Direct Measurement of Adsorption of Radiostearic Acid onto Vapor

20 Direct Measurement of Adsorption of Radiostearic

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch020

Acid onto Vapor-Deposited Metal Films Effect of Moisture D O N A L D C. W A L K E R and H E R M A N E. RIES, Jr. Research and Development Department American Oil Co., Whiting, Ind.

Radiostearic acid was adsorbed from n-hexadecane onto mica and thin vapor-deposited films of iron, gold, and copper that had been exposed to dry and water-saturated helium or a i r . Adsorption was measured directly and continuously by a recently developed technique. The mica substrate showed essentially zero adsorption. None of the metals adsorbed more than one stable compact monolayer. Iron and gold showed a large difference in adsorption in dry helium or a i r , but adsorbed about the same amount, 0.2 to 0.5 monolayer, when exposed to either wet helium or a i r . Copper adsorbed 0.3 to 0.7 monolayer in all atmospheres except wet air, in which it showed a weak adsorption of nine monolayers; rinsing with hexane removed all but one monolayer.

In most techniques for studying adsorption on metals, uniform, clean, and reproducible metal surfaces are difficult to prepare and the adsorption process cannot be followed continuously [2, 3,4, 7,10,11,16, 18, 21], Clean and reproducible metal surfaces are also difficult to prepare and maintain in methods that measure adsorption continuously and directly on a metal-coated window of a Geiger tube [ l , 6,7,13]. A recently developed apparatus and technique provide controlled conditions for the production and maintenance of relatively clean metal films and the precise measurement of adsorption [20]. Metal is evaporated onto a mica window supported within a high-vacuum apparatus; adsorption onto the metal film is measured directly and continuously by a counter tube below the window. Reported previously were results of the adsorption of radiostearic acid from dilute n-hexadecane solution onto the mica substrate and 295

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

ADVANCES IN CHEMISTRY SERIES

296

100-A. films of iron, gold, and copper that had been exposed to purified dry helium or air [20]. Because the presence of moisture is important in many surface phenomena, corresponding experiments have been conducted in which the metal films were exposed to water-saturated helium or a i r .


Experimental Radiostearic acid was adsorbed from a 0.0005 molal solution in n-hexadecane onto mica and 100-A. films of iron, gold, and copper that had been exposed 15 minutes to water-saturated helium or a i r . A l l experiments were conducted at 35.0° ± 0.2° C. Seventy hours was a l lowed for adsorption; duplicate determinations agreed within 0.03 monolayer. Apparatus and Procedure. The metal films were deposited and adsorption thereon was carried out in a high-vacuum apparatus, as shown in Figure 1. The mica window was supported between two chambers that were evacuated simultaneously to prevent breakage; the window was coated by evaporating metal from a heated filament in the upper chamber. The gas was then admitted to both chambers until the pressure was 1 atm. Valve 3 was closed to isolate the upper chamber, the bottom plate was removed, and the counter tube was positioned under the window. After the film had been exposed 15 minutes to the gas, the solution was admitted and counting was started with an automatic counter-printer system. The apparatus was calibrated by determining the counting rate of a compact monolayer of the radiostearic acid on the film balance. Such a layer should have the same counting rate as that of a compact monolayer adsorbed on a relatively smooth metal film. The smoothness of

Figure 1. Adsorption

apparatus


20.

WALKER

AND

RIES

Adsorption

on

vapor-deposited metal films, with respect to adsorption of long straightchain polar molecules, is apparently close to that of water if optimum rate of deposition and film thickness are used, and if the films are de posited on a flat substrate [20]. Surface potentials of tightly packed monolayers on metals are the same as those of corresponding mono layers on water [8]. Backscattering of the n-hexadecane was assumed to be the same as that of the water. Vapor deposition of metal films, calibration, and related details of apparatus and procedure have been reported [20]. Materials. The purchased radiostearic acid, C H C O O H , was of high purity, as shown by pressure-area isotherms on the film b a l ance. It had a specific activity of 1.5 millicuries per millimole. The n-hexadecane was distilled twice and passed through a silica gel column under nitrogen to remove polar impurities. A 0.0005 molal solution of radiostearic acid in n-hexadecane was stored under nitrogen in Teflon bottles. Five milliliters of solution was used—enough to cover the mica window to a depth of about 1.5 mm. A depth of 0.015 mm. contained enough radiostearic acid for a continuous monolayer. F o r each experiment, a new mica window was prepared from a thin sheet (5 to 7 microns) of cleaved Indian ruby mica. The window was mounted in the adsorption chamber and thoroughly washed with twicedistilled benzene. Weighed amounts of the metals, in the form of 7- to 15-mil wire at least 99.95% pure, were evaporated from heated tungsten filaments to give a film thickness of 100 Α., by assuming the density of the film to be the same as that of bulk metal [12,19]. The air was purified by passage through concentrated sulfuric acid, Drierite, and a trap cooled with solid carbon dioxide. The helium was passed through two coiled 15-foot borosilicate glass tubes cooled with liquid nitrogen and containing alumina and charcoal, which had been activated previously by heating at 500° C. under vacuum for 24 hours. Immediately before entering the apparatus, each gas was bub bled through water that had been deionized and twice distilled in an a l l quartz system. 1 7


