RIDEAL, ERIC KEIGHTLEY

(b. Sydenham, England, 11 April 1890; d. London, England, 25 September 1974)

physical chemistry.

Rideal was the first of four children born to a London consulting chemist, Samuel Rideal, and his Irish wife, Elizabeth Keightley. As a financially successful and leading public analyst, author of many textbooks on water and sewage purification, and a deviser of the Rideal-Walker quantitative test for disinfectant activity (1903), Samuel Rideal could afford to give his children an excellent education. After attending Farnham Grammar School in Surrey, Rideal boarded at Oundle School in Northamptonshire, a boys’ secondary school that, under its brilliant headmaster, F. W. Sanderson, was unusual among English public schools for its obligatory engineering and workshop practice as part of the curriculum.

In 1907 Rideal entered Trinity Hall, Cambridge, on an open scholarship in natural science. There, although he studied formally in Sir William Pope’s chemistry department, with its very strong emphasis upon inorganic and organic chemistry, it was the physics classes in the Cavendish Laboratory that excited his interest in the physical chemistry then being systematized by Wilhelm Ostwald and Walther Nernst in Germany and by James Walker in Great Britain. An even greater formative influence on Rideal at Cambridge was the stimulating lectures of the physiologist William Bate Hardy, whose interest in osmosis was to prompt Rideal’s lifel ong passion for surface chemistry, which he always viewed as the boundary discipline between mechanical and biological systems.

Rideal graduated with first-class honors in chemistry in 1910. Then, like most British chemists before World War I, he spent two years studying in Germany-first, at his father’s suggestion, with the electrochemist A. Fischer at the Technical University in Aachen, and then with Kekulé’s pupil Richard Anschütz at the University of Bonn. He obtained his doctorate, with a dissertation on the electro-chemistry of uranium salts, from Bonn in June 1912.

On returning to London in 1912, Rideal spent his first two years doing electrochemical consulting in his father’s office suite. He was testing water supplies in Ecuador when war broke out in Europe in 1914. After serving with the Royal Engineers in Belgium, he contracted dysentery in 1916 and spent the remainder of the war in the department of munitions inventions. Here he spent most of his time developing a catalyst for the industrial production of ammonia that used the synthetic laboratory method Fritz Haber had described in 1905. Rideal’s co-worker on this project was Hugh Stott Taylor, with whom he published Catalysis in Theory and Practice in 1919. For his work during the war period, he was awarded the M.B.E.; he also found time to produce, with (and for) his father, four textbooks on industrial chemistry.

As visiting professor of chemistry at the University of Illinois, Urbana, from 1919 to 1920, Rideal had the opportunity to become friends with such leading American physical chemists as Gilbert N. Lewis, Richard C. Tolman, and Irving Langmuir; the influence of the latter’s work proved particularly stimulating. Rideal returned to England in 1920 to take up the Humphrey Owen Jones lectureship at the University of Cambridge, where he also became a fellow of Trinity Hall.

Rideal married Peggy Jackson, sister of the American writer Schuyler Jackson, in 1921; they had one daughter. He was a devout Anglican and, but for the year in America, would probably have played an active role in the Christian Socialist campaign of the Anglo-Catholic Reverend Conrad Noel, which he joined in 1918. An internationalist in science and politics, Rideal believed fervently that Christianity should be the basis of political life.

In 1930, through Hardy’s influence, Rideal was elected to the chair of colloidal physics, but within a year he moved to the Plummer chair of colloid science. Although he would undoubtedly have been elected professor of physical chemistry at Cambridge in 1937 (the post went to Ronald G. W. Norrish), a serious operation for an intestinal tumor the year before, which left him with a permanent colectomy, decided him against entering the competition.

