Coadsorption of n-Octadecane and Stearic Acid on Metal Surfaces

18 Coadsorption of n-Octadecane and Stearic Acid

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on Metal Surfaces W. P. D O Y L E AND A. H. E L L I S O N Texaco Research Center Beacon, Ν. Y. Radioisotopic tracer techniques were applied to study the coadsorption of n-octadecane and stearic acid on a metal surface immersed in a n-octadecane solution of stearic acid. Dual labeling was employed for determining the surface concentrations of both n-octadecane and stearic acid. n-Octadecane was labeled with tritium and stearic acid with carbon-14. The results of half-hour adsorption experi ments provide direct proof of coadsorption of polar and nonpolar materials on iron, copper, silver, and platinum surfaces. The films produced on silver and copper by 19-hour adsorption consisted of approximately one molecular layer of stearic acid and two m o l ecular layers of octadecane. A new model is proposed to describe the structure of this thick coadsorbed film. The composition of the adsorbed film produced on a metal or metal oxide surface as a result of contact with a dilute solution of a long-chain polar solute in a long-chain nonpolar solvent has been the subject of several recent papers [1, 2, 4, 5], A l l of these workers conclude that such films are not, as originally believed, monolayers consisting en tirely of vertically oriented polar molecules but rather mixed monolayers containing major amounts of solvent coadsorbed with the polar solute. Zisman and coworkers [2, 5], as a result of their contact potential and wetting angle measurements of films formed by octadecylaminecetane adsorption, suggest that a mixed monolayer of polar and nonpolar molecules is in a nonequilibrium state and that with time the surface concentration of polar molecules increases and reaches 100%. On the other hand, Ries [4], in studies of the stearic acid-cetane a d sorption on metals (oxide) and mica, finds that surface reactivity is a factor. Thus, after extended adsorption time the surface concentration of vertically oriented polar molecules was about 0.3 monolayer on unreactive surfaces and several monolayers on reactive surfaces. 268

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

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While the concentration of polar molecules in these films has been determined directly by the tracer technique, no similar direct determination of solvent concentration has been carried out. This work was undertaken, first of a l l , to measure directly by the radioactive tracer method the amount of n-octadecane solvent present in an adsorbed stearic acid f i l m . It was also our desire to elucidate the structure of these coadsorbed films and determine the effect of a d sorption time upon it. Critical to work in this area is the ability to isolate these films from the generating solution. Zisman's [3] early work provided the "oleophobic" or retraction method for this purpose. The application of this method in this work yielded metal samples after adsorption having broad areas of oleophobic or dry surface. However, the few droplets of solution adhering to the area of surface to be counted and the larger amount of solution adhering to the edges and backs of metal samples, which would grossly contaminate the counting equipment, had to be removed by a solvent rinse before radioactive analysis could be carried out. This solvent rinse had little effect on the ability of the tracer technique to provide a clear answer to the main objective of the work. M


Co-adsorption

Tt

Experimental Stearic a c i d - l - C was purchased from the Volk Radiochemical Corp. It was purified by paper chromatography and diluted with Eastman White Label stearic acid to a specific activity of 0.218 millicurie per millimole. Its purity was determined by isotopic dilution. n-Octadecane1, 2 - H was prepared by low pressure hydrogénation of n-l-octadecene over Pd on asbestos, using tritium-labeled hydrogen. Except where noted, the n-octadecane was purified by percolation through silica gel prior to each adsorption experiment. The melting point of this product was 27.5-28°C. and its specific activity was 0.124 mc. per mmole. The metal samples, 2 x 1 1/2 χ 1/8 inches, were polished to a m i r r o r finish on a polishing wheel covered with kitten-ear polishing cloth wet with a suspension of Linde alumina (Type B-5125). Each p o l ished metal sample was covered with an evaporated film of the same metal (1000 A . thick) just prior to use. These films were hydrophillic, indicating the absence of organic contamination. The purpose of depositing this metal film onto the metal substrate was to cover the alumina embedded in the metal substrate during the polishing operation. Electron diffraction patterns obtained from the metal surfaces showed that the surface was contaminated with alumina before the evaporation step and essentially pure metal after the evapora tion. This technique would have shown up any contaminant present in the surface layer (100 A . deep) in excess of about 10%. It is acknowl edged that a film of oxide is present on these surfaces. In each experiment films were produced on two m i r r o r s by i m mersing them for a measured time in a rectangular cell containing the solution of stearic a c i d - l - C (10~ gram per ml.) in n-octadecane-1, 2 - H . A l l adsorption studies were carried out at 40°C. It was desired to produce surfaces comparable to those obtained by Bartell [1] and Zisman [2]—that is, dry films. However, in no case did we obtain a completely dry surface. After the m i r r o r s had been in the 1 4

