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The reaction of oleic acid with copper surfaces
J O U R N A L OF COLLOID AND I N T E R F A C E SCIENCE 9.4, 252--257
(1967)
The Reaction of Oleic Acid with Copper Surfaces M. J. D. LOW,* K. H. B R O W N , AND H. I N O U E School of Chemistry, Rutgers, The State University New Brunswick, New Jersey
Received January 3, 1967; revised April 18, 1967 The reaction of oleic acid with copper plates was studied in air, using infrared emission and reflection spectroscopy. The reaction occurs slowly even at room temperature. The spectra indicate the formation of an ester-like species, tentatively identified as a copper bi-ester, which is the precursor of the binuelear copper oleate complex. The latter is the main product of the reaction. There is some decomposition of oleic acid and oleate above 100°C. The complex is more mobile over the oxide surface than free acid or ester species. Speculations on the mechanism of boundary lubrication as well as other surface phenomena have caused much interest in the adsorption of f a t t y acids on metals, and have prompted a variety of fruitful studies dealing with the formation and nature of monolayers on gold, silver, platinum, chromium, nickel, and other surfaces. However, less information has been obtained with more "reactive" metals such as copper or aluminum because of the occurrence of bulk reactions in addition to ehemisorption. With copper, H a t h w a r and Smith (1) reported t h a t more t h a n one monolayer was formed when oleie acid was taken up from eyelohexane solution, and Yoshioka and Y a m a m o t o concluded t h a t multilayers of oleate formed on copper (2). Daniel noted t h a t benzene solutions of oleic or stearie acid developed a blue coloration when in contact with copper powder, and found t h a t copper soaps were formed (3). Beiseher, using a tracer teehnique, found t h a t a monolayer of stearic acid was more rapidly converted to stearate on oxidized t h a n on clean copper (4). Gaines found multilayer formation on copper in radiostearie acid-nitromethane solution, probably due to soap formation (5). Walker and Ries studied the adsorption of radiostearie acid from dilute n-hexadeeane solution on copper films which * Department of Chemistry, New York University, New York N.Y. 10453.
had been exposed to dry helium or air, and found by chemical analysis that some copper stearate was formed (6). Also, E b e r h a r d t and Mehliss (7) mentioned preliminary work on the infrared spectra of stearie acid adsorbed on air-oxidized copper film, and noted the formation of absorption bands at 1585, 1560, and 1520 em -I due to unidentified reaction products. Such results indicate that oleie or stearic acid reacts readily with copper (or oxidecovered copper). However, little attention seems to have been paid to the nature of the reaction products, although terms such as salt, soap, stearate, or dioleate have been used to describe them. As the nature of the reaction products and intermediates m a y be of mechanistic interest and importance, we are studying such "reactive" systems with infrared spectroscopic methods, and report some observations on the reaction of oleie acid with copper surfaces. EXPERIMENTAL Copper plates 100 X 55 X 1 mm. were used as samples. T h e plates were polished with extra-fine flint paper and cleaned chemically by dipping in chromic acid clearing solution for 5 minutes. The plates were then polished with clean glass wool, washed thoroughly with water, and air dried. Bright, shiny surfaces resulted. However, these were oxide coated, because all work was carried out in Mr. A pair of plates were
252
REACTION OF OLEIC ACID WITH COPPER SURFACES installed in an emission device (8) or in a "homemade" reflection device similar in principle and operation to the reflection attachment marketed by the Perkin-Elmer Corp. Spectra were recorded in air with a PerkinElmer Model 521 spectrophotometer fitted with a Reeder thermoeouple. 1 Samples were placed between salt plates or in commercial cavity cells for absorption-transmission spectra. Most of the emission and reflection work was carried out with fixed slits of 200 , , corresponding to a theoretical slit width of 3 em. -1 at 1600 cm. -1. The transmittance and emission ordinates of the various figures are arbitrary. All spectra were obtained by double-beam operation, using one copper plate as sample and one as reference. The plates oxidized during emission work, but there were no significant differences in emission between bright and tarnished plates. 0leic acid was applied to a sample plate by wiping the plate rapidly with a glass wool pad moistened with acid, and then immediately wiping the plate with dry pads so that the plate appeared to be dry. Other methods, such as depositing oleic acid by covering the plate with a fine spray of droplets of oleic acid solution using an atomizer or depositing, spreading, and evaporating a known amount of solution, did not produce more reproducible results than the wipe-onwipe-off technique. It is estimated that several milligrams of acid were retained after a cold plate had been wiped dry. Slightly larger amounts were retained by plates at 200 ° and 300°C. A few experiments using pure acids obtained from the Hormel Institute gave essentially the same results as those carried out with c.p. oleic acid, suggesting that trace impurities did not affect the bulk reaction to a significant extent. Consequently, most of the work was carried out with c.p. oleic acid.
RESULTS AND DISCUSSION A large number of emission measurements were made at temperatures from i00 ° to 300°C. Emission spectra recorded at sample i C. M. Reeder Co., 173 Victor Avenue, Detroit, Michigan.
