, a molecule with a permanent dipole moment will polarize an otherwise nonpolar surface, thereby increasing the attraction. [Pg.1503]

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 ... [Pg.39]

Interfacial dipole moment induced by contact adsorption... [Pg.153]

The polarity of a solid surface can be regarded as the strength of its average electrostatic field F. This field interacts with permanent or induced dipoles of adsorbed molecules, whereas its gradient interacts with permanent or induced quadrupoles. These interactions give rise to components of the adsorption energy such as EFfl (with permanent dipoles), Ef (polarization contribution) or PQ (with permanent quadrupoles). Therefore, the selection of the appropriate immersion systems, differing mainly in the molecular dipole moment n of the immersion liquid, can be expected to provide information on the value of EPfl since ... [Pg.135]

It would thus seem strongly probable that this atypical variation in the differential enthalpy of adsorption corresponds to the presence of an ordered fluid. This could result from the interaction of the cationic field with the permanent moments of nitrogen or carbon monoxide and even with the induced dipole of the methane molecules. [Pg.295]

For the moment, let us focus our attention on gas-phase reactions catalyzed by solid surfaces. For a catalytic reaction to occur, at least one and frequently all of the reactants must become attached to the surface. This attachment is known as adsorption and takes place by two different processes physical adsorption and chemisorption. Physical adsorption is similar to condensation. The process is exothermic, and the heat of adsorption is relatively small, being on the order of 1 to 15 kcal/mol. The forces of attraction between the gas molecules and the solid surface are weak. The.sc van der Waals forces consist of imcraciion between permanent dipoles, between a permanent dipole and an induced dipole, and/or between neutral atoms and molecules. The amount of gas physically adsorbed decreases rapidly with increasing temperature, and above its critical temperature only very small amounts of a substance are physically adsorbed. [Pg.650]

The interaction of a Pd4 (C4v) cluster with the oxide surface was analyzed in more detail with the help of electron density difference plots and other theoretical tools, such as population analysis, core level shifts as well as induced and dynamic dipole moments [175]. Three interaction mechanisms were found to contribute to different extent metal polarization with the subsequent electrostatic attraction, Pauli repulsion, and covalent orbital interactions. Electrostatic interactions make up a sizeable fraction of the adhesion energy the polarization of the metal adsorbate by the surface electric field provides an important bonding mechanism. For the adsorption of Pd on-top or in the vicinity of the surface Mg " cations this electrostatic interaction accounts for almost the entire adsorption energy, albeit counteracted by Pauli repulsion. For adsorption on-top 0 , on the other hand, mixing of adsorbate and substrate orbitals becomes noticeable. This hybridization or covalent bonding at the interface with the oxide anions is complemented by electrostatic polarization. Further work is required to establish in a more quantitative way the relative importance of electrostatic and chemical bonding contributions. However, in line with our other studies of... [Pg.409]

Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...