THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in U. S. Patent Office)
(Copyright, 1953, by the American Chemical Society)
Founded by Wilder D. Bamroft FEBRUARY 18, 1953
VOLUME57
NUMBER2
APPLICATIONS O F RADIOTRACERS T O THE STUDY OF SURFACES BYJOHN E. WILLARD Department of Chemistry, University of Wisconsin, Madison, Wisconsin Received June 8, 1958
This paper discusses ways in which radiotracers can be useful as tools in the study of reactions of surfaces. Illustrations are taken from several types of investigations. New information is presented on the reaction of bromine vapor with glass, on a method for the determination of sticking coefficients of gaseous particles striking surfaces and on the reaction of unsaturated vapors of organic compounds with surfaces in evacuated systems.
Introduction Surfaces are two dimensional, or very thin three dimensional systems. As such their chemical content per square centimeter is extremely small, i.e., of the order of g. or mole. Yet it is the properties of this thin layer which, in many cases, make a substance scientifically interesting or practically useful. Radioisotopes with specific activities high enough to give thousands of Eounts per minute-from the number of atoms i n a square centimeter of surface areal are now available2,3for a majority of the elements. These have made it possible to study conveniently and quantitatively many chemical reactions of surfaces which it was previously impossible even to detect. Such studies may be designed to yield new information on surface forces and the kinetics of reactions involving surface atoms, and to aid in the solution of problems in fields such as metal plating, detergent action, mirror formation, corrosion prevention, catalysis, adhesion of protective coatings, degassing of surfaces, classification of minerals and others. Many experiments4 have been done on the exchange between ions in precipit,ates and in the ~~
(1) J. W. Hensley. A. 0. Long and J. E. Willard, Ind. Bag. Chew., 41, 1415 (1949). (2) U. S. Atomia Energy Commission Isotopes Catalogue No. 4, 1951. (3) J. E. Willard, Reo. Chem. Prog., 19, 163 (1951). (4) For a comprehensive list of references see: 0. E. Meyera and R. J. Prestwood, “Radioactivity Applied to Chemistry,” edited by A. C. Wahl and N. A. Bonner, John Wiley and Sons,Inc., New York, N. Y., 1951, Table 1B.
supernatant solution. A few4 have been done on exchange reactions at the surface in other two phase systems. Brief exploratory investigations have also been carried out on reactions of tagged ions in solution with minerals and other surfaces not containing the same species of ion.5 It is the purpose of the present paper to illustrate the potentialities of the tracer method for the study of reactions of surfaces, by discussing some examples of recent work in the field.
General Considerations The general experimental requirements for such investigations are simple. If the surface to be studied is a flat solid it may be immersed for the desired length of time in the medium containing the tagged element or compound with which it is to react, and then be removed, rinsed and counted. The tagged species must, of course, have a high enough specific activity so that the quantity which it is desired to detect in the surface will give a measurable counting rate. It is also necessary that the form in which the tagged compound is bound on the surface be sufficiently stable so that it is not removed when the unreacted tagged substance is rinsed off. Counting of such samples is simplified by the fact that the layer i i i which the activity is present is so thin that self absorption is negligible. Thus the CI4 and S36 beta particles with ranges of about 30/mg./~m.~ and half thicknesses of about 3 (5) (a) T. Westermark and L. G. Erwall, Research, 4, 290 (1951); (b) T.G.Church, Can. J . Research, AaS, 164 (1950); (c) F. A. Paneth, “Radioelements as Indicators,” McGraw-Hill Book Co., Inc.. New York, N. Y., 1928.
