Peter Joseph Wilhelm Debye

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Debye, Peter Joseph William

(b. Maastricht, Netherlands, 24 March 1884; d. Ithaca, New York, 2 November 1966)

chemical physics.

Debye, the son of Wilhelmus and Maria Reumkens Debye, went to school in Maastricht until he left for the nearby Technische Hochschule in Aachen, across the border in Germany. Here he was an assistant from 1904 to 1906, obtaining a degree in electrical engineering in 1905. When Arnold Sommerfeld, the eminent German mathematical physicist, was called from Aachen to the University of Munich in 1906, he took Debye with him. Debye remained in Munich as an assistant for five years, obtaining a Ph.D. in Physics in 1908 and serving as Privatdozent in his last year. In 1911, at the age of twenty-seven, he succeeded Einstein as professor of theoretical physics at the University of Zurich. After only a year in Switzerland he returned to his native country for a year as professort of theoretical physics at the University of Utrecht, only to leave again for Germany, where he stayed from 1913 to 1920 as professor of theoretical and experimental physics at the University of Göttingen. On 10 April 1913 he married Matilde Alberer. Their son, Peter Paul Ruprecht, who was later to collaborate in some of the light-scattering researches, was born in 1916; their daughter, Mayon M., was born in 1921. The period from 1911 to 1916 was perhaps the most productive for Debye. In spite of holding three professorships in three countries during the first three years of this period, he produced his theory of specific heats, the concept of permanent molecular dipole moments, and the related theory of anomalous dielectric dispersion. With Paul Scherrer he developed the powder method of X-ray analysis.

In 1920 Debye returned to Zurich as professor of experimental physics and director of the Physical Institute at the Eidgenössische Technische Hochschule, where he was surrounded by an able group of students and assistants, one of whom, Erich Hückel, collaborated with him in his next great basic contribution, the Debye-Hückel theory of electrolytes published in 1923. The work on X-ray scattering and dipole moments continued through the Zurich period along with that on electrolytes. In 1927 Debye moved to the University of Leipzig as professor of experimental physics, a professorship reported to be the most lucrative in Germany. Physical chemists now flocked to Leipzig to Debye’s institute as they had to Ostwald’s a generation earlier. Although good work continued through this seven-year period at Leipzig, no great basic discoveries were made. In 1934, the second year of the Nazi regime in Germany, Debye moved to the University of Berlin as professor of theoretical physics and supervised the building of the Kaiser Wilhelm Institute of Physics, which he named the Max Planck Institute. Here, as in Leipzig, good work continued; but nothing of outstanding importance emerged. In 1936 Debye received the Nobel Prize in chemistry and, in 1939, had the unusual experience of seeing a bust of himself unveiled in the town hall of his native city.

Debye was not only a brilliant and original scientist but also a wise and shrewd man of the world. In the performance of administrative duties in Berlin, he had to spend a great deal of time dealing with Nazi bureaucrats. He had retained his Dutch citizenship when he came to Berlin, having been told by the minister of education that he would not be required to become a German citizen. However, not long after World War II broke out, he was informed that he could not enter his laboratory if he did not become a German citizen. He refused to do so and soon succeeded in getting to the United States, where he became a citizen in 1946. He had lectured many times in the United States and had declined offers of professorships at many leading universities; but he now gave the Baker Lectures at Cornell University and was appointed professor of chemistry and head of the chemistry department there, positions which he held from 1940 to 1950, when he became professor emeritus. He continued active in research and consultation until the end of his life. The first ten years at Ithaca produced the work on light scattering, his last great contribution. Debye’s many achievements were recognized by his election to membership in some twenty-two academies throughout the world and the award of twelve medals and eighteen honorary degrees.

Debye’s physical vigor equaled his vigor of mind and, in middle and old age, his appearance was that of a man at least ten years younger. The extraordinary clarity of his thinking made it possible for him to revise and develop a previously incomplete or inadequate treatment of a phenomenon into an important generalization or new method of investigation. The power to convert this same clarity of thought into words made him a lecturer capable, to a remarkable degree, of making a difficult or obscure subject clear to an audience.

