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elasticity, plasticity, or tenacity of the fiber, should be able to furnish some measure of the quality of silk a t any given stage in such an exposure. Conclusion
It is thus evident that the sum total of our knowledge concerning silk is woefully little, and that each bit of information only points the way t o a whole field remaining to be explored.
Vol. 22, No. 4
Not only can such varied tools as organic and physical chemistry, physics, and mathematics, and their composite forms of colloid chemistry, physiological chemistry, biochemistry, and biology add to our knowledge of the fundamental nature of silk, but in doing so these sciences should themselves be enriched. As a material which is attractive in its form and cleanliness for handling and one which is allied with so many fields of pure and applied science, we believe silk has no rival.
CHANDLER LECTURE The Chandler Lecture for 1929 was delivered a t Columbia University on December 13, 1929, by Irving Langmuir, assistant director of the Research Laboratory of the General Electric Company, and a t the time President of the AMERICANCHEMICAL SOCIETY. I n presenting the medal awarded a t the close of the lecture, Dean George B. Pegram, of Columbia, spoke in part as follows in praise of the medalist:
To try to state all his scientific researches and discoveries would take more time than we have. His experimental and theoretical investigations of the emission of electrons by hot bodies, of ionic conduction in high vacua, of chemical reactions a t very low pressures, of molecular shapes, of the structure of atoms and of the forces between them put him in the front rank of those who have contributed to knowledge in these fields. You have just heard how clearly and definitely he visualizes phenomena a t low pressures in terms of individual atoms. No one has made friends with atoms more successfully. You have seen that he does not crowd them, he gives them plenty of room to be themselves, he keeps them a t a temperature they like so that they are neither too sluggish nor too excited, and they perform for him with wonderful fidelity. But besides having this mastery of scientific research, Doctor Langmuir has applied his knowledge to practical invention with the greatest success. His invention of the concentrated filament gas-filled electric light doubled the efficiency of electric light. I t is said to save the people of this country a million dollars a nightthough they probably spend most of that same million for more light, which is a good thing. Then there are his numerous inventions applied to electron tubes for radio and other purposes, his high-vacuum pump that is now used in every laboratory, and others. His great chief, Doctor Whitney, himself an early recipient of a Chandler medal, has said of Langmuir: ”The modern prophet is a doer. No one can fix the best ratio between thinking and ......
doing. The pure thinker is apt to think too much, and the active man to be too active. I know of no one who seems to combine these two characteristics more effectively than Langmuir.” The Charles Frederick Chandler Foundation was established in 1910, when friends of Professor Chandler presented to the trustees of Columbia University a sum of money, and stipulated that the income was to be used to provide a lecture by an eminent chemist and also a medal to be presented to this lecturer in further recognition of his achievements in the chemical field. The previous lecturers and the titles of their lectures are as follows: 1914
L. H. Baekeland
1916
M’. F . Hillebrand
1920
W. R. LVhitney
1921 E‘. G. Hopkins
-
E. F . Smith
1025
1023 R. E . Swain 1925
E. C . Kendall
1926
S.W.Pari
1927
Moses Gomberg
1925 J. A. Wilson
Some Aspects of Industrial Chemistry [Vol. 6 , 7 6 9 (19141 I Our Analytical Chemistry and I t s Future [Vol. 9, 170 (1917)l T h e Littlest Things in Chemistry [Vol. 12, 599 (1920) 1 Newer Aspects of t h e Nutrition Problem [Vol. 1 4 , 6 4 (192211 Samuel L a t h a m hIitchill-A Father in American Chemistry [Vol. 14, 556 (1922) I Atmospheric Pollution b y Industrial Wastes [Vol. 15,296 (192311 Influence of the Thyroid Gland on Oxidation in t h e Animal Organism [Vol. 17, 525 (1925) 1 T h e Constitution of Coal-Having Special Reference t o t h e Problems OF Carbonization [Vol. 18, 620 (1926)l Radicals in Chemistry, Past a n d Present IVol. 20, 159 (192811 Chemistry and Leather [Vol. 21, 150 (1929)!
