2040
CORNELIUS E. KLOTS,Y. RAEF,AND RUSSELLH. JOHNSEN
Commentary on Scavenging Plots in Radiation Chemical Studies of Liquid Cyclohexane1
by Cornelius E. Klots, Y. Raef, and Russell H. Johnsen Department of Chemistry and the Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida (Received April 16, 1964)
Hydrogen yields from X-irradiated liquid cyclohexane in the presence of a variety of potential hydrogen atom scavengers have been measured and analyzed within the framework of an assumed scavenging mechanism. Only oxygen- and iodine-containing solutions are amenable to this interpretation, other solutes showing a more complicated concentration dependence. The role of a molecular detachment process is delineated.
The work of Dewhurst2 and Burton8 has strongly implied that thermally diffusing hydrogen atoms are important in the radiation chemistry of the liquid aliphatics. From this premise and its corollary that much of the radiolytically produced hydrogen originates in the reaction H
+ R H + H z + .R
(1)
Hardwick4 has developed techniques of seemingly unprecedented potency for determining both the yields of such hydrogen atoms and the rate of their abstraction reaction relative to scavenging by a suitable additive (see ref. 4 and intermediate papers). During the course of some earlier work in this laboratory,6 it appeared worthwhile to exploit this technique to determine the relative scavenging efficiencies of various aromatic agencies. The initial results, in a cyclohexane medium, were of sufficient interest to justify their extension to a larger number of scavengers. The data are presented in this paper and permit an operational commentary on scavenging studies of this type. I n brief, they suggest that mechanistic conclusions are not to be so easily reached as has been previously suggested.
Experimental Eastman Kodak spectral grade, Fisher spectral grade, and Phillips research grade cyclohexane were all used, both as received and after purification by sulfuric acid or gas-liquid chromatography. Radiolytic yields of hydrogen were identical from all sources T h e Journal of Physical Chemistry
except when chromatography indicated an obvious olefinic impurity. Five-milliliter samples were degassed and sealed off in Pyrex ampoules, mounted on a turntable, and irradiated at room temperature with Xrays generated by 3-Rfev. electrons. Typical doses were 0.1 RIrad and were monitored by a sample of pure cyclohexane, for which a Go(Hz)= 5.55 was assumed.6 Electron density corrections were made when significant. Hydrogen yields were measured, as gas volatile a t - 196O, with a McLeod gauge, except in the case of added oxygen; here hydrogen yields were determined by injecting the volatile gases onto a 8-)k charcoal column at room temperature, using i'?hemal conductivity detector and nitrogen as a carrier gas. Solutions were prepared volumetrically. The solutes were Eastman Kodak spectral or research grade, when available. In the case of oxygen, a Henry's law constant, K = 7.3 X lo2 (atm.), was estimated from analogous solubilities in the literature.
Results and Discussion The qualitative influence of free-radical scavengers on radiolytic hydrogen yields led early workers to the (1) This work was supported in part by the U. S. Atomic Energy Commission under contract No. AT-(40-1)-2001, and the Division of Biology and Medicine, U. 9. Atomic Energy Commission. (2) H. A. Dewhurst, J . P h y s . Chem., 62, 15 (1958). (3) G. Meshitsuka and M. Burton, Radiation Res., 10, 499 (1959). (4) (a) T. J. Hardwick, J . P h y s . Chem., 64, 1623 (1960); (b) T. J, Hardwick, ibid., 66, 1611 (1962). (5) C. E. Klots and R. H. Johnsen, ibid., 67, 1615 (1963). (6) P. J. Dyne and J. A. Stone, Can. J . Chem., 39, 2381 (1961).
