group@. Chemical methods are usually limited to polymers of low molecular
weight determinations of LO-gram samples of oxyethylene compounds is shown in the following table:
ly is the conceptration of the
sing the sample size to overcome problem. The determination of
Approx. No. Av. Mol. Wt. 2 , 000
8,000 13,000
17,000
30,000
HO-[CBaCHzOCH&-] "OK
Net Titer, Ml, 20 6.0 3.1 2.4 1.3
e,
7%
0.3 1.1 2 2
2.5
3 1
reagent. Therefore, the reaction proceeded rapidly and no difficulty \vas experienced in determining the unreacted hydrochloric acid. A technique such as this could conceivably be applied to other problems dealing with insoluble polymers; however, sample size would no doubt be limited because of the high viscosity of the swollen polymer-solvent mixture. I n 8ome cmes the methods cannot be
These data show that as the molecular weight increases the
since this causes so For practical purposes this technique is of little value above a molecular
reactivity of the functional group in the polymer.. LITERATURE C l E D
tion of approximately 10 ml, is desirable
net titration as low
A studv of the nrecisi
prevent excessive errors. One unique solution to solubility problems was recently reported for determining unreacted epoxy groups sins (1). These resins were e m the NCl-diox cleave the oxirane nely divided polymer swelling occurred and intimate contact was obtained between the sample and the END
(1) Dannenburg, H., Harp, W. R., Jr., ANAL.CHEM.28,236 (1956). (2) Gorsuch, T. T., Analyst 135,84(1959).
(3) Johnson, D. P., unpublished data,
Union Carbide Chemicals C Charleston, W. Va. ( 4 ) Johnson, D. P., Critchfield, F. E., J.Agr. Food Chew., in press. RECEIVEDfor review July 10, 1961. Accepted September 13, 1981. Division of Analytical Chemistry, 139th Meeting, ACS, 5t. Louis, Mo., March 1961.
OF SYMPOSIUM
ity of Condense
b Condensed 14-labeled palmitic a down by the bangmuir-B on gold surfaces which had previously been treated electrolytically in on oxidizing or reducing environment. The stabilities of the resulting monolayers were determined by measuring double layer capacities as a function of time and by radiochem terminations. The most stab1 A disruption of the surface, either by forming or removing a monolayer of gold oxide caused the adsorbed layer to become less stable.
connection with a study of the anodic oxidation of gold surfaces (7),i t was desired to compare the stabilities of condensed monolayers of palmitic acid on oxidized and reduced gold N
1836
0
ANALYTICAL CHEMISTRY
this purpose, a slightly uir-Blodgett (& 10) electrode through a solution-air interface at which a condensed monolayer of carbon-14-labeled palmitic acid was present. By suitable application of a controlled potential to the gold electrode prior to its withdrawal, its surface could conbe placed in a reduced or o electrode with rbed lmitic acid could then be r either oxidizing or reducns, and the double layer observed as a func. While the double layer e regarded as a n abthe amount of palsurface, i t is a conndestructive method of following surface changes as a function of conclusion of a n experiunt of adsorbed palmitic
acid could be determined by dissolving it from the surface and using scintillation counting to determine carbon-14. EXPERIMENTAL
Prior Electrolyte Treatment of Surrevious studies on the electrolytic oxidat old in aqueous hloric acid (6, as kno potential of 1.55 volts (us. h electrode) in 0.01M HCIOl or 1.65 volts in 1M HC104, a sur responding approximat ld(II1) oxide could be put face by anodic treatment. A t higher potentials, a dee oxidation to form a massive 1 Au208 occurs. The
1
Present address, The Dow Chemical
Co., Midland, Mioh.