253
temperatures near 100°C. were of poor quality because of the low thermal emission of the plates, but acceptable results could be obtained at higher temperatures. An example of an emission spectrum obtained at 150°C. is given in Fig. 1. The spectrum of the acidcovered surface differs from that of the pure oleie acid (spectrum A, Fig. 1) over the spectral range shown, indicating that a reaction producing species other than oleie acid had occurred. The emission spectra changed with the passage of time. Figure 2 shows a sequence of spectra typical of those observed at 200°C. In general, emission spectra recorded immediately after application of the acid to plates at 200°C. or higher showed a prominent band in the 1710-1700 era. -1 range. The band declined rapidly in intensity and broadened. This behavior is attributed to the evaporation of oleie acid from the hot plates. Spectra showing structure much like spectrum B of Fig. 2 were recorded after
°1 B
17'bo
I~'oo
13bo cry'
absorption-transmission spectrum of absorption-transmission spectrum of copper oleate. C: emission spectrum of oleie acidtreated copper plate at 150°C., recorded immediately after acid addition. FIG. l. A: oleie acid. B:
254
LOW, BROWN, AND INOUE
Z O (D (D W
18oo 16oo 14oo crS' FiG. 2. Emission spectra at 200°C. A pair oe plates were installed and the background spectrum A was recorded. Oleic acid was then applied and spectra were recorded. The number next to each spectrum gives the time elapsed in minutes since the acid addition. The ordinates are displaced. several minutes. Slower changes were then observed. The band near 1710 cm. -1 shifted to 1735 cm. -~, all bands including a prominent one at 1585 cm. -~ gradually declined in intensity and became less well defined, and a band near 1535 cm. -1 remained constant or increased slightly in intensity. Results which were essentially identical were obtained over the entire temperature range, except t h a t the changes in the spectra occurred more rapidly at the higher temperatures. The degree of oxidation of the sample plates did not have a marked effect on the emission spectra, because essentially the same results were obtained with "bright" plates bearing invisible oxide films and with plates deliberately oxidized to various extents. In all cases the oleic acid reacted rapidly with the oxide film. For example, if oleic acid was applied at 200°C. to a plate which had been aged b y heating i n air so t h a t a brown oxide film had formed (the brown color corresponds to an oxide film
about 400 A thick (19)), the film disappeared within a few seconds and the plate assumed its original bright, coppery sheen. The plate remained bright thereafter. However, if a bright or a heavily oxidized plate was treated with excess acid at 300°C, a translucent, shellack-]ike coating was formed, indicating t h a t the acid a n d / o r the reaction products decomposed at the highest temperature. Reflection spectra recorded at lower sample temperatures showed the same structure as emission spectra, with the exception t h a t there was no decomposition of acid or reaction products. Some results are given in Fig. 3. As with the high temperature emission results, the reflection spectra recorded at or near room temperature showed t h a t slow reactions were occurring. The results m a y be summarized as follows. A reflection spectrum recorded immediately after application of the acid at or near room temperature was similar to the absorption spectrum of oleic acid. The spectra changed slowly with the passage of time. A band originally observed at 1703 cm. -1, due to the carbonyl of the undissociated acid, slowly declined in intensity and was replaced b y a less intense band at 1735 cm. -I. Simultaneously, a band appeared at 1610 cm. -1, grew in intensity, and shifted to 1585 cm. -I. Also, a broad band in the 1500-1400 cm. -1 region, which is due to C-H modes in oleie acid, grew in intensity at the low wavenumber side of the band.
~0
~-r--
~rw
30 sec. A
,8oo'
,~bo \\'
232 rain. B
438 min. C
612 rain. D
17bo,sbo \\' r/ooI~oo\\,goo r;ooc~-"
FIG. 3. Reflection spectra. The spectra were recorded after the addition of oleic acid at the times shown. The sample was at 27°C. for 277 minutes (spectra A, B) and at 70°C. thereafter (spectra C, D).
