1151
C O M M U N I C A T I O N S TO T H E EDITOR
Density of Liquid Uranium
Sir: The density of liquid U was examined as part of a continuing study of the physical properties of liquid actinide elements. A knowledge of the volume change associated with the solid to liquid transformation is of fundamental importance in the study of liquid metals. The majority of metallic elements exhibit approximately 3% volume expansion during the melting process,' except for two classes of elements which contract during melting. One class consists of the semimetallic elements with open or layer-like solid structures, such as Bi, Sb, and Ga, which apparently transform to a more densely packed liquid phase. The other class consists of only the rare earth element Ce and the actinide element Pu which exhibit volume contractions for the melting of rather dense bcc solid phases. These changes are 0.8 and 2.4% volume contraction for Ce2 and Pularespectively. A change in the electronic configuration of the atoms in metallic Ce has been proposed by Jayaraman4to explain its volume contraction. The volume change associated with the melting of the bcc solid phase of 7-U has not been measured directly, although the density of liquid U has been reported previously by Grosse, Cahill, and Kirshenb a ~ m . When ~ their results for the density of the liquid are compared with the calculated density of the solid a t the melting point, a volume contraction during the melting of U is implied. Because of the significance of this implication on the electronic structure of metallic U, the density of liquid U was redetermined and is reported here. A pycnometric technique, previously described, was utilized for this investigation. I n this technique small tantalum pycnometers were filled a t test temperatures. An especially reduced zirconia crucible was used as the liquid metal container in order to minimize the oxidation of the U. The substoichiometric zirconia crucible was formed when a commercially available, impervious ZrO2 crucible was heated in a high vacuum furnace in the presence of zirconium metal for 4 days a t 1450" and approximately Torr residual pressure. The U was of high purity, 99.96 wt % U, with the major impurities (in ppm) being: Al, 12; Fe, 64; Nil 14; Si, 43; C, 146; other metallics, 152. The isotopic composition was 0.24 wt yo 235Uand 99.76 wt % 238U. The density values were not adjusted to any other isotopic composition. The measured density values, listed in Table I, were fitted to a straight-line function of temperature by the least-squares method. The equation of the line was
p
(g/cm3)
=
(19.520
- 16.01)10-4T("K)
with a standard deviation of 10.016 g/cm3. The present values are approximately 3.5% lower near the melting point and the temperature coefficient is slightly larger than the values given by the previous investigator^.^ The deviation between the two sets of data is greater than the standard deviation given for either measurement. No explanation of this deviation is obvious, especially since experimental data were not given in the original publication; however, surface tension forces on the suspension wire of the sinker often cause an apparent density increase in the Archimedean method used by Grosse, et al. Table I : Density of Liquid Uranium Temp, OC
Density, g/amg
1137 1162 1180 1206 1245
17.252 17.215 17.225 17.149 17.082
The significant result of the present measurement is that the density of liquid U is less than the density of the solid, as shown by the following calculations. Although the density of r-U has not been measured at the melting point, an estimate based upon thermal expansion and X-ray data6 indicates it would have a density of 17.65 g/cm3 and a molar volume of 13.48 cm3/g-atom. The present data indicate a liquid density at the melting point of 17.27 g/cm3 and a molar volume of 13.77 cm3/g-atom. A volume expansion during melting of approximately 2.2% is indicated, therefore, very similar to the average of 3% found for most metals and unlike the volume contraction shown by Ce and Pu. These calculations were supported by examination of the pycnometers after filling which indicated a volume (1) A. R. Ubbelohde, "Melting and Crystal Structure," Clarendon Press, Oxford, 1965, pp 170-171. (2) L. J. Wittenberg, D. Ofte, and W. G. Rohr in "Rare Earth Research 11," Karl 5. Vorres, Ed., Gordon and Breach Science Publishers Inc., New York, N. Y.,1964,pp 257-275. (3) C. 2.Serpan, Jr., and L. J. Wittenberg, Trans. A I M E , 221, 1017 (1961). (4) A. Jayaraman, Phys. Rev., 137A, 179 (1965). ( 5 ) A. V. Grosse, J. A. Cahill, and A. D. Kirshenbaum, J. Arne?. Chem. isoc., 83, 4666 (1961). (6) C. R. Tipton, Jr., Ed., "Reactor Haqdbook," Vol. 1, 2nd ed, Interscience Publishers, New York, N. Y.,1960, pp 111 and 119. Volume 74,Number 6 March 6 , 1970
COMMUNICATIONS TO THE EDITOR
1152 contraction during the freeeing process. This conclusion was based upon the observation that for metals like Ce and Pu, with known volume expansions during solidification, a drop of liquid was forced to the tip openings of the pycnometers. No such liquid droplets were noted for the pycnometers used for U. The conclusion of volume expansion during melting of U is in keeping with two direct observations made of this phase transformation. During interfacial tension measurements Rosen, Chellew, and Feder' estimated a 3% volume expansion. Recently, the phase diagram8 of U has been published and an initial positive slope of 4.1°/kbar noted for the change in the melting point as a function of pressure. By use of the Clapeyron equation and the heat of f u ~ i o n2900 , ~ cal/g-atom, the solid -P liquid volume change calculated from the phase diagram is $-0.35 cm3/g-atom, in good agreement with the value of +0.29 cm*/g-atom, calculated from this work.
