Place: United States of America
Subject: biography, chemistry
US physical chemist who is best remembered for his studies of adsorption at surfaces and for his investigations of thermionic emission. For his work on surface chemistry he was awarded the 1932 Nobel Prize for Chemistry. Unlike most scientists of world renown he did most of his research in a commercial environment, not in an academic institution.
Langmuir was born in Brooklyn, New York City, on 31 January 1881. He attended local elementary schools before his family moved to Paris for three years, where he was a boarder at a school in the suburbs. His interests in the practical aspects of science were fostered by his brother Arthur. In 1895 the family returned to the USA to Philadelphia, and Langmuir went to the Chapel Hill Academy and later to the Pratt Institute in Brooklyn. After high school he entered the School of Mines at Columbia University and graduated in 1903 with a degree in metallurgical engineering. His postgraduate studies were undertaken at the University of Göttingen under the guidance of Hermann Nernst; he gained his PhD for a thesis on the recombination of dissociated gases. After a brief period as a teacher in New Jersey, he joined the General Electric Company in 1909 at their research laboratories at Schenectady and remained there until he retired in 1950; he was its assistant director 1932-50. He died in Falmouth, Massachusetts, on 16 August 1957.
Langmuir's work at Columbia concerned the dissociation of water vapour and carbon dioxide around red-hot platinum wires. His first studies at General Electric involved the thermal conduction and convection of gases around tungsten filaments. Langmuir showed that the blackening on the inside of ‘vacuum-filled’ electric lamps was caused by evaporation of tungsten from the filament. The introduction of nitrogen into the glass bulb prevented evaporation and blackening, but increased heat losses which were overcome by making the tungsten filament in the form of a coil.
At the same time, Langmuir was carrying out research on electric discharges in gases at very low pressures, which led to the discovery of the Child-Langmuir space-charge effect: the electron current between electrodes of any shape in vacuum is proportional to the 3/2 power of the potential difference between the electrodes. He also studied the mechanical and electrical properties of tungsten lamp filaments to which thorium oxide had been added. He showed that high thermal emissions were caused by diffusion of thorium to form a monolayer on the surface. One consequence was the development of an improved vacuum pump based on the condensation of mercury vapour. This work also initiated his 1934 patent for an atomic welding torch, in which the recombination of hydrogen atoms produced by an electric arc between tungsten electrodes generated heat at temperatures in the order of 6,000°C/11,000°F.
For three years Langmuir considered the problems of atomic structure. Building on Gilbert Lewis's atomic theory and valency proposals, Langmuir suggested that chemical reactions occur as a consequence of a desire by an atom to achieve a full shell of eight outer electrons. He was the first to use the terms electrovalency (for ionic bonds between metals and non-metals) and covalency (for shared-electron bonds between non-metals).
During the 1920s Langmuir became particularly interested in the properties of liquid surfaces. He went on to propose his general adsorption theory for the effect of a solid surface during a chemical reaction. He made the following assumptions:
(a) the surface has a fixed number of adsorption sites; at equilibrium at any temperature and gas pressure, a fraction θ of the sites are occupied by adsorbed particles and a fraction 1 − θ are unoccupied;
(b) the heat of adsorption is the same for all sites and is independent of the number of sites occupied;
(c) there is no interaction between molecules on different sites, and each site can hold only one adsorbed molecule.
From this model he formulated Langmuir's adsorption isotherm, which can be expressed as
1/θ = 1 + 1/bP
where θ = fraction of sites occupied, b is a constant based on rate constants of evaporation and condensation, and P = pressure.
During World War II Langmuir was responsible for work that led to the generation of improved smokescreens using smoke particles of an optimum size. He later applied this knowledge to particles of solid carbon dioxide and silver iodide which were scattered from aircraft to seed water droplets for cloud formation, in an attempt to make rain.
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