Rideal’s fears that the disability would diminish his energy for research and administration were misplaced. In 1946, following much government war work at Cambridge, Rideal was appointed Fullerian professor of chemistry and director of the Davy-Faraday Laboratory at the Royal Institution in London; he resigned in 1949 because he found the social commitments that went with the position too taxing. From 1950 to 1955 Rideal was professor of chemistry at King’s College, London, where for the third time in his career he built up a surface chemistry research school from virtually nothing. He spent his long and active retirement, begun in 1955, at Imperial College, London, as senior research fellow in the chemistry department of his pupil R. M. Barrer.

Knighted for his services to chemistry in 1951, Rideal was elected a fellow of the Royal Society in 1930 and played particularly active roles in the Faraday Society (now part of the Royal Chemical Society), of which he was the president (1938-1945), the Chemical Society (president, 1950-1952), the Royal Institute of Chemistry, and the Society of Chemical Industry (president, 1945-1946), which created the Rideal Lecture in his honor. The triennial Chemisorption and Catalysis Conference, which he launched in 1962, was renamed the Rideal Conference in his honor in 1971.

Rideal, in great demand as an industrial consultant, suffered considerable financial difficulty, as well as embarrassment, in the early 1930’s when he had to pay a sizable tax on previously undeclared consultancy income. His Cambridge research student C. P. Snow, who disliked Rideal, was to base the character of the lawyer Herbert Getliffe in Strangers and Brothers (1940) on this financial episode; earlier, Snow portrayed Rideal in The Search (1934) as the irrepressibly effervescent and slightly bogus Professor Desmond, “the supreme commercial traveller, the salesman of science and thermometer of scientific gossip.”

Rideal published nearly 300 scientific papers in his sixty years of active research; 70 percent of them were written in collaboration with research students. Although such collaborative efforts have become normal in the twentieth century, a few of his students were disturbed by a certain casualness in his proprietorship and transmission of their intellectual and experimental creations. On the other hand, there is no doubt that Rideal was, with good students, one of the century’s outstanding research supervisors. At Cambridge, the Royal Institution, and King’s College, he almost single-handedly developed large and successful research schools that attracted students from overseas as well as from Great Britain. Among his pupils were R. G. W. Norrish, a future Nobel Prize winner, who began work on photolysis with Rideal in 1922; Frank P. Bowden, the Australian expert on lubrication; C. P. Snow, who pioneered infrared spectroscopy with Rideal before becoming a civil servant and successful novelist; G. van Praagh, a chemistry teacher who played an important role in the development of the heuristic Nuffield syllabi, which transformed the teaching of science in British secondary schools in the 1960’s; and Daniel D. Eley, one of the fourteen fellows of the Royal Society and fifty professors of chemistry Rideal is estimated to have trained for industrial or academic careers.

Rideal was neither a good experimentalist nor a profoundly original theoretician, but he had charisma, organizing ability, good judgment on how a new technique could be exploited, and above all what he himself described as “the God within”: enthusiasm. His work, although given unity by the theme of surface chemistry, was exceedingly diverse in character. The papers are uniform in style: an opening discussion of the problem, together with a thorough survey of earlier literature, a mathematical and theoretical analysis of the problem that involves experimentally determinable variables, an experimental section, and a summary of the findings. Since five or six research projects were commonly in progress simultaneously and continuously, any strictly chronological treatment would prove hopelessly complex.

Following his doctoral studies in Germany, Rideal published work on electrochemistry and was concerned with fuel cells throughout his life. His principal contribution as a physical chemist, however, was to provide an understanding of the chemistry of surfaces. Given his father’s interest in disinfectants and their collaboration on the text Chemical Disinfection and Sterilization (1921), it is hardly surprising that Rideal’s interest in surface phenomena arose from the biochemical/bacteriological problem of how germicides act on bacteria. If they were akin to Ehrlich’s “magic bullets,” how, exactly, did they penetrate the bacterial wall? Rideal saw that this behavior was part of the general physical phenomenon of the mechanism of adsorption by charcoal, fibers, and metal surfaces, and that a good starting point was the study of capillary penetration by liquids.

