May, 1958
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519
SOME PROPERTIES OF SURFACE FILMS FORMED BY ADSORPTION OF n-NONADECANOIC ACID ON MECHANICALLY ACTIVATED METAL SURFACES BY HILTONA. SMITHAND TOMLINSON FORT, JR. Chemistry Department, The Unihersity of Tennessee, IOnoxville, Tennessee Recezved November 89, 1867
Fresh surfaces prepared by machining metals under an inert, non-polar solvent are for a short period chemically hyperactive. Such surfaces have been shown to adsorb n-nonadecanoic acid from cyclohexane solution to a limiting value of just one monomolecular layer. Attainment of this saturated coverage is the result of a chemical reaction between the fatty acid and the atoms of the fresh metal surface, activated in the presence of low energy electrons from the surface. The result is the formation of the appropriate metal soap. Adsorption is then a function of the activity of the metal, degree of activation of the machined surface, duration of the activated state, and the free energy requirements of the soap-forming reaction. It is shown that, although the saturated surface coverage may be maintained for long periods of time, adsorbed films of the type described are not static on the metal surfaces. Soap molecules are continually desorbed. The vacancies thus created in the adsorbed la er are filled by diffusion of additional fatty acid to the surface, and then reaction in situ to form additional molecules of a&orbed metal soap. The rate of this desorption and adsorbed film replenishment decreases with age of the metal surface in a fashion which strongly suggests that it, as well as the initial chemisorption, is a function of the activation of the surfaces. This idea is substantiated by the fact that the “exchange” described does not take place at a uniform rate across the machined surfaces. At certain immobile “sites” exchange is many times more rapid than a t other, less active areas. It is then postulated that the sites on the metal surfaces where exchange is rapid are identical with the sites from which the low energy electrons are emitted which are responsible for the Kramer effect. Several conclusions are then drawn about the nature of the electron emission itself. An adsorption picture is outlined which is able to account’ satisfactorily for all the experimental data described.
Previous studies’ have shown that adsorption of n-nonadecanoic acid onto metallic surfaces is strongly influenced by the extent of surface oxidation. I n all experiments, adsorption of the acid was shown to be higher on oxidized than on unoxidized surfaces. The difference in adsorption was attributed to the more favorable free energy change associated with the reaction of metal oxide and acid than that associated with the reaction of fatty acid and metal. I n recently reported work,2 it has been shown that metal surfaces prepared by machining under an inert cover liquid of cyclohexane were able to adsorb the fatty acid from cyclohexane solution to a higher degree than surfaces which had been oxidized by exposure to the air for short periods. Further, adsorption on all the unoxidized surfaces of metals more active than silver reached the same limiting value,oa population density of 4.16 molecules per 100 A2. of geometric surface area. This is the population density which would be obtained if the metal surfaces were of unit roughness factor, and adsorbed exactly one monomolecular layer of acid, in a close packed array limited by the cross section of the carboxyl group. Smith and McGil12 showed that in every case where this saturated adsorption layer was attained, adsorption was the result of a chemical reaction between fatty acid and metal surface atom. They were able to correlate the high adsorptive ability of the freshly machined surfaces with the “mechanical activation” described by Shaw3 and the phenomenon of low energy electron emission from abraded metal surfaces first described by Kramer.4 The object of this work was to extend the adsorption studies described to additional metals in order to determine their adsorption behavior and (1) H. A. Smith and K. A. Allen, THIBJOURNAL.68, 499 (1954). (2) H. A. Smith and R . M . McGiIl, i b i d . , 61, 1025 (1957). (3) M. C. Shaw, J . A p p l i e d Mechanics, 15, 37 (1948). (4) J. Kramer, “Der Metallische Zustande,” Vandenhoeck and Ruprecht, Gottingen, 1950.
