Title: Scavenger kinetics in the radiolysis of cyclohexane-methyl iodide solutions
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 Material Information
Title: Scavenger kinetics in the radiolysis of cyclohexane-methyl iodide solutions
Alternate Title: Radiolysis of cyclohexane-methyl iodide solutions
Physical Description: vii, 94 l. : illus. ; 28 cm.
Language: English
Creator: Mani, Inder, 1928-
Publisher: s.n.
Place of Publication: Gainesville
Publication Date: 1966
Copyright Date: 1966
 Subjects
Subject: Radiochemistry   ( lcsh )
Kinetic theory of liquids   ( lcsh )
Chemistry thesis Ph. D
Dissertations, Academic -- Chemistry -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis - University of Florida.
Bibliography: Bibliography: l. 87-89.
Additional Physical Form: Also available on World Wide Web
General Note: Manuscript copy.
General Note: Vita.
 Record Information
Bibliographic ID: UF00097865
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000421865
oclc - 11020709
notis - ACG9863

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SCAVENGER KINETICS IN THE RADIOLYSIS

OF CYCLOHEXANE-METHYL IODIDE

SOLUTIONS












By

INDER MANI













A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY













UNIVERSITY OF FLORIDA


June, 1966

























































UNIVERSITY OF FLORIDA


3 1262 08552 2513














ACKNOWLEDGMENTS


The author extends his sincere appreciation to his supervisory

committee, the faculty of the Chemistry Department of the University

of Florida and his colleagues for their interest and friendly cooper-

ation in pursuance of this work. He is especially grateful to his

research director, Dr. R. J. Hanrahan for his constant help and

guidance in all the phases of this dissertation.

Also many thank to Mrs. Philamena Pearl for the typing of

the dissertation.

I want to give my heartfelt thanks to my wife, Vinod Agrawal,

without whose help and understanding this would not have been possible.












TABLE OF CONTENTS


ACKNOWLEDGMENTS . . . . . . . .

LIST OF TABLES . . . . . . . .

LIST OF FIGURES . . . . . . . .

Section

I. INTRODUCTION . . . . . .

Summary of the Previous Work. . .
Summary of the Present Work . . .

II. EXPERIMENTAL METHODS. . . . . .

Apparatus . . . . . . .
Purification of Materials . . .
Preparation of Samples. . . . .
Irradiation of Samples. . . . .
Post-Addition of Iodine to Irradiated
Determination of Hydrogen Iodide. .

III. EXPERIMENTAL RESULTS. . . . . .

Effect of Additives on the Radiolysis
Cyclohexane . . . . .
Effect of Additives on the Radiolysis
Cyclohexane-Methyl Iodide Solutions


Page

ii

iv

v


Samples .





of Pure

of
. . .
. . .















. . .


IV. DISCUSSION AND INTERPRETATION . . . . .

Mechanism of the Gamma-Radiolysis of Cyclohexane
and Cyclohexane-Methyl Iodide Solutions . .
Mathematical Analysis . . . . . . .
Assignment of Parameters. . . . . . .
Comparison with Experiments . . . . .
Concentration Dependence of Primary Yields in
Cyclohexane-Methyl Iodide Solutions . . .
Conclusion . . . . . . . . .

BIBLIOGRAPHY . . . . . . . . . . .

APPENDIX . . . . . . . . . . . .

BIOGRAPHICAL SKETCH . . . . . . . . . .