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mg./cm.2, or even the 18 kev. beta from H3, with a range of less than 1 mg./cm.2 can be counted without self absorption correction. In the case of the H3 a windowless counter must be used and absorption by the sample may become important if the activity has penetrated the surface to a depth of many atomic layers. In cases where the solid is present as a mass of particles rather than a flat surface, it is sometimes convenient to determine the amount of the radioactivity which has entered the solid phase by dissolving the latter and counting it in a solution type counter. Alternatively the decrease in activity in the gas6 or liquid may be determined in a solution counter (annular jacket type) if the fraction reacted is sufficient to allow the change to be determined with the required accuracy. Rather than starting with the radioactive species in the gas or solution it is sometimes more informative or convenient to tag the solid. The surface may be tagged by pretreatment with a radioactive substance with which it reacts, or by bombarding it with charged particles of low penetrating power’ which produce radioactive atoms in the surface by transmutation processes. Alternatively the whole volume of the solid may be made radioactive by synthesizing it from tagged material or by inducing activity in it by neutron irradiation.8 When the activity is produced by irradiation of the sample it must be remembered that the tagged atoms are usually formed with high energies and may therefore be present in abnormal chemical states, and that the state of chemical combination of other atoms in the system may have been changed by the radiation. The extent of such effects will depend on the nature of the solid used and the type and extent of irradiation. Among the problems which can be attacked most effectively by the use of a tagged solid is the determination of the rate of dissolution of a very slightly soluble substance, such as glass. In carrying out surface studies it is particularly important to be aware of any foreign radioactive species in the tracer used. For example, NaZ4(15 days) produced by the NaZ3(n,.7)NaZ4reaction in sodium carbonate may contain a much Iongerlived radiospecies produced from small amounts of impurity in the carbonate. This radioactive contaminant has been observed‘ to be much more strongly sorbed on glass and metal than is sodium ion. There is usually no completely satisfactory way of expressing the extent to which reaction b e h e e n a surface and the surrounding medium has occurred. In order to give an indication of the depth to which the reagent has penetrated, the results are sometimes expressed in “atom or “monolayers.”l If the surface, area, density and composition of the solid sample are known,‘j such a unit may have an exact physical significance. If the composition is not known, the unit must be defined arbitrarily as for examplel: “a monolayer is the number of ions required to cover the macro surface area of
the sample if each ion covers an area equal to the square of its ionic diameter.” If the surface area is not known it is usually most convenient to express the results in terms of per cent. of the solid reacted, although when expressed in this way results from different experiments in which the solid has different surface to volume ratios are not directly comparable. I n the case of reactions of ions with surfaces it is sometimes of importance to determine whether the cation and anion are sorbed simultaneously. This may be accomplished by tagging each with a radioactive tracer. The initial counting rate of the sample will be the sum of the counting rates of the two sorbed species. The contribution of each may be determined by a second count made after the shorter lived species has decayed, or by a second count taken with an absorber over the sample to cut out the weaker of the two radiations. I n other cases it is useful to employ two different radioisotopes of the same element, as for example NaZ2(3 yr.) and NaZ4(15 days). If one isotope is present in the surface to be studied and one in the medium to which it is exposed simultaneous rates of sorption and desorption may be determined.8 It has been speculateds that it may be possible to carry out substitution, addition, and other synthetic reactions on surface molecules containing organic groups. Such procedures might make it possible to prepare surfaces with properties designed to‘meet a variety of specific needs. Quantitative investigation of the surface reactions involved in such studies is made possible by radioactive tracers.
(6) G. B. Kistiakowsky and J . R. Van Wazer, J. Am. Chem. Soc.. 65, 1829 (1943). ( 7 ) J. V. Fitzgerald, Glass Ind.. 30, 259 (1949). (8) A. 0. Long and J. E. Willard, Ind. Eng. Chem., 44, 910 (1952).
(9) G. L. Vandervort and J. E. Willard, J . A m . Chem. Soc., 7 0 , 3148 (1948). Studies of related interest have been reported by H. Deul, Clay Minerals Bull., 1, 205 (1952), and Rolloid Z.,124, 164 (1951). (10) J. W. Hensley, J . A m . Cer. Soc., 84, 188 (1951).
