ADSORPTION OF POLAR ORGANIC MOLECULES ON CHROMIUM
3731
-2RTIF) is different from that at the prereduced electrodes (bV/b In i = -R T / F ) . Acknowledgments. We wish to thank the U. S. Army Electronics Research and Development Laboratory, Fort Monmouth, N. J., for financial support (Contract No. DA-36-039-SC88921) and Dr. H. F. Hunger and Mr. J. Wynn for discussions. We also thank Mr. 0. Shannon for machining the rotating disk apparatus and Dr. E. Yeager for the design.
as an intermediate. The major reaction is the reduction to water. At the prereduced rhodium electrodes in alkaline solution, the main path for oxygen reduction is that in which oxygen is reduced to hydrogen peroxide. Hydrogen peroxide reduces further to water. Reduction to water without hydrogen peroxide is a minor reaction. The mechanism of oxygen reduction a t the preoxidized rhodium electrodes in acid solution (bV/b In i E;:
Adsorption of Polar Organic Molecules on Chromium
by B. J. Bornong and P. Martin, Jr. Rock Island Arsenal, Labmatory Branch, Rock Island, Illinois 61,901 (Received January $0,1967)
~~
~
Ellipsometric and surface potential (AV) data were obtained, at 25' and 40% relative humidity, on retracted monolayers of the homologous series of amines, amides, acids, and alcohols on chromium. Curves of AV vs. N , where N is the number of carbon atoms per a,dsorbed molecule, reached an asymptotic maximum at N 1 14 for the amines and at N 2 18 for the acids and alcohols. AV was constant for the amides up to N = 18. Surface clipole moments (pP) were calculated from the Helmholtz equation using AV and surface coverage data.
Introduetion Studies have been made using ellip~ometryl-~and surface potential t e c h n i q u e ~ ~to- ~measure ~ the adsorption of certain polar organic compounds on metals. A recent work" combined the two methods to perform parallel experiments on the growth of oxide films on metals. The work reported here, using these methods, was intended to improve our understanding of molecular packing and dipole orientation in adsorbed films and to develop specific models .for the systems studied.
Experimental Section Materials. The following reagent grade or White Label compounds were used in this investigation : acetic acid (Du Pont); formic, palmitic, and stearic acids, 1-octanol, butylamine (Fisher Scientific Co.) ; octanoic, decanoic, lauric, and myristic acids, butyl, decyl, and dodecyl alcohols, 1-tetradecanol, l-octa-
decanol, hexylamine, stearamide (Eastman Kodak) . Compounds of other purity designations were: docosanoic acid 99%, cerotic acid of undesignated purity (Eastern Chemical Corp.) ; acetamide, butyramide, hexanamide, octanamide, lauramide, myristamide, pal(1) J. R. Miller and J. E. Berger, J. Phya. Chem., 70, 3070 (1966). L. S. Bartell and J. F. Betts, ibid., 64, 1075 (1960). (3) W.T. Pimbley and H. R. MaeQueen, ibid., 68, 1101 (1964). (4) R. R. Stromberg, E. Passaglia, and D. J. Tutas, J. Res. Yatl. Bur. Std., A67, 431 (1963). (5) K. W.Bewig and W. A. Zisman, J. Phys. Chem., 67, 130 (1963). (6) C.0.Timmons and W. A. Zisman, ibid., 69, 984 (1965). (7) M.H.Gottlieb, ibid., 64,427 (1960). (8)F. M. Fowkes, ibid., 64,726 (1960). (9) B. J. Intorre, T. K. Kwei and C. M. Peterson, ibid., 67, 55 (1963). (10) D.A. Haydon, Kolloid Z.,188, 141 (1963). (11) J. E.Boggio and R. C. Plumb, J. Chem. Phys., 44, 1081 (1966). (2)
Volume 71, Number 19 November 1967
3732
mitamide, and stearamide, 95-99% (K & K Laboratories, Inc.) ; 1-hexadecanol, Fisher N.F. grade, and a commercial sample from Archer Daniels Midland Co. Hexyl alcohol, 1-eicosanol, octylamine, decylamine, dodecylamine, tetradecylamine, hexadecylamine, and octadecylaniine were practical or technical grade (Eastman Kodak). All the alcohols, liquid acids, docosanoic, and cerotic acids were percolated through alumina or silica gel columns while molten or in their normal liquid state to remove more adsorbable impurities. The amides, except the White Label samplewhich wasused as received, were recrystallized from ethanol after treatment with activated carbon. The remaining compounds were used as received. Amines used probably contained carbon dioxide because they melted over wide temperature ranges. Other compounds used had melting points within 1-2’ of their literature values. Thickness Measurements. Ellipsometer measurements were made with a Rudolph Mode1436/200 E with photoelectric attachments. Procedures for use of the ellipsometer have been described.12-14 The instrument was calibrated for this study with Langmuir-Blodgett barium stearate films deposited on chromium (ferrotype plate) surface^.'