traces of adsorbed material from the surface. Upon shifting the potential to 0.45 volt in 0.01M HClO4 or 0.65 volt in 1M HClOd, the oxide could be reduced from the surface. A potentiostat constructed b y Enke (6), capable of delivering a maximum current of 50 ma., with a rise time of 50 psec. and a noise level of 1 mv., was used. All potentials were measured against a saturated sodium chloride calomel electrode using a perchloric acid salt bridge and referred to a standard hydrogen electrode. Placement of Monolayer on Surface. An electrode was prepared in the form of a bright gold foil of 99.9% purity (Goldsmith Brothers Smelting Co.), welded t o a thin gold wire, welded in turn t o a platinum wire which was sealed into soft glass tubrng. Only the foil and part of the gold wire were immersed into solution. A snugly fitting guide, machined out of a Lucite rod 11/*inch long and 1 inch in diameter permitted raising the electrode without lateral motion. The electrode was raised at the rate of 0.1 cm. per minute b y using a slowly turning pulley to draw a thread attached at one end to the electrode and a t the other end to a counterweight. To provide a condensed monolayer a t the solution-air interface, the following procedure was employed. A Petri dish was filled with 0.01M perchloric acid solution. Into the Petri dish were placed two sections of glass tubing to enclose the reference electrode bridge and counter electrode to prevent their contamination, but to permit electrolytic contact through their notched lower ends. Two glass arcs, prepared by longitudinal sawing of a short length of glass tubing were placed near the center of the Petri dish. The arcs served to keep in position a nylon thread ring 3 cm. in diameter. The thread had previous1 been dipped in a 5% solution of para& in benzene and thoroughly dried. The thread ring served as a barrier between a monolayer of palmitic acid inside the ring and a layer of oleic acid outside the ring, the latter acting as a “piston oil” to keep a pressure on the palmitic acid layer as it was being displaced to the gold surface. Before adding the palmitic or oleic acids, the indicator electrode was lowered until about 2 mm. of its stem was submerged. About 0.02 ml. of 0.04m-M solution of C14-palmitic acid in benzene was introduced on top of the solution inside the thread ring, taking care to place the solution a t a spot removed from the stem of the electrode. The palmitic acid solution would form a small lens, the disappearance of which signified the completion of spreading. If the lens moved and wetted the thread, the amount of palmitic acid adsorbed would be too low. -4 drop of oleic acid was then added to the solution down the rim of the Petri dish. The thread would immediately shrink, but care would be taken not to allow it to touch the electrode stem nor to be directly above
any part of the electrode. After 3 minutes of standing, the electrode was slowly raised by means of the pulley. Vibrations and air currents were carefully avoided. By several preliminary experiments in which the carbon-14 count was made without subsequent treatment, i t was ascertained, as described below, that a reproducible monolayer could be placed in this manner. Radioactive Counting Technique. T h e simplest technique, using a windowed Geiger counter directly on the solid samples, was rejected because of its low efficiency. Flow counting, in which a solid sample could be directly introduced into t h e sample chamber was tried, b u t was abandoned for several reasons, including the following: uncertainty in the backscattering coefficient would have necessitated an empirical factor for our particular system; available sizes of planchets (1 em. diameter) were inconvenient for allowing the same electrodes to be used for electrochemical measurements; and the flow system used was poor in reproducibility (ca. 20% variations with repeated counting of the same sample). Scintillation counting, although a destructive method for our samples, was advantageous in being an absolute method of high precision (standard deviation 1% in the present measurements). A Tri-Carb liquid scintillation spectrometer (Packard Instrument Co.), a fully automatic instrument, was used for the counting. The electrodes