2281
COMMUNICATIONS TO THE EDITOR I
culated pressures. Tc, was evaluated from the relation2 Tox = PO A,zlzz by assuming A, = A 2 = TC2[(el~/~22) - 11 and by using previously obtained values2 of the force constants q'k. In our convention, component 1 is always that with the smaller value of t/k ( A , 6 0). The results for eight systems are shown in Figure 1. The curve for the fifth system, belonging to the series with n-CJ?16, is drawn separately to avoid mutual crossing of the curves. The curves of Poxwere not drawn for the same reason. They are always more or less concave (for the fourth system PcSdiffers from Po by 6.5 psia only, at 2 = 0.5). The calculated curves reproduce well the characteristic asymmetry, convexity, or concavity of the experimental data. The results show that the theory of pseudocritical constantsa is valid also in cases of very large differences in sizes and t/k of the components.
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1000
500
(4) K. S. Pitrer, J. Amer. Chem. SOC.,77,3427 (1955).
DEPARTMENT OF CHEMICAL ENGINEERING THEOHIOSTATEUNIVERSITY COLUMBUS, OHIO 43210
n "
ALEKSANDERKREGLEWSRI
1 0
0
0.5
0.5
0
RECEIVEDFEBRUARY 21, 1968
1.0
XI
XI
Figure 1. The critical pressure POof binary mixtures. The points are experimental and the curves were calculated from eq 3 and 1. z1 is the mole fraction of component with the smaller value of elk: (1) n-C71316 -k CzHs;" (2) n-C7F16 CaHs;O ( 3 ) n-C7F16 n-C4H10;a ( 4 ) n-C7Fl6 4- n-C6H14;a ( 5 ) n-C7Fis $- a-CsHzo;" (6) n-CbHiz n-CgHw~: ( 7 ) n-C4Hlo n-C7H16;' (8) C2H6 n-C7Hlsnd L. W. Jordan, Jr., and W. B. Kay, Chem. Eng. Progress Symp. Ser., 59, 46 (1963). * W. B. Kay and D. Hissong, privately circulated report No. 46-67, Department of Chemical Engineering, The Ohio State University. c W. B. Kay, Znd. Eng. Chem., 33, 590 (1941). d W . B. Kay, ibid., 30, 459 (1938).
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The Thermal Decomposition of Lithium Azide
Sir: The thermal decomposition of barium,2 and strontium azides3 is accelerated by preirradiation + + with y rays (CosO),X-rays, and ultraviolet light. It was of interest to extend such studies to include the alkali metal azides. We report here the thermal decomposition of unirradiated and preirradiated lithium azide. The absence of any previous exhaustive examination of the kinetics of decomposition of LiN3 is probably associated with the fact that the salt is relations for ( T c - T',) and TC,, given previously2) deliquescent and this presents handling difficulties. holds also in cases of large differences in T c and Pc Bowden and Singh4 observed that the isothermal of the components. Since, by definition, a mixture at decomposition in vacuo is accelerated by preirradiation the pseudocritical point fulfills the conditions of critical with slow neutrons at room temperature, but made no state of a pure substance, the following assumptions detailed study of the effect. are allowed and were made. (i) The derivative is Lithium azide was prepared by the neutralization equal to Riedel's constant, expressed in terms of acenof lithium hydroxide solution with a 3% solution of tric factors4 of the pure components w1 and w2 hydrazoic acid. The salt was crystallized from a ( b In P x / b In T ) v X= 5.808 4- 4.93(wlSl 4-WZZZ) ( 2 ) and dehydrated in vacuo slightly acid solution 2"() over Pz05. Abnormal difficulties were experienced in (ii) The quantity Pc,Vcx/(RTcx)is equal at any comobtaining reproducible pressure-time plots for the of a reference substance position to PCoVCo/(RTCo) thermal decomposition of the unirradiated salt because which, as before,2 is characterized by Poo,Vco,and TCo of its sensitivity to light and deliquescent nature. linear in mole fraction z. For V'1 = V'Z = Voxwe obtain (1) E. G . Prout and M. E. Brown, Nature, 205, 1314 (1965).
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pox = TCx(pclzl ~c2~2)/(Tc1X1TC2z2)
(3)
For VO1# V'2 and nonadditive V c ,an unknown parameter appears; eq 3 was, however, used in these cases without noticeable loss in precision of the cal-
(2) E. G. Prout and D. J. Moore, ibid., 203, 860 (1964); E. G. Prout and D. J. Moore, Special Technical Publication, No. 400, American Society for Testing and Materials, 1966, p 45. (3) E. G . Prout and D. J. Moore, Nature, 205, 1209 (1965). (4) F. P. Bowden and K. Singh, Proc. Roy. SOC. (London), A227, 22 (1954).
Volume '72, Number 6 June 1968
COMMUWCATIONS TO THE EDITOR
50
100 150 Time, min.
200
Time, min.
Figure 1. Curve A, pressure-time plot for decomposition of unirradiated lithium azide a t 185': line B, p'/* us. t ; line C, (1 - a)'/a us. t.
Figure 3. Pressure-time plots showing the effect of preirradiation (ultraviolet light) on the thermal decomposition of lithium azide a t 185': curve A, unirradiated; curve B, irradiated for 150 min a t 10 cm from source.
I
I
25 0
50
100 Time, min.
150
200
Figure 2. Pressure-time plots showing the effect of preirradiation ( y rays) on the thermal decomposition of lithium azide a t 170" : curve A, unirradiated (the decomposition ceased after 10 hr); curves B-I, preirradiated by y rays with doses of 10, 50, 250, 500, 1500, 5000, 15,000 rads, and 0.7 Mrad, respectively.