The American physical chemist Edward W. Washburn suggested in 1921, from an analysis of the forces involved, that the distance x, advanced by a liquid penetrating a capillary tube, obeyed the relationship

where γ is the surface tension, η the viscosity, r the radius of the tube or fiber, t the time taken to reach distance x, and θ the angle of wetting (meniscus angle). In 1922 Rideal pointed out that the net forces involved varied with the velocity of flow as well as the length of the tube wetted. The time equation was therefore more accurately represented by

where d is the density of the liquid. For small radii, neglecting all but the first term, this reduced to the Rideal-Washburn equation, which was to be used extensively in the growing detergent industry.

Investigations of the adsorption of liquids by Irving Langmuir during World War I had shown that adsorption usually rendered the surface concentration of a solute much higher than the bulk concentration in the solvent. Rideal surmised in 1923 that if a similar effect were found for a membrane-water interface, then the germicidal efficiency of a disinfectant could be determined by its ability to lower surface tension. Experiments confirmed that the strongest bactericides were those with the highest ability to lower the surface tension of water. Both Hardy and Langmuir had shown previously that surface films are molecular monolayers, with the molecules so oriented that polar groups (such as –OH, –COOH) pointed into the water and nonpolar groups (such as hydrocarbon chains) reached above the water surface. Rideal was the first to apply this insight to microorganisms and their destruction, pointing out that their growth could be inhibited by attaching to them a lipid-soluble molecule with heavy side chains.

Although Rideal was to return intermittently to the study of bacterial surfaces, by 1925 the more general study of surfaces had acquired its own fascination and momentum. There were, moreover, many potential industrial and commercial applications to be investigated and exploited. In 1925, for example, Rideal showed that evaporation from a water surface could be retarded by as much as 50 percent by spreading the surface with a monolayer of long-chain fatty acids. Further extension of this principle by Langmuir two years later led to important applications in the conservation of reservoirs, particularly in equatorial regions of the globe.

The state of contemporary knowledge of surface chemistry was summarized by Rideal in An Introduction to Surface Chemistry (1926). This text, which was rigorously presented in the formal language of chemical thermodynamics, included a discussion of colloids. Assuming that colloids could adsorb gases or liquids as monolayers analogous to the classical Langmuir Surface, Rideal sought to overturn the so-called empirical Schulze-Hardy Rule (an extension by W. B. Hardy in 1900 of nineteenth-century observations by Hans Schulze), which claimed that in the coagulation of a colloid, the coagulating power of an electrolyte increased with increasing valence.

Rideal showed that these valence rules were not exact and that univalent metal ions, for example, could be made to disperse colloids differently. In joint work with J. H. Schulman in 1937, Rideal found that ions had very specific effects on colloids. Whereas a monolayer of a protein or of cholesterol could be easily penetrated by a dilute solution of saponin (a plant glycoside), in other cases the penetrating substance was trapped or “anchored” below the monolayer film. If the substance injected had the general formula CH₃(CH₂)₁₁X, then the degree of penetration varied as X:

This demonstration of specific ion interactions had important biochemical implications for the understanding of cell chemistry as well as pharmaceutical applications.

Earlier, in 1927, following Langmuir’s and Neil K. Adam’s development of a trough for the investigation of surface films, Rideal and H. Mouquin measured the rigidity of monomolecular layers. By applying a torque beneath the surface, they were able to determine the displacement by the optical examination of dust particles sprinkled upon the surface. Although Rideal did not pursue such studies further, the investigation was the beginning of the quantitative surface rheology that was developed by engineers in the 1940’s. The Langmuir-Adam trough was also used fruitfully with Schulman in 1931 to derive values of surface potentials for long-chain fatty acids. According to Helmholtz, the change in surface potential ΔV is given by 4πnμ, where n is the number of molecules per square centimeter of film, each molecule being credited with an average electric moment of effective vertical component μ.

