Hydrogen bonding, adsorbate-adsorbent

Physical and chemical interactions involving adsorption are referred to as phy-sisorption and chemisorption, respectively, with the distinction between the two based primarily on energetic differences. Physisorption often has little or no energetic activation barrier, and involves relatively weak, long-range van der Waals interactions. It is, therefore, a relatively low-energy process on the order of 5-12 kcal/mol, comparable to the heat of condensation of covalent, non-hydrogen-bonded adsorbates. As a result, physisorption is typically quite reversible at room temperature, but relatively nonselective. 

Figure 5.1 Silica surfaces contain high concentrations of hydrogen-bonded adsorbed water, siloxane (Si-O-Si) groupings and intramolecular hydrogen bonds via silanol (SiOH) groupings.

Figure 4.5 shows the energies of the initial weak hydrogen-bonded adsorbed state of propylene, the proton-activated transition state and the final alkoxy product state of the protonated propylene. The structures and energies are established from DFT cluster calculations using the model structure shown in Fig. 4.5a and periodic DFT calculations using the unit cell of chabazite and the zeolitic protons (Fig. 4.5b). The cluster used in Fig.

All theories have problems for some systems, especially for polar/hydrogen bonding adsorbates and complex solids. 

The hydroxylatlon of the sllazane surface species to a level of 1.5 wtZ, results In a calculated SI2-NH site density of 4 s1tes/nm. Beyond 1.5 wtX, a slower weight gain regime results. This Is attributed to physically (or hydrogen bonded) adsorbed water. The amount of physically axlsorbed water Is seen to be proportional to the relative humidity. 

The commonly accepted mechanism of heterogeneously catalyzed hydrogenation involves activation of both the hydrogen and the C—C multiple bond adsorbed on the metal surface. First one hydrogen atom is transferred to the least hindered position of the multiple bond to give a half-hydrogenated adsorbed species. This reaction is fully reversible and ac-... 

The most common hydrophobic adsorbents are activated carbon and siUcahte. The latter is of particular interest since the affinity for water is very low indeed the heat of adsorption is even smaller than the latent heat of vaporization (3). It seems clear that the channel stmcture of siUcahte must inhibit the hydrogen bonding between occluded water molecules, thus enhancing the hydrophobic nature of the adsorbent. As a result, siUcahte has some potential as a selective adsorbent for the separation of alcohols and other organics from dilute aqueous solutions (4). 

Hydrogen Bond Formation. This faciUtates adsorption if the mineral and the adsorbate have any of the highly electronegative elements S,0,N,F, and hydrogen. A weak (physical) bond is estabflshed between the sohd wall and the reagent through the alignment of the cited elements. 

The well-known DLVO theory of coUoid stabiUty (10) attributes the state of flocculation to the balance between the van der Waals attractive forces and the repulsive electric double-layer forces at the Hquid—soHd interface. The potential at the double layer, called the zeta potential, is measured indirectly by electrophoretic mobiUty or streaming potential. The bridging flocculation by which polymer molecules are adsorbed on more than one particle results from charge effects, van der Waals forces, or hydrogen bonding (see Colloids). 

Separation of enantiomers by physical or chemical methods requires the use of a chiral material, reagent, or catalyst. Both natural materials, such as polysaccharides and proteins, and solids that have been synthetically modified to incorporate chiral structures have been developed for use in separation of enantiomers by HPLC. The use of a chiral stationary phase makes the interactions between the two enantiomers with the adsorbent nonidentical and thus establishes a different rate of elution through the column. The interactions typically include hydrogen bonding, dipolar interactions, and n-n interactions. These attractive interactions may be disturbed by steric repulsions, and frequently the basis of enantioselectivity is a better steric fit for one of the two enantiomers. ... 

The orientational structure of water near a metal surface has obvious consequences for the electrostatic potential across an interface, since any orientational anisotropy creates an electric field that interacts with the metal electrons. Hydrogen bonds are formed mainly within the adsorbate layer but also between the adsorbate and the second layer. Fig. 3 already shows quite clearly that the requirements of hydrogen bond maximization and minimization of interfacial dipoles lead to preferentially planar orientations. On the metal surface, this behavior is modified because of the anisotropy of the water/metal interactions which favors adsorption with the oxygen end towards the metal phase. 

The adhesive macromolecules are adsorbed on to the surface of the substrate and are held by various forces of attraction. The adsorption is usually physical, i.e., due to van der Waals forces. However, hydrogen bond-... 

During the reaction, protons are extracted from the brucite lattice. Infrared spectra [24, 25, 31] show that during charge the sharp hydroxyl band at 3644 cm" disappears. This absorption is replaced by a diffuse band at 3450 cm"1. The spectra indicate a hydrogen-bonded structure for ft-NiOOH with no free hydroxyl groups. ft-NiOOH probably has some adsorbed and absorbed water. However, TGA data... 

Kagel (29) found that whereas pyridine is hydrogen bonded to a silica gel surface 2-chloropyridine is not, the spectrum of 2-chloropyridine adsorbed on silica gel being identical with that of the liquid, and concluded that steric hindrance probably prevents hydrogen bond formation in this case. 

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