Biographical Memoirs Volume 46 (1975) / Chapter Skim
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2. Peter Joseph Wilhelm Debye
2. Peter Joseph Wilhelm Debye
Pages 22-69
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W WILLIAMS VITAE PETER JOSEPH WILHELM DEBYE was born on March 24, 1884, at Maastricht, the Netherlands.
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THE SCIENTIST In the Nobel Prize citation to Debye ( 1936) one reads, "for his contributions to our knowledge of molecular structure
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The main subject areas are four in number: "X-Ray Scattering," "Dipole Moments," "Electrolytes," and "Light Scattering." A fifth unit is made up of "Miscellaneous Contributions." In the development and description of the researches, the reports are invariably replete with that same skill for which the author came to be known as a speaker and lecturer, namely, consummate proficiency in the description of a difficult and intricate subject in a lucid and well-organized fashion. A study of these and the other Debye contributions is indeed a rewarding experience.
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Incidentally, this factor is essential to an understanding of the Mossbauer effect. A consideration of the atomic scattering factor, Debye's second of the two refinements discussed in x-ray analysis, is of vital importance in structure determination.
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The. atomic scattering factor, the quantity calculated, is defined as the ratio of the actual amplitude to that which would be produced by a single Thomson electron under the same experimental conditions.
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For liquids on exposure to x rays a small number of broad and diffuse halos are produced in scattering. Two factors determine the outline of these halos; Debye early called them "inner" and "outer" interferences, with those of the first kind being between waves scattered by atoms belonging to the same molecule, while those of the second kind derive from intermolecular interferences.
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Each such distance produces an intensity that increases and decreases as the angle of scattering is increased; the importance of the several interatomic distances is measured by the product of the scattering factors. The results of the first experiments with gases, those from Debye's laboratory, were reported from Leipzig (1929~.
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For reasons that need not be here described, electron diffraction became at once the preferred experiment. Though it is true that the actual number of molecules to which these methods may be applied remains small, still with modern computational devices and vastly improved equipment, electron diffraction has become a method of great utility and high precision for the evaluation of molecular structure.
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The volume in which this article appears provides a good idea of the enormous amount of work that has been done in structural studies of various types of amorphous materials, an area in which Debye was the pioneer. DIPOLE MOMENTS It is as a consequence of their asymmetrical (electrical)
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An earlier Methuen pocket-size monograph, Dipole Moments, by R
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At the time of its inception the dipole moment was the principal source of information about molecular structure. Now such data for small molecules of the rigid type have become of lesser significance because of the incidence of the x-ray and electron diffraction techniques (of Debye)
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It will be indicated later that one may gain information about the average size of coiled polymer molecules in solution from light scattering measurements. However, the average coil diameter found by this experiment is usually much larger than would be calculated for a "random-coil" molecule of the same molecular weight and with unhindered rotaion about the bonds that link the monomeric structural units.
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The range of frequency over which the real dielectric constant is variable extends from the static field to one that oscillates so rapidly as not to provide for any rotational motion of the polar molecule at all; the theory thus describes a typical molecular relaxation process. The accompanying constant, called the time of relaxation, is made available from measurements of the frequency variation of either the real dielectric constant or the energy absorption for the system; in solutions this time constant may be related to molecular size and shape.
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The answer was found in the assumption that the strong electrolytes are completely dissociated when dissolved in water. This representation had been considered by others, notably by Bjerrum and Sutherland and, using it, Milner ~ had actually computed the osmotic coefficient (a quantity that is simply related to the activity coefficient)
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The solvent was a medium of uniform dielectric constant, a quantity unchanged by the addition of the solute Ions. The ions in solution might be expected to be in random thermal motion.
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It is the equivalent of this second derivation of the limiting thermodynamic law that is almost universally reproduced in the modern physical chemistry texts. The title of this article is, in translation, "Osmotic Equation of State and Activity of Dilute Strong Electrolytes." In the introduction to this report one finds the German equivalent of the sentences, "Besides, I , .
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While this explanation remains correct to a good approximation for weak electrolytes, it could not account for the square root of concentration decrease in equivalent conductance with increasing concentration that had already been found experimentally by Kohlrausch and others for the strong electrolytes. Here, per equivalent of electrolyte, the number of carriers of electricity remains substantially constant (in dilute solution)
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at length that his simple limiting law for the activity coefficient of a strong electrolyte can be directly applied in the explanation of the change in solubility of a difficultly soluble salt caused by the addition to the solution of a salt without a common ion. The quantity log s/sO versus square root of the ionic strength is linear, and log s/sO is a direct measure of -log By+, for the difficultly soluble salt.