SCAVENGING PLOTS IN RADIATION CHEMICAL STUDIES OF LIQUIDCYCLOHEXANE
204 1
-
.
concepts of scavengeable and nonscavengeable sources of h y d r ~ g e n . The ~ ~ ~ distinction between these two sources is besl illustrated by the employment of a socalled scavenging p l ~ t in ~ ,which ~ the reduction in hydrogen yield (AG) is plotted as a function of solventscavenger concentration ratio. A steady-state kinetic treatment provides the analytically convenient form (AG)
=
A(I
+ B [hydrocarbon]/ [scavenger])
(2)
Hydrogen reductions must be corrected for the fraotional dose absorbed by the additive. In this paper, the cust0mar.y method utilizing the electron fraction approximation has been followed, in the absence of the information necessary for a more appropriate procedure.$ The graphically-derived parameter ( A ) then provides a measure of G(H), the hydrogen atoms which are presumed to react according to eq. 1 in the absence of scavengers. The rate constant for this reaction (k1) relative t o that for the assumed scavenging meclianism
H+S-%HS
(3)
is then given by parameter B. Finally, from the derived value of G(H), and the observed Go(H2),am estimate of the "nonscavengeable" hydrogen is obtained. Before embarking upon a discussion of the present results, it will be useful to establish an upper limit for G(H) in the cyclohexane system. Dicyclohexyl is obtained with a G = 1.95 as a primary product.6 This is most plausibly attributed to the combination of cyclohexyl radicals since its appearance can be quenched by the addition of a variety of scavengers.10-12 [f we make the nont~iviall~?~4 assumption that the recently measured disproportionation-combination ratio (= 1.31) for cyclohaxyl radicalsI5 is applicable here, the yield of cyclohexene via disproportionation is easily obtained. From the observed initial yield,6 G(CeHl0)= 3.27, it follows that an additional source of cyclohexene exists with a G = 0.72. A yield of this magnitude is in excellent agreement with the observed cyclohexene production in heavily scavenged solutions6,10!16 and consistent with the first-order yield of D P from CSI912." Dyne18 has argued forcibly for a molecular detachment mechanism CeH12*
--f
Ce"o
mechanisms involving an ion-molecule condensation directly to dimeric product in liquid cyclohexane. The above prelude serves to establish that only -15% of the total hydrogen yield is to be assigned to reaction 4. The source of the remaining hydrogen, which might conceivably arise entirely through reaction 1, is now to be considered. In Fig. 1, the hydrogen yields from oxygen- and iodine-scavenged solutions are presented. They take the correct analytical form with intercepts indicating a G(H) 3i 1.96. This is in excellent agreement with, and confirms, previous studies using iodine.3~~~ If the scavenging mechanism is accepted, then the slopes yield k l / k 3 (oxygen) = 4.4 X k l / k 3 (iodine) = 2.6 X Such an interpretation would, however, be at odds with hydrogen yields from isotopic mixture^,^^,^^ in which a
+ Hz
(4)
The rather satisfactory convergence of three quite independent pieces of evidence, aside from delineating the extent of this reaction, lends further justification to our assumption concerning the origin of dicyclohexyl. It thus seems unnecessary to p o s t ~ l a t e
1000
so0 t$H,,l/bcavinceQ
20 0
4000
Figure 1. Hydrogen yields from oxygen- a n d iodine-scavenged liquid cyclohexane. (7) M. Burton and W. N. Patrick, J . Phys. Chem., 58,421 (1954). (8) G. E. Adams, J. H . Baxendale, and R. D . Sedgewick, ibid., 03, 854 (1959). (9) C. E. Klots, J . Chem. Phus., 39, 1571 (1963). (10) H. A. Dewhurst, J . Phys. Chem., 63, 813 (1959). (11) E. S. Waight and P. Walker, J . Chem. Soc., 2225 (1960). (12) L. J. Forrestal and W. H. Hamill, J . Am. Chem. Soc., 83, 1535 (1961). (13) It is probable, but not imperative,14 that the reactions of cyclohexyl radicals formed adjacent to each other, as may well be the case here, will be identical with those of freely diffusing species. (14) P. S. Dixon, A. P. Stefani, and M .Szwarc, J . Am. Chem. Soc.. 85, 2551 (1963). (15) C. E. Klots and R. H. Johnsen, Can. J . Chem., 41, 2702 (1963). (16) A. MaoLachlan, J . Am. Chem. Soc., 82, 1005 (1960). (17) P. J. Dyne and IT'. M. Jenkinson, Can. J. Chem., 39, 2163 (1961). (18) P. J. Dyne, J . Phys. Chem., 66, 767 (1962). (19) F. Williams, Trans. Faraday Soc., 57, 755 (1961). (20) L. Kevan and W. F. Libby, J . Chem. Phys., 39, 1288 (1963). (21) M. Burton and J. Chang, 137th National Meeting of American ~ Chemical ~ ~ ~ ~ Society, Cleveland, Ohio, April, 1960.