see whether the adsorption mechanism suggested held for these materials. Attention was to be directed particularly to aluminum metal, which had been shown2 to adsorb a monolayer of fatty acid only with difficulty, in spite of the activity of the metal. From these studies it was hoped that additional information would be gained about the whole adsorption process and the activated state of the freshly machined metal surfaces. It was expected that the findings would find application to the subjects of heterogeneous catalysis, boundary lubrication, and corrosion. Experimental Materials.-n-Nonadecanoic acid tagged in the carboxyl position with carbon-14 was synthesized by a Grignard reaction. The Grignard reagent, prepared from n-octadecyl bromide in the usual manner, was treated with radioactive carbon dioxide, labeled to the extent of 1.44 mole %, in a vacuum system similar to that described by Dauben, Reid and Yankwich.6 The product, after recrystallization from acetone, melted at 67-68’. A quantity of unlabeled acid was prepared in similar fashion except that the Grignard reagent was simply poured into a slurry of Dry Ice and dry ether, to effect carbonation. Cyclohexane was used exclusively as solvent. The technical grade material was first fractionally distilled to remove most of the contaminants, then passed through a 25-mm. X 4-ft. adsorption column. The upper half of this column was packed with Davidson 100-200 mesh silica gel, and the lower half with Alcoa F-20 grade activated alumina. Solvent was collected in all glass receptacles a t the rate of one drop every six seconds, and used immediately. The efficiency of the purifying technique was checked periodically by examining the solvent spectrophotometrically for the characteristic absorption maxima of benzene. Several liters of solvent could be purified with a single charge of adsorbent. Stock solutions of the fatty acid were prepared by dissolving 67.76-mg. samples in 500 ml. of purified cyclohexane. These were then diluted to produce a series of working solutions. The primary solutions had an acid concentration of 2.63 bg./ml. and were designated solution A or solution U, indicating radioactive or non-radioactive acid, respectively. Other solutions were fractions or multiples of these acid concentrations, and were so designated. ( 5 ) W. G. Dauben, J. C. Reid and P. E. Yankmich, Anal. Chem., 19, 828 (1947).
5 20
HILTONA. SMITHAND TOMLINSON FORT, JR.
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I n the course of this work, adsorption experiments were run on copper, magnesium, silver, aluminum, tin, indium and cobalt. The samples were analyzed spectroscopically for impurities, and t.he assays are given in Table I. All concentrations of impurities are given in per cent. and unlisted impurities were undetectable. All the samples were in the form of 1.5-inch cylinders and were mounted for machining as previously described.1 For the experiments run at constant temperature the metal sample blocks were hollowed so that water from a constant temperature reservoir could be pumped through them and forced, by means of
carbon tetrachloride solution. The colors were then compared with standard silver samples similarly treated. Differences in silver content of 0.05 pg. easily could be distinguished. Aluminum was determined using aluminon rcagent and a modification of the method suggested by Sandell.? Hues of solutions of standards and unknowns were compared in 450 X 11-mm. tubes, using 0.2 ml. of a 0.2y0 solution of the reagent. Differences in aluminum content of 0.1 pg. could be clearly distinguished. Copper was determined spectrophotometrically using dithizone as the color generating agent. The determination was made in cyclohexane solution using copper nonadecanoate solutions as standards. These standards could not be prepared directly, TABLE I as the saturation solubility of the soap in this solvent (1 x SPECTROGRAPHIC ASSAYSOF THE PURITY OF THE METAL lo-’ g./ml.) is too low for accurate weighing. Accordingly, SAMPLES~ , a standard solution of the soap in chloroform was prepared, and measured aliquots were admitted to 50-ml. graduated ImMetal sample purity Sn Co In Cu Ag Mg AI mixing cylinders. The solvent was then completely evapoAg 0.001 . . . . . . . . 0 . 0 0 2 Strong . . . . . . . . rated. About 25 ml. of cyclohexane then was added to each AI .... 0.005 .... 0.001 0.001 0.001 Strong cylinder. The copper unknowns were cyclohexane solutions Bi 0.01 ................ .... of the soap in the adsorption cups, which have been preCo .... Strong . . . . . . . . . . . . .... viously described.2 Two milliliters of O.OOl’j?odithiaone in Cr ........................ 0.02 cyclohexane now was added to each soap solutioh, and Cu 0.03 0.1 0.001 Strong 0.01 0.002 0.03 these were vigorously stirred for one hour. At the end Fe 0.03 0.1 .... 0.05 0.03 0 . 0 2 0 . 5 of this period the unknown copper solutions were poured In . . . . . . . . Strong . . . . . . . . . . . . . . . . into other mixing cylinders, and then the volume of each Mg ........ 0.0005 .... Strong 0 . 5 was brought to 40 ml. with more cyclohexane. The cylinders Mn .... 1 . 0 . . . . . . . . . . . . 0.002 0.05 next were shaken and examined as quickly as possible with Ni 0.002 1.0 . . . . 0.003 . . . . . . . . 0 . 2 the spectrophotometer. Determinations were run using 5-em. absorption cells at a wave length of 510 mp against a Pb 1.17’ .... ............ Si .... 0 .... 0 . . . . . . . . 0.4 cyclohexane blank. Beer’s law waa followed and results Sn Strong; 0 . 2 .................... were fairly precise. This indicates that the chelating agent is strong enough so that reaction was quantitative. Color Ti ........ . . . . . . . . . . . . 0.03 was not particularly stable a t these low concentrations and a The concentrations of the impurities are given in per cent. All unlisted metal impurities were undetectable. the reagent used as described loses all specificity for copper. All the impurity levels listed are correct to a factor of three. This was no handicap in the present analysis, however. * The percentage of lead in the tin sample was determined by Results a more sensitive method and should be correct to &O.l%.
