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liquid should lose all its configurational entropy on reference were listed in one column in units of calories cooling in equilibrium. per (mean) gram-at,om, as was the fashion a t that time. Apparently, in computing the data in ref. 1 (3) J. E. Kunzler and W. F. Giauque, J . Am. Chem. Soc., 74, 797 the mean gram-aton1 mas taken for the gram molecular (1952). weight, which contains 3 mean g.-atoms. Application (4) J. H. Gibbs and E. A. Di-Maraio, private communication. of this factor gives a value of 2.75 cal. deg.-’ for AI;, per bead for Bz03. This value is well within the stated limit of uncertainty of Wunderlich’s “universal” value. I n addition to any inherent variation between The Wetting of Gold Surfaces by Water‘ the configurational heat capacities for the diff ererit substances, the applicable uncertainty includes any contributions associated with any inadequacies in the by Malcolm L. White treatment of measurement and interpretation problems Bell Telephone Laboratories, Incorporated, Allentown, P e n n w l v a n i a such as: (1) heat eFfects arising from potentially iso(ReceGed M a y 7 , 1964) thermal configurational equilibration during the course of a heat capacity measurement, ( 2 ) the specification of the value of T , to which the heat capacity values are Past work on the wetting of metal surfaces by water extrapolated, and (3) the specification of the “bead” has yielded some apparently conflicting results. For unit in the case of molecules usually considered ais example, Harkins2 reported that water does not spread monomers. Without attention to such problems, the on clean mercury. Bartell and Smith,3however, found several different curves given in ref. 2 can lead to values that water had a low contact angle on vacuum-evapoof Acp for BzOa varying a t least from 2.3 to 4.4 cal. rated films of gold and silver. Trevoy and Johnson4 deg.-‘. The selection of the (here corrected) value in found that a number of different metal surfaces could ref. 1 represents the application of current understandbe made hydrophilic if properly cleaned, but this cleaning of the nature of the glass transformation process i n ing involved vigorous chemical and electrocheniical order to treat the above-mentioned problems in such oxidation, so that the metal surfaces dere undoubtedly a way as to evaluate a t T , the heat capacity of the equjcovered with an unknown amount of Dxide. Bewig and libriuni supercooled liquid (including configurational Zisman5 obtained low contact angles of water on gold heat capacity) and of the glass in a Configurational and platinum by a n HK03-H2S04etching treatment state of fixed deviation from equilibrium (thus excluding which would result in oxide formation, even on these configurational heat capacity). The original curve noble metals. thus selected for use happens to be the same as that Sonie recent observations in this laboratory have indesignated (without detailed explanation) by Thomas dicated that the oxide on metal surfaces has a very and Parks as “the normal or standard values for a striking effect on the wettability of the metal by water, carefully annealed boron trioxide.” The above coran oxide-free metal surface being hydrophobic and the rection changes the conclusion from Wunderlich’~ oxidized surface hydrophilic. original compilation that B20a is a n exceptional Gold is a metal of particular interest in surface studies, substance in the above respect. It is now shown to because it is the only metal which does not forin a n behave exactly like other glass-forming substances oxide on heating in air or oxygen. This is apparent not investigated in this respect. only from thermodynamic data,6 but also from experiWe have also examined the original reference3 for mental studies in which gold was heated in air and oxyHSSOI.3H20, the other exception in Wunderlich’s gen to 900” with no evidence of oxide formation as deoriginal tabulation. No tabulation errors were found. termined by X-ray and electron diffraction I n ref. 1 the universality of Ac, was discussed in __ terms of glass transformation based on the theory of (1) Presented a t the 145th National Meeting of the American Chemical Society, New York, N. Y., September, 1963. Hirai and Eyring for molecular rearrangement in (2) W. D . Harkins, “Physical Chemistry of Surface Films,’’ Reinhold liquids, and reference was made to the desirability of a Publishing Co., New Pork, N.Y . , 1952, p. 12. solution to the heat capacity equation based on the (3) F. E. Bartell and J. T. Smith, J . P h y s . Chem., 57, 165 (1953). second-order thermodynamic transition theory of (4) D. J. Trevoy and H. Johnson, Jr., ihid., 6 2 , 833 (1958). Gibbs and DiMarzie for glass transformation. Ac(5) K. W. Bewig and W. A. Zisman, Advances in Chemistry Series, No. 33, American Chemical Society, Washington D. C., 1961. cording to Gibbs and Di,llarzio,* this theory does also (6) A. Glassner, “The Thermodynamic Properties of the Oxides, lead to universality of Ac, a t T z , the temperature a t Fluorides and Chlorides to 2500°K.,” Argonne National Laboratory which they predict that an equilibrium supercooled Report ANL-5750. ~~