Although much of Debye’s work was concerned with the interaction of radiation with matter, it did not include spectroscopy as such. Th evolution of his interests and work from mathematical physics to physical chemistry is illustrated by the subjects of his first paper, a theoretical treatment of eddy currents (1), and of one published sixty-one years later, after his death, “Direct Visual Observation of Concentration Fluctuations in a Critical Mixture” (29).

Debye’s first major contribution (2, 4, 14) was based on an explanation of the temperature dependents of the dielectric constant. The dielectric constant ∈ of a substance had, for many years, been written in the Clausius-Mosotti equation

in which n is the number of molecules per cubic centimeter and α is the molecular polarizability, the electric moment induced in a molecule by a unit electric field. Since α was supposed to be a constant characteristic of the substance and n decreases only very slowly with rising temperature as the density decreases, it would appear that ∈ should decrease very slowly with rising temperature. The equation represented exactly the behavior of the dielectric constant of most substances with small dielectric constants; but liquids with large dielectric constants showed a rapid decrease of dielectric constant with temperature, far greater than that predicted by the equation.

The polarization of the substance had been attributed wholly to the induced shift of the electrons within the molecules, giving each, molecule a very small electric moment in the direction of the electric field E. Debye proposed that the molecules of some substances had permanent electric doublets, or dipoles in them of moment μ which would contribute to the total polarzation when an external field was apoplied. The molecule would tend to rotate so as to orient its dipole in the field, but this orientation would be reduced by the thermal motion of the molecules. Using a treatment analogous to that developed by Langevin for magnetic moments, Debye showed that the average moment per molecule in the direction of a unit field would be α + μ2 /3kT. The equation for the dielectric constant was, therefore,

in which k is the molecular gas constant and T the absolute temperature. This equation not only represented the behavior of the dielectric constant satisfactorily, but also established the existence of a permanent electric dipole in many molecules and provided a means of determining the moment of the dipole and, from this, the geometry of the molecule. After many years of use in molecular structure investigations, the unit in which the dipole moment was expressed came to be called the “Debye.”

In his second outstanding paper (3), following the first dielectric paper by only a few months, Debye treated a solid as a system of vibrating atoms and modified Einstein’s theory of specific heats, which had been only partially successful. He showed that the solid could be characterized by a complete spectrum of eigen-frequencies and that the specific heat of a monatomic solid was a universal function of the ratio θ/T, where θ is a temperature characteristic of the particular solid and T is the absolute temperature. Now commonly called the Debye temperature, θ could be calculated from the elastic constants of the solid. The Debye equation, involving the then recently developed quantum theory, gave quantitative agreement with observed specific heat values. Aside from a numerical factor, it differed from the Einstein equation in containing both the compressibility and Poisson’s ration.

Debye (4, 14) showed how the orientation of molecular dipoles in a very high frequency alternating field or in a very viscous medium absorbed energy and gave rise to anomalous dielectric dispersion and dielectric loss. His equations containing the dielectric constant, dielectric loss, frequency, and relaxation time give the classical representation of dielectric behavior, which is often referred to as “Debeye behavior.” The dependence of the molecular relaxation time upon molecular size and structure, and upon inter-molecular forces, makes it of use in the investigation of these properties. The general applicability to liquids of the Debye equations for dielectric constant and loss was improved twenty-five years and more later by the revised treatment of the effect of the internal field in liquids developed by Onsager, Kirkwood, and others.