’
a baffle, directly against the under side of ‘the thin metal sheet whose upper surface was to be machined. Jackets were also prepared for the stirred adsorption cups, so that they could be thermostated in similar fashion. Twenty minutes was allowed for the attainment of temperature equilibrium prior to machining a new surface. The machining and adsorption cups were covered with a ground glass plate during this period to prevent solvent evaporation. Temperature could be kept constant within &0.lo. The technique of measuring the amount of radioactive acid which had been adsorbed on the metal surfaces has been described previously.z Measurements were made with a Nuclear Instrument Company Model 165 scaler. The Geiger tube was a Tracerlab TGC-2 end-window tube having a mica window thickness of 1.8 mg./cm.2 and was filled with helium and a quench gas a t 720-mm. pressure. The tube was shielded with a 1/8-inch lead jacket. The tube window was equipped with an aluminum mask having a 1inch hole in its center against which the samples were pressed to obtain a reproducible counting geometry. Technique’s.-The machining techniques used have been described previously.2 Radiochemical standardization of the labeled n-nonadecanoic acid was done as described by Smith and Allen.1 Variations used for ‘individual experiments are described in conjunction with the experimental results. Analytical Methods.-In the course of this investigation, microanalytical determinations were made of cyclohexane solutions of a variety of metal soaps. In most cases, the analysis was effected by bringing the metal into the aqueous phase by three extractions with 5-ml. portions of approximately 0.1 N sulfuric acid. Cobalt then was determined spectrophotometrically, using nitroso-R salt and the method suggested by Dean.B Iron was also determined spectrophotometrically using 1,lO-phenanthroline and the method outlined by Sandell.? Silver was determined using copper keto dithiaonate as reagent. The unknowns were determined by shaking, in small vials, the aqueous extracts containing the silver with 0.2-ml. aliquots of the reagent in (6) J. A. Dean, ibid., 23, 4096 (1951). (7) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 2nd Ed., Interscience Publishers, Idc., New York, N. Y . ,
1950.
Adsorption experiments were run on cobalt, indium and tin surfaces with a variety of concentrations of fatty acid. The metals were solutionmachined and the wet surfaces were quickly transferred to a stirred adsorption cup and allowed to remain in contact with the fatty acid solutions for periods ranging from three to one thousand minutes. The beginning of the initial cut was taken as zero time. Experiments were run a t room temperature (23-28’) and the solutions were stirred at 140 r.p.m. Transfer of the indium sample from the machining cup to the adsorption cup was slowed slightly due to the fact that the “shavings” produced on machining this metal adhered to the cut surface. These had to be removed with tweezers. Results of these experiments are shown graphically in Figs. 1,2and 3. Desorption experiments were run on the two metals which had been found to adsorb a monolayer of fatty acid. A redetermination of the desorption rate from a copper surface also was made. The metals were solution-machined under solution A (indium was machined under solution 2A), then quickly transferred to 25-ml. adsorption cups for 15 minutes total acid exposure time. During this period a monolayer of fatty acid was adsorbed on each metal surface. The samples then were quickly transferred to stirred 75-ml. desorption cups which contained pure cyclohexane. The adsorbed fatty acid was allowed to desorb for measured time intervals after which the surfaces were removed, blotted dry and counted. The transfers were made very quickly so that the surfaces were always kept wet with solvent. Experiments were carried out at from 23-28’, solutions were stirred a t 140 r.p.m., and results are given graphically in Fig. 4.