3

1 4

3

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solution for a specified time they were removed and given a 5-second rinse with freshly percolated cyclohexane. Cyclohexane was chosen because its molecular structure was not similar to that of n-octadecane. It was believed that this dissimilarity would reduce the chance for exchange of the unlabeled molecules in the rinse solvent with labeled n-octadecane. The amounts of n-octadecane and stearic acid on the metal surface were then measured by radioactivity assay. Following this, the m i r r o r s were given two additional 1-minute rinses with a good deal of agitation, and were counted after each rinse. The object of the rinses was to see how readily the adsorbed species were removed. In these rinsing procedures the same volume of solvent was always used and the time was closely controlled. To simplify presentation of the data the values of the surface concentrations of n-octadecane and stearic acid on the duplicate m i r r o r s were averaged. The standard deviation for the surface coverages by acid was 14%; for coverages by octadecane it was 7% after the 5-second rinses and 50% after the other two rinses. Although the 50% standard deviation appears large, the absolute error involved is small, since the surface coverage by octadecane was small after the second and third rinses. A gas flow Geiger tube was used to count the C and H radiation from the films. The radiation from C and H can be distinguished because the energies of the beta-particles emitted in each case, and thus their penetrating powers, are different. C radiation was detected through a thin Mylar window on the Geiger tube. H radiation does not penetrate this window. The window was removed to count the H and C radiation combined. The net counting rate for H radiation was obtained by subtracting the C counting rate (corrected for greater detection efficiency with the window removed) from the combined counting rate. The counting rates were measured to a standard deviation of 3%. Blodgett monolayers of stearic a c i d - l - C and stearic acid-9, 1 0 - H had been previously prepared on silver, platinum, copper, and iron and counted in order to convert counting rate data to monolayers of stearic acid. To calculate surface coverage by n-octadecane it was assumed that vertically oriented n-octadecane molecules would occupy the same cross-sectional area as vertically oriented stearic acid molecules. 1 4

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Results and Discussion The first adsorption experiments furnished some interesting results regarding solvent purity. A stock solution of stearic a c i d - l - C in n-octadecane-1,2-H was prepared using n-octadecane-l,2-H which had been percolated through silica gel. Ten days later it was observed that the adsorption data did not reproduce the results obtained when the solution was new. Apparently the concentration of labeled polaroxidation products of n-octadecane had built up during this period to a level where they contributed significantly to the composition of the adsorbed film. To check this point an adsorption experiment was conducted in whichsilver m i r r o r s were placedfor 19 hours in n-octadecane1 , 2 - H containing no stearic acid. The date from this adsorption experiment, along with results of subsequent cyclohexane rinses, are shown in Table I. 1 4

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Table I. Adsorption of Polar Impurities from Stored Octadecane Fraction of Monolayer of Octadecane-1, 2 - H Adsorbed


