Surface Chemistry

It is a measure of the amount of gas which a surface contains; more specifically, it is the product of the pressure and time of exposure (normal unit is the Langmuir, where 1 L = 10-6 Torr s).

Adsorption of gases on solids depends on all the above factors.

Desorption is a phenomenon whereby a substance is released from or through a surface. This occurs in a system being in the state of sorption equilibrium between bulk phase (fluid, i.e. gas or liquid solution) and an adsorbing surface (solid or boundary separating two fluids).

Due to strong chemical bonds formed between adsorbent and adsorbate, the heat of adsorption is generally high.

Heterogeneous catalysts act in a different phase than the reactants. Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture. For example, in the Haber process, finely divided iron serves as a catalyst for the synthesis of ammonia from nitrogen and hydrogen.

Some chemicals are produced on large scale via catalytic oxidation, often using oxygen. Examples include nitric acid (from ammonia), sulphuric acid (from sulphur dioxide to sulphur trioxide by the chamber process), terephthalic acid from p-xylene, and acrylonitrile from propane and ammonia.

A small quantity of the catalyst is generally sufficient to catalyse a reaction.  (i) For example, in the decomposition of hydrogen peroxide, one gram of colloidal platinum can catalyse 108 litres of hydrogen peroxide.

Enzyme catalysis is the catalysis of chemical reactions by specialized proteins known as enzymes. Catalysis of biochemical reactions in the cell is vital due to the very low reaction rates of the uncatalysed reactions.

Biocatalysis is the use of natural catalysts, such as protein enzymes, to perform chemical transformations on organic compounds. Both the enzymes that have been more or less isolated and enzymes still residing inside living cells are employed for this task.

All are applications of colloids.

Covalent catalysis involves the substrate forming a transient covalent bond with residues in the active site or with a cofactor. This adds an additional covalent intermediate to the reaction, and helps to reduce the energy of later transition states of the reaction. This mechanism is found in enzymes such as proteases like chymotrypsin and trypsin, where an acyl-enzyme intermediate is formed.

Numerous reactions occur in the bodies of animals and plants to maintain the life process. These reactions are catalysed by enzymes. The enzymes are thus, termed as bio-chemical catalysts and the phenomenon is known as bio-chemical catalysis.

Zeolite have high porosity due to the presence of one, two, or three dimensional networks of interconnected channels and cavities of molecular dimensions.

Some gels are known to liquify on shaking and reset on being allowed to stand. This reversible sol-gel transformation is called thixotropy.

Colloidal sols are heterogeneous in nature. They consists of two phases; the dispersed phase and the dispersion medium.

Electro-osmosis is a phenomenon in which dispersion medium is allowed to move under the influence of an electrical field, whereas colloidal particles are not allowed to move. The existence of electro-osmosis has suggested that when liquid is forced through a porous material or a capillary tube, a potential difference is setup between the two sides called as streaming potential. So the reverse of electro-osmosis is called streaming potential.

Substances which destroy the activity of the catalyst by their presence are known as catalytic poisons. (i) For example, the presence of traces of arsenious oxide (As₂O₃) in the reacting gases reduces the activity of platinized asbestos which is used as catalyst in contact process for the manufacture of sulphuric acid.

The sol particles are prevented from setting out under the action of gravity by kinetic impacts of the molecules of the medium. The setting force can be enhanced by using high speed centrifugal machines having 15,000 or more revolutions per minute. Such machines are known as ultra-centrifuges.

Emulsion are the colloidal solutions in which both the dispersed phase and the dispersion medium are liquids. A good example of an emulsion is milk in which fat globules are dispersed in water. The size of the emulsified globules is generally of the order of 10^-6 m. Emulsion resembles lyophobic sols in some properties.

Chemisorption is a sub-class of adsorption, driven by a chemical reaction occurring at the exposed surface. A new chemical species is generated at the adsorbant surface e.g., corrosion, metallic oxidation. The strong interaction between the adsorbate and the substrate surface creates new types of electronic bonds - ionic or covalent, depending on the reactive chemical species involved.

Pressure swing adsorption (PSA) is a technology used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperatures and so differs from cryogenic distillation techniques of gas separation.

In this type of colloids sols, the dispersed phase has great attraction for the dispersion medium. In such colloids, the dispersed phase does not precipitate easily and the sols are quite stable. If the dispersion medium is separated from the dispersed phase, the sol can be reconstituted by simply remixing with the dispersion medium. Hence, these sols are called reversible sols. Examples of lyophilic sols include sols of gum, gelatine, starch, proteins and certain polymers in organic solvents.

A number of metals such as silver, gold and platinum, have been obtained in colloidal state by treating the aqueous solution of their salts, with a suitable reducing agent such as formaldehyde, phenyl hydrazine, hydrogen peroxide, stannous chloride etc..

A miniemulsion is a special case of emulsion. A miniemulsion is obtained by shearing a mixture comprising two immiscible liquid phases, one surfactant and one co-surfactant. Typical examples are hexadecane or cetyl alcohol.

Micelles are the clusters or aggregated particles formed by association of colloids in solution.