Non-specific dispersion forces

Adsorption of SOC by activated carbon may involve various combinations of chemical, electrostatic, and physical (i.e. non-specific dispersion forces) interactions [59]. The overall adsorption interactions can be very complex for some SOCs. One good example is the adsorption of phenolic compounds, probably the most widely studied class of adsorbates in the activated carbon literature. Several possible mechanisms have been proposed for phenol adsorption [60-69]. These incluile (i) n-n dispersion interactions between the basal plane of activated carbon and the aromatic ring of the adsorbate, (ii) electrostatic attraction-repulsion interactions, (iii) hydrogen bonding between adsorbate and surface functional groups of activated carbons, (iv) electron acceptor-donor complex formation mechanisms between the carbonyl... 

Entropic contributions, one source of the inequality of AH(X) and AG(X), are virtually eliminated by comparing rigid molecular structures of similar size (3). Application of Eqn. 1 in this system allows the direct resolution of specific in situ interaction energies because a) aqueous solvation contributions are nearly eliminated in the hexane system (3), and b) hexane can interact favorably with ligands only through weak, relatively non-specific dispersion forces. 

For the three zeolites of 29 Si/Al ratio, only one volume, Wq, is determinated from the D-R plot and its value is of the same order of magnitude for the three samples (Table 1). The absence of the volume Wqi indicates so, a weakening of the electrical field due to the extraction of the structural aluminium ions and wholly, the strong specific interactions H20-if fade out in favor of the non-specific dispersion forces. At high pressures the strong adsorption characterized by the hysteresis loop results thereafter, from capillary condensation into mesopores. The volume of water condensed into these mesopores is 74 % of the total volume of adsorbed water. On the other hand, Wqi and Wg are defined for the sample with a Si/Al ratio of 24.5. 

The differentiation between effects due to specific solute/solvent interactions and bulk dielectric solvent effects is not easy to visualize and is often a matter of debate [367]. The experimental data indicate that the solvent sensitivities of vx-o vibrations are complex functions of several factors, including contributions from bulk dielectric effects, non-specific dispersion and induction forces, specific HBD/HBA interactions, as well as steric effects [134]. Solvent effects on the vc o IR stretching absorption have been line-... 

The effect of polarity in enhancing the energy of interaction has been discussed by Kiselev and his associates who distinguish between non-specific adsorption, where only dispersion and repulsive forces are involved 4>d and and specific adsorption, where coulombic contributions (some or all of (p, [Pg.11]

First, it is important to appreciate that all liquid crystal mesophases exist due to non-covalent interactions between molecules, namely the anisotropic dispersion forces mentioned earlier. However, this section will address more specific non-covalent interactions that have been used either to induce liquid-crystalline behaviour or to generate a new species that is liquid crystalline. 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

The physical adsorption is characterized by weak intermolecular forces of the van der Waals type. The adsorbed particle must get close to the solid surface, since the van der Waals energy is proportional to the sixth power of reciprocal distance. The main feature of this interaction is its non-specificity, a considerable velocity and reversibility. An example of the physical adsorption is the adsorption of apolar molecules on an apolar surface resulting form disperse forces. Beside these forces the dipol-dipol interactions occur when molecules of the adsorbent or adsorbate can form permanent or induced dipoles (adsorption of gases or dipol liquids on apolar surfaces). 

An apolar aprotic solvent is characterized by a low relative permittivity (sr < 15), a low dipole moment [ju < 8.3 10 Cm = 2.5 D), a low value ca. 0.0... 0.3) cf. Table A-1, Appendix), and the inability to act as a hydrogen-bond donor. Such solvents interact only slightly with the solute since only the non-specific directional, induction, and dispersion forces can operate. To this group belong aliphatic and aromatic hydrocarbons, their halogen derivatives, tertiary amines, and carbon disulfide. 

With the use of solvatochromic probes, other non-specific forces (dispersion, dipole-induced dipole, and dipole-dipole) and specific acid-base forces have been explored in SCF solvents. In an effort to compare liquid and supercritical carbon dioxide, Hyatt(ll) measured UV-visible spectra of several solvatochromic probes. There was little difference between the Ex in the liquid and SCF states however, the data can not be interpreted fully since the density and the pressure were not given at the supercritical condition. The results indicated that the... 

Physical adsorption (physisorption) occurs when an adsorptive comes into contact with a solid surface (the sorbent) [1]. These interactions are unspecific and similar to the forces that lead to the non-ideal behavior of a gas (condensation, van der Waals interactions). They include all interactive and repulsive forces (e.g., London dispersion forces and short range intermolecular repulsion) that cannot be ascribed to localized bonding. In analogy to the attractive forces in real gases, physical adsorption may be understood as an increase of concentration at the gas-solid or gas-liquid interface imder the influence of integrated van der Waals forces. Various specific interactions (e.g., dipole-induced interactions) exist when either the sorbate or the sorbent are polar, but these interactions are usually also summarized under physisorption unless a directed chemical bond is formed. 

In this context, interactions between ionic liquids and solutes are understood as intermolecular (and in extension interionic) solvation forces, which can be categorised according to Reichardt [4] (Fig. 2) as non-specific induction and dispersion (Coulomb) forces, and specific directional stoichiometric forces (hydrogen bond acceptor and donor, electron pair acceptor and donor) [4],... 

Physical adsorption occurs due to van der Waals (dispersion) or electrostatic forces. The attraction depends on the polar nature of the fluid component being adsorbed as well as that of the adsorbent. Van der Waals forces are directly related to the polarizability. An estimate of the relative strength of interaction is based on the sorbate size and polarizability. Electrostatic forces include polarization forces, field-dipole interactions and field gradient-quadrupole interactions. These forces arise when the surface is polar. In the case of a polar solvent like water with non-polar organic impurities, the organic molecules will prefer to stick to a non-polar adsorbent such as activated carbon rather than remain in the polar solvent. Physical adsorption is reversible. Physical sorption is sensitive to temperature, relatively non-specific regarding sorbates, relatively fast kineticaUy, and has a low heat of adsorption (<2A//vap)- Multiple sorbate layers can form on the sorbent surface. 

The physically active sites of carbon black should produce an increase in interfacial adhesion and hence an increase in reinforcement. Furthermore, this increase in reinforcement should be non-specific for different non-polar polymers, since it still reflects a dispersion force interaction. While its existence cannot be questioned, it clearly cannot be assumed to be responsible for the entire observed increase in reinforcement over and above that displayed by graphitized carbon black if other, more energetic bonds are also formed. 

Non-aqueous solvents are more of a problem than aqueous acids. We expect their medium effects to be less uniform and more unpredictable, especially in non-hydrogen-bonding solvents. It may be recalled that one-colour indicators are just those substances whose medium effects may include a sizeable contribution from London dispersion forces. For acidic solvents such as formic acid H functions have been investigated and reference will be made to these later. Current interest " centres more on H acidity functions which are useful for strongly basic media, such as ethylenediamine or mixtures of methanol and sodium methoxide. More details on indicator acidity functions will be given when we consider specific solvents later in this chapter. 

---