The ingenious measurements, which utilized a radium bromide/air electrode technique that had been developed by Guyot and A. N. Frumkin, allowed Rideal to determine the actual orientation of adsorbed molecules and to show that the value of μ depended upon the state of aggregation of the film in the Langmuir trough. Thus, when the film surface was compressed, μ rose considerably, while below the surface monolayer Rideal could detect an ionic double layer of molecules that contributed to the interfacial potential. Information on the orientation of the surface could be obtained easily for complex proteins, and “the effects of alteration of inclination as well as of adhesion of the polar groups to the substrate were capable of quantitative measurement.”

Although this technique and its results were of considerable interest to biochemists, more significant was the research program Rideal began with A. H. Hughes in 1933, in which he showed how surface potentials could be used to follow the kinetics of chemical reactions taking place on a surface film—a technique that also had bearing on Rideal’s interest in catalysis. Rates of oxidation of fatty acid films were found to depend upon surface area, not thickness;and if the surface was compressed, there were marked effects of steric hindrance as preferred reaction sites were pushed above the surface. Rideal saw that what he called “a shielding or accessibility factor,” φ, should be added to the rate equation for surface reactions:

K′=ϕz′p′e^-E/RT,

where K is the rate constant, p is a function of entropy of activation, z is the collision number, and R is the gas content. This factor represented “the influence of change of molecular area on the fraction of reactive groups in the monolayer which are shielded from the reactant on compression of the film.”

The study of reactions in compressed monolayers, which was the topic of Rideal’s Royal Society Bakerian Lecture in 1951, was pursued with a large number of collaborators and employing different techniques, such as infrared spectroscopy and ultraviolet irradiation of surfaces. While gaining new insights into heterogeneous catalysis from the program, Rideal also provided a model that could be used for understanding cell biology.

Rideal’s fascination with surface penetration made him an ideal investigator for Imperial Chemical Industries when they wished to improve their understanding of the mechanism whereby dyestuffs adhered to fabrics. Between 1939 and 1941, Rideal and G. A. Gilbert made quantitative measurements of the adsorption of chlorine ions by woolen fibers (published in 1944). The study was premised upon a theoretical analysis of the dissociation of proteins and upon a statistical theorem developed by R. H. Fowler concerning the chemical potential μ of an uncharged adsorbed substance distributed at random among a limited number of sites,

where R is the gas constant and μ° (TP) is the chemical potential of the substance at pressure P and temperature T when θ (the fraction of sites occupied) is 0.5. When the adsorbed substance was charged (ionized), the chemical potential was increased by a factor ΨF, where Ψ is the site potential and F is the Faraday constant.

This analysis led to the Rideal-Gilbert titration equation for proteins,

where θ_H is the fraction of possible sites at which protons can be absorbed and Δμ=μ° (TP)_fibre−μ° (TP)_solution. This theoretical and experimental study was to lead to further industrial understanding and applications in the postwar dyeing industry, and it is a good example of the close familiarity Rideal had with practical problems that interested chemical industrialists.

Rideal’s work on ammonia synthesis during World War I stimulated a lifelong interest in catalysis, which is, of course, another problem in surface chemistry. While investigating ways of purifying hydrogen produced from water gas (a mixture of hydrogen and carbon monoxide), Rideal and Hugh Taylor found that a mixed oxide of iron and chromium catalyst would selectively oxidize carbon monoxide at a temperature of 200°C. In a detailed investigation in 1919, Rideal discovered that, of the two simultaneous reactions involved−2CO + O₂ → 2CO₂ and 2H₂ + O₂ → 2H₂O—the former was selected by an iron-cobalt catalyst, whereas a nickel-palladium catalyst favored the latter. In a study with W. W. Hurst in 1924, Rideal used palladium as a “promoter” with copper. In this case it was found that up to 175°C there was a tendency for CO to be oxidized preferentially to H₂. This tendency could be maximized with a palladium-copper proportion of 0.2 percent. If more palladium was added, the ratio of CO/H₂ oxidized decreased, owing to the specific action of the promoter, and by 1.7 percent palladium the activity was reduced to the level of unpromoted copper.

