Dispersive and specific interactions

The objective of this study is to evaluate which features of activated carbon surfaces are important for adsorption of acetaldehyde. The evaluation is based on the values of isosteric heats of adsorption, which reflect the strength of molecule interactions with the sorbent surface. After oxidation and urea modification the changes in surface chemistry and porous structure occurred. Those changes are expected to affect the dispersive and specific interactions of acetaldehyde with the activated carbon surfaces. 

Dispersive and specific interactions are considered to contribute independently to the adsorption of probe molecules at the adsorbent surface. It was presented that the adhesion of the fibre-matrix interface depends clearly on the measured strength of acid/base interactions of both fibre and polymer-matrix. Fowkes [2,3] indicated also that the surface of fillers can be chemically modified to enhance acid-base interaction and increase adsorption. 

For test substance free energy of adsorption (AG ) is the sum of energies of adsorption attributed to dispersive and specific interactions. Adsorption of non-polar probes as n-alkanes is caused by dispersive interactions, whereas for polar probes both London and acid-base interactions contribute to AG ... 

The improvements in properties noted may be linked to several factors the reduction made possible in the water/cement ratio, a reduction in gross porosity, the entrainment of air as a fine dispersion, and specific interactions or bonds with the cement gel. Using electron microscopy, Wagner (1965, 1966, 1967) and Aignesberger et al (1969) have shown that melamine and vinyl resins form networks that interpenetrate the network formed by the cement gel. Also, using similar techniques, Pierzchala (1969) concluded that poly(vinyl acetate) effectively becomes a constituent of the cement gel, and makes it chemically resistant, even to hydrofluoric acid. 

Here, AGF and AGf are the free energy contributions corresponding to dispersive and specific interactions. The free energy of adsorption is obtained from chromatographic data such as the net volnme of retention (Vn) through Equation 3.3 ... 

This observation by Coleman et al. [92] has also been discussed by Walsh and Cheng [93], where the heat of mixing is proposed to be the sum of dispersive and specific interaction contributions ... 

In the absence of chemisorption and interdiffusion, the work of adhesion is the sum of the different intermolecular forces involved and can be related to the surface free energies, where a is the compound and the superscripts D and S denote dispersive and specific interactions ... 

In Eq. (6) Ecav represents the energy necessary to create a cavity in the solvent continuum. Eel and Eydw depict the electrostatic and van-der-Waals interactions between solute and the solvent after the solute is brought into the cavity, respectively. The van-der-Waals interactions divide themselves into dispersion and repulsion interactions (Ed sp, Erep). Specific interactions between solute and solvent such as H-bridges and association can only be considered by additional assumptions because the solvent is characterized as a structureless and polarizable medium by macroscopic constants such as dielectric constant, surface tension and volume extension coefficient. The use of macroscopic physical constants in microscopic processes in progress is an approximation. Additional approximations are inherent to the continuum models since the choice of shape and size of the cavity is arbitrary. Entropic effects are considered neither in the continuum models nor in the supermolecule approximation. Despite these numerous approximations, continuum models were developed which produce suitabel estimations of solvation energies and effects (see Refs. 10-30 in 68)). 

It is known that the possible interaction between two materials 1 and 2 is determined by their surface energies, which consist of two components, dispersive, and specific or polar, y . When hydrogen bonding and acid-base interactions are also involved, the adhesion energy between the two materials, Wa will be" ... 

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. 

Typically, solute-solvent interactions are divided into two broad categories Specific and nonspecific interactions. Specific interactions include phenomena such as hydrogen bonding and ji-ji interactions, which depend on the presence of particular functional groups or steric structures. They are short ranged, and are specific in the sense that they involve individual solvent species within the first solvation shell of the liquid. In contrast, nonspecific interactions represent interactions that are not associated with the presence of individual functional groups. In molecular liquids, these include dispersion and electrostatic interactions, such as dipole-dipole forces. We will discuss the nature of each type of interaction in ionic liquids in the sections that follow. 

Dispersion and repulsion are the fundamental forces present during the adsorption of nonpolar molecules in silica because the dipole moment of this molecule is null, the quadrupole moment is very low, and interactions with the hydroxyl groups do not exist. In the case of polar molecules, dispersion and repulsion interactions are present. But, specific interactions between the silica surface and the polar molecule, such as the dipole interaction, and, fundamentally, the interactions with the hydroxyl groups [124-126] are responsible for a more intense interaction of the silica surface with the polar molecules in comparison to nonpolar molecules [4],... 