coated with monolayers were extracted by 30 minutes of soaking in toluene with occasional shaking. Residual activity, as measured by flow counting, was only about 1.5% of that initially present. A solution of the scintillator 2,5-diphenyloxazole (DPO) was added t o make the sample 3% in DPO and 15 ml. in total volume. Double Layer Capacity Measurements. T h e differential capacity of the double layer was measured by a constant current pulse method. This method is identical in principle with the square wave method used by Brodd and Hackerman (4) except that a single pulse rather than a continuous square wave was used. The magnitude of the current pulse was adjusted to give a linear voltage time trace (after an initial voltage jump due to iR drop), with a potential change of a few millivolts in a time of the order of 10 to 100 psec. The current pulse was generated from a Tektronis Type 161 pulse generator, with an adjustable pulse length of 10 psec. to 180 msec. The current pulse and the potential-time trace were displaypd simultaneously on a Tektronix Type 502 dual beam oscilloscope. The oscilloscope sweep and the current pulse were both triggered simultaneously by a signal from a 45-volt battery. Preliminary experiments using a high pressure mercury relay (6) used to delay the signal to the generator so that the sweep would start fractions of a millisecond earlier than the pulse showed te pulse thus observed in shape or length than that observed without the delaying
action. Since both the generator and the oscilloscope had fast rise times (less than 1 psec.), it was possible to dispense with the relay. The calibrated voltage deflection was accurate to 3%, the time base to better than 3y0,giving a maximum accuracy of differential double layer capacity of the order of 3%. Reagents. Perchloric acid was used as a supporting electrolyte because the oxidation-reduction behavior of gold electrodes in this medium was known. Some samples of reagent grade perchloric acid were found t o contain trace amounts of surface active material, the effect of which was evidenced by a gradual lowering of double layer capacity, a t low applied potentials. It was purified by adding about one third its volume of concentrated nitric acid and distilling Water was distilled from alkaline permanganate. Benzene and toluene were distilled from reagent grade chemicals. Oleic acid was distilled under reducpd pressure. 2,5-Diphenyloxazol and C14-palmitic acid (activity: 2.04 mc. per mmole) were used as such. Cleaning of Electrodes and Glassware. Drastic methods of cleaning the gold electrodes, such as strong anodization or prolonged heating were avoided for fear of excessive alterasurface. Reproducible obtained by keeping the 10M nitric acid, washing, and heating in air at 300’ C. for 1 t o 2 minutes before use. Glassware was cleaned by immersion in alkaline permanganate followed by rinsing in concentrated hydrochloric acid. Dichromate cleaning solution was shunned deliberately, because it has been found diflicult to remove the last traces of chromium from the glass surface (11), Monolayer Coverage of Surface. Taking 20.5 x 10-16 sq. cin. as the apparent area of a palmitic acid molecule (IO), 1 sq. cm. of area would be covered by 4.88 X 1014 molecules. Using the activity of palmitic acid given by the supplier, 1 sq. cm. of fully covered surface would yield an activity of 3670 d.p.m. This value wae used in calculating the surface coverage. The percentage of complete coverage as a function of potential was 90 to 99% (randomly variable) over ~t range of potentials from 0.55 t o 1.75 volts. The fact that the theoretical surface coverage calculated from the geometric area was achieved within a few per cent indicates either that the surface was actually quite smooth, or that palmitic acid applied by the LangmuirBlodgett technique responds to the geometric area. In this connection, Adam (1) has pointed out that if a wire gauze was dipped. the area of the film deposited was equal to the ze of the gauze, not to the of the wires forming it. On the other hand, the adsorption of VOL. 33,
NO. 13. DECEMBER 1961
1837
30 90 *..e..
*--
60
Figure 1.
I
eo 100 TIME,MIN.
120
83 9b
I
140
180
d
Desorption of monolayer at 1.65 volts
0’
20
40
ek
80
IbO
,io
d
51ME.MIN.
