By Kenneth S. Pitzer, D. H. McLaughlin, G. C. Pimentel, and J. M. Prausnitz
Joel Henry Hildebrand (b. November 16, 1881, d. April 30, 1983) was born in Camden, New Jersey. His ancestors came to America before the revolution from the upper Rhine valley. When asked about his longevity, Joel replied, “I chose my ancestors carefully” and frequently added that most, if not all, lived well past 80. His father, Howard Onid Hildebrand, was in the insurance business near Philadelphia and Joel attended local schools. His intellectual interests were particularly stimulated by a grandfather who, although of limited schooling, had read widely and accumulated an excellent library. With his interest in natural phenomena aroused, Joel acquired personally and studied Dana's Geology, Newcomb's Astronomy, and similar books. After his high school mathematics was completed with solid geometry and trigonometry, he discovered independently the power and beauty of calculus. After he had learned as much chemistry as his teacher (the principal) knew, he was given the key to the laboratory, a college laboratory manual, and encouragement to learn more on his own. Joel told with justified pride about his experiment proving that nitric oxide gas was NO rather than N2O2--a result which demolished a theory in a book by a Harvard professor which he had been given. It is clear that this high school principal was a great source of encouragement also for opening to Joel broader horizons of interest in various cultural areas including music.
Hildebrand entered the University of Pennsylvania in 1899 and wisely chose a double major in chemistry and physics in the College of Arts and Science rather than a more “professional” course in chemistry which emphasized recipes for analysis and similar details. He thereby had the opportunity to learn, not only more physics, but also history, literature, and mathematics while avoiding details of chemistry which were unimportant and sometimes even untrue.
After receiving his Ph.D. in chemistry from the University of Pennsylvania in 1906, Hildebrand was encouraged to spend a postdoctoral year in Germany learning the new science of physical chemistry. He went to Berlin where he attended lectures by J. H. van't Hoff and by Walter Nernst. He also did some research under Nernst and then returned to the University of Pennsylvania to serve on its faculty teaching physical chemistry until 1913. In that year, he was invited to join the remarkable group of young physical chemists which Gilbert N. Lewis had selected and which led in transforming the Chemistry Department of the University of California into a center of international eminence.
Hildebrand's doctoral thesis of 1906 was entitled, The Determination of Anions in the Electrolytic Way, and he continued with several papers on electrochemical methods in analysis. Herbert S. Harned was his first research student and Harned's thesis was in this area. But Hildebrand soon shifted his primary interests to physical rather than analytical topics (as did Harned, who proceeded to a very distinguished career at Yale).
The color of iodine solutions fascinated Hildebrand throughout his career; his first paper on that topic, “Über die Farbe von Jodlösungen,” was published in 1910. He soon noted (1920) that the deviations from Raoult's law of various violet solutions of I2 formed a regular pattern. However, the curve for I2 in benzene differed from this pattern and that solution had a somewhat different color. This color difference suggested a more intimate interaction of the iodine with benzene.
These ideas were extended in many directions through the years. The concept of a regular pattern of positive deviations from Raoult's law grew into a general theory of “regular solutions.” Such systems involve no specific solvation or association and the mixing of their molecules is essentially random. Equations for the activities of the components of such solutions had already been developed by several scientists but these suffered either from the absence of relationships to the properties of the pure components or, in van Laar's case, to making these connections through an approximate equation of state. While the van der Waals equation was a great advance at the time and gives a reasonable representation of gas imperfection, the quantitative deviations in the liquid region are large, and it is the liquid region that is pertinent to liquid solutions.
George Scatchard published a paper in 1931 which, in his words, “may be regarded as a quantitative development of the treatment of Hildebrand, although it disagrees with his ideas in some important details, or as a method of freeing the van Laar treatment from the inadequacies of the van der Waals equation.” Hildebrand and Wood derived the same equation two years later by a very different and modern method--by integrating the intermolecular pair potentials throughout the liquid weighted by the radial distribution function.
Both Scatchard's and Hildebrand's results yield the same working equation relating the deviation from ideal solutions (Raoult's law) to the cohesive energy density of the pure components, i.e., ΔE/V where ΔE is the energy of vaporization of a volume V of the pure liquid. More precisely, it is the square of the difference in the square roots, [(ΔE1/V1)1/2 - (ΔE2/V2)1/2]², which determines the departure from ideality.