REACTION OF OLEIC ACID WITH COPPER SURFACES
The band near 1535 era. -~ found in emission spectra was not observed. Some experiments were also carried out with sodium oleate. A solution of sodium oleate in ethanol was placed on a bright copper plate and the solvent was allowed to evaporate. Reflection spectra were recorded at room temperature at time intervals up to 3 hours after the sodium oleate addition, and after heating at 120°C. for 2 hours. The spectra showed only the presence of sodium oleate, showing that the sodium oleate was unable to react with the oxide film on the plate. The bands at 1585 em. -~ and in the 15001400 cm. -t region were observed in all emission and reflection spectra of acidcoated plates and closely match similar bands found in the absorption spectrum of copper oleate (spectrum B, Fig. 1). Studies employing magnetic, microwave, diffraction, electronic, and infrared spectroscopic as well as resonance techniques (10-21) indicate that copper acetate, butyrate, valerate, caproate, eaprylate, palmitate, oleate, and
,../o
/..o\ /, i
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i
CU
c
stearate have the structure. Satake and ~,iatuura assigned the bands of the oleate at 1585 era.-I and at 1420-1410 cm. -I to the antisymmetrie and symmetric stretehin~ vibrations, respectively (16). The 149(I1410 em. -I band falls in the region of som~'. C-H modes and is consequently not clearly defined, but would produce an increase at the low wavenumbcr side of bands in the 1500-1400 em. -~ region. The shift from 1610 to 1585 cm. -~ observed with reflection spectra prompted a series of measurements to test the hypothesis that the shift was caused by the presence of free oleic acid. A series of solutions of various concentrations of copper oleate in eyclohexane, and also of various concentrations of copper o]eate plus oleic acid in cyelohexane, were prepared and absorption
255
spectra were recorded. The antisymmetrie stretching band shifted to higher wavenumbers with increasing dilution (e.g., 1585 em. -I, 1587 era. -1, and 1595 em. -1 at 1.0, 0.035, and 0.005 mole % oleate, respectively, in the absence of oleic acid) and also with increasing concentrations of oleie acid (e.g., 1593 em. -~ for a solution containing 0.035 mole % oleate and 0.0275 mole % acid, or 1605 em. -I for a solution containing 0.005 mole % oleate and 0.1375 mole % acid). The nature of the interaction between copper oleate and oleic acid molecules causing the band shift is uncertain. The various results, however, suggest that the binuclear copper oleate was formed on the samples ag all temperatures. At low temperatures, when the reaction had not occurred to a great extent, the oleate was associated with unreacted acid; as the reaction proceeded, the degree of association decreased and the band shift to 1585 cm. -1 occurred. In emission spectra the band was observed at or near 1585 em. -1, suggesting that little unreacted oleie acid remained on the surfaces at higher temperatures. The 1735 em. -~ band observed in emission and reflection spectra would thus not be due to free acid. In order to obtain additional information, further experiments were made. Pure copper oleate was heated in air and absorption spectra were recorded. A small band was formed at 1735 era. -~ after 5 minutes at 147°C. The band increased slightly on heating for an hour at 160°C. A minor band also formed near 1535 em.-L Such a band was not found in low temperature reflection spectra, but occurred in all cases when pure copper oleate, oleie acidcopper oxide mixtures, or acid-treated samples were heated above 100°C. The band did not disappear when a specimen was cooled. Such results, as well as the general, slow decline in the intensities of emission bands as shown in Fig. 2, indicate that a slow and minor decomposition of the copper oleate occurred above about 100°C. The degradation of oleie acid in air produced weak bands in the 1700 em. -~ region; after 48 hours at 140°C. a small band was formed at 1730 em. -1 and minor bands elsewhere
256
LOW, BROWN, AND INOUE
(22). The nature of the decomposition products is not known. Although the decomposition products give rise to absorptions in the 1700 cm. -1 region, it is unlikely that they were the sole cause of the band at 1735 em. -1. The reflection spectra showed that the band was formed at low temperatures and relatively quickly, so that it is likely that an additional species was formed. The earbonyl band in esters has been reported to be in the 1730 era. -1 region for many compounds (23). Sinclair et al. reported 1720 em. -1 for the trans methyl esters of oetadeeanoie acid and 1741 em. -1 for methyl esters of elaidie, oleie, linoleie, linolenic, and araehidic acids, for example (24). The similarity in position of the 1735 em. -~ band and the earbonyl band of esters therefore suggests that a species having an ester configuration was formed on the copper surfaces. A similar interpretation was advanced for the behavior of earboxylie acids on iron and cobalt (22). Some experiments were carried out with emission techniques at 150°C. in order to gain information about the mobility of the materials on an acid-covered plate. The oleie acid was applied in a narrow, U-shaped band at the sides and bottom of a sample plate, so that most of the plate was initially bare. The portion of the plate facing the monoehromator slit was bare, so that if an emission band due to acid or oleate were detected, it would mean that acid had moved from the side to the center of the plate. A disc of Irtran-2 was placed in the center of the plate and removed after some time. Absorption spectra of the disc recorded at 10-fold ordinate scale expansion showed no trace of the acid, indicating that acid evaporated from the edges of the hot plate had not condensed on the slightly cooler front surface of the Irtran disc. Spectra typical of those obtained in similar experiments (without the Irtran disc) are shown in Fig. 4. In each case, with spectra recorded approx. 5 minutes after the addition of acid to the sides of the plate, a band appeared at 1585 em.-L The band increased in intensity and other bands formed at 1535 era. -~ and between 1450 and 1350 em. -~. Successive
E
,.,, ~k
/ ;
I
,,j
/ 5N 16bo
'
;4bo
16'oo
'
FIG. 4. Surface mobility at 150°C. Emission spectra after acid had been applied to the sides of a sample, after A, 5; B, 42; C, 113; D, 144; and E, 174 minutes.
