Forces of adsorption

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

Loaded Adsorbents. Where highly efficient removal of a trace impurity is required it is sometimes effective to use an adsorbent preloaded with a reactant rather than rely on the forces of adsorption. Examples include the use of 2eohtes preloaded with bromine to trap traces of olefins as their more easily condensible bromides 2eohtes preloaded with iodine to trap mercury vapor, and activated carbon loaded with cupric chloride for removal of mercaptans. 

We have shown that the main driving force of adsorption of surfactants in monolayers on latex surfaces is the (cooperative) inter-... 

The table reveals that the adsorption free energy is dominated by the cmc term. Thus, the dominating driving force of adsorption is of the same origin as for the micellization, i.e. it depends on the... 

Chemisorption and Physisorption. One classification of adsorption phenomena is based on the adsorption energy the energy of the adsorbate-surface interaction. In this classification there are two basic types of adsorption chemisorption (an abbreviation of chemical adsorption) and physisorption (an abbreviation of physical adsorption). In chemisorption the chemical attractive forces of adsorption are acting between surface and adsorbate (usually covalent bonds). Thus, there is a chemical combination between the substrate and the adsorbate where electrons are shared and/or transferred. New electronic configurations are formed by this sharing of electrons. In physisorption the physical forces of adsorption, van der Waals or pure electrostatic forces, operate between the surface and the adsorbate there is no electron transfer and no electron sharing. 

Types III (e.g. bromine on silica gel at 352 K) and V (e.g. water vapour on charcoal at 373 K) show no rapid initial uptake of gas, and occur when the forces of adsorption in the first monolayer are relatively small. These isotherms are rare. 

Selective adsorption of ions on an electrode surface contributes to the formation of an electrical double layer. This layer is due to the electrostatic forces of adsorption between the charge of the electrode and the ions of opposite charge present in the solution. The electrical double layer generates a difference in potential and forms an electric condenser as a first approximation. 

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. 

CLASSIFICATION OF ATTRACTIVE FORCES OF ADSORPTION CHROMATOGRAPHY BETWEEN SOLUTES AND STATIONARY PHASES... 

The second factor is simply the high effective concentration of the charged groups attainable at the interface by virtue of the inability of the long-chain ions to escape into the bulk, due to strong forces of adsorption. The ions may consequently be packed more closely together than is ever possible in bulk solution. 

Classical liquid-liquid chromatography (LLC) originated from the Nobel Prize winning work of Martin and Synge in 1941. LLC systems consist of a mobile phase and a stationary phase which is held on the support by physical forces of adsorption. The column materials are commonly prepared using the solvent evaporation technique developed for GC. In order to avoid solvent stripping of the stationary phase, it is necessary for the eluant to be saturated with the stationary phase component. The mobile and stationary phases should have contrasting polarities and be immiscible. 

By contrast, in the alumimun-rich zeohtes, there are strong intracrystalhne electric fields, so that electrostatic forces of adsorption are very important, particularly for polar or quadrupolar sorbate molecules. Such adsorbents are classified as hydrophihc because they adsorb polar molecules such as water very strongly. Control of the Si/Al ratio in a zeolite adsorbent thus provides a useful means of adjusting the selectivity of an adsorbent for a particular separation. 

Forces of adsorption are electrostatic forces, the valance energy force and the cohesive energy force. The force of relevant interest here is the van der Waals force, which exists between all atoms and molecules. The van der Waals forces can be classified into three groups ... 

The preferred technique for sampling organic vapors is collection on a porous sorbent by the van der Waals forces of adsorption (particularly the London dispersion force), with later desorption and instrumental analysis. The rate of adsorption at the sorbent surface is extremely fast and is not normally considered a rate-controlling step, and the adsorption equilibrium is normally shifted far enough in the direction of the adsorbed phase that the concentration at the sorbent surface can be regarded as insignificant. All molecules that arrive at the surface are therefore adsorbed. The adsorbed vapors are extracted from the sorbent by means of a solvent or heat, and analysis is normally carried out by gas chromatography (GC), or less often by LC. 

The adsorption of BSA is insensitive to pH in the neighbourhood of the i.e. point indicating that the apolar air phase influences adsorption in the same way as hydrophobic solid substrates (where the main driving force of adsorption are hydro-phobic interactions). 

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