COMMUNICATIONS TO THE EDITOR
94 TABLEI
SOLUBILITY OF HYDROGEN IN URANIUM AT P , P s, cc./lO $2. S / T P P,# cc./lO g .
s,
150 135 100
79 69 57 46 35 30 26
Run 1 0.41 0.0335 .35 .0301 .31 ,0310 .28 .0315 .25 ,0301 .23 .0305 .21 ,0310 .20 .0338 .19 .0347 .18 ,0353
103 82 56 47 38 34
295'
s/dF
Run 2 0.34 0,0335 .29 .0320 .28 ,0374 .23 ,0335 .22 .0357 .20 ,0343
In this work, the solubilities at various low pressures were determined a t 295" using 10 g. of powdered uranium. For this low hydrogen concentration region the solubility may be expressed by s=K*
(2)
where S is in standard cc./g. of uranium, P in microns. The data presented below show that equation 2 is obeyed.
VOl. 59
This square root relation has been shown by BattelleZbto hold for a,(3,y-forms of uranium at temperatures from 600-800" a t pressures up to 100 mm. The average value of A/@ for 10 g. of uranium is 0.033 with a standard deviation of fO.0022. Thus for 1 g. of uranium at 296" the hydrogen solubility obeys equation 2 and K has a value of 3.30 f 0.22 X Hydrogen must go into solution in the dilute region as H atoms or negative H ions or as UH. Experiments at other temperatures, though not designed to find the true heat of solution, show the heat of solution of hydrogen in a uranium (solid phase 2) at infinite dilution is approximately- 13 kcal./mole Hz. The heat of solution of hydrogen in a-uranium (solid phase 2) at the composition where the uranium hydride phase begins to separate is calculated from equation 1 to be about -20 kcal./ mole Hz. The Battelle work2bindicates that for (3, y- and liquid uranium the heat of solution of hydrogen becomes positive in contrast with the negative heat of solution found for the a uranium in this study.
COMMUNICATIONS TO THE EDITOR MONOLAYER STRUCTURE AS REVEALED BY ELECTRON MICROSCOPY
Sir: Little is known about the detailed structure of monolayers, the process of monolayer collapse and the nature of collapsed films. Films of fatty acids on water, before and after collapse, have recently been transferred from a Langmuir-Adam-Harkins film balance'J to a collodion support, shadowcast with chromium, and examined in an RCA Type EMU electron microscope. Remarkable structures were revealed. Monolayers of n-hexatriacontanoic acid2 give pressure-area isotherms similar to those of stearic acid. At zero pressure, the molecule has an extrapolated cross-sectionat area of 20.4 8.2, essentially identical to the 20.3 A.2 for stearic acid.4 It differs principally in that the film collapses a t 58 dynes per cm., as compared with 42 dynes for that of stearic acid. Samples of n-hexatriacontanoic acid films were taken6 a t 15 dynes per cm., a t 25 dynes per em., (1) N. K. Adam, "The Physics and Chemistry of Surfaces," Third Edition, Oxford University Press, Oxford, 1941. (2) W. D. Harkins, "The Physical Chemistry of Surface Films," Reinhold Publ. Corp., New York, N . Y., 1952. (3) A 36-carbon acid prepared in 99.99% purity by Dr. R. J. Meyer of our laboratories. (4) H. E. Ries, Jr., and H. D. Cook, J . ColEoid S c i . , in press. ( 5 ) A small ring wa8 raised through the filni and lowered over a collodion support. Two more-refined techniques have reproduced the iliain faatiirea uf tlie iiiicroyraphs. In 0 1 1 1 , sluallow cirps containing
and after collapse. They were shadowcast with chromium at an angle of 15" to the surface. Electron micrographs are shown in Fig. 1. A blank water sample, which had been transferred from the film balance to a collodion support, is shown in A. Throughout B, the widths of the sha$ows correspond t o a film thickness close to the 50 A. expected for a monolayer of vertically oriented n-hexatriacontanoic acid. Many islands or aggregates of irregular shape and size characterize the partly compressed monolayer. Movement of islands may account for the unstable pressures in this region of the isotherm often observed for fatty acid monolayers. In C, a transition stage between monolayer islands and collapsed film is shown. The monolayer has become a continuous phase; the bare portions are now discontinuous. Islands have sometimes been seen a t this pressure, possibly because the film had been disturbed during the transfer process. In both B and C, the monolayer surface appears coarser than that of the collodion support. The texture may reflect the structure of the condensed monolayer. Bare portions may contain film molecules that are less closely packed. Across D, several long flat fiber-like structures appear. They rest on a continuous monolayer substrate and are about 100 8. or two molecules thick. collodion supports are raiaed through the film. In the other, metalcoated collodion supports are raised iwtically tlirongh the f i l r r t as in the Lauyiiu~ir-Brodgett teoliuiqiie.
