Biography of Peter Josephus Wilhelmus Debye

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Peter Debye was Dutch-American physicist greatly contributed to the theory of electrolyte solutions. He also studied dipole moments of molecules, advanced knowledge of the arrangement of atoms in molecules and of the distances between the atoms.

In 1916 he showed that solid substances could be used in powdered form for X-ray study of their crystal structures, thus eliminating the difficult step of first preparing good crystals. Debye won Nobel Prize in Chemistry, 1936, “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”.

Peter Debye (born Petrus Josephus Wilhelmus Debije) was born in Maastricht, Holland, on March 24, 1884, son of Wilhelmus and Maria Reumkens Dibje.

Picture at left shows Smedenstraat street in Maastricht where Peter Debye spent his childhood.

He completed the Hoogere Burger School at Maastricht in 1901 and for the next 4 years studied electrical engineering at the Konigliche Technische Hochschule in Aachen (only 20 miles from Maastricht), graduating in 1905. His first degree was in electrical engineering.

Figure from Peter Debye’s diplom work, Aachen 1905.

While still an undergraduate at Aachen, Debye became an assistant in mechanics to Arnold Sommerfeld and completed a study of the diffraction of light by cylindrical and spherical objects. He became so enraptured with physics and chemistry that he was permitted to use the school’s laboratory after hours for experimentation.

It was at Aachen that Debye’s first original work, a theoretical analysis of Foucault currents in a rectangular conductor, was published in Zeitschrift fur Mathematik und Physik in 1907. When Sommerfeld moved to Ludwig-Maximilian University in Munich in 1906 as professor of theoretical physics, he took Debye along as his assistant. Receiving his Ph.D. in physics in 1908 for his thesis on the effects of radiation presure on spheres of arbitrary electrical properties, Debye continued on at the University as a lecturer (privatdozent).

In 1911 Debye, then a young lecturer in physical chemistry at the University of Munich, deputized for Albert Einstein professor of theoretical physics at the Federal Institute of Technology in Zurich, for one year when Albert Einstein took an appointment as a professor at Prague. It was here that he developed his theories on polar molecules and the specific heat of solids. In 1912 Debye became professor of theoretical physics of Utrecht.

Peter Debye married Mathilde Alberer, April 10, 1913, and two children were born – Peter Paul Ruprecht (b. 1916), later a physicist collaborated with Debye in some of his researches, and Mathilde Marie Gabriele (b.1921), later Mrs. Gerhard Saxinger.

The right photo shows Peter Debye with Kathrin and Dorothee Simon (probably kids of his friends).

After 2 years at Utrecht, Debye accepted the professorship of theoretical and experimental physics at University of Gottingen, where he remained until 1920. The research facilities at Gottingen enabled him to test his theory of permanent dipoles, and in 1916 with the Swiss researcher Paul Scherrer he published the powder method of X-ray diffraction, now known as the Debye-Scherrer method of identifying crystalline substances by photographing the diffraction pattern of a beam of X-rays directed onto the powdered crystalline material. This method is still used widely today in laboratories.

These two pictures were taken using X-ray directed through two different crystals. The atomic structure of the crystals can be determined by analysing the different diffraction of the X-ray.

Debye returned to Zurich in 1920, as Professor of Physics, and Principal of the Eidgenossische Technische Hochschule. Debye developed here a concept of magnetic cooling and an interionic attraction theory of electrolytes. Together with Erich Huckel, one of his assistants, he published two fundamental treatises in 1923 concerning electrolytic solutions, ions in solutions that carry electric current.

He suggested that the deviation of solutions of electrolytes from the laws of ideal solutions is due to interionic attractions. The ideas put forward in these publications enabled chemists to make enormous progress in the field of electrochemistry.