Electrochemical Interactions of Tungsten, Thorium, Caesium, and Oxygen’ Irving Langmuir GEKERAL E L E C T R I C C O M P A N Y , S C H E N E C T A D Y , N. Y.
HE atomic theory of electricity, in the form of the
T
electron theory, has done much to make Faraday’s laws of electrochemical action seem obvious t o us. The Laue-Rragg methods of determining the arrangement of atoms in crystals by using x-rays have proved that even in solid rock salt there are no molecules of KaC1, but the crystal consists of a space lattice of Na+ and C1- ions, and the electric attractions between the ions account for the cohesive and elastic properties of the solid. We qee that the salt dissolves in water primarily because the attractive force between the ions is decreased greatly in a medium of such high dielectric constant. Thus the electrolytic dissociation theory also becomes obvious. 1
Received February 14, 1930.
In a state of thermal equilibrium, at any temperature T , the molecules in a gas are moving with velocities obeying probability laws, giving a Maxwell’s distribution of velocities. In a field of force, such as a gravitational or electric field, the molecules or ions vary in concentration in accordance with Boltzniann’T law nl,fn? = e w p ( U ‘ / k T ) Soie-The
(1)
symbol e x p l r ) has t h e same meaning as ez.
where W is the work that must be expended in transferring a molecule or ion from a region 1 to a region 2, while nl and n2 are the concentrations in these two regions; k is the Boltzmann constant 1.37 x 10 16 erg. deg. 1 If we are dealing with ion? having charges e , W becomes equal to lie, where L7 is the electric potential difference between 2 and 1.
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391
The Nernst equation giving (the electromotive force of tirely charged anode. The saturation current, in amperes concentration cells, or between electrodes having certain per square centimeter, is given by the Richardson-Dushman hypothetical solution pressures, is obtained by muely equat- equation (5) I = AT2exp(-b/T) (2) ing the logarithms of both sides of the Boltzmann equation. The greatest recent advance in our understanding of electro- where h and d are, in general, constants that are characchemical phenomena has resulted from the work of Debye teristic of the nature of the surface of the filament. For and Huckel (3). They conceived the idea of using the Bolta- clean tungsten A = 60 amperes per square centimetw and iriann equation together with the Poisson equation (which b = 52,600” K. According to this equation loglo I , when deals with the potentials set up by space charge.) to calcu- plotted as ordinate against l / T as abscissa, gives very cloqely a straight line whose slope is -ih lnte the extent to which positive ions 2T)/2.30. Figure 1 gives the electron TT ould tend to gather around negative emission from filaments having variions, and vice versa. This made it ous kinds of surfaces. The electron powble to understand even quantitaemission is plotted as ordinate on a tively the numerous discrepancies that logarithmic scale while the ab&-as had previously been found between are proportional to the reciprocals of obqervations and the older theories of the temperatures indicated a t the botelectrolytic diswciation. tom of the figure. The straight line All these electrochemical laws h a w marked “Pure-JJ‘” corresponds to the to do primarily with equilibrium conelectron emission from a clean tungditions and are subject to thermoiten surface, calculated from the Du-hdynamic treatment. Not much light mail equation with the ~ a l u e sof A can be thrown o n the mechanism of and b given above. electrochemical reactions b y t he.