Within a year of the discovery of X-ray diffraction by crystals by von Laue and the Braggs in 1912, Debye published three papers proving that the thermal movement of the atoms in the crystal affected the X-ray interferences. Here he was examining from a different point of view the atomic lattice treated in his specific heat work. Late in 1913 he sent in for publication a long paper (5) deriving a factor now called the Debye factor, which gave the decrease of intensity of the diffraction spots as a function of wavelength, diffraction angle, and absolute temperature. The best-known paper among Debye’s many theoretical investigations of X-ray scattering was that with Scherrer (6) on the X-ray interference patterns of randomly oriented particles, which became the basis for the structure analysis of crystal powders, polycrystalline metals, and colloidal systems by the Debye-Scherrer method, possibly the most powerful tool for the determination of the structures of crystals of high symmetry.

Debye and Scherrer (7) investigated the the electron distribution inside the atom by analysis of intensities, introducing the atomic form factor, which was later to prove of importance. Although his long study of X rays had been concerned mainly with the classical wave theory of scattering of radiation by matter, Debye (8) used the quantum theory in 1923 to develop independently a quantitative theory of the Compton effect, which evidenced the dualism of the wave and particle theories of light. When he later extended the work on atomic structure and X–ray scattering (7) to molecules and liquids (15–17), he developed tools for structural investigations that, because of the dualism of the wave and particle theories, provided a foundation for the electron diffraction method, a major method of molecular structure determination.

Physical chemistry could almost be said to date from the quantitative formulation of the theory of ionic conduction by Arrhenius in 1887, but this theory of partial dissociation into ions proved inadequate. A number of investigators proposed a complete dissociation into ions, but it was not until 1923 that Debye abd Hückel (9, 10), by mathematical analysis, developed the fundamental thermodynamics of electrolytic solutions and solved the problem of electrolytic conductance. They treated the solution as having a structure somewhat analogous to that of a crystal of sodium chloride, in which each sodium ion is surrounded by six chloride ions and each chloride ion by six sodium ions, as shown ten years earlier by X-ray analysis. However, the extent of this ordering in the solution was determined by the equilibrium between the thermal motion and the interionic attractive forces, which were dependent upon the dielectric constant of the solvent and the concentration of the solute. Each ion, instead of being regard as in an actual lattice, was treated as surrounded by an ionic cloud whose thickness and relaxation time, reminiscent of that involved in dielectric loss, were important in determining the properties of the solution. The development of the theory of their behavior was a major contribution to our understanding of electrolytes and, in particular, predicted and explained (12, 13) the effect of very high field strengths on conductivity observed by Max Wien. Other publications by Debye and his co–workers during this period extended and applied the basic ideas contained in these papers. Like many of Debye’s other pioneering investigations, this work provided the theoretical basis for most of the extensive work subsequently done in the field.

A common thread runs through many of Debye’s papers, however diverse the subjects may seem. In an isolated paper (11) on magnetization, the Langevin function, used in the derivation of the equation for the dielectric constant, was shown to be not entirely correct but was employed, nevertheless, in the calculation of the approximate temperature change produced by an adiabatic magnetic process. Debye then raised “the question whether an efforts should be made to use such a process in approaching absolute zero” and concluded the paper with a sentence typical of his thinking: “Only experiments can decide, and the above analysis should stimulate the carrying out of these.” Such experiments led later to a very close approach to absolute zero.

Having dealt with long electromagnetic waves in his work on dielectric constant and loss, and with short waves in his work on X-ray scattering, Debye developed a suggestion by L. Brillouin and showed both theoretically and experimentally that sound waves in a liquid could form an optical grating, in which the wavelength of the sound waves in the liquid played the same part as the grating constant of a ruled grating (18, 19). Twelve years later Debye was considering light scattering in solution (20), building on the much earlier work of Lord Rayleigh and Einstein on gases and liquids. His previous work on X-ray scattering contributed now to his development of light scattering as a tool for the absolute determination of molecular weights of polymers and the spatial extension of macromolecules in dilute solutions, based essentially on determination of the turbidity of the solutions (20–22). The increase in light–scattering power or turbidity of a solution over that of the pure solvent was found to be proportional both to the number and to the weights of the molecules. The molecular weight was determined by combining measurements of the excess turbidity with the excess of refractive index of the solution over that of the solvent. The averaging process used in calculating the intensity of the scattered light was identical with that used in obtaining the scattering of X rays from a gas molecule (17). In the course of this development of the light–scattering method Debye became so interested in polymers that he worked on their viscosity, diffusion, and sedimentation rate (23); but he also continued the light-scattering attack upon colloidal solutions in the investigation of the size and shape of micelles (24, 25).

