COMMUNICATIONS TO THE EDITOR
2806
-
2’50BEFORE IRRADIATION 2.40---*-- 3.05x lOl9 ev/ml
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For such a process, dv/dt = -ka and the particle radius would decrease linearly with dose. (2) A constant rate of reaction, for particles above a certain size, would correspond to a linear change in volume with dose. The observed particle size-dose relationships indicate that a more complex mechanism is involved and that there is an increasing reactivity of the colloidal material with a decrease in particle size. I n the series represented by curve D the colloidal system was made 0.16 M in n-propyl alcohol before irradiation. Since the alcohol is an efficient scavenger for both H and OH radicals, the marked acceleration in the rate seems most likely due to a modification in the interfacial properties of the sulfur-water system. The colloidal suspensions do not respond well to the usual degassing techniques and bubbling with gas evidently disrupts the narrow particle size distribution. Results in deaerated systems at this time are qualitative, but in every case the rate was faster than in the systems exposed to the atmosphere. Current experiments are concerned with a more detailed study of the kinetics and include measurements of salt effects and those of additives which alter the interfacial characteristics of the colloidal system.
6
0.4 060 0 20 30 40 50 60 70 80 ‘90100 110 120 130
SCATTERING ANGLE Figure 1. Polarization ratio spectra of a sulfur hydrosol before and after exposure to ionizing radiation. The initial spectrum corresponds to particles of 0.30-p radius. After receiving a dose corresponding to 3.05 X 1019 e.v./ml. the radius has been reduced to 0.19 p.
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0.34 0.32
3 0.28
Acknowledgment. This work was supported by a grant from the Division of Radiological Health of the U. S. Public Health Service. Appreciation is expressed to the Department of Food Science for the use of their facilities and light scattering photometer. DEPARTMENT OF CHEMISTRY THEUNIVERSITY OF GEORGIA ATHENS,GEORGIA 30601
0.26
0.20 C
F. J. JOHNSTON
Evidence for Bromine-82m Isomeric Transition
0.18
Activated Reactions in Saturated Hydrocarbons
0.16
and Alkyl Halides
Figure 2. Effect of ionizing radiation on particle size in monodisperse sulfur hydrosols. The series represented by curve D was 0.16 M in n-propyl alcohol.
(TI,> 5-6 min.)lS2 has helped to resolve apparent discrepancies in the literature concerning experimentally correct values of BFJmand Brs2 organic retentions resulting from the radiative neutron-capture reactions in condensed organic systems. Isotope separations of
picture Of radiaWithin the framework Of the tion-induced reactions in dilute aqueous systems, one may consider two limiting cases, (1) The rate of disappearance of elemental sulfur depends on the surface area exposed to a constant radical “flux.”
(1) 0. U. Anders, Abstracts, 148th National Meeting of the American Chemical Society, Chicago, Ill., Sept. 1964, p. IOR. (2) J. F.Emery, ibid.9 P. 10R. (3). For a review of hot atom reactions in condensed phases, see J. E. WJlard, Nucleonics, 19, No. 10, 61 (1961); -4nn. Reu. Phys. Chem., 6, 141 (1955).
DOSE, ev/ml x
The Journal of Physical Chemistry
COMMUNICATIONS TO THE
EDITOR
Brmm and BrB2 attributable to Br79(n,y)Brsomand Br81(n,y)Br82reactions in several liquid and solid organic systems have been reported and refuted by various auth01-s.~ The disagreement may be due in part to BrE2"(IT)BP reactions. We studied the systems listed in Table I with particular attention given to experimental parameters. The detailed experimental procedures and equipment used were described previ~usly.~All thermal neutron irradiations were performed a t a flux of loll n. sec.-l in the Triga reactor a t the Omaha, Neb., V. A. Hospital. One-milliliter samples were frozen by immersing in liquid nitrogen, resulting in opaque crystalline solids in the saturated hydrocarbon-Br2 and C C l r Br2 mixtures, and glasses in the alkyl bromides. All samples were irradiated for 3 see. in the frozen state. Unusual and totally unexpected results were that neither BrSomnor BrE2organic retentions were over 1% in saturated Cghydrocarbons and CC14when the samples were kept frozen and extracted directly from the frozen state into organic and inorganic phases (Table I). When the samples were melted and maintained at 25" for 10 min. immediately following irradiation, BrS2 organic retentions increased markedly while those of BrSomremained below 1%. When samples were kept frozen for 5 hr. after irradiation (allowing decay of B$2m before thawing) and then thawed and left a t 25" for 10 min., both B P m and BP2 organic retentions remained below 1%. Similar experiments showed that BrE0(18 min.) in cyclohexane acted exactly as BrBom. I n the alkyl halides studied (Table I), B F m and Br8*organic retentions were finite in the pure systems and decreased when Br2 scavenger was added. An isotope separation was noted in the solid systems with Brs2 organic retentions being greater than those of Brmm. However, when the samples were melted immediately after irradiation (as described above) BrBom organic retentions remained constant while those of BrB2were lowered. When the samples were kept frozen 5 hr. following irradiation, before melting, Br82 organic retentions remained constant. Thermal reactions were shown not to produce a change in organic retentions, as noted by the constancy of BrEomretentions while B P retentions changed.
reaction produced Bf12 retention changes. It is interesting that Br82m(IT)B92reactions did not cause any chemical reactions in the frozen state. Obviously, Br80m and B?z isotope separations (or lack of them) in liquid organic systems may partially
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Table I : Organic Retentions" of BFm and B F in Solid Saturated Hydrocarbons and Alkyl Halides Produced in 3-Sec. Thermal Neutron Irradiations a t - 196' organic retention, %-Frozen after Thawed after irradiationb irradiationC Brsom Brsz Braom Bras
--Av.
Mole fraction of Br2
0.137 0.096 0.120 0.106 0.107
0.6 0.7 0.3 0.6 0.8
0.6 0.7 0.3 0.6 0.8
0.6 0.7 0.3 0.6 0.8
7 12 12 8 10
0.074 0.172
0.7 1.0 74 43 63 26 61 39
0.8 1.0 85 57 80 36 79 53
0.7 1.0 74 43 63 26 61 39
10
... 0.179 0.164
... 0.140
19 75 40 64 25 48 25
Maximum experimental error was f 0 . 5 . Samples were stored under liquid nitrogen before and after irradiation. They were melted into the extraction mixture while extracting. Samples were thawed immediately after irradiation and brought to 25' for 10 min. before extraction.
result from isomeric transition reactions. The combination of the short half-life of BrsZm(5-6 min.) and relatively long half-life of BP2 (35.9 hr.) makes it impossible to obtain, directly, absolute data for BrE1(n,y)BrS2and BrE1(n,y)BrS2" activated reactions. The BPZmisomeric transition activated reactions must be prevented or evaluated to obtain data for the (n,y) activated reactions. I n the case of Brmmdata, one can eliminate contributions from Brmmisomeric transition reactions by proper counting technique^.^ The magnitude of observed isotope separations should be a function of the time of irradiation and time between irradiation and extraction. Even in solid systems this reaction may affect isotope separations when samples are thawed before the BrBzm has decayed.
Acknowledgment. This work was supported by the Minnesota Mining and Manufacturing Company through a research fellowship.
(5) J. A. Merrigan and E. P. Rack, J . Phw. Chem., 69,2795 (1965).
D~~~~~~~ op c~~~~~~ JOSEPH ANDREW MERRIGAN UNIVERSITY OF NEBRASKA EDWARDPAULRACK LINCOLN, NEBRASKA RECEIVED JUNE 10, 1965
Volume 69, Number 8
August 1966