Volume 68, Number 8
August, 2964
2042
CORNELIUS E. KLOTS,Y. RAEF,AKD RUSSELL H. JOHNSEN
role for iodine other than scavenging is indicated. The present authors are not prepared to resolve this conflict; we note, however, that previously established rate constants22for these solutes indicate that if bulkdiffusing hydrogen atoms are produced in these solutions, they will be scavenged with at least the indicated efficiency. A G(H) 'v 2.0 must then represent a maximum yield for such species. Reference to Fig. 2 will reveal a somewhat more complex situation. Despite the variety of conventional scavengers used, in no case is the expected
One might reconcile the above difficulties with a strict free radical interpretation by invoking a role for spur penetration at high solute concentrations, as suggested by indications of nonhomogeneous effects in alkane radiolyses. 2 4 , 2 5 Our present results, however, are equally consistent with evidence that solutes may act in a manner other than through their assigned scavenging role. As evaluated above, the molecular detachment process, together with freely diffusing hydrogen atoms (presumably formed by simple bond rupture), is not sufficient to account for the observed hydrogen yields. An additional mechanism, involving kinetically hot hydrogen atoms, has often been inked,^^,^^,^^ but has been criticized recently by one of us.28 Such considerations do not exclude the possible role of electronically excited hydrogen atoms. * $ There is by now, however, considerable evidence that significant yields of charge separation exist momentarily or are readily induced in the liquid alkane^.^^^^^^^^ This suggests imniediately the often-considered possibility of ai1 ion-molecule mechanism, written in the (perhaps naive) stoichiometric form RH+
+ RH --+
RH2+
+ R --%2R + H2
(sa)
or more discreetly RH" Figure 2. Hydrogen yields from cyclohexane solutions containing a variety of organic scavengers.
analytical form obtained. Tangents to the observed curves a t concentrations -1% do show a remarkable tendency to extrapolate to a G(H) 3.2, as previously suggested. The curvature observed a t high solute concentrations, however, persists throughout the concentration range studied, introducing an element of arbitrariness into any kinetic analysis. This is most dramatically illustrated in the case of benzene. Additional data of ours, a t lower benzene concentrations (not shown), indicate a prolonged curvature and eventually merge with those given by Stone.28 The resultant plots are thus ambiguous with respect to a mechanistic interpretation. The reader may convince himself that the more obvious sources of systematic errorscavenger depletion and residual ,impurities in the cyclohexane-will lead to curvature of a sign opposite to that obtained. We are therefore of the opinion that hydrogen yield data do not warrant the simplicity of interpretation previously accorded them. Indeed, it is quite possible that the seemingly well-behaved data of Fig. 1 merely reflect the higher dilutions used. I n a i y case, no special significance can be attached to any intercept derived from the data of Fig. 2.
-
4b18
The Journal of PJhpjhyaical Chemistry
+ R H --+
2R
+ H,
(jb)
We note that because of the only transient nature of the charge separations (as evidenced by the low ion yields from such media32), any distinction between these two mechanisms becomes operationally blurred. a s They do have the virtue of generating the cyclohexyl radicals necessary for our previously assumed disproportionation-combination reactions. They also provide a basis for the apparent mobile character of primary excitations in aliphatic media26sSo by virtue of a charge migration process. More immediately, a t least some of the results of Fig. 2 may then be under(22) J. H. Sullivan, J . Chem. Phys., 30, 1292 (1959); J. K. Thomas, J . Phys. Chem., 67, 2593 (1963). (23) J. A. Stone and P. J. Dyne, Radiation Res., 3, 353 (1962). (24) J. W. Falconer, Nature, 198, 985 (1963). (25) J. W. Falconer and M.Burton, J . Phys. Chem., 67, 1643 (1963). (26) P. J. Dyne, J. Denhartog, and D. R. Smith, Discussions Faraday Soc., 36, 135 (1964). (27) V. V. Voevodskii and Y. N. Molin, Radiation Res., 17, 366 (1962). (28) C. E. Klots, J . Chem. Phgs., 41, 117 (1964). (29) R. Platzman, Radiation Res., 17, 419 (1962). (30) T. J. Hardwick, J . Phys. Chem., 66, 2132 (1962). (31) U '. H. Hamill, et al., Discussions Faraday SOC.,36, 169 (1964). (32) G. R. Freeman, J . Chem. Phys., 39, 988 (1963). (33) Hamill has mentioned, however (ref. 121, that reaction 6a has not been observed for cyclohexane in the mass spectrometer.