In further light-scattering studies (26) Debye investigated visible light scattered by a one-component homogeneous liquid in the vicinity of its critical point and by a homogeneous mixture of two liquids in the vicinity of their critical mixing temperature. He showed that the angular dissymmetry of this scattered light could be used as a measure of the range of molecular forces for ordinary molecules and as a means of measuring the size of coils in polymer molecules too small for measurement by the previously developed light-scattering method. He continued along these lines in his last work (27, 28), exploring the effect of a strong electric field upon critical opalescence by theoretical and experimental methods, the latter involving ingenuity and experimental skill of a high order.


I. Original Works. The Collected Papers of Peter J. W. Debye (New York, 1954), consisting of fifty-one papers; a brief biography by R. M. Fuoss; and comments by Fuoss, H. Mark, and C. P. Smyth, was published in honor of Debye’s seventieth birthday. The papers were selected by Debye himself, translated into English if published originally in another language, and grouped according to subject matter. Many of these papers, together with additional references, are listed below, numbered for reference.

(1) “Wirbelströme in Stäben von rechteckigem Querschnitt,” in Zeuschrift för Mathematik und physik, 54 (1907), 418–437.

(2) “Einige Resultate einer kinetischen Theorie der Isolatoren,” in Physikalische Zeitschrift, 13 (1912), 97–100.

(3) “Zur Theorie der spezifischen Wārmen,” in Annalen der Physik, 39 (1912), 789–839.

(4) “Zur Theorie der anomalen Dispersion im Gebiete der langwelligen elektrischen Strahlung,” in Berichte der Deutschen physikalischen Gesellschaft, 15 (1913), 777–793.

(5) “Interferenz von Rötgenstrahlen und Wärmebewegung,” in Annalen der Physik, 43 (1914), 49–95.

(6) “Interferenzen an regellos orientierten Teilchen im Röntgenlicht. I ,” in Physikalische Zeitschrift, 17 (1916), 277–283, written with P. Scherrer.

(7) “Atombau,” ibid., 19 (1918), 474–483, written with P. Scherrer.

(8) “Zerstreuung von Röntgenstrahlen und Quantentheorie,” ibid., 24 (1923), 161–166.

(9) “Zur Theorie der Elektrolyte. I. Gefrierpunktserniedrigung und verwandte Erscheinungen,” ibid., 185–206, written with E. Hückel.

(10) “Zur Theorie der Elektrolyte. II. Das Grenzgesetz für die elektrische Leitfähigkeit,” ibid., 305–325, written with E. Hückel.

(11) “Einige Bemerkungen Zur Mgnetisierung bei tiefer Temperatur,” in Annalen der Physik, 81 (1926), 1154–1160.

(12) “Dispersion von Leitfâhigkeit und Dielektrizitätskonstante bei starker Elektrolyte,” in Physikalische Zeitschrift, 29 (1928), 121–132, 401–426, written with H. Falkenhagen.

(13) “Dispersion der Leitfähigkeit starker Elektrolyte,” in Zeitschrift für Elektrochemic, 34 (1928), 562–565, written with H. Falkenhagen.

(14) Polar Molecules (New York, 1929).

(15) “Zerstreuung von Röntgenstrajlen an einzelnen Moleklen,” in Physikalische Zeitschrift, 30 (1929), 84–87, written with L. Bewilogua and F. Ehrhardt.