The most important his paper on this subject is this one: P. Debye and E. Huckel, Zur Theorie der Elektrolyte. I. Gefrierpunktserniedrigung und verwandte Erscheinungen (On the Theory of Electrolytes. I. Freezing Point Depression and Related Phenomena), Physikalische Zeitschrift, Vol. 24, No. 9, 1923, pp. 185-206. Translation of this paper.

The Debye-Huckel theory starts from the assumption that the electrical potential fluctuations due to the ions in an electrolyte are small. At each point, a potential shift f will give rise to a charge density opposing the fluctuation:

r = (e2f/kT)Scizi2

where the c’s are the concentrations in ions.m-3. The charge density affects the potential via Poisson’s equation, expressed here in the spherically symmetric case:

N2f = r-2(d/dr)(r2(df/dr)) = -r/e

which has the solution for an ion of charge ze:

f = (ze/4per)(e-r/l )

i.e. the solution without ions multiplied by the exponential term e-r/l. l is called the Debye length. Much closer to the ion than the Debye length, the potential is essentially what it would be in pure water. Much further away than l, the potential is screened. The Debye length is also symbolised rD, and is given by:

l = o(ekT /e2Scizi2)

In water it is approximately 0.32 I-1/2 nm. The stabilisation of each ion by its accompanying cloud of opposite charge is given by:

U = (z2e2/8pel)

so that the log activity coefficient for the ions of charge z is given by:

ln gz = (U/kT) = (z2e2/8pelkT)

Note that the activity coefficient of individual ion species cannot be measured separately, because all the ions always change energy together. The Bronsted-Bjerrum formula is derived from the difference in stabilisation between the ground-state reactants and the activated complex, and the charge of the activated complex is always the sum of the individual charges. Since

(zA + zB)2 – zA2 – zB2 = 2zA zB

the change of reaction rate constant depends on the product of the two charges. This is usually specified in terms of half the change:

ln g+ = (zA zB e2/8pelkT) = A zA zB I1/2

Summary. The ions in an electrolyte have a screening effect on the electric field from individual ions. The screening length is called the Debye length and varies as the inverse square root of the ionic strength. The resulting relative stabilisation of charge concentrations fully explains the Bronsted-Bjerrum formula for reaction kinetics.

Debye-Huckel in Electrochemistry

The Debye-Huckel theory can be tested most accurately by changes in electrode potentials in electrochemistry. When current passes through a cell, ions are discharged at the anode and cathode with conservation of charge, so the overall change of energy brought about by screening corresponds to the the Bronsted-Bjerrum kinetic case with zero charge on the activated complex. These precise measurements show that the Debye-Huckel theory is only a first approximation, and that deviations from it cannot be neglected even at quite low concentrations. An extension of the Debye-Huckel law is given by:

ln g+ = -(A |z+z- | I1/2 )/(1 + BI1/2 )

The coefficient B can be explained in terms of specific short-range interactions between the ions and water or each other but is best regarded as an empirical parameter.

Becoming director of the Physical Institute of University of Leipzig in 1927, Debye experimented with the measurement of interatomic distances in molecules by X-ray scattering and continued his work on dipoles and electrolytes, publishing a number of books, some of which were translated into English, including “Quantum Theory and Chemistry” (1928), “Polar Molecules” (1929), “The Dipole Moment and Chemical Structure” (1931), “The Structure of Molecules” (1932), “Magnetism” (1933), “The Structure of Matter” (1934), and “Nuclear Physics” (1935). In 1915 Professor Debye became Editor of Physikalische Zeitschrift and continued to act in this capacity until 1940.

Whether as classroom teacher or as special lecturer he was renowned for his facility of expression. This apparent ease of expression must have required concerted effort at organization. Nowhere were his abilities to explain scientific principles better demonstrated than in his lectures for large introductory physics courses presented during the Zurich and Leipzig periods. The concomitant lecture table displays were correspondingly pertinent and skillful; here again it was obvious that much thought and time gone into their preparation.

