COMAIUSICATIOXS TO THE EDITOR
1847
perturbation of the more-correct liquid-phase results by isotope effects in rates of secondary decomposition reactions; or (b) the liquid phase results indicate a condensed phase limitation upon the motions necessary for completion of some substitution reactions in the tertiary position. I n any event, the primary replacement isotope effect is independent of phase, and thereby of thc choice between these alternate mechanisms.
of a bidimensional condensation in the nth layer, me write
and
(15) This research has been supported by AEC Contract No. AT-
With an appropriate thermodynamic definition of the adsorbed layer, utn) and constitute approximate THOMAS SM.41L DEP LRTMENT O F CHEhllSTRY15 values of the molar energy and entropy of the outer F. S. ROWLAND UKIVERSITY OF CALIFORNIA condensed layer; u(")and s(") represent the same I R V ~ NCALIFORNIA E, 92664 quantities relative to the outer layer of the solid adRECEIVED FEBRUARY 20, 1968 sorbat e. Formula 1 differs from that of Singleton and Halsey2r3by the addition of the entropic term -1/R(s(") - de)), I n this preliminary report on our experimental Bidimensional Condensation n-ork, we wish to sholy the importance of this term. in Adsorbed Layers We have studied the adsorption of lirypton and xenon on a certain number of halides having a layerSi?: I n a paper to appear,l we have derived formulas like structure, namely SiClz, CoC12, FeC12, CdC12, expressing the logarithm of the ratio of the vapor presCdBr,, CdI,, and PbI,. The results reported here sure of the adsorbate P ( - ) to the transition pressure in concern only the condensation of the first layer of an adsorbed film P ( n )and its first derivative as a funckrypton on the basal face of these substrates: typical tion of the inverse absolute temperature. I n the case isotherms for nickel chloride are plotted in Figure 1. The coefficients of the regression lines (eq 1) are detwmined by least-squares analysis. Thus the ratio P(m)/P(ljcan be split into its energetic and entropic CI 71. contributions. The results thus obtained are given in Table I, which also contains the crystallographic parameter a of the hexagonal or pseudo-hexagonal lattices of the adsorbents and, for comparison, the diameter of the krypton atom (distance between two krypton atoms in the (111) plane of the fcc solid krypton). From these data two conclusions can be drawn. First, the entropic term of formula 1 may be as im(11-1)-34, Agreement N o . 126.
(1) Y. Larher, J . Chim. Phys., in press. (2) J. H. Singleton and G. D. Halsey, Can. J . Chem., 33, 184 (1955). (3) L. J. Slutsky and G. D. Halsey in "Physical Chemistry, An
Pressurr ( t o r r )
Figure 1. Adsorption isotherms of krypton on nickel chloride.
Advanced Treatise," E. H. Eyring, D. Anderson, and 117, Jost, Ed,, Academic Press, New York, N. Y., 1966.
Table I : Respective Contributions of Energy and Entropy to the Ratio P ( m ) / P ( of l ) the Vapor Pressure of the Adsorbate to the Condensation Pressure of the First Layer of Krypton on Different Adsorbents
pW
Adsorbent
Temp, OK
p 0
KiClz COClp FeClz CdClz CdBrz CdIz PbII
75.69 75.34 75.13 82.52 88.44 88.35 88.52
26.9 22.4 19.4 21.3 35.7 53.1 38.2
Energetic contribution to P@)/P(l)
5.13 4.78 4.85 9.20 26.9 41..8 12.45
Entropic contribution to P ) / N
a of t h e adsorbent,
Diameter of the krypton atom, A
5.25 4.68 4.00 2.32 1,325 1.27 3.07
3.543 3.544 3,579 3,854 3.95 4.24 4.555
4.07
A
volume 7% .\'umber
6
May 1968
1848 portant as the energetic one: it is certainly not to be neglected a priori. Secondly, there is clearly a cor- uc-1) and (d') relation between the quantities (u(') dm)) on the one hand, the compatibility of the lattices of the adsorbent and adsorbate on the other (of course energies are comparable only for adsorbents with the same anion). A high compatibility of the lattices favors a high value of the energy difference ( ~ ( - 1 u(')) and a low value of the entropy difference (dl).dm)).Such results are quantitatively satisfying and can be used for a deeper understanding of the molecular structure of the adsorbed phase. I n the cases studied here, there is a rough cancelling
The JOUTnal of Physical Chemistry
COMMUNICATIONS TO THE EDITOR of the effects of the energetic and entropic terms on the ratio P(m)/P(l)j so that the application of the SingletonHalsey formula would have hidden the structural information contained in the experimental results. We conclude that a cautious attitude toward approximate molecular theories of adsorption, particularly those underestimating the role of entropy, is a t least sometimes worthwhile. Y. LARHER SERVICE DE CH~MIE PHYSIQUE C.E.N.SACLAY GIF S/YVETTE,FRANCE ACCEPTED AND TRANSMITTED BY THEFARADAY SOCIETY (NOVEMBER 28, 1967)