(16) “Röntgeninterferenzen und Atomgrösse,” ibid., 31 (1930), 419–428.

(17) “Röntgenzerstreuung an Flüssigkeiten und Gasen,” ibid., 348–350.

(18) “On the Scattering of Light by Supersonic Waves,” in Proceedings of the National Academy of Sciences, 18 (1932), 409–414, written with F. W. Sears.

(19) “Zerstreuung von Licht durch Schallwellen,” in Physikalische Zeitschrift, 33 (1932) 849–856.

(20) “Light Scattering in Solutions,” in journal of Applied Physics, 15 (1944), 338–342.

(21) Angular Dissymmetry of Scattering and Shape of Particles, Rubber Reserve Company, Technical Report no. 637 (9 Apr. 1945).

(22) “Molecular-Weight Determination by Light Scattering,” in Journal of Physical and Colloid Chemistry, 51 (1947), 18–32.

(23) “Intrinsic Viscosity, Diffusion, and Sedimentation Rate of Polymers in Solution,” in Journal of Chemical Physics, 16 (1948), 573–579, written with A. M. Bueche.

(24) “Light Scattering in Soap Solutions,” in Annals of the New York Academy of Sciences, 51 (1949), 575–592.

(25) “Micelle Shape from Dissymmetry Measurements,” in Journal of Physical and Colloid Chemistry, 55 (1951) 644–655, written with E. W. Anacker.

(26) “Angular Dissymmetry of the Critical Opalescence in Liguid Mixtures,” in Journal of Chemical Physics, 31 (1959), 680–687.

(27) “Electrical Field Effect on the Critical Opalscence,” ibid., 42 (1965), 3155–3162, written with K. Kleboth.

(28) “Electric Field Effect on the Critical Opalscence. II. Relaxation Times of Concentration Fluctuations,” ibid., 46 (1967), 2352–2356, written with C. C. Gravat and M. Leda.

(29) “Direct Visual Observations of Concentration Fluctuations in a Critical Mixture,” ibid., 48 (1968), 203–206, written with R. T. Jacobsen.

II. Secondary Literature. Articles on Debye are Karl Darrow, in Annual Year Book of the American Philosophical Society; and Mansel Davies, in Journal of Chemical Education, 45 (1968), 467–473.

Charles P. Smith

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Peter Joseph William Debye

The main contribution of the Dutch-born American physical chemist Peter Joseph William Debye (1884-1966) was the development of methods based on induced dipole moments and x-ray diffraction for the investigation of molecular structures.

Peter Debye was born on March 24, 1884, in Maastricht, Netherlands, the son of William and Maria Reumkens Debije. At the age of 17 Debye entered the Technical Institute of Aachen and earned his diploma in electrical engineering in 1905. He immediately obtained the position of assistant in technical mechanics at the institute. At the same time his interest in physics received strong promptings from Arthur Sommerfeld, then serving on the faculty. Debye followed Sommerfeld to the University of Munich and obtained his doctorate in physics by a mathematical analysis of the pressure of radiation on spheres of arbitrary electrical properties.

The dissertation and a 1907 paper on Foucault currents in rectangular conductors gave clear evidence of Debye's ability to produce the mathematical tools demanded by his topics. A fitting recognition of Debye's youthful excellence was his succession in 1911, at the age of 27, to Albert Einstein in the chair of theoretical physics at the University of Zurich. While in Zurich he worked out, on the basis of Max Planck's and Einstein's ideas, the first complete theory of the specific heat of solids and the equally important theory of polar molecules. Debye was professor of theoretical physics at the University of Utrecht from 1912 until 1914, when he received the prestigious post of director of the theoretical branch of the Institute of Physics at the University of Göttingen. In 1915 he became editor of the famed Physikalische Zeitschrift and served in that capacity for 25 years.