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Due to the instability of gold oxide it is possible to clean gold surfaces by thermal or chemical techniques that will remove organic Contamination, but will not oxidize the gold, so that a clean oxide-free metal surface can be obtained. An all-Pyrex and metal system was built so that a sample of gold foil could be placed in it, heated to 500' in clean air, and then cooled to about 5' without removing it from the system. Air, saturated with water a t room temperature, was then passed over the sample and condensed on the surface. By observing the appearance of the condensed water under 50-100 x niagnification, it was possible to determine the wettability of the surface. Water condensing on a completely hydrophilic surface, such as clean glass, spreads immediately so that a thin film of water is formed which gradually builds up in thickness, as evidenced by a continually changing pattern of interference colors. When a thick enough film of mater js formed, the interference colors disappear, and the sample surface is seen through a transparent film of water. With a less wettable surface the water does not spread as completely, this being easily detected by the lack of broad interference colors and the appearance of nonwetting areas. Strongly hydrophobic surfaces show droplets formed from the condensation. A very effective technique for obtaining clean air was to pass it through a stainles steel tube (packed with stainless steel wool) heated to 550-600'. This heating treatment served to oxidize organic contaminants, presumably to CO, and HzO which were then condensed in a Dry Ice-alcohol trap following the furnace. The cleanness of the system was checked as follows: a piece of polished aluminum (99.0+yo) was oxidized at 500' in air for 10 min. in the apparatus to give a completely hydrophilic surface. The sample was kept in the system with the cleaned air flowing over it and the wettability periodically checked. It mas found that the oxidized aluminum surface stayed completely hydrophilic for a period of at least 23 hr., showing that no hydrophobic contaminants were present in the air. When the oxidized aluminum surface was removed from the clean system and exposed to the laboratory air for 5 min., a detectable change in wettability was seen, so the gas system used can be considered as very clean. The gold used was 0.010-in. foil of 99.997, purity which was degreased with trichloroethylene and acetone before using. The samples were polished with 0.3-fi particle size alumina on billiard cloth and then ultrasonically agitated to remove any adhering abrasive pauticles. The vacuum-evaporated gold films were prepared on cleaned microscope glass cover slips by evaporation of 99.99yo gold from a tungsten filament in a T h e Journal of Physical Chemistry
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Varian VI-4 unit at a pressure of lo-' min. to give about a 1000-A. film of gold. This is considered to be a grease-free system, because no lubricants are used. I n order to test the contamination associated with the polishing process, a microscope cover slip was polished in the same way as the gold samples. It was found that the surface was slightly hydrophobic after the polishing, but could be made completely hydrophilic by heating in clean air in the apparatus at 250' or higher for 10 min., showing that the contaminants picked up during the polishing are easily removed by thermal oxidation. The polished gold surface was found to be strongly hydrophobic, showing drop-type condensation of water vapor. Heating the gold to 500' for 30 min. in the clean air in the apparatus caused no change in the wettability of the surface. The gold could even be heated to 950' for 2 hr. in a muffle furnace with no change in wetting characteristics, showing that organic contamination was not causing the hydrophobic character of the surface, since any organic material would be burned off a t this temperature. The gold can also be cleaned by a wet oxidation procedure using hot 30% hydrogen peroxide, this treatment having previously been shown to be very effective for removing all traces of oxidizable impurities without leaving any metallic or inorganic residue^.