Reactions of Glass with Aqueous Solutions Exploratory investigations have been made of the effect of several variables on the reactions of several different ions in solution with glass surfaces.lvstlo I n general, one square inch sections of microscope slides have been used as the test specimens. One problem has been the choice of a cleaning procedure which would give a typical and reproducible glass surface. A sequence of scrubbing under distilled water, drying between filter papers, rinsing in the condensing vapors of carbon tetrachloride, and flaming until sodium color appears in the flame has been used for many of the tests. Even after this treatment, it is found that the two sides of a single specimen may show differences of 20% or more in the amount of sodium ion sorbed during immersion in an aqueous solution of a sodium salt. Likewise, radioautographs show that the sorbed activity is not uniformly distributed over the surfaces. For these reasons highly quantitative evaluations of the effects of variables on the sorption process have not yet been attainable. However, it has been found that the sorptionprocess is sufficiently sensitive to a number of the important variables so that largc effects which may furnish important clues as t o the nature of the surface and of its reactions can be observed. Among such observations are the following: soda-lime-silica glass is 1. Rate of Reaction.-When immersed in neutral solutions of radioactively tagged sodium ion some of the latter becomes bound by the surface a t a rate which decreases rapidly with time during the first minutes of immersion. The rate a t any time after immersion, up to a t least five hours, is dependent on the temperature, the 10” coefficient being about 1.35. The rate appears to be insensitive to the sodium ion concentration at the concentrations from ca. 0.005 to 0.05 M which were tested. The amount of sodium removed from the solution by the surface is in the range from a few tenths monolayer’ to a few monolayers. In contrast to glass, fused silica shows a
Feb., 1953
APPLICATIONS OF RADIOTI~ACEHX TO THE STUDY OF SURFACES
131
much lower sorption of sodium ion and the process seems which had been washed with water and condensing carbon tetrachloride vapors and had been lightly flamed. The to have a negative temperature coefficient. 2 . Pretreatment.-Glass which has been flamed to the bromine remained on the samples even after evacuation for fusing point shows little or no so tion of any ions tested. half ail hour and standing in air for 24 hours. Less bromine However, glass which has been amed only to the point (0.3 monolayer) was picked up if the plates were prepared where the sodium color appears in the flame shows fourlold by flaming nearly to fusion and still less (0.03 monolayer) higher sorption of sodium ions than glass which has been if the vacuum system was flamed while evacuated after inscrubbed in water and degreased in carbon tetrachloride troducing the flamed plates. but not flamed. The temperature coefficient for the reSticking Coefficients action of the flamed and unflamed glass is the same, suggesting that the effect of the flaming is to make more area avail111 experiments involving ion collection on metsl able for the sorption process rather than to produce areas receivers, and in other vacuum system problems, it with higher reactivity. 3. Evidence as to Ion Exchange.-When glass is allowed is frequently important to know what; the probabil,to sorb Na24+ from solution and is subsequently immersed ity is that a certain type of particle will stick to a in a solution of Na2a+ the Na24+ appears in the latter solution at the same time that the NaZ2+ enters the glass. The collector or wall when it strikes. This can be derelative rates of the two processes have been interpreted to termined13 if the particles are radioactively tagged means that there is a continual net loss of sodium from the and if a uniform beam of them is allowed to fall on glass, that is, that it is dissolved faster than the exchange an area which is divided into four quadrants. TWO process occurs. An attempt has been made to demonstrate that a typical opposite quadrants are covered with the material ion exchange process occurs at glass surfaces by soaking for which data are desired and the other two quadsoparate glass specimens for 330 hours in 0.2 N K+, B a + + rants open into long tubes which are closed at the and H +,with the intent of displacing all the sodium from the far end. These tubes are designed to serve as accessible surface layers, The samples were then irradiated with neutrons to induce radioactivity in the sodi.um which traps. They are long enough so that nearly every they contained. A subsequent 10-minute leachin of the particle which enters will undergo many collisions samples with water removed about equal amounts o