Oct., 1959
ADSORPTION OF FLUORINATED COMPOUNDS ON SOLIDPLANAR SURFACES
correlates observed rate data with a model based upon monodisperse cavities of uniform size; whereas, actually, a distribution of initial cavity aizes probably exists. Throughout, we have ignored molecular entanglements between polymer molecules which have been considered important by various investigators. Our justification is that the polymer concentrations ape moderately low and that the proposed mechanism gives satisfying results without considering this additional complication. Allen, Burnett, et dl6 discuss at some length the limiting degree of polymerization below which a polymer will not degrade in a cavitating fluid and
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the discrepancies in the determination of this quantity by various investigators. Since this discrepany involves a range in the limiting degree of polymerization of 200-2000, it is obvious from the scatter in Fig. 3 that our data will not aid in answering this question. It seems worthwhile to point out, however, that the mechanism we propose would predict a strong dependency of this limiting value upon the intensity and characteristics of the cavitational process. It is not surprising, on this basis, that the different experimental conditions of the various investigators lead to different values of the limiting degree of polymerization.
A RADIOACTIVE TRACER STUDY OF THE ADSORPTION OF FLUORINATED COMPOUNDS ON SOLID PLANAR. SURFACES. I. PERFLUOROOCTANOIC ACID’ BY J. W. SHEPARD AND JOHN P. RYAN Contribution No. 146from Central Research Department, Minnesota Mining and Manufacturing Company, St. Paul, Minnesota Received March 10, 1060
A method for measuring areas of flat surfaces involving solution adsorption techniques is described and its limitations discussed. A rarbon-14 labeled fluorochemical acid ( C T F I ~ C ~ Owas H ) used. Isotherms for the adsorption of the acid onto plane surfaces of glass, quartz, aluminum and platinum have been determined. The adsorption is not reversilde. Desorption studies showed that the rate and extent of desorption was a function of the polarity of the desorbing solvent. Contact angles using hexadecane were found to be a poor measure of the extent of surface coverage by the adsorbate. Surfaces of platinum and quartz were unreactive and surface area measurements corresponded closely to the geometric area. Soft glass and aluminum showed signs of chemical reaction with the perfluoro acid. An exchange phenomenon was observed between the adsorbed acid molecules and those in solution. The rate and extent of exchange for the surfaces studied increased in the order: glass, aluminum, platinum.
Introduction The use of radioactive tracer techniques in studying surface phenomena has expanded considerably in recent years.2 One technique which has considerable merit since i t lends a quantitative aspect to the usual measurements of surface properties is the adsorption of long chain polar organic compounds labled with C-14 or H-3 onto solid surfaces. These “oleophobic” films, when properly prepared, have the interesting property of not being wetted by the solution or the pure solvent.* When the film-covered surfaces are withdrawn from the solution, the liquid recedes, leaving a dry surface made up, in most cases, of a closely packed monolayer of the organic compound oriented with the polar group on the surface of the solid and the hydrocarbon chain perpendicular to the substrate. Preliminary work in our laboratory with a benzene solution of C-14 labeled stearic acid indicated that this adsorbed acid imparted only a low degree of “oleophobic” character to the surface and the samples emerged from the adsorption cell wet with (1) Presented at the Symposium on “Surface Chemical Propertiea
of Fluorochomioals,” 134th Meeting of the American Chemical Society, Division of Colloid Chemistry, Chicago, September, 1958. (2) F. P. Bowden and A. C. Moore, Trans. Faradav Sor., 47, 900 (1951); J. E. Willard. THIS JOURNAL, 67, 129 (1953); D. E. Reischer, ibid., 67, 134 (1953); E. Rideal and J. Tadayon, Proc. Roy. Soc. (Lond o n ) , 226A, 346 (1954); J. E. Young, Aslr. J . Chem., 8, 173 (1955); H. A. Smith and T. Fort, Jr., THISJOURNAL, 62, 519 (1958); H. D. Cook and H. E. Ries, Jr., Miami ACS Meeting, April, 1957. (3) W. C. Bigelow, D. L. Pickett and W.A. Zisman, J . Colloid Sci., 1, 513 (1946).