LIST OF TABLES


Table Page

1. Physical properties of cyclohexane-methyl iodide
solutions as a function of composition. . . . ... 18

2. Experimental G values for the radiolysis of
cyclohexane-methyl iodide solutions . . . .... .. 27

3. G(H') as a function of scavenger concentration. . . 74

4. Calculated G(R-), G(12), G(HI), and rate constant
ratios . . . . . . . . . . . 77

5. Cyclohexane-methyl iodide solutions: comparison
between observed and calculated yields. . . . ... 79










LIST OF FIGURES


Figure Page

1. Vacuum manifold. . . . . . . . .... 11

2. Iodine consumption: pure cyclohexane with added 12. .. 19

3. Iodine production: pure cyclohexane with added HI. . 20

4. Iodine production: pure cyclohexane with both'
12 and HI added. . . . . . ... . . . 23

5. Iodine production: cyclohexane-methyl iodide
solutions without additives. . . . . . ... 26

6. Net iodine production or consumption: cyclohexane-
methyl iodide solutions with added 12 . . . ... 28

7. Iodine production: cyclohexane-methyl iodide
solutions with added HI. . . . . . . ... 30

8. Iodine production: 99.9 volume percent cyclohexane-
methyl iodide solution with added HI . . . ... 31

9. Role of H atom yield as an adjustable parameter:
pure cyclohexane with added 12 ........... 45

10. Role of H atom yield as an adjustable parameter: pure
cyclohexane with added HI. . . . . . ... 46

11. Role of kHI/kl2 as an adjustable parameter: pure
cyclohexane with added 12 .. . ....... .. 47

12. Role of k I/ki2 as an adjustable parameter: pure
cyclohexane with added HI. . . . . . . ... 48

13. Role of kHI/kI as an adjustable parameter: 20 volume
percent cyclohexane-methyl iodide solution with
added 12 . . ............... 51

14. Role of kHI/kI2 as an adjustable parameter: 40 volume
percent cyclohexane-methyl iodide solution* with
added I2 .................. .... 52

15. Role of kHI/kI as an adjustable parameter: 60 volume
percent cyclghexane-methyl iodide solution, with
added 12 ................... ... 53











16. Role of kHI/kI2 as an adjustable parameter: 80 volume
percent cyclohexane-methyl iodide solution with
added 12 . . . . ...................... 54

17. Role of kHI/kI2 as an adjustable parameter: 90 volume
percent cyclohexane-methyl iodide solution with
added 12 .... .... ...... .. . 55

18. Role of kHI/k1 as an adjustable parameter: 95 volume
percent cyclohexane-methyl iodide solution- with
added 12 . . . *... ... ..... 56

19. Role of kHI/k1 as an adjustable parameter: 99.9 volume
percent cyclohexane-methyl iodide solution with
added 12 .. ............... ... . 57

20. Role of kHI/k2 as an adjustable parameter: 20 volume
percent cyclohexane-methyl iodide solution with
added HI . . . . . . . . .. . .. . .58

21. Role of kHI/k12 as an adjustable parameter: 40 volume
percent cyclohexane-methyl iodide solution with
added HI .. . . . . . . . . 59

22. Role of kHI/kI2 as an adjustable parameter: 60 volume
percent cyclohexane-methyl iodide solution with
added HI . . . . . . . . .. . . 60

23. Role of kHI/k12 as an adjustable parameter: 80 volume
percent cyclohexane-methyl iodide solution with
added HI . . . . . . . . .. . . 61

24. Role of kHI/kI2 as an adjustable parameter: 90 volume
percent cyclohexane-methyl iodide solution with
added HI . . . . . . . . . .. . 62

25. Role of kHI/k12 as an adjustable parameter: 95 volume
percent cyclohexane-methyl iodide solution with
added HI . . . . . . . . .. . . 63

26. Role of kHI/ky as an adjustable parameter: 99.9 volume
percent cyclohexane-methyl iodide solution with
added HI . . . . . . . . .. . . 64

27. Role of HI production from spurs as an adjustable
parameter: 95 volume percent cyclohexane-methyl
iodide solution with added 12' . ............ 66

28. Role of HI production from spurs as an adjustable
parameter: 95 volume percent cyclohexane-methyl
iodide solution with added HI. . . . . . ... 67










29. Role of H atom yield as an adjustable parameter: 99.9
volume percent cyclohexane-methyl iodide solution
with added 12 . . . . . . . . . .. 69

30. Role of H atom yield as an adjustable parameter: 99.9
volume percent cyclohexane-methyl iodide solution
with added HI. . . . . . . . ... 70

31. Role of HI production from spurs as an adjustable
parameter: 99.9 volume percent cyclohexane-methyl
iodide solution with added 12 . . . .. . . 71

32. Role of HI production from spurs as an adjustable
parameter: 99.9 volume percent cyclohexane-methyl
iodide solution with added HI. . . . . . ... 72

33. G values as a function of composition: cyclohexane-
methyl iodide solutions. . . . . . . ... 82

34. Flow chart: computer main program. . . . . ... 92

35. Flow chart: computer sub-programs. . . . . ... 93













SECTION I


INTRODUCTION


Radiation chemistry is the study of the chemical effects pro-

duced in a system by the absorption of ionizing radiation. The radi-

ations may come from radioactive nuclei (alpha, beta, or gamma rays),

from particle accelerators (electrons, protons, deuterons, etc.) and

from X-ray machines. The radiation dissipates energy by interacting

with electrons in the system and each photon or particle can ionize or

excite a number of molecules (via secondary electrons in the case of

photons).

