Co MMUN~CATIONS TO THE EDITOR 2809

whelmed. Reaction 6 is well known from studies of the hydrogen-oxygen reaction; it inhibits so that hy- drogen and oxygen will not combine at atmospheric pressure and temperatures much below 500°, except in the presence of a suitable catalyst. Although the H02 radical is sufficiently stable that it inhibits hydro- gen-oxygen below -500°, at higher temperatures chains may be continued through the slow reaction

EO2 + H2 - H2Oz + H

Since the hydrogen-fluorine reaction continues, albeit at a reduced rate, it seems reasonable that HO2 must react with fluorine, as in reaction 7. Reaction 8 is a reasonable bimolecular chain-termination step, while reactions 2’, 3’, and 1’ are familiar links from the hydrogen-oxygen branched chain. (Reactions 1 ’, 2’, and 3’ must be included to account for the fact that in the presence of a large amount of oxygen the reaction rate is independent of oxygen concentration.) Water is not detected as a reaction product since it further reacts to yield H F and ~ x y g e n . ~

By applying the steady-state approximation to the concentrations of hydrogen, fluorine, and oxygen atoms and also the OH and H02 radicals and remembering that k6[M@)] >> k2’, the following rate expression is ob- tained

Actually the various third-body efficiencies in reaction 6 should be considered

h [ M @ ) ] = k6,~,([H2] + 0.43[N21 + 0.35[02] f fFz[F2l + fHF[HFI)

Here fF, and fHF are the third-body efficiencies of F2 and HF relative to hydrogen (the efficiencies for nitrogen and oxygen are from Lewis and Von Elbe4). We might guess that fF* - 0.3 (by analogy with nitrogen and oxygen) ; based on this assumption an analysis of the data of Levy and Copeland’s2 Figure 3 indicates that fHF - 1. Thus, the mechanism predicts a rate pro- portional to about the first power of the fluorine con- centration and a somewhat lower order with respect to hydrogen. An over-all rate constant can be obtained by equating the theoretical rate expression with the empirical formula of Levy and Copeland

klk2‘k, ksks

[&I + 0.43[N21 + 0.35[02] [Hzll”

Values so calculated are shown in Table I. The over- all rate constants show slightly less variation, per- centagewise, than the empirical 1.5-order rate constants.

kover-all = - - - kV%

Table I: Rate Constants for the Hydrogen-Fluorine Reaction at 132”O

-Rate constants-

Fa HZ Oz Nz m h - 1 (ref. 2) kover-all, m h - 1 -Initial mole fraotions- ka/,, a.u.-’/Z

0.032 0.032 0.794 0.142 0.14 0.38 0.066 0.066 0.659 0.209 0.14 0.28 0.129 0.129 0.659 0.084 0.14 0.22 0.066 0.132 0.659 0.143 0.16 0.25 0.132 0.066 0.659 0.143 0.15 0.29 0.066 0.264 0.659 0.011 0.18 0.23 0.066 0.396 0.527 0.011 0.20 0.24 0.066 0.527 0.396 0.011 0.26 0.31

Av. 0.17 i= 0.03 0.27 & 0.04

Total pressure 695 torr; from Table I11 of ref. 2.

In summary, the mechanism proposed here seems to provide an adequate description of the oxygen-in- hibited reaction between hydrogen and fluorine. The experimental data for smaller oxygen additions can perhaps help to elucidate further the combination of the mechanisms set forth here and in the preceding communication.

(5) V. A. Slabey and E. A. Fletcher, N.A.C.A. Technical Note 4374, Sept. 1958.

LEWIS RESEARCH CENTER RICHARD S. BROKAW NATIONAL AERONAUTICS AND SPACE

CLEVELAND, OHIO 44135 ADMINISTRATION

RECEIVED JUNE 22, 1965

Parachor and Surface Tension of

Amorphous Polymers

Sir: Knowledge of the surface tension of polymers is of interest, on the one hand, as a means of elucidating the nature of the bulk and surface structure of polymeric liquids‘ and, on the other hand, because of its control- ling influence on such practical applications of poly- mers as spinning, adhesion, and stability of dispersions. The critical surface tension yc of wetting of solid poly- mers, obtainable from contact angle data by a proce- dure proposed by Zisman12 has often been employed as an estimate of the surface free energy of polymers, but there always remained the question of the precise rela- tion between yc and the true surface free energy. For this reason, there has recently been a growing interest

(1) H. W. Starkweather, Jr., SPE Trans., 5 ,5 (1965). (2) W. A. Zisman, “Advances in Chemistry Series,” No. 43, American Chemical Society, Washington, D. C., 1964, p. 1.

Volume 69, Number 8 August 1966