SURFACE FILMS FORMED BY ADSORPTION
May, 1958
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521
The cobalt(I1) and iron(II1) nonadecanoates mere prepared by the methods outlined by Elliott* and by Whitmore and L a ~ r o . The ~ cobalt soap 4 was obtained from benzene as a lavender-colored o2 solid which decomposed when heated to about 0 200". The iron soap was a salmon-colored solid 2 3 which melted at 112-113'. The indium and tin soaps could not be prepared by the method described because of their extreme tendency toward 3 hydrolysis. 22 The solubilities of the cobalt and iron soaps were .determined a t 25" in water and in cyclohexane. +;, eP Saturated solutions were assured by equilibrating excesses of the soaps with the solvents a t an ele31 vated temperature, and then at 25", for periods of several days. These were then filtered and analyzed for their heavy metal content. The water 0 solubility of the nonadecanoates obtained a t 25' 1 5 10 50 100 500 were: iron soap, 4.75 x 10-8 g./ml.; cobalt soap, Acid exposure time, min. 6.83 X 10-7 g./ml. The cyclohexane solubilities Fig. 1.-Adsorption isotherms on cobalt: 0, solution 2A; at 25" were: iron soap, 3.92 X g./ml., cobalt 0, solution A; 0 , solution A/2; - - - -, monolayer adsorpsoap, 3.16 X 10-6 g./ml. These may be com- tion. pared with other solubility determinations given previously. The values obtained for the water solubilities of the two soaps and the values reported previously2 4 for the solubility and dissociation constant of n-nono$ adecanoic acid were used to calculate the free 0 energy changes involved in poap formation from 2 the metals and fatty acid a t 25" and under a re-33 ducing atmosphere. If (Nd) represents the n-nonadecanoate radical, and H(Nd) represents n-non3 adecanoic acid
I I
3
e
c3
'/&o(s)
+ H(Nd)(s) +' / z C O ( N ~ ) ~ (+S )'/zHz(g)
AFzss0 = -3.2 kcal./mole
I/sFe(s)
4-H(Nd)(s) +1/3Fe(Nd)3(s)
+ l/zHz(g)
A F z s ~= $1.6 kcal./mole.
These values may be compared with the results of similar calculations published previously.2 The results given in Figs. 1 4 show quite different behaviors for the three metals studied. Cobalt may be said to behave "normally" when one considers its position in the electrochemical series and the results for other metals previously reported.2 The saturated population density of 4.16 molecules per 100 A.2 of geometric surface area was finally attained when it was machined under three different solution concentrations of fatty acid. The difference in times required to reach this level may be attributed simply to a concentration effect. It has been shown previously for other metals2 that when the saturated adsorption level was attained adsorption was due to a chemical reaction between fatty acid and metal surface atom. The desorption rate from the cobalt surface supports this. If rate of desorption is a function of solubility of the soap in the desorbing solvent, cobalt(I1) nonadecanoate should be more soluble in cyclohexane than the corresponding copper, silver and lead soaps,2 but much less soluble than n-nonadecanoic acid. The solubility determinations showed that this was indeed the case. (8) 5. B. Elliott, "The Alkaline-Earth and Heavy Metal Soaps," American Chemical Society Monograph No. 103, Reinhold Publ. Corp., New York, N. Y.. 1946. (9) W. F. Whitmore and M . Laura, Ind. Eng. Chem., 22, 646 (1930).
8 2
E? *a
8
2 1
0 5 10 50 100 500 Acid exposure time, min. Fig. 2.-Adsorption isotherms on indium: 0, solution 2A; Q, solution A; Q, solution A/2; 0 , solution A/8; - - - -, monolayer adsorption. 1
Calculation shows that the cobalt soap formation reaction has a negative free energy change associated with it. This is a more favorable situation than has been found for copper, lead and silver2 for which the formation of the soap has been proven. This further verifies the chemical reaction hypothesis. It is interesting t o note that for the five metals for which these calculations have been made, the free energy changes associated with the soap formation reactions exactly parallel the order of free energies of formation of the metal ions. Hence, further determinations are probably unnecessary. For all metals more active than cobalt the change will almost surely be negative, and positive for those less active. Indium was found to adsorb a monolayer ,of nnonadecanoic acid only when machined under the most concentrated solution, 2A. Progressively wider departures were noted at lower acid concentrations, and an actual decrease in adsorption was
HILTON A. SMITHAND TOMLINSON FORT, JR.