(> theories. For this purpose the kinetic6 The slope of the straight line5 tliiib niubt be studied. Debye and Hrielrel obtained is proportional to the latent i n t h e i r second .paper ($) made a heat of evaporation of the electrons, notable contribution in this field by and by dividing h by 11,600 thib may their analysis of the problem of the be expressed in volts. Thus for pure mobility of ions involving the rate a t tungsten this heat of evaporation corwhich the redistribution of chargeresponds to 4.52 volts; that i., the around a moving ion took place. work that must be expended to renioi e Irving Langmuir a n electron from the tung-ten iy tlie Some of the m0.t important electrosame R S the work that must be done chemical phenomena, such as pwsivity in moving an electroil against a reaiid orervoltage. are verv imuerfectlv understood. These phenomena involve conditioni which do tarding potential difference of 3.52 volts. Schottky (20) and others hare shovn that a large, or not correspond to equilibrium. TVe n e d more detailed knowledge of t,he factors which determine the velocit,!, of surface even major, part of this work is expended in nioring the reactions; the rates of formation of adsorbed films; the electron against the force of attraction that results from rates a t which electrons can pass from a metal through a the positive charge that, the electron induces in the tungsten layer of adsorbed atoms to interact with atoms or ions in i.urf~ce-the so-called electric iinage force. This t’heory the solution, or the rate7 a t which positive ions can pass from indicate., in agreement with experiment, that the electron the metal to the solution. The electric forces a t the surface emission from a metal increafes if strong external acceleratnaturally play a n esential part in these processe.. ing electric fields are applied. Thus, if I , is the electron T have made no direct experimental study of these electro- enii.+ion with a n-eak field as given 113‘ Equation 2 , the eleccliemica! ~~hcnoniena, but some of the work that I have tron enii*.;ioii xith a btrong field, E (volt. per centimeter), is done during tlie la>t fifteen years on reaction,s of heated I = I,, e x p ( 4 . i t E ! T ) (3) filaments of tungsten or other material> with gases at low Although for extreniely strong field. Schott,ky’s theory pressures seeins to tiiron- light on the mechanism of many surface actions which should he applicable to tlie study of requires iiiodification in accordance Jvith the neIv wave meelectrochemical plienoniena. Tliis i. especially imie of our clianics, yet it agree; with these newer theories aiid with studies of elcctron eriii,ssion a i d po,sitive-ion emission and of recent experiment. in indicating that with accelerating field the changes in thew that are caused by absor1)ed films of btrengths of the order of 10; or 10s volts per centimeter, atoiiiic thickne~s. K e have stiitlietl films of oxygen, thorium. clectrons may lie pulled out of inetals at rooni temperature. ( w h i n , barium, etc., 011 tungsten; and composite or double These “field currents” increase extremely rapidly nith the filnis eucli as monatomic films of caesium firmly glued to a field strength and may reach enormous current den.ities of tungsten surface by an iriternietliate nionatoinic film of oxygen. millions of amperes per equarf centimeter. The field reSucli plienoineiia are essentially within the field of electro- c4uired to dram- such currents i. approximately independent cliemistry, although electrochemist. have usually confined of temperature. In adsorbed films of atomic thickness there may exist their studies to phenomena of electrolytic solutions. I shall attempt to give a brief summary of the rezults of our potential drops of several volts, so that fields of lo8 volts studies of gas reactions and of the electrical properties of per centimeter may well exist a t the surfaces of electrodes adsorbed filnis o n filaments, and to discuss particularly their inimersed in electrolytes, or a t the interfaces between solids bearing on the electrochemi~tryof electrodes in solution. in contact. Thus the mechanism of the passage of current from electrodes to electrolytes is probably similar to that of currents observed in vacuum in presence of high accelerating Reaction for Pure Tungsten fields. Electrons evaporate from a hot tungsten surface in high The electric image theory furnishes us also with a relation vacuum. This rate of evaporation is determined by rneasiir- between the heat of evaporation in volts and the size of the ing the current of electrons which can be drawn to a poai- atoms of the metal surface. As a first approximation, b
+
cI
I
.