In 1935 Debye became professor of physics at University of Berlin and director of the Kaiser Wilhelm (now Max Planck) Institute for Physics in Berlin-Dahlem, with its excellent research facilities.

Debye won Nobel Prize in Chemistry, 1936, “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”. His Nobel Lecture, December 12, 1936, “Methods to determine the electrical and geometrical structure of molecules” is available in the Internet.

In 1938 the Nazi government began to insist that Debye give up his Dutch citizenship and become a German citizen to continue as director of the Institute of Physics. He refused and left Germany for Italy, shortly afterwards finally taking up residence in the United States. He came to the U.S.A. two months before the German invasion of his native country (1940), and went to Ithaca, N.Y., U.S.A., where he had been invited to deliver the Baker Lectures at Cornell University.

From 1940 to 1952 Debye served as head of the Chemistry Department at Cornell, which soon became a leader in solid state research, largely due to his influence. During World War II he became a consultant in the synthetic rubber program. He became an American citizen in 1946. Unlike the European phase of his life, where Debye moved from city to city every few years, in the United States he remained at Cornell for the whole remainder of his career. Retiring from Cornell in 1952 as professor emeritus, he continued his research in the field of high polymers, showing that degree of light scattering is an accurate indication of molecular size.

In his years in the United States Debye became an inveterate traveler. He gave lectures and seminars outside of Ithaca almost weekly. At meetings his appearances invariably meant large audiences, for from his discussions at them the new and unexpected was the rule. He possessed the ability to explain scientific ideas and principles to a wide variety of audiences, and wherever he went he was received as a desirable and agreeable lecturer.

Known as the “Master of the Molecule” because of his pioneering work in molecular structure, Debye was a dominant figure in physical chemistry and chemical physics during the first half of the 20th century. He came to chemistry late, via training in electrical engineering, mathematics and physics.

The number of scientific concepts named after him attests to the originality of his work. These include the Debye theory of specific heat, the Debye-Huckel theory, the Debye-Scherrer method of X-ray diffraction, the Debye-Sears effect in transparent liquids, the Debye theory of wave mechanics, the Debye temperature, the Debye shielding distance, and the Debye frequency. The American Chemical Society also has an award named in his honor.

The unit of measurement for dipole moment – debye (D) – bears Debye’s name in recognition of this achievement

Debye (D) is a CGS unit of electric dipole moment used in chemistry and physics. A charged molecule can be regarded as a tiny bar with opposite charges at the ends. Such a bar is called a dipole, and its dipole moment is the amount of the charge multiplied by half the length of the bar. Thus the appropriate unit of dipole moment in the SI would be the coulomb meter (C m). Since this is much too large for molecules, the debye is defined as an electric dipole moment of 10-18 statcoulomb centimeter or 3.33564×10-30 coulomb meter.

Professor Debye was wedded to Physics and Chemistry and his devotion to his work gained him many distinctions, and Honorary Doctorates have been conferred upon him by the following universities and learned institutes: Brussels and Liege; Oxford; Sofia; Mainz; Technische Hochschule, Aachen; Eidgenosissche Technische Hochschule, Switzerland; and in the United States: Harvard; St. Lawrence; Colgate; Notre Dame; Holy Cross; Brooklyn Polytechnic; Boston College; Providence College.

He holds the Rumford Medal of the Royal Society, London, and the Franklin and Faraday Medals, the Lorentz Medal of the Royal Netherlands Academy, the Max Planck Medal (1950) awarded by the West Germany Physical Society, the Willard Gibbs Medal “Chicago (1949), the Nichols Medal (1961), the Kendall Award (Miami, 1957), and the Priestley Medal of the American Chemical Society (1963); and was appointed Kommandeur des Ordens Leopold II in 1956.

Debye’s most treasured award is his bust, a gift of the natives of his birthplace, Maastricht, was there unveiled in his honor to adorn the town hall. It has been noted by others that this distinction probably pleased Debye above all others.

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