522
Vol. 62
behavior. Indium is, like aluminum, trivalent but is a larger atom and steric limitations should consequently not be as severe. If the difficulty of 4 crowding three acid molecules around a single & metal atom were the thing that made the soap film 0 unstable, adsorption should have been higher on 2 -23 the former metal. Actually, the reverse was found 3 to be the case. Adsorption on tin metal was always much less than the calculated monolayer value. Adsorption E2 declined after about ten minutes acid exposure time ._ $ in a manner which closely paralleled that of the .+a 2 adsorption isotherms on gold and platinum stirs faces. This behavior was clearly anomalous, and 41 an explanation was not immediately apparent. It appeared that all of the inconsistencies mentioned could be explained if it were assumed that the adsorbed fatty acid film was not static, but that 0 there was continual desorption of metal soap. 1 5 10 50 100 500 Essentially complete surface coverage would then Acid exposure time, min. Fig. 3.-Adsorption isotherms on tin: 0, solution A: be maintained by diffusion of additional fatty acid 0, solution A/2; - - - -, monolayer adsorption. to the surface and reaction there with the newly exposed metal surface atoms to form more soap. The 100 low adsorption on aluminum, indium and tin could then be due to an exceptionally fast rate of desorption from these metal surfaces, due perhaps to relatively high solubilities of these soaps in cyclo80 hexane. This hypothesis was subjected to a variety of e; M tests. Most of the tests were actually carried out 2 on copper metal, as this material is easily machin$ 3 60 able, adsorbs a monolayer of fatty acid readily, shows no oleophobic property with regard to the solution a t any fraction of surface coverage, and because solubility measurements had already been made on the copper(I1) soap in both water and cyclohexane. However, results should be applicable to the other metals also. The copper soap formation reaction has been shown to have a positive free energy change.2 20 Reaction is then only possible because of the activated copper surface from which the “Kramer electrons” are being emitted. This electron emission is known1° to decrease with time. If desorption and monolayer replenishment are dependent on 0 solubility of the desorbed soap, the Kramer effect 0 0.5 1.0 1.5 2.0 must remain operative until solution saturation is Desorption time, hours. Fig. 4.-Desorption isotherms from metal surfaces: 0, accomplished. This was tested by machining a copper; @, cobalt; 0, indium. copper surface, allowing it to adsorb a monolayer of acid, then transferring the wet surface to new noted for this metal a t the higher acid exposure adsorption cups containing additional radioactive times under the most dilute solutions. Desorption rate from the indium surfaces was faster than that acid solution. These, of course, were not saturated from any of the metals for which soap formation with desorbed copper soap, and it was expected had been proved, but much slower than desorption that a decrease in surface coverage would be noted from gold and platinum surfaces on which it has due to the inability of the decaying Kramer effect activate sufficient sites to keep pace with the debeen shown that chemical reaction does not take to sorption. No decrease in adsorption was noted, place. The free energy requirements of the inhowever. dium-fatty acid reaction should be such that reacT o prove that the (‘exchange” rate was not simtion could occur. In view of these facts, reaction was considered likely. The rapid departures from ply a function of soap solubility, a saturated solumonolayer adsorption then had to be explained. tion of non-radioactive copper(I1) nonadecanoate Indidm is a more electropositive material than in cyclohexane was prepared, and a solution maeither copper or silver, both of which have been chined copper surface covered by an adsorbed film of shown2 to form monomolecular layers more easily radioactive acid transferred t o it 15 minutes after than indium does. It is also doubtful that steric (10) 0. Haxel, P. G. Houterrnans and IC. Seeger, 2. Phveik, 130, limitations could cause the anomalous adsorptive 109 (1951).
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SURFACE FILMS FORMED BY ADSORPTION
May, 1958
523
100
80
d
8?
60
8 m
d!
2
40
g 20
0
0
0.5 1.0 1.5 2.0 Desorption time, hours. Fig. 5.-Desorption isotherms from copper: 0, into cyclohexane; 0, into cyclohexane saturated with copper nonadecanoate. 100
ai
s
0 200 400 600 800 Sample age a t beginning of exchange, min. Fig. 7.-Decay of exchange rate with age of copper surfaces.
3500
210
3000
180
500
30
0
0
80
2
20
0
0
0.5 1.0 1.5 2.0 Desorption time, hours. Fig. 6.-Fatty acid-soap exchange on copper surfaces: 0, exchange of labeled monolayer under solution of unlabeled acid; 0 , exchange of unlabeled monolayer under solution of labeled acid.
machining. After various time intervals the sample was removed, blotted dry and counted. A new surface was prepared for each experimental point. Results are given in Fig. 5, where they are compared with the rate of desorption of a similarly prepared film into pure solvent. It is evident that solution saturation does not prevent desorption, as a marked departure from the saturated adsorption level was noted. The question now was whether “exchange” ac-
100 200 300 400 500 600 Scale for 0 and 0 4 8 12 16 20 24 Scale for Exchange time, minutes. Fig. S.-Exchange rates determined from analytical measurements: 0,copper; 0 , aluminum; 0, siIver.
tually occurred while the copper surface was equilibrated under the fatty acid solution, or whether the adsorbed film was kept static due to some property of the acid. To settle this point, a copper surface was machined under solution A, transferred to a stirred adsorption cup for 15 minutes total acid adsorption time, then transferred, while wet, to another cup which contained stirred solution U where it was kept for various time periods. It was then removed, and the fraction of the initial radioactive monolayer determined. The concomitant curve was also developed by measuring the radio-
HILTON. A. SMITHAND TOMLINSON FORT, JR.