spectra were recorded for periods of up to 4 hours. No bands were detected in the 1700 era. -1 region where the free acid on the ester species would show a strong absorption. It may be concluded that only the copper oleate was significantly mobile over the surface; free oleie acid and the ester-like species being more strongly bound to the surface. The various spectra thus show that the reaction of oleie acid with the samples was complex. As the work was carried out in air, even bright plates were oxide covered. Tanner et al. worked with "bright polished" copper, cleaned chemically in a more vigorous manner than that of the present work, and reported the presence of oxide, although the film was less than 380 A thick (25). The initial stage must therefore have involved a reaction of acid, but not of oleate ions, with a copper oxide of indefinite stoiehiometry. The main product was the binuelear copper oleate which, along with oleie acid, was subject to some degradation at high temperatures. In view of the low temperature reflection results it seems likely that an ester species was the precursor of the binuclear complex. Such a species would come about through the ehemisorption of oleic
REACTION OF OLEIC ACID WITH COPPER SURFACES
acid to form an immobile surface ester which R
I I
0=0 0
I
Y
O C u O Cu 0
/////i~///i///~N////i///
could be the intermediate of a covalent species RCOO-Cu-OOCR. In view of the mobility data of Fig. 4, this bi-ester may itself be adsorbed on the surface and be immobile. Association of the bi-ester would produce the binuclear complex. The fate of the acid hydrogen would be to form surface hydroxyls; water would form upon further reaction and be desorbed at high temperature. It. is uncertain to what extent, the over-all reaction would proceed after the removal of the oxide layer, although the results of Walker and Ries with clean, evaporated film (6), and those of Hathwar and Smith with freshly g e n e r a t e d a c t i v e surfaces p r o d u c e d b y c u t t i n g copper i m m e r s e d in solution (1), i n d i c a t e d t h a t some r e a c t i o n w i t h oleic acid does occur. F o r such eases of a d s o r p t i o n f r o m solution, t h e f o r m a t i o n of a " m o n o l a y e r " m i g h t i n v o l v e covering t h e surface w i t h t h e p o s t u l a t e d i m m o b i l e ester, while multilayer formation may involve a build-up of a d s o r b e d b i - e s t e r a n d t h e b i n u c l e a r complex. ACKNOWLEDGMENTS Support for this work by grants PRF-1247-A3 from the Petroleum Research Fund of the American Chemical Society, and GP 143~ from the National Science Foundation, is gratefully acknowb edged. REFERENCES 1. ]=IATH~VAR, G. S., AND SMITH, H. A., "The Adsorption of Polar Organic Compounds on Freshly Machined Metal Surfaces," D. ]). C. Document AD 412423; Hathwar, G. S., Ph.D. thesis, The Univ. of Tennessee, 1963. 2. YOSttIOKA,S., ANDYAMAMOTO,H., Oyo B~ttsuri 24, 155 (1955).
257
3. ])ANIEL, S. G., Trans. Faraday Soc. 47, 1345 (1951). 4. BEISCHER,D. E., J. Phys. Chem. 57,134 (1953). 5. GAINES, G. L., JR., J. Colloid Sci. 15, 321 (1960). 6. WALKER,]). C., ANDRIES, H. E., JR., J. Colloid Sci. 17, 789 (1962). 7. EBERHARDT, E., AND MEHLISS, G., Z. Chem. 1961, 248. 8. LOW, M. J. ])., AND INOUE, H., Anal. Chem. 36, 2397 (1964). 9. EVANS, U. R., "The Corrosion and Oxidation of Metals : Scientific Principles and Practical Application," p. 55. Arnold, London, 1960. 10. LIFSCHITZ,J., AND ROSENBOHM,E., Z. EIektrochem. 21, 499 (1915). 11. GugA, B. C., Proe. Roy. Soc. (London) A206, 353 (1951). 12. FOEX, G., KARANTASIS,J., AND PERAKIS, N., Compt. Rend. 237,982 (1953). 13. FIGGIS, B. N., AND MARTIN, R. L., J. Chem. Soc. 1956, 3837. 14. ABE, H., AND SHIMADA,J., Phys. Rev. 90, 316 (1953). 15. BLEANY, B., AND BOWERS, K. D., Proc. Roy. Soc. (London) A214, 451 (1952). 16. SATAKE,I., AND MATUURA, R., Kolloid-Z. 176, 31 (1961). 17. BALLHAUSEN, C. J., "Studies in Crystal Spectra," ]).D.C. document AD 455103, 1964. ]8. YAMADA,S., NAKAMURA,H., AND TSUCHIDA, R., Bull. Chem. Soc. Japan 31, 303 (1958). 19. TSUCHIDA, R., AND YAMADA, S., Nature 176, 1171 (1955). 20. MARTIN, R. L., AND WATERMAN,H., J. Chem. Soe. 1957, 2545. 21. Ross, T. G., Trans. Faraday Soc. 55, 1057 (1959). 22. Low, M. J. D., AND INOUE, ]:t.~ Can. J. Chem. 43, 2047 (1965). 23. t~ELLAMY, L. J., "The Infra-red Spectra of Complex Molecules," pp. 178ff. Wiley, New York, 1960. 24. SINCLAIR,R. G., MCId~AY,A. F., MYERS, G. S., AND JONES, R. N., J. Am. Chem. Soc. 74, 2578 (1952). 25. TANNER, D. W., POPE, D., POTTER, C. J., AND WEST, ])., J. Appl. Chem. (London) 14, 361 (1964).