3

Rinse Time, Seconds

F r e s h octadecane

Stored octadecane

5 65 125

0.3 0.1 0.0

0.5 0.4 0.3

Results of an identical adsorption experiment carried out immedi ately after the initial silica gel treatment of the η-octadecane-1,2-H are included in Table I for comparison. It is obvious that tritium-labeled polar impurities were formed in the stored solvent. On the basis of these results it was decided to con duct all subsequent adsorption experiments with solutions prepared not more than 2 days before. The authors are aware that experimenters in this field appreciate the general requirement for solvent purity in ad sorption experiments. However, it was thought that these data should be presented to emphasize the special importance of solvent purity when studying coadsorption of solvent and solute by tracer techniques and to point out the relatively short time required for impurities to develop. 3

0.5-Hour Adsorption In Table II the coverages in equivalent Blodgett layers are given for silver, platinum, copper, and iron samples following a 0.5-hour adsorption in the stearic acid-n-octadecane solution and the subsequent rinse steps. Given also are the coverages of n-octadecane on silver Table II. Stearic Acid and Octadecane Concentrations in F i l m s Produced by 0.5-Hour Adsorption at 40°C. (Expressed as equivalent Blodgett monolayers) Metal Rinse T i m e , Seconds

n-Octadecane Stearic acid Total

5


65


125

a

Silver

Platinum

Copper

Iron

Solvent

Solution

0.3

1.2 0.5 1.7

1.3 0.7 2.0

0.9 0.7 1.6

0.4 0.6 1.0

0.2 0.5 0.7

0.1 0.5 0.6

0.2 0.6 0.8

0.2 0.5 0.7

0.0 0.4 0.4

0.0 0.4 0.4

0.1 0.5 0.6

0.1 0.5 0.6

-

0.3 0.1

-

0.1 0.0

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O n e sample.




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following a 19-hour adsorption in the pure solvent and the subsequent rinse steps. This run was made for reference, to show the efficiency with which this 5-second rinse removes excess liquid. In comparing these reference data with the data for acid-containing films, it is impor tant to realize that the latter are oleophobic surfaces. Thus the excess liquid being removed consists of loosely adhering droplets which should be more easily removed than the liquid layer produced in the reference run. It can be seen from Table II that the films remaining on all the metals after the 0.5-hour adsorption and 5-second rinse contain more n-octadecane than can be accounted for on the basis of incomplete r e moval of excess solution. This is taken to mean that n-octadecane is coadsorbed with stearic acid. The significant removal of n-octadecane by the second and third rinses, compared to the almost negligible r e moval of stearic acid, indicates that n-octadecane is rather weakly bound on the surface. 19-Hour Adsorption The results, obtained in the 19-hour adsorption studies using silver and copper m i r r o r s , are tabulated in Table III, with reference data from Table Π. These data provide the same evidence of coadsorbed but loosely bound n-octadecane as seen with the 0.5-hour films. The amounts of both acid and solvent after adsorption and 5-second rinse are, however, significantly greater in the 19-hour films. Higher coverages by solvent only might be attributed to poorer rinsing technique, but the higher coverages by both components must be attributed to increased adsorp tion of acid and solvent as a result of the longer adsorption time. That an increase in the adsorption of acid is accompanied by an increase in the amount of n-octadecane in the film is additional evidence of solvent coadsorption. It is unlikely that the observed high total coverages of 3.2 and 3.6 equivalent Blodgett layers can be attributed to surface roughness. P o l ished surfaces of this type usually have roughness factors of about 1.5. Table ΙΠ. Stearic Acid and Octadecane Concentrations in F i l m s Produced by 19-Hour Adsorption at 40°C. (Expressed as equivalent Blodgett layers) Metal Silver Copper Rinse T i m e , Seconds