However, with appropriate precautions, highly reproducible results were obtained with both powdered and pelleted lithium azide. The results reported here are for the powdered salt. A typical pressure-time plot is shown in Figure 1 with the mathematical analysis. The power law, p = kltn, with n = 3, fits the acceleratory period in the temperature range 140170". The decay reaction conforms to the contracting c, where a is the sphere formula (1 - a)'/' = lcd fractional decomposition. The decomposition is greatly altered by preirradiation with y rays (Co60) or ultraviolet light (Hanovia low power lamp) at room temperature. The induction period is shortened (with a y-ray dose of 15,000 rads it is eliminated), and the acceleration of the reaction is increased. Figures 2 and 3 illustrate these effects.
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I
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100
Time, min.
Figure 4. Curve A, pressure-time plot for decomposition of irradiated (y rays, 15,000 rads) lithium azide at 1.55"; line B, p'la us. t ; line C, (1 us. t.
Figure 2 also shows the effect of varying the y-ray dose. The decomposition of LiN3 is more sensitive to preirradiation by y rays than the alkaline earth azides. The mathematical analysis of the pressure-time plot for the decomposition of preirradiated ( y rays) LiN3 is shown in Figure 4 where it can be seen that the same equations apply as for the unirradiated salt (in the range 180-200'). The activation energies for the decomposition of irradiated and unirradiated LiN3 are listed in Table I.
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The Journal of Physical Chemistry
Table I : Activation Energies (kcal mole-') for the Decomposition of LiNa ( I = induction period)
Unirradiated Irradiated (15,000 rads)
E1
Ekt
Bk2
22.1
30.4 25.3
31.1 29.5
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CONnlUNICATIONS TO THE
EDITOR
2283 Comments on "Temperature Dependence of Contact Angle and of Interfacial Free Energies
150
in the Naphthalene-Water-Air System" 2 X
8 100 0
d
g F5
50
50
100 Time, min.
150
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Figure 5. Pressure-time plots showing the effect of exposing irradiated ( y rays, 15,000 rads) lithium azide to water vapor prior to decomposition at 195' : curve A, irradiated, no exposure to water; curve B, unirradiated salt, no exposure to water; curve C, irradiated salt exposed to water vapor prior to decomposition.
A study of the effect of a brief exposure of the preirradiated ( y rays and ultraviolet light) salt to water vapor prior to decomposition showed that the accelerating effect was destroyed (Figure 5 ) . Similar preexposure of the unirradiated salt did not alter the subsequent decomposition. Exposure of the unirradiated salt to water vapor at room temperature at the end of the induction period followed by a resumption of heating resulted in a new induction period followed by the usual decomposition. The irradiation effect could, however, be restored by irradiating again after exposing the irradiated salt to water vapor. Conformity of the pressure-time plots to the power law, with n = 3, indicates that during the acceleratory period reaction takes place at a fixed number of nuclei and that growth of these reaction centers occurs in a three-dimensional manner. During the decay period the spherical nuclei touch and reaction occurs at an approximately spherical interface which contracts as the decomposition proceeds. The effect of irradiation is to increase the number of nuclei which develop on heating, although the effect of water vapor indicates that the initiating species in the creation of a nucleus is different for the irradiated salt. The results and conclusions will be presented in full elsewhere.
AclcnowZedgnzent. We thank the South African Council of Scientific and Industrial Research for financial assistance and the African Explosives and Chemical Industries for a research fellowship held by one of us. DEPARTMENT O F CHEhlISTRY OF CAPETOWN UNIVERSITY C.4PE TOWN, SOUTH AFRICA RECEIVED MARCH25, 1968
Sir: I n a recent article, Jones and Adamson' reported values for the interfacial free energy in the naphthalenewater-air system at 82". They used this result to test the Fowkes relationship2 and reported that this relationship predicts an interfacial tension which is too high or that the London-dispersion-force component of the surface tension of water (ywd) must be 33 ergs/cm2 instead of 22 ergs/cm2 in order to fit the data. The purpose of this communication is to point out that the data of Jones and Adamson cannot be used as a direct test of the Fowkes relationship. It can be used as an indirect test and, when so applied, appears to be in reasonable agreement. This agreement is more evident when the results are compared with those obtained with the benzene-water system, which Jones and Adamson remark to be quite similar to the naphthalene-water system at 82". The interfacial tension for an aromatic hydrocarbonwater interface can be written in Fowkes' terminology as d
yow
+ -towp
(1) where yowd is the London-dispersion component of the interfacial tension and TOW" is the component due to the interaction of the n bonds of the aromatic hydrocarbon with water. This approach makes the reasonable assumption that any dipole-dipole or dipoleinduced dipole interactions are negligible in the systems under consideration. The first term of eq 1 is given by Fowkesl as TOW =
Towd = yw
+ yo - 2 4 w d y o d
(2)
It is this term (yo\vd) which Jones and Adamson attempted to calculate and to compare with the measured value of the naphthalene-water interfacial tension at 82". As expected their calculated value is too high, since a-bond interactions at the naphthalene-water interface would have the effect of making the interfacial tension lower than if dispersion forces alone were operative at this interface. Table I shows a comparison of components of the benzene-water interfacial tension at 20 and 80" with those of naphthalene-water at 82". An additional conjugated aromatic system of styrene-water is included for comparison purposes. (All values for surface and interfacial tensions were taken from the International Critical table^.^) The interfacial tension for
E. G. PROUT (1) J. B. Jones and A. W. Adamson, J . Phys. Chem., 72,646 (1968). V. C. LIDDIARD (2) F. M. Fowkes, ibid., 67, 2538 (1963). (3) "International Critical Tables," 1'01. 4,McGraw-Hill Book Co., Inc., New York, N. Y., 1928.
