Adsorption
Adsorption is the accumulation of atoms, molecules, or ions at the surface of a solid or liquid as the result of physical or chemical forces. It differs from absorption, in that an adsorbed substance remains at the surface while an absorbed substance spreads throughout the absorbing material. An adsorbed substance is termed an adsorbate while the material on which adsorption occurs is the substrate. The release of an adsorbate is termed desorption.
Chemists make a distinction between chemical adsorption, or chemisorption, characterized by the formation of chemical bonds with the substrate, and physical adsorption or physisorption, which results from the van der Waals force. Chemisorption plays an essential role in corrosion, heterogeneous catalysis, and electrochemistry. Physisorption is a factor to be dealt with in the design of vacuum systems and is also used as a tool to study highly irregular surfaces. Further distinctions can be drawn between localized adsorption, in which the adsorbed molecules remain at the sites at which they were initially adsorbed, and delocalized adsorption, in which the adsorbed molecules can move along the surface. A distinction can also be made between monolayer adsorption and multilayer adsorption.
Adsorption is a very widespread phenomenon. In the open atmosphere many solid materials are covered by a layer of molecules adsorbed from the gas. Our understanding of the adsorption of gases has largely been a by-product of the early vacuum-based electronics technology. Beginning with the electric light, which involved heating thin metal filaments in an evacuated enclosure, through the development of electronic vacuum tubes and the television tube, the adsorption of gases and their subsequent desorption under reduced pressure became an important factor limiting the performance of electronic components. One of the first major researchers in the field, the American physical chemist Irving Langmuir, began his work on adsorption as an employee of the General Electric Company, seeking to extend the life of light bulbs with tungsten filaments. It was Langmuir who first realized that the surfaces of metals and other solids were generally characterized by unsatisfied valences and that adsorption could be the result of covalent bond formation with the substrate. In 1932, Langmuir received the Nobel Prize for his work in surface chemistry.
The adsorption of gases on solids is generally described by an equilibrium relation called an isotherm, which describes how the amount of adsorbed material varies with the partial pressure of the gas (with the temperature held constant). The amount of material adsorbed is sometimes expressed as the coverage, that is, as a fraction of the number of adsorbing sites that are occupied by an adsorbed molecule. One of the simplest isotherms, now generally known as the Langmuir isotherm, is obtained by assuming a rate of adsorption proportional to the product of the gas partial pressure and the amount of unoccupied surface balanced by a rate of desorption proportional to the amount of occupied surface. A slightly more complicated isotherm, the Brunauer, Emmett, Teller (BET) isotherm, allows for the possibility of multilayer adsorption. The latter isotherm is frequently used to determine the total surface area of a finely divided powder, by measuring the amount of nitrogen or another gas it absorbs as a function of pressure.
The adsorption of reactants is the first step in the heterogeneous catalysis. Many of the transition metals are adsorbers of hydrogen, oxygen, hydrocarbon molecules, and carbon monoxide.Nickel is used, for instance, to add hydrogen to oils with multiple bonds, referred to as unsaturated or poly- unsaturated oils, to produce a solid form for cooking use.Iron is used in the synthesis of ammonia from nitrogen and hydrogen. Amorphous alumina-silica composites are used to crack or split hydrocarbon chains in petroleum refining. A dispersion of platinum on alumina particles is used in petroleum reforming, a combination of reactions that turn a mixture of hydrocarbons into a fuel better suited to use in internal combustion engines.
Adsorption is also a key factor in electrochemical reactions. When a metal electrode is immersed in an electrolyte solution it is generally covered by physically adsorbed water molecules. Anions like chloride and bromide also adsorb on electrode surfaces while smaller cations like lithium, sodium and potassium are generally too strongly bound to a small number of water molecules to bind directly to the electrode. The number and arrangement of adsorbed ions and molecules is a function of the electrode charge and may strongly affect the approach of ions in solution to the electrode, and thus the rates of electrochemical reactions.
An important case of adsorption on liquid surfaces is that of surface-active or surfactant molecules at the water-air or water-oil interface. These molecules typically involve a charged or polar hydrophillic group and a nonpolar hydrophobic group. By aggregating at the water-air interface, the molecules are able to obtain an energetically favorable configuration with their hydrophobic ends protruding from the water. The development of this type of adsorbed layer acts to lower the surface tension of the water, an effect predicted in the Gibbs adsorption isotherm, introduced by the American Physicist J. Willard Gibbs (1839-1903) in 1875. The ability of detergents to remove oily material from fabric is due to the presence of surfactant molecules which stabilize the oil-water interface, reducing its surface tension and enabling the oil to break up into small droplets which disperse through the water.
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