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the process involves labeled or unlabeled adsorbed molecules, and hence the presence of radioactivity serves no function other than that of its use as an 4 analytical tool. A copper surface was machined under solution U 0 and exposed to this solution in a n adsorption cup 4 3 3 for various time intervals. At measured time intervals it was transferred, for 30-minute periods, 0, t o another cup containing solution A. It was then removed, blotted dry, and the pick up of radioi 2 active acid noted. Results of a series of such .$ e experiments are given graphically in Fig. 7, and are Y i? another confirmation of the exchange hypothesis. The exchange also was verified by another type 31 of experiment. If soap molecules are continually desorbed from the metal surfaces, there should be a gradual build up of metal in the fatty acid solu0 tions. Accordingly, these solutions were analyzed 0 5 10 50 100 500 1000 for copper after various metal surface exposure Acid exposure time, minutes. times. Results are given in Fig. 8. Zero time was Fig. 9.-Adsorption isotherms at elevated temperatures 15 minutes after machining, when the surface with on copper surfaces: 0,25’; a, 48’; Q45’ without stirring; its adsorbed film was transferred t o a new fatty 0, 60”; - - - -, monolayer adsorption. acid solution. Figure 8 shows that there is an increase in copper concentration in the cyclohexane 80 solution with time which verifies the exchange hypothesis. 70 The results of the experiments given in Figs. 7 and 8 point up the interesting feature that the ex93 60 change rate is not constant but decreases with age of the metal surface. This decrease cannot be due 0 E to saturation of the acid solutions with soap as it 50 has been shown (Fig. 5 ) that desorption occurs 4 even into a completely saturated soap solution. 0 Accordingly, several tests were made of factors 40 which it was thought might influence the exchange E a rate. 2, Y 30 The effect of different concentrations of acid s solution was first determined. A series of experi20 ments was run in which copper metal was machined under a variety of non-radioactive acid solution ts concentrations, allowed to adsorb a monolayer of 10 acid, then was transferred to stirred cups which contained radioactive acid of various concentrations. 0 Rate of exchange was determined as a function of 0 2 4 6 8 10 12 time. Exchange was found t o be unaffected by Exchange time, hours. the concentration of the acid solutions, as long as Fig. 10.-Exchange rates determined from radiochemical this was great enough to maintain a monolayer of measurements: 0,aluminum; a, copper; a, magnesium; fatty acid on the surfaces. 0, silver. A series of adsorption measurements was run on activity of a surface on which an inactive layer had copper metal a t various temperatures. Results been adsorbed after various exposure times to are given in Fig. 9. It is evident that high temsolution A. The experiments were run in thermo- perature suppresses the adsorption and this may stated equipment a t 25”, and solutions were stirred probably be attributed to a higher rate of desorpwith a magnetic stirring bar a t 140 r.p.m. A new tion of soap molecules at 48 and 60” than a t 25”. surface was machined for each experimental point. This predominates over the higher number of colResults are given graphically in Fig. 6. These two lisions of fatty acid molecules with the surface and experiments showed conclusively that “exchange” the higher average energy of the potentially reacting adsorption was indeed occurring, as it had been acid molecules. It is interesting to note the necesshown that transferring a surface with adsorbed sity of stirring the acid solutions in the adsorption acid film from one cup t o another did not affect experiments. One of the curves of Fig. 9 is an adtotal surface coverage. The changes noted could sorption isotherm on a copper surface in which the then only have come from desorption of soap from fatty acid solutions were not stirred. The partly the surface and filling of the resulting vacancy in covered surface forms a barrier to further adsorpthe adsorbed layer by other molecules of fatty acid. tion by depleting the solution of fatty acid in the It may be noted from Fig. 6 that there is no dif- vicinity of the surface. As desorption occurs from ference between the rate of “exchange” whether the partly covered surface total coverage declines. m
0-4
-e
W
3
-
SURFACE FILMS FORMED BY ADSORPTION
May, 1958
3.2
I n the stirred solutions this barrier is overcome and surface coverage increases to the levels indicated. The rate of “exchange” from surfaces of three other metals was measured by both the radiochemical and analytical techniques used for copper. Results are included in Figures 8 and 10. Zero time was always 15 minutes after machining, a t which time the metal samples were transferred to new adsorption cups and the exchange experiments run in the thermostated equipment at 25”. Solutions were stirred a t 140 r.p.m. The results given in Figs. 8 and 10 verify that exchange occurred on each metal surface tested. The exchange rates decreased in the order aluminum > copper > silver.
2.8 2.4 oi
8 2.0
2
3
.a
(11) I t inay be recalled that the original radioactive n-nonadecanoic
acid is actually tagged to the extent of only 1.44 mole %. However, i t has already been shown t h a t there is no difference in behavior in tagged and untageed molecules, and no error is made if the radioactive acid is treated as though all inolecules were tagged with carbon fourteen of 1.44% of its normal activity.