A
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of Dushman's equation is inversely proportional to the atomic diameter, or proportional to where V is the atomic volume. Roughly speaking, the temperatures a t which a given electron emission can be obtained from various metals is thus inversely proportional to the atomic diameter. Thorium Films
A single layer of adsorbed thorium atoms covering the surface of a tungsten filament increases the electron emission 100,000 fold a t a temperature of about 1500" K. The line marked "Th-W" in Figure 1 gives the emission as a function of temperature. Films of this kind ( 7 ) may be produced when desired on filaments of thoriated tungsten-that is, tungsten filaments into which thorium oxide (0.5 to 2 per cent) has been incorporated during manufacture. After the filament is mounted in the bulb and exceptionally good
Figure 1
vacuum is obtained, preferably by use of magnesium or calcium vaporized on to the bulb, the filament is heated for about a minute to 2800" K. or more. This causes a slight reduction of the thorium oxide by the tungsten, so that a few parts per million of metallic thorium are then present as a solid solution in the tungsten. This thorium diffuses slowly through tungsten crystals and more rapidly along the grain boundaries between crystals, and thus reaches the surface, where it tends to accumulate as an adsorbed film because it lowers the surface energy just as an oil film does on water. However, a t temperatures above about 2300" K. it usually evaporates more rapidly from the surface than it diffuses from the interior to the surface. The thorium film is therefore best prepared by maintaining the filament a t about 2100" K. for about half an hour. The temperature may then be lowered to 1800" or 1900" K., which is a good operating temperature-that is, a temperature a t which an electron emission of more than 0.1 ampere per square centimeter may be obtained without appreciable loss of thorium from the surface by evaporation, the diffusion being still rapid enough to maintain the surface completely covered in spite of a slight loss that may occur from positive-ion bombardment due to residual gas. The electrical and physical properties of adsorbed films and of the phenomena involved in their formation may be studied with remarkable ease and accuracy. This is the kind of information that should be particularly valuable to the electrochemist. The condition of the surface of the filament may be determined a t any time by measuring its electron emission under standard conditions at a suitably chosen testing temperature, such as 1450" K. This temperature is so low that no measurable diffusion or evaporation of thorium
VOl. 22, No. 4
occurs and yet the currents are of convenient magnitude amperes), being not so large that to measure ( l o - * to space charge influences them or that positive-ion bombardment produces changes in the surface. I n this way the rate a t which thorium diffuses to the surface a t any higher temperature, such as 2000" K., or the rate a t which evaporation occurs at, say, 2300" or 2400" K. can be measured. It is found that the value of b in Dushman's equation varies approximately linearly with 8, the fraction of the surface covered with thorium atoms. This means that the logarithms of the electron emissions are linear functions of 6. For example, in Figure 1, where we have used a logarithmic scale, the emission of a tungsten filament having only half of the thorium on the surface that is needed to cover the surface-i. e., 8 = 0.5, would be given by a line half way between those of "Pure-W" and of "Th-W." The upper end of the line "Th-W" in Figure 1 shows a deviation from a straight line corresponding to the loss of adsorbed thorium by evaporation; a t temperatures approaching 2400" K. the emission becomes that of pure tungsten. If the temperature is then suddenly lowered below 1900" K. the emission remains that corresponding to "Pure-W." This result, which may be simply interpreted theoretically, should be applicable to electrodes in solution. The changes in electrode polarization caused by adsorbed films should be proportional to 8, the fraction of the surface covered by adsorbed atoms. I n the case of thorium films on tungsten, effects analogous to electrode polarization are observed. A fully thoriated surface (8 = 1) in electrical contact with a pure tungsten surface is found to have a potential 1.5 volts positive with respect to the latter. This contact potential is approximately the difference of the heats of evaporation of electrons from the two surfaces (expressed in volts), or