f sodium with the walls before returning to the opening. activity from each of them. The amount removed was 50% Consequently, if it has an appreciable sticking coas much as obtained from a sample presoaked in Na+ solution. About seven times as much activity was removed efficient for the walls, its chance of escaping is negfrom a sample with no presoaking. The results suggest ligible. The ratio of the activity captured by the that the replacement of sodium ions in the surface by foreign surface of each covered quadrant to that found in ions was not as rapid as the rate of dissolution of the surface each tube gives the probability that sticking occurs to expose new sodium. Other evidence that ion exchange is not the uniquely on the first collision. In carrying out this expericontrolling factor in the rate of entrance of ions from solu- ment the source of particles and the quadrant astion into surface structures lies in the fact that a change in sembly are mounted in a vacuum system in such a the hydrogen ion concentration of the solution by a factor way as to minimize spurious effects due to scattered of lo6changes the rate of Na+ sorption by a factor of only particles. ten or twenXy.1,a In one such experiment13 a square 4 cm.2 deposit 4 . Leaching of Neutron-irradiated Glass Samules.8It may be c a l h a t e d that a 1.63-g. glass s uare of {he type of ceresin wax which had been photobrominated used in this work will give 1.9 X 10'0 dis.Tmin. when bom- with Brso was placed 1 cm. from another square of barded with a thermal neutron flux of 10'2 neutrons/cm.2/ sec. for 4 hours. By immersing such samples in water the same size containing two silver surfaced quadfollowing irradiation and observing the amount of sodium rants and two trap tubes. As s result of the isoactivity leached out the depth of leaching may be eEtimated. meric transition, Br80 (4.4 hr.) -+ Br89 (18 min.), Preliminary experiments show i t to be about 50 A. fdr an the brominated ceresin served as a source of the initial 10-minute leaching period. latter isomer. Following the nuclear process many 5 . Radiophosphate as a Test for Alkaline Attack.loof the Brso (18 min.) atoms broke their chemical Most of the rather meager evidence available'*" indicates bonds and entered the gas phase. The ratio of such that anions are not appreciably sorbed by clean glass surnuclides which stuck to the silver quadrants to those faces. It has recently been shown,I0 however, that treatment of which entered the traps was 0.2: soft glass for a few minutes, or of Pyrex brand glass tor two It should be noted that such ratios may be exhours, with sodium hydroxide solution at 80" changes the pected to be very sensitive to the method of prepacharacter of the surface so that rapid sorption occurs. This ration of the receiving surface. They may also be suggests that the glass normally has a surface layer, altered by deposits of vapors from "non-volatile" of silicic acid, which is different chemically from of t.he glass and protects the latter from contact, with the substances present in the system, such as those desolution. Measurement of the sorption of radiophosphate scribed below. affords a unique test for the removal of this surface layer.
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Reaction of Glass with Bromine Vapor With the aid of radiotracers i t is sometimes possible to demonstrat,e a reaction between gases and surfaces which are normally considered t o be unreactive. Th;l," has been done for bromine vapor in contact with glass. Sections of microscope slides one square inch in area were placed on a rack in a vacuum system and exposed t o bromine vapor: (a) in the dark at room temperature for 30 minutes; (b) with intense illumination for 30 minutes, and (c) in the dark a t 150-175" for 30 minutes. It was found that about one molecular layer of bromine became attached t o the glass in the fist two cases and three times as much in the last case. This w~kstrue both for samples which had been prepared only by thorough wiping with cleansing tissue and for those (11) W. A. Weyl. Glass Ind., 28 (8),408, 428 (1947). (12) F. J. Johnston, Ph.D. thesis, University of Wisconsin, 10.52.
Reactions of Surfaces with Vapors of Compounds of Low Volatility It has been observed repeatedlyla that when thiu samples of organic compounds which are usually considered to be non-volatile are mounted in a vacuum system, amounts of these compounds several monolayers thick appear on other surfaces in the system. This is the case even when the system is continually pumped a t a pressure of less than lou4 mm. Some experiments show that the tendency for the adsorbed layers t o form is styongly dependent on the nature of the receiving surface. Thus surfaces of silver arid Pyrex which received equiva(13) W. E. Rice, Ph.D. thesis, Uiriversity of Wisconsin, 1952.