In recent years, this quantity (ΔE/V)1/2 has been called the solubility parameter and given the symbol . The Scatchard-Hildebrand equation is quite successful--better than any other equation of comparable simplicity and generality. But it is not surprising that there are departures from perfect agreement and Hildebrand has presented from time to time tables of adjusted solubility parameters which yield improved agreement. These were always discussed in relation to aspects of the intermolecular forces which might explain the need for adjustment.
Hildebrand, always the effective teacher, summarized the current status of knowledge about nonelectrolyte solutions in monographs designed to interest and to instruct chemists. Initially, these were general reports on the status of knowledge in the field and carried the title, The Solubility of Nonelectrolytes. The successive editions of 1924, 1936, and 1950 (the last with R. L. Scott) grew in size along with the rapid advance of knowledge in this area. Opposite the title page of the third edition is a picture of a tube containing seven incompletely miscible liquids (heptane, aniline, water, perfluorokerosene, phosphorus, gallium, and mercury)--a beautiful example of Joel's flair for generating interest in and enjoyment of his topic for discussion. After 1950, Joel left to others the task of general review of knowledge concerning non-electrolyte solutions and prepared smaller books concentrating on the areas of his particular interest. These were Regular Solutions in 1962 with R. L. Scott and Regular and Related Solutions: the Solubility of Gases, Liquids, and Solids in 1970 with J. M. Prausnitz and R. L. Scott.
The relationship of the color of iodine solutions to their other characteristics has been noted. Hildebrand's most important discovery in this area came in a series of papers with Benesi in 1948-50 which related an intense ultraviolet absorption to the formation of electron-donor-acceptor complexes. This type of complex, now more commonly called a charge-transfer complex, has been investigated extensively by others, is well-understood theoretically, and is an integral part of our body of organized knowledge.
The “rule” which carries the Hildebrand name concerns the entropy of vaporization of a normal liquid. In 1915 he showed that, for a typical group of “normal” liquids boiling near or below room temperature, the entropy of vaporization was more nearly constant if compared at a constant vapor volume rather than on the constant pressure basis of Trouton's rule. With this considerably higher precision of agreement, the Hildebrand rule became a much more useful criterion of a normal liquid. In comparison hydrogen-bonded or other highly polar liquids have larger entropies of vaporization than do “normal” liquids. Another idea of Hildebrand's which has great practical as well as theoretical importance concerns the use of helium in deep diving. A diver at depth experiences high pressure and correspondingly increased solubility of breathing gases in his blood. The problem of the “bends”--the release of this gas as a bubble in a blood passage when the diver emerges--was well-known. In the mid-twenties Hildebrand suggested that this problem could be ameliorated by substituting helium for nitrogen in mixture with oxygen for the diver's breathing gas. Not only is the solubility of helium much less than that of nitrogen at a given pressure but also the diffusion rate is faster. These basic ideas have had a major role in improvement of diving capability and safety ever since.
Through the years Joel took pleasure in demolishing concepts which he regarded as spurious or misleading. He was not fooled by “polywater.” He was severely critical of theories of liquids which were based on complex assumptions about structural features for which there was no direct verification. With the deeper insight of molecular dynamics calculations these complex assumptions have now been disproven in many cases. But Joel had refused to accept these theories even if they were reasonably successful in representing the experimental data available at the time. Several of these situations are described in his 1977 paper, “Operations on Swollen Theories with Occam's Razor.”
Another case of this type is the “hydrophobic effect,” or worse, the “hydrophobic bond.” Joel objected to these terms because “phobic” implies repulsion. It is true that in aqueous solution a solute containing both alkyl (or other nonpolar) groups as well as polar groups will arrange itself in a manner to favor water contact with the polar groups of the solute and alkyl group contact with other alkyl groups. But this does not mean that an alkyl group is actually repelled by a water molecule. Rather, as Hildebrand concluded, “there is no hydrophobia between water and alkanes; there is only not enough hydrophilia to pry apart the hydrogen bonds of water so that the alkanes can go into solution without assistance from attached polar groups.”
