July, 1957
THEHEATOF SUBLIMATION OF BORON
na and nw are the indices of refraction for the sodium D line of adsorbate and water. I n the case of 2,4-pentanedione, an index of refraction was calculated assuming additive molar refractions for the keto form. This was done because pure 2,4pentanedione is largely (78y0)in the enol form, whereas a t infinite dilution in water it is largely (85y0)in the keto form6 and we are concerned with its behavior at infinite dilution. It may be noted that in every case the adsorbate molecule is adsorbed in such a manner that the positive end of its dipo1.e is oriented toward the surface: this orientation tends to place carbons toward the surface and functional groups away from the surface. .. Marked deviations of 4a/ln f o from unity, in the case of octanoic acid, and of -AF:/V’/lAP from 4.7 kcal./cm.2 in the case of phenol invite speculation as t o their cause. We suggest that the first deviation is due to a marked change in orientation of the octanoic acid between infinite dilution and complete monolayer states. If the adsorption forces were significantly less in the complete monolayer than a t infinite dilution this would be reflected in a n abnormally small value of a,and this would be expected if the octanoic acid molecules tended to “stand up” a t high concentrations and to “lie down” a t low concentrations in the monolayer. (6) G. S. Hammond, “Steric Effects in Organic Chemistry,” Chap. 9, M. 8. Newman, Ed., John Wiley and Son, New York, N. Y., 1956, esp. Tables XIV, XVI, discussion page 452.
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Such behavior is quite in accord with observed curvatures of capacitance-potential curves which suggest markedly lower molecular areas a t high concentration. The deviation in the case of phenol may be due t o its anisotropic polarizability or to a greater than normal distance between phenol and surface. The desorption peaks in the 2,4-pentanedione system are anomalously small. This may be due to electrolytic oxidation and reduction of the dione on anodic and cathodic sides, or to stabilization of an oxidation product of mercury, presumably Hg(I1), through formation of an acetylacetone complex. Such phenomena would destroy the (essentially) ideally polarizable character of the electrode. An alternate explanation is interesting in its possible application to field effects on equilibrium. The enol form is more polarizable, but the keto form is stabilized by the polar solvent. Polarization of the electrode causes displacement of adsorbate (low dielectric constant) by water (high dielectric constant) but if the adsorbate has two forms the high electric field should stabilize the more polarizable form. This form is also more strongly held to the surface (eq. ll), and if this stabilization occurs it will cause the desorption peaks to be reduced in height and to be spread out, in accord with experiment. Acknowledgment.-We are indebted to Dr. L. S. Bartell for a helpful discussion of the ideas embodied in eq. 11 and 12.
THE HEAT OF SUBLIMATION OF BORON AND THE GASEOUS SPECIES OF THE BORON-BORIC OXIDE SYSTEM’ BY ALANW. SEARCY AND CLIFFORD E. MYERS Contribution f r o m the Ceramics Laboratories, Division of Mineral Technology, University of California, Berkeley, Calif,, and the Department of Chemistry, Purdue University, Lafayette, Indiana Received March 9, 1967
The sublimation pressure of boron has been measured in three kinds of crucibles. The heat of sublimation a t 2RS01