^^^^ Polarizer and compensator were placed before reflection. A mercury light source giving the 5461-.4 line was used. The change in A, the phase difference caused by reflection, in going from filmfree to film-coated surface was taken to be proportional to film thickness in the thickness range measured here.17 Each thickness was obtained by a difference measurement on every chromium specimen; the calibration factor of 6.32 A/deg change in A was used. It was assumed that refractive indexes of calibration films and measured films were the same. Comparison of the calibration procedure with the Drude linear approximati or^'*^'^ gave the same film thickness when a film refractive index of 1.52 was substituted into the approximate equation. The angle of incidence was 70”. Surface Potential Measurements. The vibrating condenser method as originally described by ZismanZ0was used to make surface potential measurements. A diagram of the apparatus is shown in Figure l. The reference electrode of platinum (1 cm diameter) was above the test electrode and vibrated at 500 cps by a permanent magnet speaker cone energized by a General Radio audio oscillator, Type 1311-A. The metal to be measured was placed on a metal platform. A bakelite for the be made to the upper electrode by three leveling screws. I n addition, the entire leveling device holding the lower electrode could be raised or lowered by means of a large screw at the bottom of the chamber. The entire assembly The Journal of Physical Chemistry
B. J. BORNONG AND P. MARTIN, JR.
m SPEAKER
POTENTIOMETER
Figure 1.
U
Diagram of the vibrating condenser apparatus.
was housed in a heavy metal chamber to minimize any electrical interference. The signal generated by the vibrating condenser was fed into a Keithley decade isolation amplifier, Model 102-B, followed by a General Radio amplifier and null detector, Type 1232A, which filtered out all frequencies except 500 cps. The amplified and filtered signal was registered on the null detector or observed on a Dumont oscilloscope 304H as a sine wave. To measure a potential, the metal specimen studied was placed on the platform directly under the platinum electrode. The specimen was raised to within approximately 0.5 mm of the reference electrode. The sensitivity of the condenser could be improved by decreasing the distance between the electrodes without affecting the potential. The magnitude of the potential was obtained by applying an opposing voltage across the two electrodes by using a 1.5-v battery and a ten-turn potentiometer. When the ntlll detector indicated zero or the sine wave on the oscilloscope disappeared, the potential was measured with a Precision Scientific titrometer, Model 8860. Measurements could be made within 2 mv. A reversing switch was used to indicate the presence of any additional electrical fields. Using this switch, the polarity of the potential would be reversed but its magnitude would not. I n this work, the surface potential (AV) represented the change from the initial to the final potential which is (12)A. B. Winterbottom, “Optical Studies of Metal Surfaces,” The Royal Norwegian Scientific Society, Report No. 1, F.Bruns, Trondheim, Norway, 1955,Chapter 3. (13)A. Rothen, Rev. Sci. Inatr., 28, 283 (1957). (14) F. L. McCrackin, E. Passaglia, R. R. Stromberg, and H. L. Steinberg, J . Rea. Natl. Bur. Std., A67, 363 (1963). (15) K.B. Blodgett, J . A m . Chem. Soc., 57, 1007 (1935). (16) K. B. Blodgett and I. Langmuir, Phgs. Rev., 51, 964 (1937). (17)See ref 12, 42. (18) p. Drude, Ann. phyaik, 36 (3),865 (1889). (19)A. c.Hall, J . Phga. Chem., 69, 1654 (1965). (20) w.A. Zisman, Rev. Sci. Instr., 3,367 (1932).
