Monomolecular film

(physical chemistry) A film one molecule thick. Also known as monolayer.

A film one molecule thick; often referred to as a monolayer. Films that form at surfaces or interfaces are of special importance. Such films may reduce friction, wear, and rust, or may stabilize emulsions, foams, and solid dispersions. The broad field of catalysis, which is basic to petroleum refining and many chemical industries, involves chemical reactions that are accelerated in the thin films of reactants at interfaces. Moreover, thin films containing proteins, cholesterol, and related compounds constitute biological membranes, the internal interfaces that control the complex processes of life. See also Catalysis.

In all of these areas, a single monomolecular layer at the interface is the most important. It is held to the adsorbing surface by forces stronger than those that hold any succeeding layer. On solid surfaces, it is the only layer that can be chemisorbed. It may be the site of enhanced chemical reactivity, or the last line of defense.

Monolayers on solids, or at liquid interfaces, may be formed by adsorption from the adjacent bulk phases; the process may show high specificity for particular chemical species. Measurements of the extent of adsorption have historically provided information on the composition and structure of monolayers formed in this way. A variety of surface-sensitive instrumental techniques, such as diffraction and scattering of low-energy electrons, neutrons, and ions, and spectroscopy of adsorbed species, have been brought to bear to obtain information about the structure of the surface layer and chemical perturbations in it. See also Adsorption; Spectroscopy.

In addition, monolayers of a wide variety of substantially insoluble substances can be formed at a liquid-gas interface by allowing them to spread over the surface. The properties of such films at the water-air interface can be manipulated, controled, and measured in simple and elegant ways. A variety of specialized experimental techniques have been developed to study these insoluble monolayers.

In order to form spread monolayers which are sufficiently stable to study, a substance must combine low solubility and volatility with some moiety which attracts it to the liquid surface; for films on water, this generally means one or more polar functional groups. Totally nonpolar substances, such as the higher-molecular-weight paraffin hydrocarbons, will not spread on water (although they can spread on liquids of very high surface tension, such as mercury). Typical among the large group of substances which do form insoluble monolayers on water are the long-chain fatty acids and their derivatives such as glycerides, sterols, and many lipid substances of biological origin, including the fat-soluble vitamins and natural pigments such as chlorophyll. Many polar synthetic polymers, including polyvinyl acetate and polymethyl methacrylate, can be made to spread as monolayers on water; so can many proteins, because their tertiary structure unfolds at the air-water interface. See also Polar molecule.