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Casassa and Eisenber~ t have discussed this problem as it relates to osmotic pressure, light scattering, and sedimentation equilibrium in such systems. LIGHT SCATTERING The final period of Debye~s life began in 1940 when he arrived at Cornell University.
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Of the two kinds of mathematical analysis for light scattering from solutions, the vibrating dipole theory of Hertz ~ and the density fluctuation theory of van Smoluchowski ~ and of Einstein,1: the latter is the more generally applicable. A basic quantity is the turbidity, T
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Debye's classical and original work on the atomic scattering factor in x-ray analysis pointed the way for him to relate the angular dissymmetry of the light scattering now involved to particle shape. The particle-scattering factor contains a size parameter, angle of scattering, and wavelength dependence.
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The presumption is that they were conceived and published independently.) In the final period of his scientific life, Debye became greatly interested ire the phenomenon of critical opalescence and lectured widely and enthusiastically on the subject.
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In the attempt to account for this value theoreticians had believed it to result from an equipartition of energy, but as more accurate data for the temperature dependence of the heat capacity were made available it became evident that this could not be the complete explanation, especially at the lower temperatures. The experimental fact is that the heavy and soft elements possess this value for the heat capacity per gram-atom at room temperature, but for the light and hard elements much higher temperatures are required for its attainment.
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In addition to the definitive article on dipole theory in the Marx Handbuch der Radiologie (1925; 1934) , his "Polar Molecules" of 1929 has served as a great stimulus to chemists.
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The overall record, of which but a small part has been herein depicted, must demonstrate that in his lifetime Professor Debye made many brilliant contributions of great value to physics, to chemistry, and to certain of their borderline disciplines. In these writings he has left a precious legacy for physical scientists.
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As a result of my own relationships with him I must note that Professor Debye did indeed have true kindness of heart, along with his rare vigor of intellect. THE AUTHOR is indebted to colleagues both here at the University of Wisconsin and at the California Institute of Technology for their advice and help.
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New York Academy of Sciences, New York American Academy of Arts and Sciences, Boston American Philosophical Society, Philadelphia Franklin Institute, Philadelphia Royal Dutch Academy, Amsterdam, Holland Royal Society, London, England Roval Institution of Great Britain, London, England Royal Danish Academy, Copenhagen, Denmark Academies of Berlin, Gottingen, Munich, Germany Academies of Brussels and Liege, Belgium Royal Irish Academy, Dublin, Ireland Papal Academy, Rome, Italy Indian Academy, Bungalore, India National Institute of Science, India Real Sociedad Espanola de Fisica y Quimica, Madrid, Spain MEDALS Rumford Medal (Royal Society, London) , 1930 Lorentz Medal (Royal Dutch Academy, Amsterdam)
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50 BIOGRAPHICAL MEMOIRS Zurich, E
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math.-naturwiss. K1.—Berichte uber die Verhandlungen der Saechsischen Akademie zu Leipzig, mathematischnaturwissenschaftlich Klasse Bull.
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1912 Zur Theorie der spezifischen Warmen.
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In: Vortrage uber die kinetische Theorie der Materie und der Elektrizitat.
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Uber die Konstitution von Graphit und amorpher Kohle. Nachr.
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Naturforsch. Journal of Mathematics and Theorie der elektrischen und magnetischen Molekulareigenschaften.
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Chem., 130: 56. Uber die Zerstreuung von Rontgenstrahlen an amorphen Korpern.
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Bestimmung der inneren Struktur von Flussigkeiten mit Rontgenstrahlen.
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Sack. Theorie der elektrischen Molekuleigenschaften.
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Belg., 21:166. Dielektrische Sattigung und Behinderung der freien Rotation in Flussigkeiten.
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Z., 38: 161. \lethoden zur Bestimmung der elektrischen und geometrischen Struktur von Molekulen.
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The determination of polymeric molecular weights by light scattering in solvent-precipitant systems.
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Light scattering investigations of carefully filtered sodium silicate solution.
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A study of crystallite sizes in polymers by a light scattering method.
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Phys., 30:843. Angular dissymmetry of the critical opalescence in liquid mixtures.
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A light scattering study of the aggregation of acidified sodium silicate solutions.
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A, 1:2387. Light scattering and molecular forces in electromagnetic scattering.
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An electrical field effect on the critical opalescence.
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Measurement of relaxation times of concentration fluctuations by the electric field effect on critical opalescence.
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