ADSORPTIONOF POLAR ORGANIC MOLECULES ON CHROMIUM
produced when a film is adsorbed on an initially clean surface. Electrical interference was minimized by using shielded cables throughout, enclosing the loudspeaker and potentiometer in metal boxes, and connecting the entire shielding system to a common ground. The aged platinum reference electrode was checked periodically with a second platinum specimen. The potential difference did not change by more than 10 mv over a period of several weeks. The aged platinum was considered suitable since it did not vary during the time for a measurement. Procedures. Commercial ferrotype plates, chromium plated steel (lip0110 Metal Works), were sheared into 20 X 25 mm pieces. Residual organic films were removed from the chromium surface by dipping the metal pieces in chromic acid and flaming, as previously de~cribed.'~This procedure probably produces a chromium-chromium oxide surface. Ellipsometer readings were taken on each surface immediately after flaming. The average A value for these film-free surfaces was 133.69" and was reproducible to within 0.48'. For a given chromium specimen, A could be reproduced to within 0.1'. Potential measurenients were made within 6-5 min after flaming; the average potential value was 265 mv and was reproducible to within 5 mv. The organic film was applied immediately after these measurements were made. Retraction from the melt was the usual method of application of n film.21 The chromium specimen was placed on the heating stage of a melt,ing point apparatus; the retraction temperature wm 5-10' above the melting point of the compound. Liquids were applied at room temperature. Homologs with less than six carbons did not retract; these compounds were placed on the chromium and allowed to evaporate for 3-5 min. Final potential and ellipsometer readings were taken immediately after application of the films. Additional retracted films of octadecylamine and stearic acid were applied at higher temperatures to determine the effect of retraction temperature on film thickness and surface potential. All potential and ellipsometric measurements were made at 25 f 1' and 40 f 10% relative humidity.
Results and Discussion
All the experimental and calculated data for the adsorbed organic films on chromium are shown in Table I. Surface potential data from Table 1 are shown in Figure 2, plotted against N , the number of carbon atoms for the homologous series of fatty acids, alcohoh amines, and amides. Markers on the graph represent the arithmetical mean for each compound. The curves rise to an asymptotic nlaximum at N 2 18 for the acids and alcohols and at N 2 14 for the amines. The amides
3733
0.6
'
0
>
r Aminoi
0.5'
Amidol
N,NUMBER OF CARBON ATOMS Figure 2. Surface potentiah for homologs adsorbed on chromium.
gave a horizontal line indicating surface potential independent of chain length. The behavior of amines on platinum as reported by Bewig and Zisman6 is analogous to that obtained here. AV for stearic acid adsorbed on chromium agrees with that obtained by Timmons and Zisman.6 However, they obtained an asymptote at N 2 14 for the adsorption of acids on Pt and NiO. This difference may be caused by the dissimilar metals, different surface preparations, and dipole orientation. Ellipsometric data from Table I on the homologous series of compounds are plotted in Figure 3. Data for the acids and amines are joined by the longest curve, the alcohols by the lower curve, and the amides by the dotted line. The curves show thicknesses somewhat lower than expected for close-packed monolayers. For example, stearic acid has ameasured thickness of 15.4 A, which is less than the expected value of 24.4 A.l5>le Thinning of the monolayers is taken to indicate that the films are not close packed. The ellipsometer, then, measures the average thickness of partially depleted films. Previous work has shown that film thicknesses of monolayers measured ellipsometrically are proportional to the number of adsorbed molecules.1.2 That the monolayers studied here are not close packed is in part caused by their method of application. Figure 4 shows the effect of the retraction temperature (21) W. A. Zisman, Advances in Chemistry Series, No. 43, R. F. Gould, Ed., American Chemical Society, Washington, D. C., 1984, pp s-11.