Solvent

Solution

0.3

2.2

2.4

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1.2 3.6

η-Octadecane Stearic acid Total

5


65

0.1

0.2 0.8 1.0

0.2 1.0 1.2


125

0.0

0.1 0.7 0.8

0.1 0.9 1.0

0.3

0.1

-

0.0


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These considerations lead us to believe that these 19-hour films of adsorbed acid and coadsorbed solvent are multilayers. In view of the r e sults with the 19-hour films, the higher than expected total coverages observed with most of the 0.5-hour films are understandable. F o r a given adsorption time there was little variation in the amount of acid adsorbed on the different metals. With the 0.5-hour films there was significant variation in the amount of n-octadecane. It is believed that this solvent variation is a result of the difficulty inherent in performing the delicate task of removing by a short rinse all excess solution without removing the very loosely bound coadsorbed solvent. Conclusions A revised picture is needed to describe the structure of films composed of coadsorbed polar solute molecules and nonpolar solvent molecules. The close-packed model of Reis implies that the coadsorbed solvent should be relatively tightly bound. That this is apparently not the case is suggested by the ease with which the n-octadecane was r e moved from all the films. The data show that n-octadecane is more firmly attached to the surfaces on which stearic acid is adsorbed than to surfaces on which no acid is adsorbed. This means that the coadsorbed n-octadecane found is not present in the film as relatively large aggregates of randomly oriented solvent molecules on the surface. These aggregates, if they exist on the surface initially, should have been removed by the 5-second rinse. This is based on the fact that solvent was removed from the silver m i r r o r s which had been immersed in pure n-octadecane containing no stearic acid. In the case of the 19-hour adsorption studies one might picture these thick films as consisting of a mixture of randomly dispersed acid or soap and solvent. However, if this were true, the rinsing treatments should have removed large amounts of the acid or soap along with the solvent. This was not the case. The fact that large amounts of acid were not removed indicates that the acid is closely associated with the metal surface. Therefore, we propose a model for coadsorption which is consistent with our results. In our picture of coadsorption nearly all the stearic acid is adsorbed at the metal surface. The rinsing data tend to bear this out. The larger size of the polar head than the hydrocarbon tail of stearic acid results in some space between the hydrocarbon tails of the acid molecules which can be penetrated by solvent molecules. These solvent molecules fit between the stearic acid molecules and are aligned with the hydrocarbon tail of the acid. However, in this picture of coadsorption it is assumed that the entire length of the solvent molecule does not associate with the hydrocarbon moiety of the acid. A portion of the solvent molecule extends beyond the hydrocarbon tail of the acid, and in turn acts as anchor for a second layer of solvent molecules. In this way several layers of solvent molecules could conceivably be built up on a monolayer of adsorbed stearic acid. The solvent molecules in these layers, as described, would tend to be oriented in the same way as the adsorbed acid—that is, vertical to the surface with methyl groups extending outward. If in the last layer out from the surface the hydrocarbon tails are drawn together by van der Waals forces, the result would be a low energy methyl group surface. This picture is consistent with the oleophobic nature of the films. Moreover, the fact that the



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n-octadecane molecules in this model would be available for association with the rinsing solvent accounts for the ease with which the solvent can be removed from the films. It is our belief that the thick (a few monolayers) oleophobic films containing a volatile component observed by Bartell [1] in his studies of octadecylamine-cetane adsorption on metals are of the same type as observed in this work, the main difference being the lower volatility of n-octadecane than cetane. The results reported in this paper support the conclusion of Zis man [2, 5] and others [1, 4], that films produced at comparatively short times contain both the polar solute and the nonpolar solvent, but that coadsorbed films are also formed at longer adsorption times. The pro posed model describes the primary adsorbed layer next to the metal surface as consisting of nearly 100% adsorbed stearic acid at long ad sorption times. Literature Cited (1) (2) (3) (4) (5)

Bartell, L. S., Ruch, R. J., J. Phys. Chem. 60, 1231 (1956). Bewig, K. W., Zisman, W. Α., Ibid., 67, 130 (1963). Bigelow, W. C., Pickett, D. L., Zisman, W. Α., J. Colloid Sci. 1, 513 (1946). Cook, H. D., Ries, Η. Ε., Jr., J. Phys. Chem. 63, 226 (1959). Levine, O., Zisman, W. Α., Ibid., 61, 1188 (1957)

Received March 27, 1963.


Coadsorption of n-Octadecane and Stearic Acid on Metal Surfaces

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