(h- b,)/11,600 The high electron emission of thorium films on tungsten
is in accord with the fact that the atomic volume of thorium is more than twice that of tungsten. However, the emission of a complete monatomic lilm of thorium on tungsten is considerably greater than that of pure thorium metal. As an approximation it is often useful to recognize that the value of b for a monatomic film is roughly that of the metal in bulk, but it must be understood that the layer of atoms underlying the surface layer has a distinct modifying influence. Caesium Films
Monatomic films of caesium on tungsten have an effect much greater than that of thorium in increasing the emission from tungsten filaments, which is to be expected from the very large atomic volume of caesium. At about 800" K. the emission is increased approximately in the ratio lozoto 1 by a film completely covering the surface, 8 = 1. To produce such a film it is merely necessary to bring caesium vapor, activated a t room temperature (10-1 mm. pressure) into contact with a tungsten filament having a temperature below about 700" K. From the low boiling point and the softness of metallic caesium it is clear that the forces between caesium atoms are relatively weak and we might therefore suspect that caesium atoms would be only weakly adsorbed. Instead of that, we see that caesium atoms actually leave a surface of this metal a t room temperature to pass over to a tungsten surface a t a temperature 400 degrees higher and to cover this surface practically completely with an adsorbed film. This remarkable case of adsorption has been found to be due to the tendency of caesium atoms to lose their valence electrons to the tungsten. The work necessary to remove an electron from a caesium atom is
IiVD L'STRIAL A N D EJVGINEERING CHEMISTRY
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measured by the ionizing potential, 3.9 volts. The affinity of a pure tungsten surface for electrons is measured by the heat of evaporation, 4.52 volts. Therefore, when a caesium atom comes into contact Kith a.tungsten surface, the tungsten captures the valence electron and leaves the caesium as an ion, which is then held to the tungsten surface because of the negative charge which it induces (electric iniage force). This is confirmed by the fact that these adsorbed films are not formed if caesium is brought into contact with thoriated tungsten having an electron affinity of only 2.9 volt!'. If the filament temperature is high enough the caesium ions evaporate from the filament as fast as the atoms strike the surface, but these ions are not formed if a fully thoriated surface is used. Thus with pure tungsten (or any material with electron affinity greater than about 4 volts, such as carbon, nickel, molybdenum, or platinum) the positiveion current that can be drawn with voltages great enough to overcome space charge is strictly independent of filament material or temperature (above a certain lower limit) or the electrode voltage, but is accurately proportional t o caesium vapor pressure, p , bcing in fact given by the theoretical equation
I
=
ep(2amkT)-'h
(4
where Z is the current density, e the charge of an electron, and m the mass of the caesium atom. The two curves marked "Cs-W" in Figure 1 give the electron emission of pure tungsten filaments in presence of caesium vapor saturated a t 20" and a t 80" C. The falling off of emission as the temperature is raised above 700' or 800" K. is due to the evaporation of caesium so that 0 progressively decreases. When the current of caesium in the surface film is greater than corresponds to about 0 = 0.20, the electron affinity of the surface is less than 3.9 volts, so the caesium evaporates as atoms. Only a t temperatures above about 1200' K., when 0 has fallen to 0.2, do all the atoms leave the filament as ions. By such studies as these, especially by using transient phenomena analogous to those employed with the thoriated filaments, it is possible to make measurements of the rates of evaporation of electrons, ions, and atoms from adsorbed caesium films on tungsten as functions of temperature, of 0, and of the strength of the electric fields at the surface. Many results of this kind have already been obtained (1, 2, 6, 9) and systematic measurements are being made by J. B. Taylor. The escape of ions from the surface is greatly facilitated by an accelerating field, and this removal of ions, unlike that of electrons, depletes the supply and decreases 0 In such ways as this it should be possible to gain fundamental knowledge that may profoundly extend our understanding of electrochemical phenomena. Oxygen