(1967)
The Reaction of Oleic Acid with Copper Surfaces M. J. D. LOW,* K. H. B R O W N , AND H. I N O U E School of Chemistry, Rutgers, The State University New Brunswick, New Jersey
Received January 3, 1967; revised April 18, 1967 The reaction of oleic acid with copper plates was studied in air, using infrared emission and reflection spectroscopy. The reaction occurs slowly even at room temperature. The spectra indicate the formation of an ester-like species, tentatively identified as a copper bi-ester, which is the precursor of the binuelear copper oleate complex. The latter is the main product of the reaction. There is some decomposition of oleic acid and oleate above 100°C. The complex is more mobile over the oxide surface than free acid or ester species. Speculations on the mechanism of boundary lubrication as well as other surface phenomena have caused much interest in the adsorption of f a t t y acids on metals, and have prompted a variety of fruitful studies dealing with the formation and nature of monolayers on gold, silver, platinum, chromium, nickel, and other surfaces. However, less information has been obtained with more "reactive" metals such as copper or aluminum because of the occurrence of bulk reactions in addition to ehemisorption. With copper, H a t h w a r and Smith (1) reported t h a t more t h a n one monolayer was formed when oleie acid was taken up from eyelohexane solution, and Yoshioka and Y a m a m o t o concluded t h a t multilayers of oleate formed on copper (2). Daniel noted t h a t benzene solutions of oleic or stearie acid developed a blue coloration when in contact with copper powder, and found t h a t copper soaps were formed (3). Beiseher, using a tracer teehnique, found t h a t a monolayer of stearic acid was more rapidly converted to stearate on oxidized t h a n on clean copper (4). Gaines found multilayer formation on copper in radiostearie acid-nitromethane solution, probably due to soap formation (5). Walker and Ries studied the adsorption of radiostearie acid from dilute n-hexadeeane solution on copper films which * Department of Chemistry, New York University, New York N.Y. 10453.
had been exposed to dry helium or air, and found by chemical analysis that some copper stearate was formed (6). Also, E b e r h a r d t and Mehliss (7) mentioned preliminary work on the infrared spectra of stearie acid adsorbed on air-oxidized copper film, and noted the formation of absorption bands at 1585, 1560, and 1520 em -I due to unidentified reaction products. Such results indicate that oleie or stearic acid reacts readily with copper (or oxidecovered copper). However, little attention seems to have been paid to the nature of the reaction products, although terms such as salt, soap, stearate, or dioleate have been used to describe them. As the nature of the reaction products and intermediates m a y be of mechanistic interest and importance, we are studying such "reactive" systems with infrared spectroscopic methods, and report some observations on the reaction of oleie acid with copper surfaces. EXPERIMENTAL Copper plates 100 X 55 X 1 mm. were used as samples. T h e plates were polished with extra-fine flint paper and cleaned chemically by dipping in chromic acid clearing solution for 5 minutes. The plates were then polished with clean glass wool, washed thoroughly with water, and air dried. Bright, shiny surfaces resulted. However, these were oxide coated, because all work was carried out in Mr. A pair of plates were
252
REACTION OF OLEIC ACID WITH COPPER SURFACES installed in an emission device (8) or in a "homemade" reflection device similar in principle and operation to the reflection attachment marketed by the Perkin-Elmer Corp. Spectra were recorded in air with a PerkinElmer Model 521 spectrophotometer fitted with a Reeder thermoeouple. 1 Samples were placed between salt plates or in commercial cavity cells for absorption-transmission spectra. Most of the emission and reflection work was carried out with fixed slits of 200 , , corresponding to a theoretical slit width of 3 em. -1 at 1600 cm. -1. The transmittance and emission ordinates of the various figures are arbitrary. All spectra were obtained by double-beam operation, using one copper plate as sample and one as reference. The plates oxidized during emission work, but there were no significant differences in emission between bright and tarnished plates. 0leic acid was applied to a sample plate by wiping the plate rapidly with a glass wool pad moistened with acid, and then immediately wiping the plate with dry pads so that the plate appeared to be dry. Other methods, such as depositing oleic acid by covering the plate with a fine spray of droplets of oleic acid solution using an atomizer or depositing, spreading, and evaporating a known amount of solution, did not produce more reproducible results than the wipe-onwipe-off technique. It is estimated that several milligrams of acid were retained after a cold plate had been wiped dry. Slightly larger amounts were retained by plates at 200 ° and 300°C. A few experiments using pure acids obtained from the Hormel Institute gave essentially the same results as those carried out with c.p. oleic acid, suggesting that trace impurities did not affect the bulk reaction to a significant extent. Consequently, most of the work was carried out with c.p. oleic acid.
RESULTS AND DISCUSSION A large number of emission measurements were made at temperatures from i00 ° to 300°C. Emission spectra recorded at sample i C. M. Reeder Co., 173 Victor Avenue, Detroit, Michigan.