Metals 2 9 7

Vapor-Deposited

Results and

3 5

1 4

Discussion

Mica was the least reactive surface studied, probably because it is nearly molecularly smooth and the basal surface of its lattice is essen tially a layer of oxygen atoms [3,9]. Furthermore, it is not readily wetted by water. It did not adsorb stearic acid in wet or dry helium or in dry air [20]. In the presence of wet a i r , it initially adsorbed about 0.2 monolayer, but adsorption decreased to zero after 40 hours. D e sorption to zero may indicate that the water and a i r , or other impuri ties on the surface, diffused or dissolved in the n-hexadecane solution. Iron exposed to the wet atmospheres adsorbed about half a mono layer of stearic acid; in the dry atmospheres, about one monolayer, as shown in Figure 2. Perhaps water reduces the activity of the surface by presenting a water film that is less attractive to stearic acid than either dry iron or iron oxide. Any oxide surface beneath the water film has little influence on adsorption; the iron film exposed to wet air ad sorbed only about 0.1 monolayer less than the film exposed to wet helium. A reverse effect to the extent of 0.3 monolayer was observed in dry atmospheres.



298

DRY ^ 5

0.5

-

•

-

•

-

•

-

•

-

•

-

•

-

•

•

AIR -

•

-

•

-

•

-

•

_

D

_

p

_

D

(-

(^2-&^-Q

"0-0-0


-

W E T HELIUM

^^9~~o—o—o-o—ο—ο—ο—ο—ο WET

n

AIR

- ο — ο I

DRY

I

30 TIME,

Figure 2.

ο - — ο -

HELIUM

L

Δ

__J

I

40

50

.

.

1

L_

60

70

HOURS

Adsorption of radiostearic onto iron films

acid

Adsorption on gold in wet helium or air atmospheres was about half of that in dry a i r , as shown in Figure 3. The stable adsorption of 0.6 monolayer in dry air may be due to a film of gold oxide [17]. In the wet atmospheres, however, adsorption may have been on a water film on the gold. As with iron, adsorption on gold in wet air was only slightly less than in wet helium. At equilibrium, gold in dry helium did not ad sorb radiostearic acid. The initial adsorption and subsequent decrease to zero are more difficult to explain. A s was shown earlier [20], high initial surface activity of the gold film [5,14] did not cause the adsorp tion. Gold films may hold a small amount of contaminants such as oxygen, water, and gold oxide, which in turn adsorb radiostearic acid. Desorption may then occur because the contaminant diffuses or dis solves in the n-hexadecane solution.

DRY AIR • — • — π — • — • — • - — • — • Ζ

ι/ /

5

ι /

Ο *

Ο

O

|/Ρ-0-0^ 0.5 l'A

DRY HELIUM — ο

λ

^ Δ _ _

Ν>^0

Δ

^ —

ο

A Δ

α

Λ

°

ο

Ο

WET HELIUM — Δ — Δ - Δ — Δ — Δ — Δ — Δ — Δ

ο~ο—ο—ο—ο—ο—Ο—ο—0 WET AIR

TIME

Figure 3.

HOURS

Adsorption of radiostearic onto gold films

acid


20.

WALKER

AND

RIES

Adsorption Ί


5

1

on Vapor-Deposited 1

1

Metals

299

Γ

•

DRY AIR

Ο

WET HELIUM

Δ

DRY HELIUM

l/^a

0

I

I

I

10

20

30 TIME,

Figure 4.