For the present study the source of radiation was a 400 curie

Co60 source, emitting gamma rays of 1.17 and 1.33 mev. The mechanisms

by which gamma rays interact with matter, namely photoelectric effect,

Compton effect, and pair production, are well known.

The radiation chemistry of a number of hydrocarbon systems has

now been studied. It is becoming evident that the product formation in

the radiolysis of organic systems results from the direct production of

product molecules as well as through the intermediate formation of ions

and free radicals. Reactive solutes such as halogens, hydrogen halides,

DPPH, 02, etc., have been used as scavengers in the measurement of free

radical yields. In most cases, the effect of scavengers has been

interpreted only qualitatively.




2




The present study deals with a quantitative treatment of HI-I2

competition kinetics in the radiolysis of pure cyclohexane and cyclo-

hexane-methyl iodide solutions. Rate equations, based on previously

known mechanisms, were set up and their integration was done by numer-

ical calculations using the second-order Runge-Kutta method on an

IBM 709 computer.


Summary of the Previous Work

Radiolysis of the mixtures of liquid organic compounds dates

back to 1931 when Schoepfle and Fellows exposed a solution of benzene

and cyclohexane to 170 kv cathode rays. They found that the hydrogen

yield from cyclohexane decreases non-linearly with the addition of

benzene. Since then cyclohexane has been the object of a number of
4-10
investigations, e.g., in the pure state, with small concentration

of solutes 8,11-16 and in mixtures.3,6-8,15

The studies on pure cyclohexane have been concerned with the

identification and yield of radiolysis products, and the role of

scavengers. Hydrogen, cyclohexene, and bicyclohexyl are the major

products.5,6,9,10,13 The use of iodine as radical scavenger in hydro-
19
carbon radiolysis has been demonstrated by Fessenden and Schuler.1

They studied the effects of radiation on solutions of iodine in cyclo-

hexane over the iodine concentration range of 5 x 10-6 M to 5 x 10- M.

They found that the yield for the formation of alkyl iodide is indepen-

-5 -
dent of iodine concentration from 10- M to 5 x 10- M but is somewhat

higher at higher concentrations. In the lower concentration region, the

radiation yield, G(RI), is found to be 5.6 for both Co6 gamma radiation










and 2 mev Van de Graaff electrons. It appears that the radiation chem-

ical processes which are responsible for the ultimate chemical reactions

are affected by the presence of high scavenger concentration in the sub-

strate. At low concentrations, the presence of iodine does not compli-

cate the physical processes resulting from the radiolysis.

The presence of hydrogen atoms in cyclohexane radiolysis has
41
been detected directly by Smaller and Matheson and indirectly by

Forrestal and Hamill and by Meshitsuka and Burton.1 Nash and

Hamill3 studied the hydrogen yield from solutions of cyclohexane-dl2

and hydrogen iodide. They observed an increase in the total hydrogen

yield, as had been reported previously. 1012'16
22
Toma and Hamill studied the mechanism of hydrogen formation

in the radiolysis of cyclohexane and other hydrocarbons. They showed

that the molecular yield of hydrogen can be accounted for by a mechanism

of the type

+
C6H12+ e- C6H10 + 2H (hot) (i)

H (hot) + CgH12--- C6H11 + H2 (ii)

When the thermal component of G(H2) in cyclohexane is suppressed by

ca 10-2M iodine, further addition of cyclohexene23 continues to suppress

G(H2). They found that addition of methyl iodide, however, continues

to depress the somewhat correlated G(H2), G(C6H10), and G(C16H11I) by

removing a common precursor of these products. They accounted for the
24
effect of cyclohexene or benzene in cyclohexane upon G(H2) by charge

transfer processes as well as H atom scavenging.

Radiolysis of mixtures is more complicated and several processes









have been found to account for the over-all yields of products. Magee

and Burton2 suggested that ion-exchange processes may play a significant

role in the radiation chemistry of mixtures. In a system consisting

of molecules A and B, in which the ionization potential of A is less

than that of B, the ionization, irrespective of the component primarily

ionized, may be transferred to A. This greatly modifies the resultant

chemical processes.