^ After 2030 min. boiling in 30y0hydrogen peroxide, the gold surface remained strongly hydrophobic, confirming the results of the thermal treatments that organic containination cannot account for the hydrophobic character of the gold. It is known that gold is not oxidized by hydrogen peroxide. lo These results were checked on gold surfaces that were degreased, but not polished, and on vacuum-evaporated films to make sure that the surface preparation techniques mere not affecting the results. In all cases the gold remained hydrophobic. It was found, however, that the gold could be made hydrophilic by anodically oxidizing it in an acid solution. When the gold \vas made an anode in 1 N HNOs (with another piece of gold as the cathode) and a potential was applied a t a current density of about 5 ma. /cm. for 5-10 min., the surface became hydrophilic, although coniplete spreading was not observed. The gold that was used as the cathode, however, remained strongly hydrophobic. During the anodization a color change (7) D Dlark, T. Dirkinson, and W N lllair, Trans Faraday Soc , 5 5 , 1937 (1959). (8) D Clark, T Dirkinson, and R r\; Max, J Phys Chem , 6 5 , 1470 (1961). (9) D 0 Feder and D. E Koontz, ASTllI STP 246, 1958, p 41 (10) hf C Sneed, J L Maynard, and R C Brasted, Comprehensive Inorganic Chemistry," Vol. 2, D Van Sostrand, Princeton, 3 . 3 , 1964.
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on the gold anode confirmed the presence of (hydrated) gold oxide." When the oxidized hydrophilic gold surface was heated to 250' in clean air in the apparatus, the oxide disproportionated to free gold and oxygen, as evidenced by the disappearance of the oxide color, and the surface again became strongly hydrophobic. This experiment was repeated using different methods of surface preparation, always with the same result that the oxidized surface was hydrophilic and the oxide-free surface hydrophobic. When the oxidized gold was kept in clean air in the apparatus a t room temperature, it also became strongly hydrophobic after 5-10 hr. , with an attendant disappearance of the oxide color. All of the above experimental evidence leads to the conclusion that it is the oxide on a gold surface which makes it hydrophilic It seems likely that there would be more interaction o f water (and therefore more wetting) with a metal oxide than with a free metal surface because of dipole-dipole effects and hydrogen bonding of water with the oxide, these effects not being present on a free metal surface. It is also very likely that other metals are similar to gold in their wettability by water, the oxide-free metal surfaces being hydrophobic, with the formation of oxide causing the surface to become hydrophilic. These observations have been confirmed by some recent calculations of Fowkes,12who has shown that water should not spread on oxide-free nieta,l surfaces because of the large relative contributions of metallic bond forces and London dispersion forces to the total surface energy, these forces not contributing significantly to any inter-. action with water. (11) L. Young, "Anodic Oxide Films," A4cademicPress, New York, N. Y . , 1961. (12) F. M. Fowkes, 68th ASTM Meeting, Atlantic City, N. J., June, 1963.
Reactions of Large Cycloalkane Rings i n Hydrocracking
by R. J. White, Clark ,J. Egan, and G. E. Langlois California Research Corporation, Richmond, California (Received J u n e 17, 1964)
It was shown previously1 that alkylcyclohexanes having four or more carbons in alkyl side chains undergo the paring reaction. In this selective cracking reaction ,the cycloalkane character of the reactant is preserved
NUMBER OF CARBONS IN PRODUCT MOLECULE
Figure 1. Product distribution from hydrocracking: A, n-hexadecane, 43.7% cracking a t 290"; B, hexamethylcyclohexane, 87.5% cracking a t 234"; C, cyclododeca,ne, 91.0% cracking a t 296"; D, cyclopentadecane, 94.9y0 cracking a t 291 '.
(both alkylcyclopentanes and alkylcyclohexanes of lower niolecular weight are formed), and the predominant alkane product from cracking is isobutane. I n this paper the behavior of cyclododecane and cyclopentadecane under similar cracking conditions is reported. These large ring hydrocarbons having no side chain conceivably could either undergo the paring reaction after contraction of the ring or could undergo (1) C. J. Egan, G. E. Langlois, and R. J. White, J. Am. Chem. Soc., 84, 1204 (1962).
V o l u m e 68, N u m b e r 10
October, 1984