1.6
42
-g
Discussion Both the measurements of pick-up of radioactive acid by the metal surfaces and the measurements of the amount of soap desorbed from these surfaces indicate the same qualitative behavior for the exchange phenomenon, a fast initial exchange rate which rapidly declines.. The rate of desorption of soap always was found to be faster than the rate of pick-up of radioactive acid. This was as expected. A surface site is probably covered, bared and recovered several times in the course of the exchange process. As individual surface sites which were originally covered by a non-radioactive layer of acid were allowed to exchange under a radioactive acid solution,ll only the first such desorption and readsorption contributes anything to the activity of the surface. Further exchange only replaces one radioactive molecule by another. It is very unlikely that any desorbed soap is readsorbed on the surface, however, and so each desorbed molecule is detected by analysis of the cyclohexane solutions. It was found that the exchange on all the different metal surfaces studied occurred in a regular and predictable manner. Plots of the logarithm of the exchange time us. the logarithm of the per cent. of the surface covered by the non-radioactive acid gave good straight lines. This relationship is summarized in Fig. 11 for magnesium, copper, silver and aluminum. An attempt was made t o find a similar relationship between total amount of metal desorbed and exposure time to the fatty acid solutions. The data were not as precise as that obtained from the radiochemical experiments, but reasonable straight lines were obtained when the logarithm of exchange time was plotted as a function of quantity of fatty acid desorbed. These relations are summarized in Fig. 12 for copper, silver and aluminum. Because it had been found possible to express both per cent. exchange and amount of soap desorbed as functions of time, it was possible to make a quantitative comparison of the two sets of data, The treatment which follows is based on the assumptions that desorption is the rate-determining step in the exchange from all surfaces where monolayer coverage is maintained, and that the exchange ability of the metal samples was uniform
525
1.2 0.8 0.4
0 1.5
1.7 1.8 1.9 2.0 2.1 log ( a - z). Fig. 11.-Kinetic treatment of radiochemical results: aluminum; a, copper; 0, magnesium; 0 , silver. 1.6
0,
3.0 2.5
8
2.0
ai
1.5
*‘
C
E
.3
+a
bb
,o 1.0
0.5 0
0
100 150 200 250 300 350 Scale for 0 and @ 0 500 1000 1500 2000 2500 3000 3500 Scale for 0 Material desorbed, % monolayer equiv.* Fig. l2.-Kinetic treatment of analytical results: 0, silver; (3, copper; 0 , aluminum. * For silver, % ’ adsorption equivalents were used. An adsorption equivalent is 93% of a monolayer. 50
across their surfaces. Then dz/dt = (R)(u - x)/(u) (1) where x = % of surface covered by radioactive mid (adsorption
equiv.) a = total yGof metal surface covered by acid (always 100 in terms of adsorption equiv.) a z = yo of metal surface covered by non-radioactive acid (adsorption equiv.) t = time (minutes) R = rate of desorption from surface (70 adsorption equiv./min.)
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526
HILTONA. SMITHAND TOMLINSON FORT,JR.
To integrate (l), R must be evaluated. It has been shown (Fig. 12) that
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too slow. If desorption were not the limiting step this would not have been expected. The two sets of data can be brought into line if metal desorbed = b ln(t) + c (2) the second premise is discarded and the surface where b and c are constants, t i s time, and metal de- considered to consist of active and inactive sites. sorbed is given in units of % adsorption equiva- A site from which desorption occurs once is then lents. Now, differentiating (2) more likely to have exchange occur upon it a d(meta1 desorbed) = $ = second time than is a site from which desorption has (3) dt t not yet occurred. This idea is by no means new. Inserting this expression for (R) in (1) there is ob- As early as 1925, Taylor12J3suggested the presence of active sites on catalytic materials. A strong tained argument for active surface sites is also furnished by the Kramer effect. The following points are (4) pertinent. or (1) Exchange rate on none of the metal surfaces remains constant with time. Always, in(5) itially fast exchange is followed by a rapid decrease in exchange rate. This general type of behavior is which is easily integrated to yield characteristic of the Kramer effect. b (2) It has been shown2 that the Kramer effect -In@ - x) = - In t const. (6) U is operative and responsible for chemisorption on Equation 8 implies that a plot of In(a - x) vs. ln(t) surfaces prepared in the manner described. (3) It has been postulated4 that the Kramer should yield a straight line with slope of - (b/a). It is significant that such graphs were found to give electrons are not emitted from the entire metal surface but from certain “sites” or “centers.” straight lines for all the metals studied (Fig. 11). Now, equation 2 implies that graphs of quantity These electron emission sites may then be the acof metal desorbed us. ln(t) are straight lines with tive spots on which most chemisorption and reacslopes of ( b ) . Since the value of (a) is constant and tion occurs. equal to 100 (4) It has been stated that the kinetics