The role of metal-support interaction on the catalytic activity of carbon-supported Pt nanoparticles toward oxygen reduction and methanol oxidation was analyzed. It was observed that both dispersion and specific activity are influenced by the interaction of the active phase with the support, determining well-defined relationships that may be used for interpreting the electrochemical behavior of new, more advanced catalytic systems. 

The interactions between molecules which produce the cohesive energy characteristic of the liquid phase are described in the section entitled Secondary Forces Between Solvent and Solute Molecules. These involve the dispersion forces, dipole-dipole and dipole-induced dipole interactions, and specific interactions, especially hydrogen bonding. If it is assumed that the intermolecular forces are the same in the vapor and liquid states, then -E is the energy of a liquid relative to its ideal vapor at the same temperature. It can be described as the energy required to vaporize 1 mole of liquid to the saturated vapor phase (Af U) plus the energy required for the isothermal expansion of the saturated vapor to infinite volume. Detailed discussion of the theory and derivations is given in the publications by Hildebrand and associates cited above. 

It should, however, be pointed out that in the above case, if one simply ascribes a single solubility parameter to each monomer, it is Impossible to predict an overall negative enthalpy of mixing. It has also been noted that a window of miscibility can be explained by a favorable specific interaction without recourse to a cross term. If one separates the normal dispersive forces from the specific interaction, then as a first approximation, when the solubility parameters of the two polymers are similar the unfavorable dispersive interactions are small and specific interactions yield miscibility. For a copolymer/polymer mixture the solubility parameters might be expected to match at some specific copolymer composition (32). A method of combining the features of both the specific interaction and the cross term is to use something similar to the UNIFAC group contribution system and model all the interactions, both favorable and unfavorable within the system. 

Intermolecular interactions in adsorbent/adsorbate system may be dispersive and specific which corresponds to the dispersive (7 ) and specific component (7 ) of free surface energy (7s) of adsorbent ... 

The origin of the critical point can be traced to the temperature effect on miscibility. Patterson [1982] observed that there are three principal contributions to the binary interaction parameter, the dispersive, free volume and specific interactions. As schematically illustrated in Figure 2.16, the temperature affects them differently. Thus, for low molecular weight systems where the dispersion and free volume interactions dominate, the sum of these two has a U-shape, intersecting the critical value of the binary interaction parameter in two places — hence two critical points, UCST and LCST. By contrast, most polymer blends derive their miscibility from the presence of specific interactions, characterized by a large negative value of the interaction parameter that increases with T. The system is also affected by the free volume contribution, as well as relatively unimportant in this case dispersion forces. The sum of the interactions reaches the critical value only at one temperature — LCST. 

The study of chemisorption requires the calculation of both dispersive and bonding interactions, because the interaction of the overall adsorbate with the framework must be considered as well as specific bond formation. For chemisorption, the electronic structure is important, so that quantum mechanical modelling is required. 

Deviations of real molecules from the reference system may occur, e.g., due to attractive interactions (dispersion), non-spherical shape of the molecules (chain formation), and specific interactions (hydrogen bonding, dipole-dipole interactions). These contributions can be accounted for by using different perturbation terms. Depending on what kinds of perturbation are considered and which expressions are used for their description, different models based on perturbation theories have been developed in the literature. 

Gii lu-Ustundag and Temelli [63] reviewed the effect of a co-solvent on the phase behaviom of lipids in SC CO2. They found that physical interactions between the solutes and co-solvent, such as dipole - dipole, dipole - induced dipole or induced dipole - induced dipole (dispersion) interactions and specific interactions such as H-bonding and charge transfer complexes, are important contributors to the co-solvent effect. The use of a cosolvent may also lead to a change in selectivity. The magnitude of the effect of the co-solvent is thus a combination of the solvent, the co-solvent, the solute and the operating conditions. 

Fig. 2.19 Interactions in polymer solutions and blends usually comprise of dispersive forces, the free volume effects, and specific interactions, (a) The yi2 of polymer solutions are typically dominated by the contributions from dispersive forces and free volume, whose T dependence can result In a UCST and LCST. (b) In polymer blends, the contributions from the free volume and the contributions from specific interactions usually control the T dependence of yi2, giving rise to an LCST...

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