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lent esposure in an evacuated system containing sodium m-hydrosytribromobenzoate tagged with BF2 (36 hr.) gave counting rates equivalent to 3 molecular layers and 0.8 molecular layer of the compound, respectively. In some experiments much.thicker layers were observed. Since the conditions of such experiments seem to preclude the presence of saturated vapors of the condensing subst.ances the receiving surfaces must act on the molecules in such a way as to reduce the vapor pressure of the solid compounds. The cause of the phenomena is presumably an inductive effect similar to that which is thought to account for the fact14 that ten to 500 molecular layers of water, benzene, or mercuric iodide adsorbed on glass or platinum may be in equilibrium with their unsaturated vapors.
had an “ultra fine structure” of the surface which resulted in some of the plated atoms being buried a t a depth of a micron or so. Mechanism Studies on Fischer-Tropsch Synthesis.-Carbon-14 has proved a valuable tool in answering the question as to whether a metallic carbide is an intermediate in the synthesis of hydrocarbons from carbon monoside and hydrogen over iron, nickel and cobalt cata1ysts.l’ By carhiding the catalyst surface with C14 before carrying out a hydrocarbon synthesis and then measuring the radioactivity in the hydrocarbon after synthesis it was shown that the majority of the product is formed by some process not involving the reduction of a carbide intermediate. Surface Exchange Reactions of Aluminum Halides.-With the aid of aluminum chloride tagged Other Applications with C136it has been found that a surface of solid aluminum chloride is necessary for the exchange of Adsorption of Surface-active Agents at a Solution-Air Interface.-The maximum range of the chlorine between aluminum chloride and carbon beta radiations emitted by S3b is only about 0.3 mm. tetrachloride.18 Subsequently it, was shown that a of water and most of them are absorbed by a much variety of gaseous organic chlorides1gand also elesmaller thickness. Salley15a-dand co-workers and mental chlorine12 and hydrogen chloridelgb will Aniansson and Lamm15eJhave made ingenious use of exchange with aluminum chloride. These observathis fact by comparing the counting rates at t,he tions led to demonstration of the fact that Friedelsurfaces of solutions containing S35 in the form of Crafts syntheses can be carried out between gasedi-n-octyl sodium sulfosuccinate in one case and ous reactants in contact with solid aluminum chloin t.he form of sodium sulfate in the other. The ride. In other work6 it has been observed that gaseous over-all concentration of S36was the same in the two cases, but in the former the compound tended methyl bromide will exchange with solid aluminum to concentrate a t the liquid-air interface and thus bromide and with barium bromide but not with poyielded higher counts. From a series of such ex- tassium bromide. Acknowledgment.-This work was supported in periments at different concentrations it was possible to obtain a well-defined adsorption isotherm. The part by the United States Atomic Energy Commisauthors state: “The method seems to he generally sion and in part by the University Research Coniapplicable to surface phenomena involving adsorbed mittee with funds made available by the Wisconsin layers, either mono- or multimolecular in nature Alumni Research Foundation. -other radioisotopes having sufficiently soft raDISCUSSION diation such as C14, Ca45,Fe59, etc., could be employed-the method appears to open considerable (1) S. C . LIND(Carbide and Carbon Chemical Corporibpossibility for examining interaction in surface lay- tion).-Will Dr. Willard please extend his remarks regarding the possible influence of diffusion of active sodium int.0 ers.” Another method for studying the concentration glass and ion exchange with sodium in glass on his results of a solute in surface layers involves removing for adsorption? J. E. b‘ILLARD.-Despite a considerable number of invessamples of the surface film on a loop of wire. It [see references given in reference (S)] of the int,eris not limited to isotopes whose radiations are of tigations action between glass and ions in solution the nature of t.he low penetrating power.16g physical and chemical processes which control this interDetermination of Depth of Surface Penetration action is still very obscure. I t is probable that several processes in addition to the ion exchange remtion play a role by Alpha Range Measurements.-Alpha particles in governing the rate and amount of sorpt>ionof sodium ion emitted in radioactive decay have very short from solution. These may include the diffusion of the hyranges in matter. Use has been madeI6 of this drated sodium ions through the film of liquid at the surface property to test the depth to which polonium atoms of t,he glasa, the dehydration of ions as a function of temper:tture, the changes in hydration and tionding strurtuw of penetrated surfaces on which they were eleotm- the which may occur HS a function of teniper;tt~ur,c,t l ~ c plated. The information was obtained by measur- rateglass of dissolution of the glass, and the process of diffusion ing the residual ranges of the alpha particles from of ions into and out of thc interior of the glass. the plated atoms. It was found that samples of (2) T. J. ZAK (Bausch and Loinb).-Was Dr. Willard’s gold, and to a lesser extent, platinum and nickel work confined only to soda lime glasses or were heavy barium (14) J. C. Henniker. Rev. Modern Phy., 21, 322 (1949). (15) (a) J. K. Dixon, A. J. Weith, Jr.. A. A. Argyleand D. J. Salley, Nature, 163, 845 (1949); (b) D. J. Salley, A. J. Weith, Jr., A. A. Argyle and J. K. Dixon. Proc. Rou. Soc. (London), 8203, 42 (1950); ( 0 ) C. R4. Judson, A. A. Argyle, D. J. Salley and J. K. Dixon. J . Chem. Phys.. 18, 1302 (1950); (d) C. M. Judson, A. A. Argyle, J. K. Dixon and D. J. Salley, ibid., 19, 278 (1951): (e) G.Aniansson and 0. Lamm. Nature, 165, 357 (1949); (f) G. Aniansson, J . Phys. Coll. Chem., 55, 1286 (1951); (g) Hutchinson. J . Colloid Sci., 4, 599 (1949). (16) hI. Haissinsky, 13. Faraggia, A. Cocha and P. Avignon. Plws. Rev.. 7 5 , 1963 (1949).