X-ray Research

In Göttingen, Debye started a most fruitful collaboration with P. Scherrer. Their first paper, "X-ray Interference Patterns of Particles Oriented at Random" (1916), gave immediate evidence of the enormous potentialities of their powder method to explore the structure of crystals with very high symmetry. It also proved very useful in work with polycrystalline metals and colloidal systems. Two years later Debye and Scherrer extended the method from the study of the coordination of atoms to the arrangement of electrons inside the atom. It was in this connection that they formulated the important concept of "atomic form factor." Debye and Scherrer formed such a close team that when, in 1920, Debye became professor of experimental physics and director of the physics laboratory at the Swiss Federal Technical Institute in Zurich, Scherrer followed him there. The two inaugurated a most influential x-ray research center which attracted students from all over the world.

In the field of x-ray research Debye's signal success in Zurich was his demonstration in early 1923 that in the collision between x-rays and electrons, energy and momentum are conserved; he also suggested that the interaction between electromagnetic radiation and electrons must therefore be considered as a collision between photons and electrons. But Debye's principal achievement in Zurich consisted in the formulation of his theories of magnetic cooling and of interionic attraction in electrolyte solutions. The latter work, in which he collaborated with E. Hückel, was closely related to Debye's pioneering research on dipole moments. Debye had already been for 2 years the director of the Physical Institute at the University of Leipzig when his classic monograph, Polar Molecules, was published in 1928.

War and Postwar Years

Debye's rather rapid moves from one university to another were motivated by his eagerness to work with the best available experimental apparatus. Thus in 1934 he readily accepted the invitation of the University of Berlin to serve both as professor at the university and as director of the Kaiser Wilhelm Institute. The latter establishment, now known as Max Planck Institute, was just completing, with the help of the Rockefeller Foundation, a new laboratory which was to represent the best of its kind on the Continent. During his stay in Berlin, Debye became the recipient of the Nobel Prize in chemistry for 1936. It was awarded to him "for his contributions to our knowledge of molecular structure through his investigations on dipole moments and on the diffraction of x-rays and electrons in gases."

Meanwhile, the Nazi government began to renege on its original promise that Debye would not be asked to renounce his Dutch citizenship while serving as director of the Kaiser Wilhelm Institute, a post with a lifetime tenure. Shortly after World War II broke out, he was informed that he could no longer enter the laboratory of the Institute unless he assumed German citizenship. As Debye refused, he was told to stay home and keep busy writing books. He succeeded in making his way to Italy and from there to Cornell University, which invited him to give the Baker Lectures in 1940.

Debye made Cornell his permanent home. He served there as head of the chemistry department for the next 10 years. His wartime service to his adopted country (he became a citizen in 1946) concerned the synthetic rubber program. In pure research he further investigated, in collaboration with his son, Peter P. Debye, the light-scattering properties of polymers, on which he based the now generally accepted absolute determination of their molecular weights. He was a member of all leading scientific societies and the recipient of all major awards in chemistry. His outgoing personality kept generating enthusiasm and goodwill throughout his long life, which came to an end on Nov. 2, 1966. Since 1913 he had been married to Mathilde Alberer, who shared his lively interest in gardening and fishing.

Further Reading

The best sources available on Debye's life and on the various aspects of his scientific work are the introductory essays in The Collected Papers of Peter J. W. Debye (1954). A detailed biographical profile of Debye is in the Royal Society, Biographical Memoirs of Fellows of the Royal Society, vol. 16 (1970). Debye is discussed in Eduard Farber, Nobel Prize Winners in Chemistry, 1901-1961 (1953; rev. ed. 1963); Aaron I. Ihde, The Development of Modern Chemistry (1964); and Chemistry: Nobel Lectures, Including Presentation Speeches and Laureates' Biographies, 1922-41, published by the Nobel Foundation (1966). □

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Debye, Peter Joseph Wilhelm (1884–1966) US chemist, b. Netherlands. He was best known for his work on molecular structure and ionization. He pioneered X-ray crystallography and received the 1936 Nobel Prize in chemistry.