Manion and Burton irradiated four mixtures, namely toluene-

benzene, cyclohexene-benzene, cyclohexane-benzene, and cyclohexane-

cyclohexene by 1.5 mev electrons. The results obtained are consistent

with an interpretation involving considerable emphasis on ion and

excitation-transfer mechanisms. In a mixture of two components A and B,

both are primarily affected by the ionizing radiation to produce A

and B and of excited molecules A* and B. If IA > I where IA and IB

are the respective ionization potentials, then the process

A + B --A + B (iii)

may occur.

Either of the excitation-transfer processes

A* + B A + B* (iv)

A + B* A* + B (v)

can occur dependent on the relative heights of the energy levels

involved. The condition for most probable excitation transfer is that

EA = E where EA and EB represent respective excitation energies.

However, reaction (iv) is possible if EA > EB, the reference being to









the lowest excited states. In the case of toluene-benzene mixtures,

the contribution of the excitation-transfer mechanism is unimportant,

for the lower excited states make a negligible contribution to the

total chemical effect. The primary chemical processes involve princi-

pally toluene, as is required by the ionization-transfer mechanism.

In the radiolysis of cyclohexene-benzene mixtures, the two effects act

in opposition, with cyclohexene playing a sacrificial.role in protec-

tion of benzene ions and benzene offering sponge-type protection to

excited cyclohexene molecules. In cyclohexane-benzene mixtures, benzene

protects cyclohexane because it has both lower ionization potential

and lower excitation energy. In cyclohexane-cyclohexene cases, limited

evidence indicates just sacrificial protection by cyclohexene.

The role of ionic processes such as electron attachment, charge

transfer and ion-molecule reactions has been emphasized by Hamill and
11
coworkers. Williams and Hamill studied the chemical effects of

electron capture by solutes in hydrocarbons during gamma irradiation.

They irradiated various samples consisting of cyclohexane, toluene and
131 CH131 C2H 131
benzene as solvents and I CH and CH as solutes. The

observed yields could not be interpreted on the basis of positive charge

transfer which may contribute whenever the ionization potential of the

solvent exceeds that of the solute. This condition holds for cyclo-

hexane as the solvent but not for benzene and toluene. They found that

electron capture does appear to provide a consistent explanation of the

observed results.
12 11
Schuler2 extended the studies of Williams and Hamill1 on

dissociative electron capture by examining the effect on the hydrogen








yield from cyclohexane of solutes similar to those which they used.

He found that low concentration of solutes having high electron

affinities such as iodine, alkyl halides, and sulfur dioxide decrease

the hydrogen yield in the gamma irradiation of cyclohexane by about

40 percent. ,This decrease in hydrogen is ascribed to a transfer of

energy to the solute, probably by an electron capture mechanism.

Forrestal and Hamill irradiated a number of'liquid mixtures

containing cyclohexane. They showed that the hydrogen yield can be

divided into three components; one due to reactions of thermal H atoms,

one to reactions of hot, high-velocity H atoms, and the third corre-

sponding to hydrogen formed by molecular processes. In the case of

radiolysis of cyclohexane-iodine mixtures with low iodine concentrations,

they found that the decrease G(H2) = -2.0 is matched by nearly equal

G(HI) and is attributed to scavenging of thermal H atoms. They reported

that the iodine concentration must be greater than 0.3 M in order to

scavenge at least 99 percent of the available H atoms in cyclohexane.

Forrestal and Hamill also presented considerable data on cyclohexane-

methyl iodide system. They explained the effect of methyl iodide on

G(H2) in a similar manner and found that their results with 0.1 electron

percent or less added methyl iodide could be explained by competitive

H atom scavenging by cyclohexane and methyl iodide. They postulated

that the behavior in the 1-10 percent methyl iodide concentration range

is due to dissociative electron capture by methyl iodide.

