Sept ., 1960
TIEAT O F L ~ D S O R P T I O XO F ,iRGON AND
psec. There was little permanent change and oxygen increased this change by approximately 15y0. In hexane (not degassed) with a similar concentration of approximately llf a very intense transient was obt,ained a t entirely different wave lengths, that is continuous absorption was found below 300 mp. There was also a large permanent change, probably to HBr and BY?. Since Br- was produced in tht: aqueous experiments by flashing (as cheeked by precipitation of AgBr), it seems very probable that the transient in water with absorption a t 370 mp is Brz.- (see ref. 9). As regards the transient in hexane, perhaps this is the same as that produced on flashing Br2 itself in hexane (see below). (IV) CCIJ3r (Absorbs below 300 mp).-CC13Br was the most effective substance used by Zeelenberg.” In oxygenated hexane and CC4 no transient absorption could be detected with this compound. (V) NaC10.-It was thought that C10-- might produce a transient spectrum analogous to that obtained from BrO-. However, despite its high photochemical r e a ~ t i v i t yand , ~ the use of concentra-
KRYPTOX ON R I O L Y B D E N U M
SULFIDE
1255
tions of it varying from to 10-5 ill both in water and 0.1 N sodium hydroxide, C10- showed only a permanent disappearance a t 300 mp and a slight transient decrease a t higher wave lengths. (VI) Further experiments were done with solutions of the halogens in water, acid, CC14and hexane both oxygenated and degassed. No absorption attributable to BrO. was detected but some very interesting observations were made 1Yhic.h are reported elsewhere.*O (VII) Conclusions.-Apart from the 450 mp absorption detected in oxygenated bromoacetic and dibromoacetic acids, no further confirmatory evidence could he found for the assignment of this hand to the BrO. radical. In view of this, despite the fact that the transient behaves in all respects as expected, it is considered that further evidence should be sought before a final assignment can be made. One of us (S.K.B.) is indebted to the British Rayon Research Association, Manchester, England for leave of absence. We are also indebted to Mr. W. A. Mulac for technical assistance. (20) N. R. Bridge, J . Chem. f ’ h y y . , 32, 915 (1960).
T H E HEAT OF ADSORPTION OF L4RGON AXD KRYPTON ON ;1/IOLYBDEKUJI DISULFIDE-SEPARATIOK OF ENTHALPIES INTO RATIONAL COMPOXENTS BY PETER CANNON General Electric Research Laboratory, Schenectady, N . Y . Received M a r c L 16, 1960
The molar integral heats of adsorption of two rare gases on oxide-free L\IoSz are presented as functions of coverage. The data are analyzed in term$ of both sorbate-solid and sorbate-sorbate interactions. The values of each separate interaction are calculahle from the fundamental physical properties of the components of the system. The calculation of the dispersion forces for the gas-solid pair is most successful using London’s form for the dispersion constant. The data cover the ranges 4 x 10-7 < p / p , < 2 x lOW3(Ar)and 4 x < p / p o < 4 X 10-YKr). The argon film exhibits a gas-liquid transition and the krypton film shows a maximum self interaction energy equal to one half the heat of liquefaction.
Introduction When we consider the changes in state of a monatomic film of an adsorbate on a uniform substrate with various degrees of coverage, it is apparent, that the anticipated behavior should be equivalent (in two dimensions) to that of a regular bulk fluid compressed isothermally from very small to quite high densities. In addition, if the temperature of the experiment is appropriate, phase transitions analogous to those which would occur in the bulk should appear. Such interpretations have been applied in a phenomenological manner to the behavior of films of insoluble long chain polar molecules on the surface of water’ and various aqueous systems, and have been invoked to rationalize the behavior of adsorbed films on, e.g., graphite.2 In the former case, objections recently have been advanced3 to the concept of actual phase changes involving discontinuous changes in molecular coarea. Only a few quantitative treatments of the (1) I. Lansmuir. Colt. Sump Monog., 3, 48 (1925). (2) G. Jura and D. Criddle. THIBJOURNAL,66, 163 (1951). (3) R. h a n o w and L. Witten, J. Chem. Phys., 28, 405 (1958).
behavior of adsorbed films have been attempted, and here the emphasis has perhaps been on forecasting the pressures a t which two dimensional condensation mill occur, and accounting for these values in terms of mutual interaction potentials between the atoms of gas and the solid.4 The evaluation and use of total equations of state for adfilms has not received a great deal of attention, perhaps due to the fact that the van der Waals equation pv = kT( 1
+
e)
has a considerable range of applicability and the virial coefficient p becomes rapidly more intractable as we attempt to use more than a very simple picture of the molecular events occurring in the system. It is therefore of considerable interest to attempt the interpretation of energies of adsorption in terms of the degree of coverage, the adsorbentadsorbate interaction, the adsorbateadsorbate (4)
B. B. Fisher and W. G. McMillan, ibid., 28,
5152 (1958).