253
temperatures near 100°C. were of poor quality because of the low thermal emission of the plates, but acceptable results could be obtained at higher temperatures. An example of an emission spectrum obtained at 150°C. is given in Fig. 1. The spectrum of the acidcovered surface differs from that of the pure oleie acid (spectrum A, Fig. 1) over the spectral range shown, indicating that a reaction producing species other than oleie acid had occurred. The emission spectra changed with the passage of time. Figure 2 shows a sequence of spectra typical of those observed at 200°C. In general, emission spectra recorded immediately after application of the acid to plates at 200°C. or higher showed a prominent band in the 1710-1700 era. -1 range. The band declined rapidly in intensity and broadened. This behavior is attributed to the evaporation of oleie acid from the hot plates. Spectra showing structure much like spectrum B of Fig. 2 were recorded after
°1 B
17'bo
I~'oo
13bo cry'
absorption-transmission spectrum of absorption-transmission spectrum of copper oleate. C: emission spectrum of oleie acidtreated copper plate at 150°C., recorded immediately after acid addition. FIG. l. A: oleie acid. B:
254
LOW, BROWN, AND INOUE
Z O (D (D W
18oo 16oo 14oo crS' FiG. 2. Emission spectra at 200°C. A pair oe plates were installed and the background spectrum A was recorded. Oleic acid was then applied and spectra were recorded. The number next to each spectrum gives the time elapsed in minutes since the acid addition. The ordinates are displaced. several minutes. Slower changes were then observed. The band near 1710 cm. -1 shifted to 1735 cm. -~, all bands including a prominent one at 1585 cm. -~ gradually declined in intensity and became less well defined, and a band near 1535 cm. -1 remained constant or increased slightly in intensity. Results which were essentially identical were obtained over the entire temperature range, except t h a t the changes in the spectra occurred more rapidly at the higher temperatures. The degree of oxidation of the sample plates did not have a marked effect on the emission spectra, because essentially the same results were obtained with "bright" plates bearing invisible oxide films and with plates deliberately oxidized to various extents. In all cases the oleic acid reacted rapidly with the oxide film. For example, if oleic acid was applied at 200°C. to a plate which had been aged b y heating i n air so t h a t a brown oxide film had formed (the brown color corresponds to an oxide film
about 400 A thick (19)), the film disappeared within a few seconds and the plate assumed its original bright, coppery sheen. The plate remained bright thereafter. However, if a bright or a heavily oxidized plate was treated with excess acid at 300°C, a translucent, shellack-]ike coating was formed, indicating t h a t the acid a n d / o r the reaction products decomposed at the highest temperature. Reflection spectra recorded at lower sample temperatures showed the same structure as emission spectra, with the exception t h a t there was no decomposition of acid or reaction products. Some results are given in Fig. 3. As with the high temperature emission results, the reflection spectra recorded at or near room temperature showed t h a t slow reactions were occurring. The results m a y be summarized as follows. A reflection spectrum recorded immediately after application of the acid at or near room temperature was similar to the absorption spectrum of oleic acid. The spectra changed slowly with the passage of time. A band originally observed at 1703 cm. -1, due to the carbonyl of the undissociated acid, slowly declined in intensity and was replaced b y a less intense band at 1735 cm. -I. Simultaneously, a band appeared at 1610 cm. -1, grew in intensity, and shifted to 1585 cm. -I. Also, a broad band in the 1500-1400 cm. -1 region, which is due to C-H modes in oleie acid, grew in intensity at the low wavenumber side of the band.
~0
~-r--
~rw
30 sec. A
,8oo'
,~bo \\'
232 rain. B
438 min. C
612 rain. D
17bo,sbo \\' r/ooI~oo\\,goo r;ooc~-"
FIG. 3. Reflection spectra. The spectra were recorded after the addition of oleic acid at the times shown. The sample was at 27°C. for 277 minutes (spectra A, B) and at 70°C. thereafter (spectra C, D).