In their work on solutions of HI in cyclohexane-dl2 which was

mentioned above, Nash and Hamill3 attributed the increase in the total

hydrogen yield to dissociative electron attachment. At high concentrations









-2
of ca 10-2 M, HI may become involved in electron capture or energy

transfer processes.
20
Croft and Hanrahan20 studied the iodine production in gamma

radiolysis of cyclohexane-methyl iodide solutions. To explain the

results, they postulated that the production of iodine in dilute

solutions in cyclohexane is due to the ion-molecule mechanism similar

to that proposed by Gillis, Williams and Hamill21


C6H12 + CH3I C6 12 + CH 31 (vi)

CH31++ CH3 --> (CH3)I+ + I. (vii)

Upon neutralization of the product ion there results a "pocket"

containing two methyl radicals and two iodine atoms which can undergo

diffusion controlled recombination, giving ethane and iodine. Between

10-100 electron percent methyl iodide, the residual free radical pro-

duction from cyclohexane would fall to zero and the efficiency of

reaction (vii) would approach its value in pure methyl iodide.


Summary of the Present Work

The present work reports a series of experiments performed by

irradiating pure cyclohexane with 12, HI, or both added initially.

Cyclohexane-methyl iodide solutions having 20 to 99.9 volume percent

cyclohexane were also irradiated with added 12 or HI and without

additives. The resulting iodine production or consumption during the

course of radiolysis was plotted against dose in each case. The

initial slopes of the graphs for those experiments in which no additives

were added are designated as "Normal Rates" of the iodine production;




8




the initial slopes with added 12 and with added HI are designated as

"Minimum Rates" and "Maximum Rates", respectively. In the case of

cyclohexane-methyl iodide solutions having 80 to 95 volume percent

cyclohexane with added 12, the graphs are more interesting in that

the iodine concentration actually decreases at first, comes to a minimum,

and then increases.

During the investigations described here, it was found possible

to predict quantitatively the curvature of graphs of iodine concentra-

tion vs dose on the basis of HI-I2 competition kinetics. A simplified

mechanism, based upon previous work, has been used. For pure cyclo-

hexane, this mechanism includes the production of alkyl radicals,

hydrogen atoms, and stable hydrocarbons within the "spurs" (hot spots)

of the radiation tracks and reaction of the thermalized alkyl radicals

and H atoms with HI or 12 outside the track. For solutions of cyclo-

hexane-methyl iodide, the mechanism includes the production of HI, 12,

alkyl radicals and stable hydrocarbons inside the track and reaction of

thermalized radicals with HI or 12 outside the track. Differential

rate equations, based on conventional kinetics, have been set up. The

analytical integration of the equations appeared very cumbersome and

has been carried out by numerical calculations using second-order

Runge-Kutta method on an IBM 709 computer. These calculations provided

a general procedure applicable to pure cyclohexane and with slight

modifications to cyclohexane-methyl iodide solutions.

Input data for a given experiment are the Maximum, Minimum, and

Normal Rates of iodine production. An initial scavenger concentration

(HI or 12, or both) is given to the computer, matching an actual











experiment. The ratio of rate constants kHI/kl2 is used as an adjust-

able parameter, where kHI and k12 are the rate constants for reactions

of radicals with HI and 12 respectively. For pure cyclohexane, the

value of the H atom yield, which cannot be established directly from

the present experimental results, has also been used as an adjustable

parameter. The computed curves so obtained fit the experimental points

quite well.

The results are summarized below:

For pure cyclohexane, variation of H atom yield affects the end

point whereas the ratio kHI/k12 does not. A value of kHI/k12 = 0.71 has

been found for the best fit. The effective H atom yield used in the

calculations depends upon the initial scavenger concentration. G(H')

has been found to be 0.70 to 1.43 for iodine scavenger concentrations

of 0.31 x 10-3 M to 1.71 x 10-3 M. For HI scavenger concentrations of

2.11 x 10-3 M-to 8.88 x 10-3 M, G(H-) is 1.40 to 1.74.

For cyclohexane-methyl iodide solutions with 20 to 95 volume

percent cyclohexane, the H atom yield does not enter into calculations

because methyl iodide is present in high concentration so that all H

atoms react with methyl iodide in or near the spurs to produce HI. The

value of kHI/k12 depends upon the concentration of the components of the

solutions. For the range of solutions studied, the values of kHI/k12

have been found to be 0.55 to 0.98. For 99.9 volume percent solution,

methyl iodide is not present in sufficient amounts to react with all

the H atoms. Therefore, the H atom yield is also to be adjusted. For

this case, G(H') has been found to be 0.70.

The calculations also provided the yield of thermal alkyl

radicals, and the yield of 12 and HI within the radiation tracks.