or Pb glasses also studied? J. E. WILLARD.-The tests were a11 made with soda-limesilica glass. (3) W. H. JOHNSTON (Pwdue University).-I
would like
(17) J. T. Kummer, T. W. DeWitt and P. H. E m m e t t , J . A m Chem.
(1948).
Soc.. 70, 3630 (18) C. Ihatthe magnitude of the activation energy (ca.10 kcal.) found for sodium ion in this work is consistent with the view that the rate-controlling process is a diffusion in and through the hydrated silica lattice or gel. If the controlling process were a diffusion through the aqueous medium in contact with the glass, much lower activat.ion (ca.4-5 kcal.) energies would have been found. (9). W. PHILIPPOFF (Franklin Institute).-Further tracer experiments were done in the flotation field by Gaudin and collaborators a t M.I.T. Dodecylamine acetate with only 5% coverage is needed for flotation, thus proving that in the field only incomplete monolayers are formed. The amount of coverage can be measured by a very simple method using the contact angle. This method is applicable to monolayers of hydrophobic materials (long-chain ions) on surfaces. J. E. WILLARB.-A discussion of such applications and further references may be found in the paper by A. M. Gaudin, and P. L. DeBruyn, Sci. Monthly, 365 (1950); and the paper by G. L. Simard, M. Burke and D. J. Salley, Transactions A.I.M.E., 190,39 (1951).
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(10) GEORGEE. BoYD.-It is suggested that the 2.8 h Sis1 could be used quite nicely to determine the extent tjo which fiolution processeR are occurring when glass ifi treated with various aqueous solutions. This silicon activity can be induced in glass by neutron bombardment as was the case with the sodium activity. I t would be of great interest to know how important this effect is in the studies that have been presented.
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J. E. \hLAm.-Yes, this might give very useful information and should be tried. Care will be required to separate the silicon from the other radioactive species preBent before counting, and the length of the ex eriments will be limited by the 2.8-hr. half-life of the Si". &ass could be synthesized from pure irradiated silica and the rate of ~ O S Sof silica to the solution measured but the experiments would have to be completed within a day or so of the end of the irradiation.
RADIOACTIVE MONOLAYERS : A NEW APPROACH TO SURFACE RESEARCH BY D. E. BEISCHER U . R. Naval School of Aviation Medicine, Pensacola, Florida Received June 2 , 196.8
The study uses monomolecular layers prepared by the method of Langmuir and Blodgettl to brie a limited upp ply of a reagentjn the form of a homogeneous monolayer of carbon-14 tagged stearic acid on the surface of diierent materials. The system is heated to a degree where chemical reaction of the coating material with the topmost layer of the surface takes place. The excess of the monolayer reagent is then removed and the distribution of the reaction product visualized in a radioautograph of the surface. The extreme sensitivity of this method allows localization and measurement of small differences in the chemical activity of a surface.