REACTION OF OLEIC ACID WITH COPPER SURFACES
The band near 1535 era. -~ found in emission spectra was not observed. Some experiments were also carried out with sodium oleate. A solution of sodium oleate in ethanol was placed on a bright copper plate and the solvent was allowed to evaporate. Reflection spectra were recorded at room temperature at time intervals up to 3 hours after the sodium oleate addition, and after heating at 120°C. for 2 hours. The spectra showed only the presence of sodium oleate, showing that the sodium oleate was unable to react with the oxide film on the plate. The bands at 1585 em. -~ and in the 15001400 cm. -t region were observed in all emission and reflection spectra of acidcoated plates and closely match similar bands found in the absorption spectrum of copper oleate (spectrum B, Fig. 1). Studies employing magnetic, microwave, diffraction, electronic, and infrared spectroscopic as well as resonance techniques (10-21) indicate that copper acetate, butyrate, valerate, caproate, eaprylate, palmitate, oleate, and
,../o
/..o\ /, i
L;
i
CU
c
stearate have the structure. Satake and ~,iatuura assigned the bands of the oleate at 1585 era.-I and at 1420-1410 cm. -I to the antisymmetrie and symmetric stretehin~ vibrations, respectively (16). The 149(I1410 em. -I band falls in the region of som~'. C-H modes and is consequently not clearly defined, but would produce an increase at the low wavenumbcr side of bands in the 1500-1400 em. -~ region. The shift from 1610 to 1585 cm. -~ observed with reflection spectra prompted a series of measurements to test the hypothesis that the shift was caused by the presence of free oleic acid. A series of solutions of various concentrations of copper oleate in eyclohexane, and also of various concentrations of copper o]eate plus oleic acid in cyelohexane, were prepared and absorption
255
spectra were recorded. The antisymmetrie stretching band shifted to higher wavenumbers with increasing dilution (e.g., 1585 em. -I, 1587 era. -1, and 1595 em. -1 at 1.0, 0.035, and 0.005 mole % oleate, respectively, in the absence of oleic acid) and also with increasing concentrations of oleie acid (e.g., 1593 em. -~ for a solution containing 0.035 mole % oleate and 0.0275 mole % acid, or 1605 em. -I for a solution containing 0.005 mole % oleate and 0.1375 mole % acid). The nature of the interaction between copper oleate and oleic acid molecules causing the band shift is uncertain. The various results, however, suggest that the binuclear copper oleate was formed on the samples ag all temperatures. At low temperatures, when the reaction had not occurred to a great extent, the oleate was associated with unreacted acid; as the reaction proceeded, the degree of association decreased and the band shift to 1585 cm. -1 occurred. In emission spectra the band was observed at or near 1585 em. -1, suggesting that little unreacted oleie acid remained on the surfaces at higher temperatures. The 1735 em. -~ band observed in emission and reflection spectra would thus not be due to free acid. In order to obtain additional information, further experiments were made. Pure copper oleate was heated in air and absorption spectra were recorded. A small band was formed at 1735 era. -~ after 5 minutes at 147°C. The band increased slightly on heating for an hour at 160°C. A minor band also formed near 1535 em.-L Such a band was not found in low temperature reflection spectra, but occurred in all cases when pure copper oleate, oleie acidcopper oxide mixtures, or acid-treated samples were heated above 100°C. The band did not disappear when a specimen was cooled. Such results, as well as the general, slow decline in the intensities of emission bands as shown in Fig. 2, indicate that a slow and minor decomposition of the copper oleate occurred above about 100°C. The degradation of oleie acid in air produced weak bands in the 1700 em. -~ region; after 48 hours at 140°C. a small band was formed at 1730 em. -1 and minor bands elsewhere
256
LOW, BROWN, AND INOUE
(22). The nature of the decomposition products is not known. Although the decomposition products give rise to absorptions in the 1700 cm. -1 region, it is unlikely that they were the sole cause of the band at 1735 em. -1. The reflection spectra showed that the band was formed at low temperatures and relatively quickly, so that it is likely that an additional species was formed. The earbonyl band in esters has been reported to be in the 1730 era. -1 region for many compounds (23). Sinclair et al. reported 1720 em. -1 for the trans methyl esters of oetadeeanoie acid and 1741 em. -1 for methyl esters of elaidie, oleie, linoleie, linolenic, and araehidic acids, for example (24). The similarity in position of the 1735 em. -~ band and the earbonyl band of esters therefore suggests that a species having an ester configuration was formed on the copper surfaces. A similar interpretation was advanced for the behavior of earboxylie acids on iron and cobalt (22). Some experiments were carried out with emission techniques at 150°C. in order to gain information about the mobility of the materials on an acid-covered plate. The oleie acid was applied in a narrow, U-shaped band at the sides and bottom of a sample plate, so that most of the plate was initially bare. The portion of the plate facing the monoehromator slit was bare, so that if an emission band due to acid or oleate were detected, it would mean that acid had moved from the side to the center of the plate. A disc of Irtran-2 was placed in the center of the plate and removed after some time. Absorption spectra of the disc recorded at 10-fold ordinate scale expansion showed no trace of the acid, indicating that acid evaporated from the edges of the hot plate had not condensed on the slightly cooler front surface of the Irtran disc. Spectra typical of those obtained in similar experiments (without the Irtran disc) are shown in Fig. 4. In each case, with spectra recorded approx. 5 minutes after the addition of acid to the sides of the plate, a band appeared at 1585 em.-L The band increased in intensity and other bands formed at 1535 era. -~ and between 1450 and 1350 em. -~. Successive
E
,.,, ~k
/ ;
I
,,j
/ 5N 16bo
'
;4bo
16'oo
'
FIG. 4. Surface mobility at 150°C. Emission spectra after acid had been applied to the sides of a sample, after A, 5; B, 42; C, 113; D, 144; and E, 174 minutes.