In the last decade Joel gave considerable attention to the viscosity of liquids, or, as he preferred, the fluidity which is the reciprocal of viscosity. These papers were collected in a small monograph, Viscosity and Diffusivity: A Predictive Treatment, published in 1977 with an introduction by J. O. Hirschfelder. In the introduction Hirschfelder wrote of Hildebrand, “somehow he has the ability to sweep away all of the complexities and discover simple relationships which will take theoreticians another generation to derive.” Indeed Joel did present new empirical relationships which are simpler and more accurate than those in common use. And he presented them in a simple qualitative theoretical framework which was free from inconsistencies or the complexities often contrived to circumvent inconsistencies. This book often elicited the comment that “Hildebrand is a genius finding ways to present data so that they fall on a straight line.” But Joel's were not merely functions yielding straight lines; he also required conformity to general ideas of molecular structure and behavior. Indeed, he was a genius in research of this type.
Hildebrand's impact as a teacher was just as important and in many respects more remarkable than this role in research. His freshman chemistry lectures, given regularly from 1913 until his “retirement” in 1952 were legendary. Thousands of alumni recall his vivid descriptions and dramatic demonstrations as well as enlivening digressions into music, art, and mountaineering.
A single course was offered at Berkeley with total enrollment usually somewhat over 1000, with lectures in a room seating about 500, but with laboratory, quiz, and discussion in groups of 25. William Bray and Wendel Latimer took primary responsibility for the laboratory and wrote the book for it. Most of the regular faculty took freshman sections (in addition to other teaching) and thereby initiated the graduate students into their teaching assistant duties in an apprenticeship pattern. Thus there was extensive involvement of most of the faculty with the general chemistry course and general agreement concerning its character. But Hildebrand gave the lectures, wrote the quizzes and examinations, and was in general charge of the course. He wrote the central text, Principles of Chemistry, which was revised several times.
The course at Berkeley, as developed by Hildebrand, Bray, Latimer, and others, departed from the pattern of that time by much greater emphasis on principles with reduced attention to memory of specific factual material. It was only after about 25 years that other textbooks began to appear which reflected similar emphasis. Of course, the “Berkeley” books were used elsewhere in the intervening years.
As is often the case, the pattern has recently shifted farther (probably too far) toward dominance of theory and general principles and the near exclusion of “factual” material. The “Hildebrand” course maintained a balance; the student learned that, while important aspects of chemistry could be related to general principles through relatively simple equations, other experimental facts were best remembered, if important enough, or looked-up when needed. To promote the habit of quick and convenient reference to this body of knowledge, Latimer and Hildebrand prepared their Reference Book of Inorganic Chemistry in 1928. It was revised several times and was available combined with Principles of Chemistry in a single volume.
Joel was superb as a lecturer and he thoroughly enjoyed lecturing. There were many lecture experiments with an entertaining aspect and lots of humorous comments which the students enjoyed. But he never lost sight of the primary purpose of the lectures, and most of these entertaining features were tied into the primary lesson of the day. His enthusiasm, combined with thorough knowledge and excellent lecture technique, were almost irresistible. There was never a problem of slack attendance at Hildebrand lectures.
From 1913 through 1952 Hildebrand had about 40,000 students in his freshman lectures. While only a moderate proportion followed chemistry professionally, many became engineers, physicists, or other scientists. Others became lawyers, business executives, leaders in various fields, and they have a clearer picture of the role of science in the modern world because of their contact with Joel Hildebrand. His impact as a teacher was great indeed.
This fame as teacher of chemistry gave him the credentials and brought invitations to influence educational matters more broadly. His former students, now in a multitude of positions of responsibility and influence, urged his inclusion in committees, boards, or conferences. A notable example was the Citizens Advisory Committee to the Joint Education Committee of the California Legislature.
Joel had all of the qualifications of a good administrator or organizational leader. He never shirked such responsibilities when they were pressed upon him, but he never let such duties draw him permanently away from his primary interests in teaching and research. His preferences in this respect fitted very comfortably with the policies of the University of California wherein academic administration was in the hands of distinguished professors, but there was no implication that a given individual would continue indefinitely as a department chairman or a dean. Indeed the status of ex-dean was most highly regarded at the Berkeley Faculty Club.