Volume 71 Number 1 B November 1967 I
B. J. BORNONG AND P. MARTIN, JR.
3734
Table I : Properties of Adsorbed Films on Chromium No. of carbon atoms
AV
No. of measurements
(8V
Film thickness (av dev),a
No. of measurements
dev),“ mv
A
Surface coverage, ff
Av dipole moments, lit. values.b
Surface dipole moment,
Amines 4 6
8 10 12 14 16 18
1 6 4 2 3 4 4 4
1 6 6 2 4 4 4 6
220 316 ( 1 5 ) 380 ( 1 2 0 ) 400 ( 1 2 5 ) 505 ( f 3 0 ) 565 ( f 3 0 ) 560 ( f 2 0 ) 560 (st30)
4.7 4.6 ( 1 1 . 8 ) 6 . 5 (st0. 9) 8 . 4( f l . 6 ) 11.4 ( 1 1 . 1 ) 13.7 ( f O .5) 14.2 ( f1 . 7 ) 18.5 ( f0.8)
0.69 0.57 0.60 0.62 0.70 0.72 0.66 0.76
1.4 1.3-1.6 1.4 1.3
0.17 0.29 0.34 0.34 0.38 0.42 0.45 0.39
Amides 2 4 6
8
1 1 2 4
12 14 16 18
12 11 21
8
3.6 3.5
450 390 385 ( f 5 ) 415 (st35) 359 ( f 4 0 ) 377 ( f 4 0 ) 385 ( f 3 0 ) 398 ( f10)
2 4
4.5(&0.9) 6.5( 1 0 . 3 ) 11.8( s t l . 5) 12.1 ( 5 1 . 1 ) 13.2 ( f 0 . 7 ) 14.6 (fO.9)
8 12 11 23
0.36 0.37 0.26 0.31 0.33 0.35
0.56 0.60 0.72 0.64 0.61 0.60
Acids 2
3
8
3 2 5 2 2 14 2 6
10 12 14 16 18 22 26
2 . 3 ( f1 . 8 ) 5 . 9 (Lt0.9) 8 . 6 ( st 1.5) 9 . 6 ( A I .0) 1 3 . 1( f 0 . 9 ) 16.0(st0.6) 18.4 ( f l . 5 ) 23.5 ( i1.4) 28.4 ( L t l . 9 )
3
20(f10) 47 ( s t 7 ) 105 ( f 5 ) 125 (st20) 14O(flO) 217 ( 1 7 ) 27O(flO) 292 ( 1 8 ) 290 ( zk 10)
3 2
8 6 5 18 18 15
0.85 0.55 0.63 0.59 0.69 0.74 0.75 0.79 0.80
1-1.5 1.2
0.74 0.35 0.41 0.42
1.7 1.65 1.7 1.65 1.5
0.8
0.012 0.045 0.088 0.11 0.11 0.16 0.19 0.20 0.19
Alcohols 4 6 8 10 12 14 16 18 20
2 2 2 5 4 6 2
3 3
Average deviation from t..j mean shown. Co., San Francisco, Calif., 1963. a
3
O(f5) O(Ltl0) 25 40(110) 75(st5) 90(110) 130(stlO) 240(stlO) 240(st10)
3 4 7 4 7 2 4 6 b
. McClellan,
on ellipsometric film thickness for stearic acid and octadecylamine. The rapid initial drop in thickness shows that close-packed monolayers could not be obtained when films were applied at 5-10’ above the melting points of the compounds. That the curves shown level out at higher temperatures suggests a strong adsorption under these conditions, probably chemisorption, on the chromium-chromium oxide subThe Journal of Phy&al Chemistry
4.0 (f l .2) 2 . 8 ( & I . 1) 4.4 ( L t l . 1) 5 . 7 (f0. 9) 8.l(Lt0.6) 11.1 ( f 1 . 5 ) 15.1 ( 1 0 . 1 ) 19.1( s t 2 . 6 ) 21.6 ( r t 3 . 0 )
0.50 0.58 0.70 0.78 0.80
0 0 0.032 0.050 0.080 0.082 0.098 0.16 0.16
“Tables of Experimental Dipole Moments,” W. H. Freeman and
strate. Incomplete desorption has also been reported for stearic acid on Ni0.6 Estimates of surface coverage (e) in Table I were made by taking the thickness (24.4 A) to correspond to a close-packed film of vertically oriented molecules for the 18-carbon compounds. It is assumed that thicknesses for close-packed monolayers of lower and higher straight chain homologs fall on a line from the origin
ADSORPTION OF POLAR ORGANIC MOLECULES ON CHROMIUM
3735
0
ci- acids
OQ
30
-
25
-
20
0
%-alcohols 6- amines 0- amides
.-LL.
-
0
’t
In In 0)
C
“0 15.r
01 50
I
I
150
I
200
1
250
Figure 4. Effect of retraction temperature on adsorbed monolayers of stearic acid and octadecylamine.
i I 10-
0’ 0
I 100
0