solution. Other workers have also noted this beh a ~ i o r . I~n view of this difficulty, a system was chosen which would eliminate this “carry-out” problem. A surface composed of closely packed, oriented -CFs groups has extreme oleophobic character5 and fluorocarbons as a class have the lowest free surface energy of any known compounds.6 A solution of C-14 labled perfluorooctanoic acid
(C7Fd26OH) in n-decane’ was chosen as the adsorp t,ion system. The initial objective of this research was to develop a simple method for measuring the specific surface area of plane solid surfaces. The importance of the actual surface area when studying adhesion, catalysis, lubrication, etc., is well known. Techniques for measuring specific surface areas of flat surfaces have found limited application. Gas adsorption techniques have been e x p l ~ r e dbut ~~~ the method is primarily limited by low sensitivity. (4) W.C . Bigelow and L. 0. Brookway, i b X , 11, 60 (1956); H. A. Smith and K. A. Allen, THISJOURNAL, 68,449 (1954). (5) F. Schulman and W. A. Zisman, J. ColZoid Sci., 7, 465 (1952); E. F. Hare, E. G. Shafrin and W. A. Zisman, THISJOURNAL, 68,236 (1954). (6) H. M. Scholberg, R. A. Guenthner and R. I. Coon, ibid., 17, 928 (1953). ( 7 ) J. W. Shepard and John P. Ryan, ibid., 60, 127 (1956). (8) P. H. Emmett, “Pittsburgh Conference on Surface Reactions,” Corrosion Publishing Co., Pittsburgh 12, Pa.,1948, p. 82. (9) C. Brown and H. H. Uhlig, J . Am. Chem. Soc., 69, 462 (1947); R. L. Burwell, P. A. Smudski and T. P. May, J . Am. Chem. Soc., 69, 1925 (1947); T. Rhodin, ibdd., 72, 4343 (1950); Rauscb, 2. phyrik. Chem., 201, 32 (1962).
W.
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J. W. SHEPARD AND JOHN P. RYAN
O‘Connor and Uhlig, however, did obtained reasonable values on stainless steel sheet and abraded iron foil.10 Solution adsorption has long been used to determine the specific surface area of powders.”-’* At maximum adsorption two general assumptions are made: (1) the surface is occupied by a close-packed unimolecular film, and (2) the area occupied by each molecule is the same as on an aqueous substrate. The results obtained have shown good agreement with values obtained by independent methods such as the photomicrographic and the BET. Experimental Materials. A. Labeled Acid.-The labeled perfluorooctanoic was prepared by electrofluorination of octan0ic-1-C’~acid. The latter was synthesized from BaC14Os (Oak Ridge National Laboratory). and CvH16MgBr. The acid was purified by fractional distdlation and the fraction boiling in the range 187-192’ was used in this study. The melting point was 38-43’. The neutralization equivalent was 414 (theo. 414) and the amount of free fluoride was found by analysis to be 0.003%. The tagged acid contained approximately 25% C8-acid isomers. The specific activity of the acid (0.12 millicuries/g.) was determined by gas counting the COZobtained by combusting the acid. This value was checked by measurements of known samples in a Liquid Scintillation Counter. A saturated “stock” solution of the perfluoro acid in ndecane was prepared by dissolving excess acid in 50 cc. of the solvent. The solutions used in this study were prepared by appropriate dilution of this “stock” solution. B. Solvents.-Polar impurities were removed from all non-polar solvents by passing the liquid through a column of activated alumina-silica gel. The solvents were checked for absence of polar impurities by placing a drop on neutral, acidic and basic substrate. surfaces used in this study were: C. Surfaces.-The 1. Glass-ordinary microscope slides cut to fit our &-gas sample counting holders. Edges were polished on a lapping machine. 2. Quartz--’/la” ground and polished quartz mil Alcoa flat sheet-2S, plates. 3. Aluminum-40 4. Platinum-20 mil rolled. Procedure.-The steps in the adsorption of tagged acid on a surface and the subsequent measurements were as follows. 1. The samples were pretreated before immersion in the active solution by heat treating a t 450’ in a pure NZatmosphere. When it was necessary to use the same samples for repeat runs, the adsorbed active acid was removed from the sample by rinsing in acetone or water before theaheat treatment. This technique was not designed to give a perfectly “clean” surface, but a reproducible one. It also was suitable for a wide variety of surfaces. 2. The active solutions of perfluorooctanoic acid, which were prepared by dilution to 100 ml. of an aliquot from the “stock” solution, were placed in a 200-cc. test-tube (adsorption cell) immersed in a constant temperature bath (28.5’). The adsorption cell was covered with a standard tapered cap containing an outlet and inlet tube so that a dry N2 atmosphere could be maintained over the solution a t all times. The concentrations of the adsorption solutions were determined by removing aliquots with a micropipet, evaporating t o dryness on stainless steel planchets and measuring the activity. T o eliminate the error caused by absorption of tagged acid on the walls of the pipet, the pipet (10) T. L. O’Connor and H. H. Uhlig, THISJOURNAL, 61, 402 (1957). (11) W. D.Harkins and D. M. Gans, J . Am. Chem. Soc., 58, 2804
(1931). (12) W. E. Ewing, ibid., 61, 1317 (1939). (13) H.A. Smith abd J. F. Fuzek, ibid., 68, 229 (1946). (14) E. B. Greenhill, Trans. Faraday Soc., 46, 625 (1949). (15) W.Hurst and J. I
