NOTES
16
1529
kaIt I h+k
(7) The solid line in Figure 1 is determined by the experimental data; it fits the mathematical form of eq 4. The ratio of the intercept to the slope of the linear part of Figure 1 is given by [&*/a I - 1/(X k)], which numerically is 13.3 hr. From the initial exponential rise, (A k ) - l is 1.1 hr. From these experimental values X and k can be evaluated; 1 / X = 14.4 hr and l / k = 1.2 hr. Expressed as half-lives of the active sites, the half-life in the absence of N 2 0 is 10.0 hr; the half-life in the presence of NzO at a pressure of greater than 50 torr is 0.83 hr. These values will probably vary with the nature of the metal surface. From the linear portion of Figure 1 one finds that the apparent G(N2) from the surface-catalyzed reaction is given by
+
+
X: 3OOTORR. asl00TORR.
It/ O
Y I 5 10 15 20 60 70 80 90 REACTION TIME IN lRRADlATOR (HOURS) Figure 1. NZyields from N20decomposition in the strontium p-irradiator. Solid line calculated from surface catalysis mechanism; 0,700 torr; X, 300 torr; A, 100 torr.
0
follows. S* is an radiation at rate specific rate, A, or reaction rate, k. surf ace.
active surface site produced by irOJ which decays naturally with by collision with N20 with specific a depends on the nature of the
+
s -+ s* s*-&
(1)
+
(2)
S* N20 --% N2 S (3) In the pressure range studied, we assume [NzO] >> [S*]. Also, we consider only very low enough conversions so that we may with little error take [N20]as a constant. Hence we may incorporate [NzO] into the rate constant of eq 3 and write the pseudo-firstorder rate equation for nitrogen formation, viz. (4) Similarly the rate expression describing the time dependence of [S*]is
dlS*I dt
-
aI
-
(A
+ k)[S*]
(5)
where I is the intensity of the radiation and a is a proportionality constant. The walls of the reaction chamber are continuously irradiated even during the absence of any gas. Hence there will be an initial concentration, [S*]O,which is determined by the steadystate level of [S*] according to reactions 1 and 2, that is
Integrating eq 4 and 5 using eq 6 and the additional condition that [N21td = 0, we obtain
“G(Nz)”cat= 4.2/PI where P = NzO pressure in atmospheres and I = dose rate in megarads per hour. For 1 atm of N20 “G(N2)”catcan be neglected with respect t o radiationproduced N2[G(Nz),,d = 10.21 only at dose rates greater than 10 Mrads/hr. We have confirmed this conclusion by irradiating NzO with 0.4-Mev electrons in an aluminum cell with and without stainless-steel wire packing at dose rates of 47-470 Mrads/hr. At these high dose rates no surface catalytic effects were significant and N20is a useful dosimeter. We suggest that the susceptibility of the NzO dosimeter to possible metallic surface catalytic effects be kept in mind when the dose rate to be measured is less than 10 Mrads/hr. An example in which complications might arise is in the NzO dosimetry of metallic sample loops in reactors.
Concerning the Surface Tension, Critical Surface Tension, and Temperature Coefficient
of Surface Tension of Polytetrafluoroethylene by R. H. Dettre and R. E. Johnson, Jr. Oroanic Chemic& Department, Jackson Laboratory, E . I . du Pont de Nemours and Co., Inc., Wilmington, Delaware 19899 (Received September 8,1966)
Within the past few years a number of have suggested that the surface tension of a low-energy Volume 71, N u h 6 April 1967
NOTES
1530
solid is equal to its critical surface tension of wetting.4 However, surface tension measurements on molten polyethylene6-8 indicate that solid polyethylene must have a surface tension a t least 4 dynes/cm greater than its critical surface tension of 31 dynes/cm. Moreover, studies on low-energy liquid surfacese have shown that, for many such surfaces, this difference is often larger than 4 dynes/cm and can be 50 dynes/cm or more. Recent surface tension measurements on a fluorinated hydrocarbon oil have enabled us to obtain an estimate of the lower limit to the surface tension of polytetrafluoroethylene. The oil was obtained from the Nuclear Division, Union Carbide Corp. It was a mixture of compounds, essentially free of residual hydrogen, having an average composition corresponding to C21F44. At 25" it was an extremely viscous liquid containing solid material. Measurements were made on a sample which had been percolated through alumina at 60-70". The average molecular weight of the purified sample, determined from the boiling-point elevation of 1,1,2trichloro-l,2,2-trifluoroethane,was 1070. A bromine mole/g indicated one double bond number of 4 X per 50 carbon atoms. The nmr measurements showed no hydrogen, but did indicate some chain branching. The infrared absorption spectrum gave no evidence of cyclic chains. By analogy with hydrocarbons, unsaturation can result in slightly higher surface tensions (compared to a saturated chain of the same length) and chain branching can cause lower surface tensions (compared to the straight-chain isomer). With respect to the conclusions of this note, both of these effects are probably negligible for the fluorocarbon oil studied here. Surface tension was measured a t several different temperatures using a modified Wilhelmy plate method.' The apparatus used has been previously described.' The results are given in Table I. The temperature coefficient of surface tension is -0.065 dynes/cm deg and the surface tension a t 20", obtained by extrapolation, is 21.5 dynes/cm. The extrapolated value of 21.2 dynes/cm at 25" is slightly lower than the value of 22.4 dynes/cm reported by Fowkes and Sawyerlo for a perfluorinated oil. The surface energy, calculated from the surface tension and its temperature coefficient, is 40.6 ergs/cm2 over the temperature range of measurement. Since surface tension increases with chain length in a homologous series, we would expect the surface tension of a fluid polytetrafluoroethylene to be greater than that of the fluorocarbon oil. Moreover, since solidification should not result in a decrease in surface tension, solid polytetrafluoroethylene should also have The Journal of P h y h l Chmiatry
Table I: Surface Tension of a Fluorocarbon Oil Surface
Temp, OC (ic0.7)
tension, dynedam (*0.06)
39 51 60 70 80 100 120
20.3 19.5 18.9 18.3 17.5 16.3 15.0
301 N
-HYDROCARBONS
a surface tension greater than 21.5 dynes/cm at 20". Its critical tension is 18 d y n e ~ / c m . ~Therefore, the surface tension of polytetrafluoroethylene must be a t least 3.5 dynes/cm greater than its critical surface tension. (1) V. R. Gray, New 8cknt