400
I
I
~
100 %
I00
E
s YI
+'
- 2
o -100
I
-200
- 300 -400b
T
-
ESTIMATED ERROR IN THIS REGION
I
Fig. 1.-The molar integral heat of, adsorption of argon on MoS? in the co-area range 460-180A.2/Ar: open cirrles, experimental points j closed circles, points calculated for the compression of two dimensional argon gas from a coarea of 310 arcording to a 6 : 12 force law; T = 833°K.
interactions and the temperature of the experiment', thus: AHs = f(0, el, €11 , T ) . The major experimental problem is to find systems in which the variables can be separat'ed, and this has been achieved partially by working with graphite,ja metal filmsj" and by using preadsorbed layers of various materials.jc The importance of E:, , which in the above cases is frequent'ly small compared with e l , lies in its significance to the theory of the liquid state : adsorption experiments have been recognized for some time6 to be one of the few available approaches t o the assessment of such self-int'eraction potentials. A complete description of adsorption energies in the above terms would be cognate with the theory of regular solutions as expounded by Hildebrand, and would be valuable for direct comparison of various syst'ems in terms of a given set of parameters. The question of whet'her these parameters could in turn be assessed from first principles is a vexed one: such a development is unlikely in vielv of the fact that it would constitute a solution to a many-bodied problem involving dissimilar species. But, the situat'ion is not as gloomy as this might suggest. Provided the adsorption involves no electron transfer, and the energy asbociated with it is comparable with the heat of liquefaction of the bulk adsorbate ( L e . , "physical" adsorption), it should he possible to assess from fundamental considerations the energy of a film of say a monatomic sorbate on a solid which could be treated as a semi-infinite continuum. These restrictions limit the experimentally available systems primarily to inert gases on single crystal metals and on highly anisotropic solids. In t,he latter rase, the solids of choice should be covalently hound, since Gaines' has demonstrated ( 5 ) (a) 3 . G. Aston and Q. Stottlemeyer, Symposiuln Proc., "The Chemistry of Solid Surfaces," D u k e Univ., March, 1958, p p . 11-14; (h) G . D. Halsey, Jr.. and J. H. Singleton. Can. J . Chern., 33, 181 (1954); (e) P. Cannon, THIEJOCRNAL, 63, 1292 (1959). (6) J. E. Lennard-Jones and A . F. Devonshire, Procz Row. Soc. (Lon* d o n ) , 163A,53 (1937).
the marked effect of ionic substitution on the mica basal plane upon the adsorption of the inert gases. Thus, graphite has been of interest in this respect, hut a molecular crystal might be expected to be w e n better behaved. In the present work, an account8 already has heen gix7en of the behavior of the adsorption of inert gases on molybdenum disulfide, in which a phenomenonological comparison with similar results on graphite was made. .1 number of striking 4milarities were evident. I n this paper, therefore, I propose to examine the data presented earlier to see whether it is possible to separate the eiithalpies calculated from the isotherm data into parts cognate with the parallel and perpendicular force fields in the adfilm, as functions of coverage and, if possible, to account for them from first principleq, within the above restrictions. Results Molar integral enthalpies of adsorption were computed from the isotherm data using the procedures described by Hill, Emmett aiid J ~ y i i e r . ~ Plots of these quantities as functions of coverage are shown on Figs. 1 and 3. I n the evaluation of the spreadiiig pressure plots for argon, it was found that the curves for spreading pressure 13s. log relative pressure crossed, showing a reversal in sign of enthalpy with respect to a three dimensional liquid standard state. I'or the purpose" of choice of limits for theoretical computation it was then necessary t o determine whether this change was associated with a change of phase in the adfilm: therefore, a plot of film compressibility LIS. log gas pressure n-as prepared,1° aiid it is d i o ~ non Tig. 3. It shows clear evidence of a discontinuous change in the adfilrn compressibility, lvith n deviation irom ideal behavior conimenciilg :It the position corresponding to the crossing of the &plots (spreading pressure, 4 = K ~ T 0 J ~ d ln ' ~p l p~o = ~ ~K T (dlds) log F ( T ) n-here F ( T ) is the grand partition function of the adfilm and the other iymbols are as defined by Hill, Emmett and .Joperg). The deviation becomes large very rapidly nith increasing pressure. up to a pressure of 251 p , where the compressibility of the film finally d r o p to a very low value. Molar quantities are used throughout, since this type of quantity is interpreted easily, and most of statistical thermodynamic reasoniiig is recorded in this form. The conditions under which the surface n-as prepared were surh that oxide contamination is unlikelv.'l Heats based on the result,s obtained from :350° outgassed, oxide-contaminated samples were calculated and found to be not very diff erent from those from the 950' material suggesting perhaps that oxide contaminat8ioilhas not too important an influence on physisorption in the systems *k-3IoS2 niid Kr-IIoSs. The heat plots yield ( 7 ) G . I,. C;aines, J I . , T H I S, J O C R N . ~ L ,62, 132Ci (19.58). (8) P. Cannon. ibid., 6 4 , 858 (1900). ( 9 ) T. L. Frill, P. H. Eiiiiiiett and L. G . . l o ~ - n r rJ. . ;im. C h i m . SOC., 73,5102 (195li. (10) S. J . G r e g g and F, P. Maggs. Trans. Faradail SOC.,44, 123 (1948). (11) P. Cannon, iyoture, 185, 1612 (1959).
Sept., 1960
I h ; . l T OF -kDSORPTIOr\r OF LiRGOK A N D I