spectra were recorded for periods of up to 4 hours. No bands were detected in the 1700 era. -1 region where the free acid on the ester species would show a strong absorption. It may be concluded that only the copper oleate was significantly mobile over the surface; free oleie acid and the ester-like species being more strongly bound to the surface. The various spectra thus show that the reaction of oleie acid with the samples was complex. As the work was carried out in air, even bright plates were oxide covered. Tanner et al. worked with "bright polished" copper, cleaned chemically in a more vigorous manner than that of the present work, and reported the presence of oxide, although the film was less than 380 A thick (25). The initial stage must therefore have involved a reaction of acid, but not of oleate ions, with a copper oxide of indefinite stoiehiometry. The main product was the binuelear copper oleate which, along with oleie acid, was subject to some degradation at high temperatures. In view of the low temperature reflection results it seems likely that an ester species was the precursor of the binuclear complex. Such a species would come about through the ehemisorption of oleic
REACTION OF OLEIC ACID WITH COPPER SURFACES
acid to form an immobile surface ester which R
I I
0=0 0
I
Y
O C u O Cu 0
/////i~///i///~N////i///
could be the intermediate of a covalent species RCOO-Cu-OOCR. In view of the mobility data of Fig. 4, this bi-ester may itself be adsorbed on the surface and be immobile. Association of the bi-ester would produce the binuclear complex. The fate of the acid hydrogen would be to form surface hydroxyls; water would form upon further reaction and be desorbed at high temperature. It. is uncertain to what extent, the over-all reaction would proceed after the removal of the oxide layer, although the results of Walker and Ries with clean, evaporated film (6), and those of Hathwar and Smith with freshly g e n e r a t e d a c t i v e surfaces p r o d u c e d b y c u t t i n g copper i m m e r s e d in solution (1), i n d i c a t e d t h a t some r e a c t i o n w i t h oleic acid does occur. F o r such eases of a d s o r p t i o n f r o m solution, t h e f o r m a t i o n of a " m o n o l a y e r " m i g h t i n v o l v e covering t h e surface w i t h t h e p o s t u l a t e d i m m o b i l e ester, while multilayer formation may involve a build-up of a d s o r b e d b i - e s t e r a n d t h e b i n u c l e a r complex. ACKNOWLEDGMENTS Support for this work by grants PRF-1247-A3 from the Petroleum Research Fund of the American Chemical Society, and GP 143~ from the National Science Foundation, is gratefully acknowb edged. REFERENCES 1. ]=IATH~VAR, G. S., AND SMITH, H. A., "The Adsorption of Polar Organic Compounds on Freshly Machined Metal Surfaces," D. ]). C. Document AD 412423; Hathwar, G. S., Ph.D. thesis, The Univ. of Tennessee, 1963. 2. YOSttIOKA,S., ANDYAMAMOTO,H., Oyo B~ttsuri 24, 155 (1955).
257
3. ])ANIEL, S. G., Trans. Faraday Soc. 47, 1345 (1951). 4. BEISCHER,D. E., J. Phys. Chem. 57,134 (1953). 5. GAINES, G. L., JR., J. Colloid Sci. 15, 321 (1960). 6. WALKER,]). C., ANDRIES, H. E., JR., J. Colloid Sci. 17, 789 (1962). 7. EBERHARDT, E., AND MEHLISS, G., Z. Chem. 1961, 248. 8. LOW, M. J. ])., AND INOUE, H., Anal. Chem. 36, 2397 (1964). 9. EVANS, U. R., "The Corrosion and Oxidation of Metals : Scientific Principles and Practical Application," p. 55. Arnold, London, 1960. 10. LIFSCHITZ,J., AND ROSENBOHM,E., Z. EIektrochem. 21, 499 (1915). 11. GugA, B. C., Proe. Roy. Soc. (London) A206, 353 (1951). 12. FOEX, G., KARANTASIS,J., AND PERAKIS, N., Compt. Rend. 237,982 (1953). 13. FIGGIS, B. N., AND MARTIN, R. L., J. Chem. Soc. 1956, 3837. 14. ABE, H., AND SHIMADA,J., Phys. Rev. 90, 316 (1953). 15. BLEANY, B., AND BOWERS, K. D., Proc. Roy. Soc. (London) A214, 451 (1952). 16. SATAKE,I., AND MATUURA, R., Kolloid-Z. 176, 31 (1961). 17. BALLHAUSEN, C. J., "Studies in Crystal Spectra," ]).D.C. document AD 455103, 1964. ]8. YAMADA,S., NAKAMURA,H., AND TSUCHIDA, R., Bull. Chem. Soc. Japan 31, 303 (1958). 19. TSUCHIDA, R., AND YAMADA, S., Nature 176, 1171 (1955). 20. MARTIN, R. L., AND WATERMAN,H., J. Chem. Soe. 1957, 2545. 21. Ross, T. G., Trans. Faraday Soc. 55, 1057 (1959). 22. Low, M. J. D., AND INOUE, ]:t.~ Can. J. Chem. 43, 2047 (1965). 23. t~ELLAMY, L. J., "The Infra-red Spectra of Complex Molecules," pp. 178ff. Wiley, New York, 1960. 24. SINCLAIR,R. G., MCId~AY,A. F., MYERS, G. S., AND JONES, R. N., J. Am. Chem. Soc. 74, 2578 (1952). 25. TANNER, D. W., POPE, D., POTTER, C. J., AND WEST, ])., J. Appl. Chem. (London) 14, 361 (1964).