Temperature, ‘C.
-l
5-
- octodecylornine -stearic acid
/4/
9’ f/ 4
I I I 8 12 16 20 No. o f Carbon Atoms
I
24
:
Figure 3. Ellipsometric film thicknesea of homologs adsorbed on chromium.
through the 2 4 . 4 4 thickness value. The stearic acid film, then, contains about 75% of the number of molecules of a closepacked monolayer. The Helmholtz relation can be used to correlate the data shown in Table I. This relation is
AV = 4 m p , where n is the number of molecules per unit area and pp is the total perpendicular component or surface dipole moment. Changes in AV from one member of the series to another should reflect differences in molecular packing and dipole orientation. AV increases with chain length for the amines, acids, and alcohols. This increase indicates an increase in the number of molecules adsorbed and an increase in dipole orientation. The constant AV for the higher homologs is indicated by both constant coverage and maximum orientation. For the amides, both the surface coverage and AV are constant; therefore, these homologs have the same packing and orientation. The last column in Table I shows the experimental surface dipole moments calculated by the Helmholtz equation using n equal to B/A , where A is the molecular area. The molecular area for a close-packed molecule was assumed to be 20 A2. The moments obtained were
less than the known values available for the compounds in the gaseous state. This is probably due to the induced polarization caused by the metal and other adsorbed molecules.22 Since p, is derived from AV and 8, relationships between it and chain length can also be explained by molecular packing and orientation. It can be seen in Table I that p, for the amides and amines at about 60% coverage is three times larger than pP for the acids and alcohols at the same coverage. For the amides, these large differences can be explained by the average dipole moments ( p ) being about three times higher than for the acids and alcohols. However, this explanation does not satisfy the p, values shown for the amines. An alternative is that the relatively large differences may be associated with the more basic property (the electron-donating capacity) of the NH2 group as compared to the OH group. Therefore, interactions of these groups with the metal-metal oxide surface appears to be more important in determining AV than does surface coverage for the compounds studied. The variation of pp with chain length, as shown in Table I, requires some discussion. The surface dipole moment iscomposedof the following dipolevector terms: those of the head group in its adsorbed state, those of adsorbed atmospheric gases, and that of the methyl group. The increase in p, observed for the amines, acids, and alcohols from the lower to higher homologs was about 0.15 D. This increase may be attributed to changes in the vertical components of the three dipole vector terms. Variations in these vectors can be brought about by changes in the angle of tilt, rearrange(22) C. 0. Timmons and W. A. Zisman, J . Phus. C h m . , 68, 1336 (1964).