L

I

I

40

50

60

70

HOURS

Adsorption of radiostearic onto copper films

acid

Adsorption on copper was about the same in wet or dry helium and in dry a i r , as shown in Figure 4. Slow desorption to about 0.4 mono layer at 70 hours was due to lack of formation of a stable monolayer. Chemical analysis of the liquid layer indicated the presence of copper stéarate. The highest initial rate of adsorption was on the copper film exposed to dry a i r . This suggests that a close-packed monolayer might have been adsorbed if the stéarate molecules had remained at the surface. Exposure to wet air for 15 minutes caused an apparently stable adsorption of about nine monolayers at 60 hours, as shown in Figure 5. The stéarate molecules were not tightly held, even though they r e mained at the surface, because rinsing with n-hexane removed all but τ

/ Ο

ζ

Ο

4

n-HEXANE

RINSE |

20 TIME,

40

BENZENE

RINSE

60

HOURS

Figure 5. Adsorption of radiostearic onto copper film in wet air

acid

Removal by solvents


300


about 1.4 monolayers and rinsing with benzene left only 0.8 monolayer. The last monolayer was evidently stable. Monolayers Adsorbed at 70 Hours Iron

Gold

Copper

Dry

0.9

0.0

0.3

Wet

0.5

0.3

0.4

Dry

1.2

0.6

0.4

Wet

0.4

0.2

9.0


Helium

Air

Excluding copper in wet a i r , the greatest adsorption was about one monolayer on iron in the dry atmospheres; the least, on gold in dry helium. In the presence of moisture, however, iron and gold adsorbed 0.2 to 0.5 monolayer, regardless of whether the metals were exposed to helium or a i r . Thus, two metals with large differences in reactivity adsorbed about the same amount of radiostearic acid when moisture was present. When adsorption was less than a compact monolayer, the stearic acid molecules may not have been vertically oriented and the effective molecular area was greater. On the other hand, a mixed film containing vertically oriented adsorbate and solvent molecules may have been adsorbed [7,15]. Initial adsorption-de sorption humps were observed in most of the experiments. Perhaps the stearic acid molecules rearranged at the surface to occupy greater area. Another possibility is that some stearic acid was slowly displaced by trace quantities of water or other polar contaminants. Conclusion Adsorption of stearic acid onto iron and gold in the absence of moisture was generally consistent with the relative reactivities of the adsorbents, but when moisture was present, the adsorption properties were nearly the same. In no case was there a stable adsorption of more than one compact monolayer. Copper weakly adsorbed nine monolayers in wet a i r ; all but one were easily removed by rinsing. Apparently the first layer of adsorbed molecules is bound more strongly than any succeeding layer; its molecules are the only ones that can be chemisorbed or can react chemically with the surface. When less than a compact monolayer is adsorbed, it may be either a loosely packed monolayer in which the molecules are not vertically oriented, or a mixed monolayer of vertically oriented stearic acid and n-hexadecane molecules. Acknowledgment The authors are grateful to B . A . Girman and Joseph Gabor for conducting much of the experimental work.


20.

WALKER AND RIES

Adsorption on Vapor-Deposited Metals 301

Literature Cited


(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)

Aniansson, G., J. Phys. Colloid Chem. 55, 1286 (1951). Bartell, L. S., Betts, J. F., J. Phys. Chem. 64, 1075 (1960). Beischer, D. E., Ibid., 57, 134 (1953). Bowden, F. P., Tabor, D., "Friction and Lubrication of Solids," p. 214, Oxford University Press, Oxford, England, 1950. Conrad, Μ. Α., Levy, S., Nature 189, 887 (1961). Cook, H. D., Rev. Sci. Instr. 27, 1081 (1956). Cook, H. D., Ries, Η. Ε., Jr., J. Phys. Chem. 63, 226 (1959). Fowkes, F. W., Ibid., 64, 726 (1960). Gaines, G. L., Jr., Ibid., 61, 1408 (1957). Gaines, G. L., Jr., Nature 186, 384 (1960); J. Colloid Sci. 15, 321 (1960). Hackerman, N., Powers, R. Α., J. Phys. Chem. 57, 139 (1953). Heavens, O. S., Brown, M. M., Hinton, V., Vacuum 9, 17 (1959). Kafalas, J. Α., Gatos, H. C., Rev. Sci. Instr. 29, 47 (1958). Kramer, J., Z. Physik 125, 739 (1949). Levine, O., Zisman, W. Α., J. Phys. Chem. 61, 1188 (1957). Shepard, J. W., Ryan, J. P., Ibid., 60, 127 (1956). Shishakov, Ν. Α., Ibid., 64, 1580 (1960). Smith, Η. Α., Fort, T., Jr., Ibid., 62, 519 (1958). Wainfan, N., Scott, N. J., Paratt, L. G., J. Appl. Phys. 30, 1604 (1959). Walker, D. C., Ries, Η. Ε., Jr., J. Colloid Sci. 17, 789 (1962). Young, J. E., Australian J. Chem. 8, 173 (1955).

Received April 3, 1963.


Direct Measurement of Adsorption of Radiostearic Acid onto Vapor

Recommend Documents