Films When a tungsten filament a t temperatures above about 1300" K. is acted on by very low pressures of oxygen, a monatomic adsorbed film of oxygen is produced on the surface. This film has remarkable properties which prove that it consists of atoms or negative ions rather than molecules. The electron emission, as indicated by the line "0-WJin Figure 1 is many thousand times less than that from "PureW." If heated to 1800' K. or more, this film gradually evaporates off the filament as atomic oxygen (un-ionized), no tungsten oxide being found. One of the most easily measurable properties of this film is its ability to hold caesium atoms more firmly than does a pure tungsten surface. When caesium vapor is brought into contact with a filament covered with an oxygen film, the filament gives an electron emission represented by the curves marked "Cs-0-W" in Figure 1,
393
which is almost lo5 times greater than from 'Ts-W" in the neighborhood of 1000" or 1100" K. Using such a temperature as a testing temperature, a measure of the rate of evaporat'ion of the oxygen film a t temperatures from 1600" to 2200" K. can be secured. It is found that a t 1790" K. it takes 25 minutes for about half the oxygen to evaporate from a complete film (0 = 1). The temperature coefficient is such as to indicate that the heat of evaporation corresponds to about 160,000 gram-calories per gram-atom, which is greater than the heat of dissociation of oxygen molecules into atoms. This illustrates the extremely powerful forces holding the oxygen atoms to the surface. Extrapolation shows that a t 1500" K. the time for half evaporation should be about 3 years. These monatomic oxygen films react readily with oxygen molecules which strike them to form tungsten trioxide. The mechanism of this reaction possesses many interesting features. From the electrochemical standpoint, however, the int'eraction with hydrogen is of most interest. Hydrogen molecules coming into cont'act with a tungsten surface a t 1500" K. are largely dissociated into atoms, but this action is entirely stopped by a complete film of oxygen, so that a minute trace of oxygen in hydrogen prevents measurable dissociation of hydrogen even in a period of '/z hour. The oxygen film itself is so stable that it would take years to evaporate a t 1500' K. and the complete film does not react appreciably with hydrogen. However, if the film is incomplete, so that hydrogen can come into cont'act with the tungsten, the whole of the oxygen film can be removed in a few seconds by even a low pressure of hydrogen. These phenomena seem to be practically identical with the well-known electrochemical phenomena of passivity of chromium and iron (8). The close relation, according to the periodic table, between tungsten and chromium suggests also that the mechanism of the two kinds of passivity is the same. A single layer of oxygen atoms on tungsten acts as a catalytic poison and inhibits the interaction of hydrogen and tungsten (giving atomic hydrogen) and also the interaction of oxygen and hydrogen. But the moment gaps of sufficient size are formed in the protective oxygen film, the interaction of the oxygen and hydrogen is actively catalyzed by the exposed tungsten and the oxygen film is almost instantly removed. Surely the same must be true of chromium in oxidizing solut'ions. Literature Cited Becker, Phys. Rev., 28, 341 (1926); 29, 364 (1927). Becker, Bell Telephone Laboratories, Reprint B-412,August, 1929. Debye and Huckel, Physik. Z., 24, 185, 305 (1923). Debye and Huckel, Ibid.,25, 49 (1924). Dushman, Phys. Rev., 21, 623 (1923). Killian, Ibid., 27, 578 (1926). Langmuir, Ibid., 22, 357 (1923). Langmuir, Discussion of paper by Bennett, Trans. A m . Electrochem. Soc., 29, 260 (1916). (9) Langmuir and Kingdon, Science, 57, 58 (1923); Proc. Roy. SOC.London, Al07, 61 (1925). (10) Schottky, Physik. Z . , 16, 872 (1914); Z . Physik, 14, 63 (1923). (1) (2) (3) (4) (5) (6) (7) (8)
New Books Enzyme Catalysts. Seventh Report of the Committee on Contact Catalysis. 1 5 pp. E. F. ARMSTRONG AND T. P. HILDITCH. Reprinted from Journal of Physical Chemistry, 33, 1441 (1929). National Research Council, Washington. Price, 25 cents. Use of Calcium Chloride or Sodium Chloride a s a Protection for Mortar or Concrete against Frost. Report 14, Department of Scientific and Industrial Research. 30 pp. H. M. Stationery Office, London. Price, 9 d., net. Water Pollution Research. Summary of Current Literature. Vol. 111, No. 1, January, 1930, Abstracts &-os. 1-119. Department of Scientific and Industrial Research. 36 pp, H . M. Stationery Oflice, London. Price, 1 s. 3 d., net.