2043
LOCALIZATION O F TORSIOSAL OSCILLATIOK ABOUT CAIEBON-CARBOS BOXDS
stood in terms of the classical charge-transfer mechianism of “protection.” Alternatively, electron at,tachment to a solute could affect (sa) in a variety of ways12 31,34 and also would be expected to modify the course of (5b). I n this last instance, we envisage - the mechanism (in the room temperature liquid) as essentially that of quenching with only transient charge sepa,ration.35 Thus, while the nonselective use of ScaW’nger
plots in radiation chemistry may be criticized on operational grounds, their failure niay also be understood, we suggest, in terms of the over-all pattern of recent experimental developments.
-
(34) J. Roberts and W. H. Hamill, J . P h y s . Chem., 67,2446 (1963); R. H. Schuler, ibid., 61, 1472 (1957); P. R. Geissler and J. E. Willard, J , Am, Chem, sot., 84, 4627 (1962). (35) D. K. Majumdar and s. Basu, J . Chem. Phys., 33, 1199 (1960).
Localization of Torsional Oscillation albout Carbon-Carbon Bonds in Long-Chain Molecules in Aqueous Solutions
by A. J. B. Spaull and,M. R. Nearn’ Chemistry Department, Brunel College, London, W.S, England
(Received A a r i l 3, 1963)
The temperature dependence of adsorption of 1-butanol and 1-hexanol at the air-water interface has been studied at. low surface pressures (between 0.2 dyne cm-l and 6 dynes cm. -l), and the thermodynamic data reported show that the adsorption is essentially an entropic process. The recent statistical thermodynamic argument of Aranow and Witten, that the origin of the entropy arises from the transition of a molecule upon a change of environment from the state of hindered internal rotation to the state of internal torsional oscillation, cannot account for the experimental value.
Introduction Recently Aranow and Witten2 have suggested that aqueous solutions of long-chain hydrocarbons can be regarded as a special class of nonideal solution, wherie hydrogen bonds between water molecules form a cagle surrounding a given long-chain hydrocarbon and prevent complete rotation around a carbon-carbon bond, thus confining a particular bond to the configuration it is in initially. They showed how this could account for a number of phenomena, in particular Traube’s rule of surface tension. The argument of Aranow and Witten predicts that the free energy of adsorption from aqueous solution to the liquid-air interface is an entropji effect and that this arises from the changed behavior of the hydrocarbon molecules. This theory has been discussed by Hansen and Smoldersaand H i g g ~who , ~ point
out that no accurate data on the temperature dependence of adsorption isotherms exist to demonstrate whether the suggestion of Aranow and Witten is a valid conclusion. We report some thermodynamic parameters for the adsorption of 1-butanol and 1-hexanol from aqueous solution to the air-water interface and discuss the theory of -4ranow and Witten in the light of our results.
(1) This work is to he submitted in partial fulfillment for the degree of LMM.Sc. of the University of London by M .R. Nearn. (2) R. H. Aranow and L. Witten, J . Phys. Chem., 64, 1643 (1960); J . Chem. Phys., 35, 1504 (1961). (3) R. S. Hansen and C. A. Smolders, J . Chem. Educ., 39, 167 (1962).
(4) M. Higgs, J . Chem. Phys., 35, 1504 (1961).
Volume 68, ,Turnher 8
August, 1964