These observations of catalytic selectivity immediately led Rideal into theoretical issues: Did the demonstrations support those, such as Paul Sabatier, who advocated a chemical, intermediate compound theory of catalysis, or a physical mechanism involving surface adsorption? In either case, what promoted or activated the reaction? In the United States between 1915 and 1917, Langmuir had shown that gaseous adsorption by monolayers varied with the pressure such that

(Langmuir’s isotherm), where x/m is the amount of substance adsorbed per gram of the adsorbent, p the pressure (or concentration), and k a constant for the system.

Following Langmuir’s demonstration that the adsorption of simple gases by a series of metals depended upon the natures of the gases, the strengths of adsorption being caused by a mixture of strong valence forces and weak van der Waals forces. Rideal supposed that catalytic selectivity could be explained coherently in terms of Langmuir’s isotherm if the radiation theory of activation was valid. According to this theory, which had been developed from Ar-rhenius’ original analysis of reaction velocity by M. Trautz and Jean-Baptiste Perrin in 1909 and 1916, respectively, the active molecules in chemical reactions were triggered or “activated” by the adsorption of radiation from the walls of the reaction vessel. By showing that the combination of gases and different catalytic surfaces possessed different free energies, the progress or inhibition of selected reactions could be satisfactorily explained. Rideal was able to maintain the essential features of this explanation despite the discrediting of the radiation theory and the establishment of the collision theory of activation in the late 1920’s by F. A. Lindemann and others.

In a rather intriguing collaboration with O. H. Wansborough-Jones (later chief scientist for the Advisory Council on Scientific Research and Development of the Ministry of Supply, of which Rideal was chairman from 1953 to 1958) in 1929, Rideal studied the kinetics of the oxidation of an electrically heated platinum filament and compared the results with Langmuir’s studies of the oxidation of tungsten and copper. He noticed that the differences in activation energies corresponded closely to differences in the known values of the metals’ work functions (free energy), and concluded that the rate-determining step in all three cases was due to collision and the transfer of an electron from the metal to an adsorbed molecule of oxygen.

Although usually well read in the current literature, Rideal seems to have been unaware that the Russian physical chemist Simon Z. Roginski had already emphasized the significance of electronic effects in surface oxidation in 1928. Postwar work on semiconductors by Nevill Mott was to lead Roginski and others to explore electronic theories of catalysis in which energy barriers are overcome by continuous changes in the chemical bonds of the reagent through the free exchange of electrons from the semiconductor surface. However, it was Rideal’s pupils, rather than Rideal himself, who were to participate in these developments.

On the other hand, by the mid 1930’s Rideal had developed powerful techniques that helped to establish that chemisorption (valence) rather than physical (van der Waals) forces were involved in the mechanism of catalysis, and that specific crystal lattices of surfaces were the preferred sites of reactivity. Such demonstrations were necessary preconditions for the establishment of the electron theory of catalysis. Following the spectroscopic discovery of deuterium (D₂), the isotope of hydrogen, by Harold C. Urey, G. M. Murphy, and F. G. Brickwedde in 1932, and its availability in heavy water. Rideal was one of the first to realize its usefulness in kinetic studies. In 1935 he and his collaborators studied the isotopic rate differences between hydrogen and deuterium in reducing copper oxide, hydrogenating ethylene, and the adsorption of the isotopes by charcoal.

In the same Cambridge laboratory, but working independently, J. K. Roberts showed in 1935 that in a hydrogen atmosphere a clean tungsten film was rapidly covered by a complete layer of hydrogen atoms. A theoretical analysis of the dissociation showed that adsorption of diatomic molecules by dissociation into free atoms would inevitably lead to about 8 percent empty sites at saturation.