Thus, Joel accepted appointments and served a few years in each case as Dean of Men (1923-26), Dean of the College of Letters and Science (1939-43), Chairman of the Department of Chemistry (1941-43), and Dean of the College of Chemistry (1949-51). He also played a major role in the Academic Senate and served as chairman of important committees of the Senate.
In making an administrative decision, Joel collected and digested the pertinent information, consulted other individuals as appropriate, and then reached his conclusion promptly without emotional trauma. When he left his administrative office, he left those problems behind and was ready to discuss a problem in chemistry or to give a freshman lecture with full vigor and enthusiasm.
Other organizations frequently called on Joel Hildebrand to take positions of leadership and he accepted when he believed he could make a significant contribution without undue interference with his work in chemistry. Thus he became interested in the Sierra Club and was soon asked to be President (1937-40). He held various positions in the American Chemical Society but declined nomination as President until after his retirement from regular teaching; he was then elected, and served in 1955. He managed the U.S. Olympic Ski Team in 1936.
In both World Wars, Hildebrand was asked to undertake special duties: in 1918-19 he directed the chemical warfare laboratory of the American forces in France, with the rank of Major and later Lt. Colonel and was awarded the Distinguished Service Medal. In 1943-44, he was a liaison officer in London for the Office of Scientific Research and Development. The British Government also took advantage of his presence in London to obtain his personal advice on many problems and awarded Joel their King's medal for Service in the Cause of Freedom in 1948.
In 1908, Joel married Emily Alexander whose continued good health and vigor is almost as exceptional as Joel's was; Emily reached age 97 in 1983. Their 70th wedding anniversary in 1978 was a great occasion for all of their many friends. They have four children: Louise, Alexander, Milton, and Roger. Two are professors in the sciences: Milton in zoology at the University of California at Davis, and Roger in physics at the University of Chicago.
In 1953, when Joel Hildebrand received the Willard Gibbs Medal, his son Roger was invited to help introduce him. The result was a most amusing and interesting insight into the Hildebrand family. Joel was always the enthusiastic teacher. As Roger told it, “we were encouraged and instructed in any worthwhile pursuit. The most confirmed blockhead could hardly have withstood the assault of intellectual enthusiasm which we enjoyed. Any flair for science, athletics, music, arts, or crafts on our part was noticed and the spark was fanned by a powerful hand. As a result, enough bonfires lit the sky to reduce any mother but mine to a cinder.”
Another paragraph from Roger's introduction: “We learned a lot by watching him. He worked and played hard. He is justly proud of his physical condition. He once entered a grandfathers' swimming race. Now it takes him a quarter mile or so to get really warmed up, so he dove in and swam a few laps each of breast stroke, back stroke, and crawl. His competitors, who were watching from the bank, gradually disappeared and it is said that by starting time not a one of them could be found.”
An interesting Hildebrand story arose when Editor of Who's Who in America decided in 1975 to transfer Joel to the compilation Who Was Who. This elicited a spirited response, of course, in which Joel listed five research publications, all from 1974, as well as others in press for 1975, together with copies of several comments about his current activities from others including President Handler of the National Academy of Sciences. He concluded by saying: “Leave me out of “Who's Who if you must--Europa still lists me, but please postpone till a more appropriate time including me in Who Was Who. People would be writing to learn what happened to me.”
Honors of various types came to Joel Hildebrand through the years. From the American Chemical Society came the award of the Nichols Medal in 1939, its teaching award in 1952, the Willard Gibbs Medal in 1953, and its highest recognition, the Priestley Medal in 1962. Joel was elected to the National Academy of Sciences in 1929 and to the American Philosophical Society in 1951. He received an honorary doctorate after retirement from the University of California in 1954. When the citation was read, the audience immediately applauded Joel so enthusiastically that President Sproul at first forgot to confer the degree. After that omission was remedied, Joel received a second ovation. The warmth and enthusiasm of that occasion symbolizes beautifully the high regard in which Joel was held by students, alumni, professional colleagues, and all others who came to know him.