Volume 71. Number 1B November 1967
V. R. PAIVERNEKER AND A. C. FORSYTH
3736
ments of dipoles in the substrate surface layer, changes in bonding of the adsorbed head group to substrate, and changes in lateral interactions between adsorbed molecules. Each of these terms can be expected to vary when the chain length and surface coverage change. The apparently constant orientation of the amides, reflected by their constant AV, can perhaps be interpreted as follows. Stuart-Briegleb models show that the aliphatic carbon chain can exist in a spiral vertical form which occupies about one-third greater surface area than the straight chain model. If the amide films were in this configuration, they would be nearly “close
packed” a t 60% coverage as calculated from a straightchain model. At other surface coverages, the amides would be expected to behave like the other homologous series studied. The results of this study show that ellipsometric and surface potential measurenients can be used to obtain reproducible surface dipole moments. More work is needed to separate the various dipole vector terms involved before the data can be analyzed further. Acknowledgment. The authors thank Dr. L. G. Wiedenmann for helpful discussions on molecular structure.
Photodecomposition of a-Lead Azide in the Solid State
by V. R. Pai Verneker’ and A. C. Forsyth Ezplosives Laboratory, Picatinny Arsenal, Dover, New Jersey
(Received March 6, 1967)
The kinetics of nitrogen evolution from a-lead azide during irradiation with ultraviolet light from a low-pressure mercury lamp has been investigated as a function of the intensity, temperature, and time of the irradiation, the method of preparation, and the age of the sample. The data clearly demonstrate the dependence of the initial photolytic decomposition rate on the method of preparation. It is further concluded that, during the aging process, the defects (resulting from the incorporation of impurities) which participate in the photodecomposition disappear irreversibly. The efficiency of the photolytic process is seen to be greater at +lo” than a t +25”. This is discussed in the light of possible electron traps and intermediate free radicals.
Photodecomposition of Lead Azide in the Solid State The thermal decomposition of a-lead azide under vacuum and in air has been extensively studiede2 The mechanism, which assumes that the final product of decomposition, lead, catalyzes the reaction, has been shown to be valid by the experiments by Reitzner, et al.3s Acceleration of the thermal decomposition of a-PbN6 as a result of prior irradiation has also been demonstrated experiment all^.^^ Recently Jacobs, et aL,* have shown how the photodecomposition of BaNa is affected by its prior partial thermal decomposition. On the other hand, no work on the photolysis of PbT\’6 is reported in the literature. The Journal of Physical Chemistry
Experimental Section PbN6 was contained in a small boat so as to keep constant the surface area exposed to the light source. (1) Research Institute for Advanced Studies, Baltimore, Md. 21227. (2) W. E. Garner and A. S. Gomm, J . C h a . Soc., 2123 (1931); W.E. Garner, A. S. Gomm, and N. R. Hailes, ibid., 1393 (1933); W. E. Garner, Proc. Roy. SOC.(London), A246, 203 (1958); W.E. Garner, “Chemistry of the Solid State,” Butterworth and Co. Ltd., London, 1955, Chapters 7 and 9; G. Todd, Chem. Ind. (London), 1005 (1958); M. Stammler, J. E. A M , and J. V. R. Kaufman, Nature, 185,456 (1960); B. Reitzner, J . Phys. Chem., 65,948 (1961). (3) (a) B. Reitzner, J. V. R. Kaufman, and E. F. Bartell, ibid., 66, 421 (1962); (b) J. V. R. Kaufman, Proc. Roy. SOC.(London), A246, 219 (1958);W.Groocock, ibid., A246, 225 (1958);J. Jach, J. Phys. Chem. Solids,24, 63 (1963).