,
COMMUNICATIONS TO THE EDITOR
Jan., 1955
95
B.
A.
C. D. Fig. 1.-Electron micrographs of monolayer filins of n-hexatriacontanoic acid: A, blank, no film; B, a t -15 dynes per cm.; C, at 25 dynes per cm.; D, after collapse. Shadows are light, and ari'ows indicate the direction of chrorniuin shadowcasting.
Shadows a t a small break Jhrough the substrate showed that it remained 50 A. thick. A mechanism for collapse is clearly suggested: as the pressure increases, the monolayer rises from the surface a line Of rupture> polar face to polar face, and Over to form lol1g flat structures, two molecules thick. When n-hexatriaconta1loic acid 'was deposited directly on the collodion support rather than on \vater, different structures Tvere observed. A solution in benzene of less hexatriacontanoic acid than lleeded for a closely packed monolayer \vas deposited on collodion. After evaporation of the benzene and shadovvcastillg, micrographs shelved platelets or flat micelles M,ith rounded edges alld a thickness of two molecu~es. T \ T ~mono~ayer structures were observed. Such observations of monolayer islands, collapsed films and platelets deposited directly from solution provide information basic to two-dimensional and three-dimensional nucleation, crystallization and micelle formation. RESEARCH DEPARTMENT STANDARD OIL COMPANY (INDIANA) HERMAN E. WHITING,INDIANA WAYNEA. RECEIVEUNOVEMBER26, 1954
T H E TEMPERATURE DEPENDENCE O F MECHANICAL AKD ELECTRICAL RELAXATIONS I N POLYMERS'
sir: The temperaturedependence of both viscoelastiC2,3and dielectric4 properties of a polymeric systern can be dWcribed by a single pendent parameter which represents the ratio of any time at temperature to its a! an arbitrary reference temperature To. When different systems are using the same T O for all (e.g., 298"K.),plots of this parameter ( a or~ K from viscoelastic measuremellts, bT frorn tric) against T exhibit little resemblance. However, we have found that, by selecting a different reference temperature T , for each system and plotting
(1) This work was supported by Picatinny Arsenal, Ordnance Corps, Department of the Army. The author wishes to express t o Prof. Jotin D. Ferry and Dr. Robert F. Landel his appreciation for many helpful discussions. (2) (a) R . D. Andrews, N. Hofman-Bang and A. V. Tobolsky, J . Polymer Sci., 3 , 6 6 9 (1948); (b) A. V. Tobolsky and J. R. MoLoughlin, ibid., 8, 543 (1952). (3) J. D. Ferry, J . Am. Chem. Soc., 72, 3746 (1950). RIES,JR. (4) (a) J. D. Ferry and E. R. Fitzgerald. J . Colloid S c i . , 8, 224 KIMBALI, 11953); (b) J. L). Ferry, RT. 1,. W i l l i a ~ ~aird t s fi:, 1 { . l~'iLegevald,Ttils JUUHNIL, in presa.