Rideal saw immediately that Roberts’ “Berthollide” compound model could be tested by the tungsten-promoted exchange equilibrium reaction between hydrogen and deuterium, H + D₂ ⇌ HD + D, and between the ortho- and para-forms of hydrogen, the mechanisms of which were revealed by the lattice deformation. As Rideal demonstrated between 1939 and 1941 with G. H. Twigg (for deuterium) and D. D. Eley (for para-hydrogen), such exchange reactions probably proceeded via “an interaction between a chemisorbed hydrogen atom and a hydrogen molecule held in the van der Waals fields over the layer and in all probability of a molecule held over a ‘hole’”

In other words, the exchanges H + D₂ ⇌HD + D involved three stages, with the middle, transition complex involving a “switchover” of valence. Other deuterium studies made with Twigg in 1939 showed conclusively that a very similar associative process occurred when olefins were adsorbed by nickel, the olefin opening its double bond on adsorption and thus becoming attached to two contiguous nickel surface atoms. Detailed theoretical calculations of bond lengths satisfied Rideal that such olefin adsorption could occur without distortion of the molecule.

This “Rideal mechanism,” in which a molecule in a second van der Waals layer reacts with a chemisorbed atom immediately beneath it, was probably Rideal’s most important contribution to catalytic and surface chemistry. The last thirty years of his life were spent exploring its ramifications, especially for an understanding of polymerization and what he called “the Mona Lisa of catalytic reactions.” The industrially important hydrogenation of ethylene (ethene). Although in 1955, in joint work with G. I. Jenkins at King’s College on the adsorption of ethene by nickel, Rideal rejected his own mechanism for a more complex one involving the immediate formation of triple-bond complexes, infrared investigations by R. P. Eischens in 1958 amply confirmed a Rideal mechanism. The irony here was that Rideal, with Sir Robert Robinson, had done much to encourage the use of infrared spectroscopy, first at Cambridge in 1929 with C. P. Snow (a study of the vibration-rotation spectra of nitric oxide and carbon monoxide) and later, in 1957, at King’s College with D. M. Adams (the use of infrared to map the differentiation of bacterial surfaces by species and with exposure to drugs).

BIBLIOGRAPHY

I. Original Works. A comprehensive list of Rideal’s dozen books and some 300 papers, based upon a privately printed (for subscribers) List of Papers Published During 1912–46 (Cambridge, 1946), accompanies Eley’s obituary of Rideal, though volume numbers and page references are sometimes inaccurate. To this should be added Rideal’s admiring edition of the Collected Papers of Sir William Bate Hardy (Cambridge, 1936) and a paper written with J. Tadayon, “On Overturning and Anchoring of Monolayers,’s Proceedings of the Royal Society, A225 (1954), 346–361. See also Rideal’s Sixty Years of Chemistry (Port Sunlight, 1970), an annotated copy of which is in the Royal Institution. Rideal’s few private papers that survive are at the Royal Institution, London, and are listed in the Catalogue of the Papers of Sir Eric Keightley Rideal, FRS (1890–1974) (Oxford, 1978), Contemporary Science Archive Centre, list 62/6/78. Snow’s barbed comments on Rideal are recorded in John Halperin, C. P. Snow. An Oral Biography (Brighton and New York, 1983); and depicted fictionally in C. P. Snow, The Search (London and New York, 1934; 2nd, abridged ed., 1958), in which Rideal is Desmond; and Strangers and Brothers (London, 1940; repr. London and New York, 1959), in which Rideal is the lawyer Herbert Getliffe.

II. Secondary Literature. The most comprehensive account of Rideal is given by his pupil Daniel D. Eley in Biographical Memoirs of Fellows of the Royal Society, 22 (1976), 381–413 (with portrait). An earlier, more anecdotal, version was given by Eley to the Rideal Memorial Symposium at Imperial College, London, on 5 June 1975 and printed as “The Life and Catalytic Work of Sir Eric K. Rideal, FRS,” in Chemistry and Industry (4 October 1975), 800–806. See also John T. Davies, “Sir Eric Rideal and His Contribution to Surface Chemistry,” ibid., 806–813.

W. H. Brock