equilibrium constants in the presence of 0.02 M solutions of a series of n-alkyltrimethylammonium ions are collected. The largest association constant found, 4800 M-I, for the reaction of the N-hexadecyl substrate in the presence of the n-hexadecyl surfactant, is more than 25,000 times larger than that for the model reaction, addition of cyanide to N-propyl-3-carbamoylpyridinium ion in surfactant-free aqueous solutions under comparable conditions. The largest second-order rate constant observed, 13.3 M-' sec-', for the reaction of the N-hexadecyl substrate in the presence of the n-hexadecyl surfactant, is 950 times greater than that for the same model r e a ~ t i o n . ~These increases are substantially greater than those usually elicited by dilute surfactant solutions.'*2 It seems likely that a principal driving force for the surfactant-dependent reactions is destabilization of the cationic substrates by the cationic surface of the micelles relative to the zwitterionic transition states and uncharged products.
(7) C. L. Rosen, N. R. Chellew, and H. M. Feder, Nucl. Sci. Eng., 6 , 504 (1959).
(8) N. Asami, M. Yamada, and S. Takahashi, N i p p o n Kinzoku Gakkaishi, 31, 389 (1967). (9) H. Savage and R. D. Seibel, USAEC Report ANL-6702, Argonne National Laboratory, Argonne, Ill., Sept 1963. (10) Mound Laboratory is operated by Monsanto Research Corp. for the U. S. Atomic Energy Commission under Contract No. AT-33- 1-GEN-53.
Table I : Rate and Association Constants for the Addition of Cyanide to a Series of N-Substituted 3-Carbamoylpyridinium Ions in the Presence of a Series of n-Alkyltrimethylammonium Bromides in Aqueous Solution a t 25'"
Substrate
Octyl MONSANTO RESEARCH CORPORATION WILLIAMG. ROHR J. WITTENBERG Decyl MOUNDL A B O R A T O R Y ~ ~ LAYTON Dodecyl MIAMISBURG, OHIO 45342 Tetradecyl RECEIVED AUGUST15, 1969 Hexadecyl
Secondary Valence Force Catalysis.
XI.
---Decyl
Surfactan+---Dodecyl Tetradeoyl
1.10; 530 2.5; 1100 0.28; 330
6.6; 3600 6.4; 4500
Hexadecyl
0.21; 135 1.35; 710 5.8; 4000 10.4; 4500 13.3; 4800
a Surfactant concentration is 0.02 M throughout. I n each case, the entries in the table are second-order rate constants in units of M-1 sec-l followed by association constants in units of M-I.
Enhanced Reactivity and Affinity of Cyanide Ion toward N- Substituted 3-CarbamoylPYridinium Ions Elicited by Ionic Surfactants
Sir: It has been established that rate and equilibrium constants for a number of organic reactions are altered in the presence of dilute solutions of ionic surfactants.'v2 We now wish to report an additional example, the addition of cyanide ion to N-substituted 3-carbamoylpyridinium ions, which is unusual in several respects. The principal features of this reaction are the following.
R
k
First, the rate and equilibrium constants for the reaction shown in eq are increased by low tions of cationic surfactants. In Table I, rate and The Jourml of Phzlsical Chemistry
Second, at a constant concentration of a given surfactant, rate and equilibrium constants for the reactions increase with increasing hydrophobicity of the substrate. This behavior is best illustrated by the rate and equilibrium constants measured in the presence of n-hexadecyltrimethylammonium ion (Table I). This behavior is, in the case of the rate constants a t least, not the consequence of incorporation of an increasing fraction of the substrates into the micelles with increasing substrate hydrophobicity. Measurement of rate con(1) (a) J. L. Kurz, J . Phys. Chem., 66, 2239 (1962); (b) C. A. Burnton,E. J. Fendler,L. Sepulveda, and K.-U. Yang, J . Amer. Chem. SOC.,90, 5512 (1968); (c) R. B. Dunlap and E. H. Cordes, ibid., 90, 4395 (1968); (d) R. B. Dunlap and E. H.Cordes, J. Phys. Chem., 73, 361 (1969); (e) M. T. A. Behme, J. G. Fullington, R. Noel, and E. H. Cordes, J . Amer. Chem. SOC.,87, 266 (1965); (f) L. R. Romsted and E. H. Cordes, ibid., 90,4104 (1968); (g) C. Gitler and A.OchoaSolano, ibid., 90,5004 (1968) ; (h) T. C. Bruioe, J. Katzhendler, and L. R. Fedor, ibid., 90,1333 (1968). (2) For a review, see E. H. Cordes and R. B. Dunlap, Accounts Chem. Res., 2 , 239 (1969). (3) R. B. Lindquist and E.HaCordes, J . Amer. Chem. SOC.,90,1269 (1968).