Figure
fatty acids has been used by many authors to estimate the roughness factor (12, IS). If such an estimate i s valid the roughness factor of the present gold electrode must be close to unity. This result is not altogether improbable, in view of the fact that the roughness factor of bright platinum foil was evaluated at 1.12 by the BET method using krypton adsorption (8). The fact that palmitic acid could be removed by simple toluene estraction from either the oxidized or the reduced surface could mean that salt formation probably did not take place between the acid and the oxidized gold surface. Bomden and Moore (3) found no evidence of chemical reaction between stearic acid and gold. Double Layer Capacity Measurements. The double layer capacity was observed by transferring the electrode into a clean, quiescent solution of 1M perchloric acid and making measurements a t intervals. .4t the conclusion of each experiment, the palmitic acid remaining on the electrode was determined by scintillation counting. I n Figure 1, results are shown for electrodes standing a t an oxidizing potential (1.65 volts in 131 HC104), and similarly in Figure 2 a t the reducing potential (0.65 volt). I n both figures, curve I represents a monolayer which had been coated on an oxidized surface (kept at 1.65 volts in 0.01;1.1 HCIOJ, and curve IJ, a monolayer which had been coated on a reduced surface (kept a t 0.45 volt). Curve I11 represents a clean electrode to serve as a comparison. The percentage figure a t the end of each curve represents the amount of palmitic acid left on the surface at the end of the experiment. Curves I and I1 in each case started with the same amount of adsorbed acid on the surface, as had been shown in the earlier counting experiments. It was also known that adsorbed palmitic acid lowers the double layer capacity, and that the change in capacity would serve as a n indication of the change of coverage. The gradual change of capacity of a clean surface is due to its gradual contamination by traces of 1838
e
ANALYTICAL CHEMISTRY
2 . Desorption of monolayer
surface active impurities. The contamination is more pronounced at low potentials than at high potestials, where desorption occurs because of the excessively high positive charge density. Both the capacily measurements and the final radioactive count show that in an oxidizing environment, the reduced surface had lost more acid than the oxidized one. Conversely, in a reducing environment, the oxidized surface had lost more acid than the reduced one. The larger loss in each case was no doubt due to the disrupting effect of oxidizing a reduced surface or reducing an oxidized one. The monolayer produced when reducing conditions were maintained throughout is the most stable that can be obtained, and it is significantly more stable on gold than on mercury under similar conditions (9). ACKNOWLEDGMENT
The authors gratefully acknowledge the help of R. F. Nystrom and George Wolf in radiochemical measurements. LITERATURE CITED
(1) Adam, N. K., “T$ Physics and Chemistry of Siirfaces, 3rd edition, p.
413, Oxford University Press, London, 194i. (2) Blodgett, K. R., J . Ana. Chem. Sac. 57, 1007 (1935). (3) Born-den, F. P., Moore, A. C., Trans. Faraday SOC.47, 900 (1951). (4) Brodd, R. J., Hackerman, IC., J.Electroehem. Soc. 104, 704 (1957). (5) Chao, M, S., Ph.D. thesis, University of Illinois, 1961.
(6) Enke, C. G., Ph.D. thesis, University of Illinois, 1959. ( 7 ) Laitinen, H. A., Chao, M. S., J . Electrochem. SOC.108, 726 (1961). (8) Laitinen, H. A., Enke, C. G., Ibid.. 107, 773 (1960). (9) Laitinen, H. A., Morinaga, K., Rept. ARL-TN-60-129 to Aeronautical Researrh Laboratories, U. s. Air Force. Wright-Patterson Air Force Rase Ohio. (10) Langmuir, I., Shaefer, V. J., j . Am. Ghem. SOC.58, 284 (1936). (11) Laug, E. P., IND.ENC. CHEnf., ANAL.ED. 6 , 111 (1934). (12) Orr, C., Jr., Dallavalle, J. M., “Fine Particle Measurement,” p. 207, Maemillan. New York, 1959.
at 0.65 volt
(13) Radlein, G., Honrath, H., Z. EEektrochem. 63, 397 (1959).
REC&WEDfor review August 16, 1961. Accepted October 4, 1961. Division of Analytical Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961. Work supported by grants from the National Science Foundation (grant 6-24201 and the U. S. Air Force [contract AF 33(616)-54461.
Correction Chrsnopotentiometry
with
Very recent experiments in the author’s laboratory have shown that the chronopotentiograms described [Anson, F. C., A I ~ A LCHEM. . 33, 1498 (1961); J . Am. Chem. SOC.83, 2387 (1961) ] were erroneously identified as arising from iron(I1) and iron(II1) adsorbed uniformly on platinum electrodes. dlthough the observed chronopotentiograms are quite reproducible and are in excellent agreement with the theoretical wave equation for true adsorption chronopotentiograms, the apparent adsorption disappeared when the platinum wire electrodes were removed from the glass tubing in which they were sealed and the experiments repeated. It was also discovered that no apparent adsorption was observed with electrodes sealed in glass when the exposed wire but not the wire-glass seal was immersed in the electrolyte. Experiments in progress to ascertain the actual source of the chronopotentiograms indicate that they arise from reactant that is contained in tiny invisible cracks on the inner wail of the glass tubing in which the electrodes ard sealed, Additional experiments are in progress to clarify more fully the nature and origin of the chronopotentiograms.