Introduction The concept of unimolecular layers has contrihuted much to the exploration of the structure and properties of the solid surface since Langmuir pointed out in 1916 that due to the rapid decrease of intermolecular forces G t h distance, the first adsorbed layer shows preferred binding to the adsorbent. The usual adsorption studies draw conclusions from the phenomena observed during the gradual build-up of a surface layer. The procedure here described approaches the probIem of surface activity in a different way. It offers to the surface A homogeneously distributed limited amount of material in the form of a Langmuir-Blodgett' monolayer of stearic acid. This "forced on" monolayer is not, a t first, in equilibrium with the surface of the supporting material. Observation of the adjustments taking place under the influence of thermal movements in the system may give valuable information concerning the binding forces extending from the surface. The u8e of carbon-14 labeled stearic acid as a monolayer substance and a photographic plate as the most convenient radiation detector will make these surface reactions in molecular dimensions visible in a direct way. Since improvements of the radiographic technique developed for this study may be useful in other applications, they will be described in detail in the following experimental procedure. Experimental Procedure The preparation of the radioactive surface layers followed the procedure described by Langmuir and Blodgettl for the formation of "built-up" layers. The application of carbon14 tagged stearic acid2 as a s reading substance changes the normal procedure only in-so-?& as precautions against radio hazards are taken. The tray (10 X 22 cm.) has a well on one end. It was made of plexiglass and covered by a thin layer of paraffin applied by dipping into a paraffin-bath. (1) K. B. Blodgett, J . A m . Chem. Soc., 57, 1007 (1935), and later Publications by Langmuir and collaborators. (2) The carbonyl labeled stearic acid with an isotope abundance OF about 1% was supplied by the Texas Research Foundation, Rennsr, Texas, on allocation from the Isotopes Division, U. 9.Atomic Energy Commission, Oak Ridge, Tennessee.
The coat of paraffin with traces of radioactive mat,erial could be easily removed by a stream of hot water and replaced by another coat for further experiments. The spreading tray was placed into a stainless steel safety tray and covered by a transparent cellulose acetate bell shield for protection against dust from the atmosphere. The stearic acid was spread on a solution which proved very satisfactory in Blodgett's experiments with monolayers: lor4 M barium chloride; 2 X 10-4 M KHCOI in double distilled water. The pH of this solution was about 7 and the monolayers consisted of a mixture of about 60% barium stearate and 40% stearic acid.8 Using the movable thread technique, the monolayers were compressed by oleic acid (29.5 dyne per cm., 25'). In experiments with monolayers consisting of salt-free stearic acid, the spreading was performed on a surface of double distilled water or 1 X 10-8 N HC1. The monolayers were transferred from the aqueous solution to a solid substratum by immersing a plate of a few square centimeters area into the above-mentioned solution before spreading the stearic acid on the surface of the bath. The plate was pulled out of the bath a few minutes after spreading the radioactive monolayer. Each succeeding dipping adds two monolayers to the previous layers, one deposited on the down trip and one on the trip out of the solution. By dipping the plate t o regularly decreasing deDths, wedge-form layers with various activities on one plate can be h i l t . Verv different materials were used as substrata. Mica in the f&m of selected freshly split Indian ruby mica sheets was aged for a day to neutralize the surface charge generated by the cleaving process. A number of the mica plates were specially treated before coating with a monolayer. I n some experiments the surface was scratched slightly by a fine needle. Another pretreatment consisted in treating the surface with a metal ion solution or in making a fingerprint on the mica plate. The minerals, molybdenite and bruoite, were also used in freshly split and aged condition. Plates of different metals like aluminum, copper or lead were polished and made fat-free before coating with a monolayer. The analytical separation of the fraction of stearic acid which remained unohanged on the surface, from the fraction which was bound chemically to atoms of the surface was carried out in one of two different ways: (1)The free stearic acid was removed by extraction, dipping the coated plate for one minute into benzene. (2) The free stearic acid was evaporized in the vacuum generated by a Cenco Hyvac Air Pump: An investigation of these analytical methods revealed that benzene dissolves not only stearic acid but also a certain percentage of the metal stearate. The percentage of dis(3) I. Langmuir and V. J. Schaefer, J . Am. Chem. 6 o c . . I S , 284 (1936)
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