of the pick-up of radioactivity are as predicted from the from (2), b = slope/(2.303) kinetics of desorption. This would be predicted if and active sites were responsible for exchange and the from ( G ) , Z, = (slope)(100). activity of each site were different but decayed according to the same rate law. The actual results Determination of the (b) values from (2) and (6) would then represent a summation of the exchange then allows comparison of the two types of data. ability of all sites across the surface. This calculation was made, and results are given in It may then be tentatively stated that all porTable 11. It is obvious that the results do not tions of the metal surfaces are not activated to the check. The value of (b) obtained from (2) is al- same degree by the machining process. The more ways greater than that obtained from (8). The energetic surface sites are not only able to activate value of the ratio bz/bs decreases in the order, more chemical surface reactions than are the less aluminum > copper > silver. energetic ones, but also desorb adsorbed metal soap at a faster rate. This implies that energy is TABLE I1 required to separate the adsorbed molecules from COMPARISON OF DESORPTION AND EXCHANGE RATES the metal surfaces, and that this energy is not Metal b ( e q . 2) b b . 0) Ratio greater than that which can be furnished by the Aluminum 929 18.7 49.7 Kramer effect. This must be so because it is deCopper 76.4 22.8 3.35 sorption and not adsorption which is the limiting Silver APP. 4 3.30 1.2 factor in the exchange equilibrium. Magnesium .... 5.10 ... By making use of the Kramer effect and its This lack of correspondence could hardly be due hypothesized influence on adsorption, desorption, to experimental error. I n the case of aluminum and metal surface activity, several other pieces of this would require an error of 5000%. Accord- experimental data may be explained. (1) It has been shown2 that a copper surface ingly, the assumptions on which the mathematical treatment was based were re-examined. The first aged ten minutes under pure cyclohexane is able of these, that desorption was the rate-determining to attain a coverage of only half a monolayer of step in the exchange, still appeared to be valid. fatty acid. It has also been shown (Fig. 10) that It was known that essentially complete coverage much more than half a monolayer of acid is adwas maintained on the metal surfaces over long sorbed by the surface after this time. This is a time periods. It was also known that exchange consequence of the varied activity of the surface. occurs during these periods. If desorption were Ten minutes after machining some of the low not the slow step in the exchange, a departure from energy sites have decayed so that they can no complete surface coverage would be expected, and longer activate surface reaction, while others can this was not observed. Also, the general kinetics still activate many reactions. of the exchange are as predicted, the only difficulty (12) H.S. Taylor, Proc. R o y . SOC.(London), AlO8, 105 (1925). (13) H. S. Taylor, THIEJOURNAL,80, 145 (1926). being that the rate of pick-up of radioactive acid is
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May, 1958
RADIOACTIVE TRACER STUDIES OF MONOLAYERS
(2) The difference in the ratio of observed to calculated exchange decreases in the order: aluminum, copper, silver. This indicates either that more active centers are produced on copper than on silver but less than on aluminum, or that the centers have more energy. Verification of these ideas will require additional work, but it is not illogical that the order should be as named, considering the activities of the metals. (3) In the adsorption measurements made at elevated temperatures, adsorption plateaus were noted a t both 48 and 60”. These plateaus occurred at less than monolayer surface coverage. To explain these plateaus as well as similar plateaus noted when most metals are machined under the most dilute acid solutions, it is necessary to postulate that a portion of the metal surfaces is inactive so far as adsorption is concerned. These “dead areas” are the result of the decaying of the Kramer effect. Now, a t elevated temperatures Kramer activity is initially very high but rapidly decays as the sites emit all their available e1e~trons.l~This rapid decay occurs before sufficient fatty acid can diffuse to the surface to form the saturated adsorbed layer, and thus adsorption plateaus are noted at less than a complete monomolecular layer. The situation is probably aggravated by a high desorption rate of the incomplete surface film. As time passes, both the desorption rate and the energy of the active centers decline. These rates are different. At forty minutes acid exposure time the Kramer effect has decayed to such an extent that it is no longer able to keep pace with the desorption, which requires a smaller amount of energy. As soon as this happens, the sharp departures from the adsorption plateaus indicated in Fig. 9 occur. (4) The low adsorption noted on aluminum surfaces now can be ascribed definitely to a very fast desorption rate of the soap from the metal surfaces. This may also be true for indium. This desorption rate probably is due partly to the solubilities of these materials in cyclohexane, and partly to the strained condition of the adsorbed soap molecules (14) I