SECTION II


EXPERIMENTAL METHODS


Apparatus

Samples for irradiation were deaerated on a vacuum line with a

manifold shown in Figure 1. Preparation of a series of solutions with

the same initial iodine concentration was facilitated by the use of a

vacuum line buret.26 The pump section of the vacuum line consisted of

a Welch Duo-Seal fore-pump connected through a liquid nitrogen trap

and a water condenser to a mercury diffusion pump. This arrangement

pulled down the whole system to a pressure of about 104 mm of mercury.

The pumps were connected to the vacuum line which consisted of the

following: several 19/38 standard taper ground glass joints for intro-

ducing samples, a small calibrated tube (T, about 2 ml volume) for

introducing measured amounts of gases, a mercury manometer (M) and a

sub-manifold (Sl) for attaching and sealing off cells (C1), all

connected to the main manifold through stopcocks. Another sub-manifold

(S2) for attaching and sealing off sample cells (C2) was connected to

the vacuum line via a vacuum line buret (B) and a by-pass line as shown

in the figure. The stopcocks above and below the buret have a teflon

plug and o-ring seal, and require no grease. Kel-F fluorocarbon grease

was used for all other stopcocks during the experimental work. Fifty

ml round bottom Pyrex flasks (F) were used as sample reservoirs.




















Cfl



























00
Ocu
CC





H







































z










For all radiolysis experiments, except for those used for HI

determination, irradiation vessels (Cl, C2) were 13 x 100 mm test

tubes, with attached spectrophotometer cells. The spectrophotometer

cells were made from square Pyrex tubing. Irradiations were performed

at 25 20C. using a modified Firestone-Willard type 400 curie Co60

source described previously.27 Iodine was analyzed spectrophotometri-

cally using a Beckman DU spectrophotometer. Hydrogen iodide was

analyzed by extracting with distilled water and the aqueous layer was

separated by a separating funnel28 of a special type.


Purification of Materials

Phillip's "pure grade" cyclohexane was passed through silica

gel before use. Methyl iodide (Eastman Organic Chemicals) was passed

through alumina, distilled on a Todd still and passed through alumina

again. "Baker analyzed" reagent grade iodine and hydriodic acid were

used without further purification. Pyrex glass wool used in the vacuum

line was purified by washing with carbon tetrachloride and then heating

to 5600C in an annealing oven. Water used during the experiments was

purified by distilling ordinary distilled water in the presence of alkaline

potassium permanganate.

The apparatus such as burets, measuring flasks, sample flasks,

and irradiation vessels were washed with distilled water and acetone

and then heated in an oven.










Preparation of Samples


General procedure

The general procedure of filling all irradiation vessels was

the following: a 4 ml volume of the required sample was measured into

a small round bottom flask (F) fitted with a female 19/38 joint.

After adding about one gram of phosphorus pentoxide, the flask was

connected to the vacuum line through a column of phosphorus pentoxide

and a stopcock. The sample was then frozen in liquid nitrogen and

evacuated. To remove air from the sample as thoroughly as possible,

it was melted, refrozen and pumped several times. The main section of

the vacuum line was isolated from the pump section by closing the stop-

cock. The sample was then transferred to the irradiation vessel. Two

or more melting, freezing and pumping cycles were performed and the

sample sealed off at a constriction.


Samples containing iodine

Samples containing iodine were prepared by diluting a stock

solution of iodine in cyclohexane to a volume of 4 ml. The solution

was then used to fill the irradiation vessels as described above.

Iodine concentration in the sample was verified spectrophotometrically.

To prepare several samples containing the same initial iodine concen-

tration, a stock solution of about 25 ml was degassed in a flask

connected to the vacuum line. This solution was then transferred to

the buret (B) and homogenized by a small stirring magnet placed inside

it. Four ml samples were then transferred to the irradiation vessels (C2).









Samples containing hydrogen iodide

Dry hydrogen iodide was produced by dehydrating hydriodic acid.

The hydriodic acid was frozen to liquid nitrogen temperature in a round

bottom flask and phosphorus pentoxide added on top of it. The flask

containing the frozen acid was then attached to the vacuum line through

phosphorus pentoxide column and degassed. The hydriodic acid was allowed

to melt and interact with P205, and the hydrogen iodide released was

collected in another round bottom flask connected to the. line. This

collected hydrogen iodide was degassed and stored at liquid nitrogen

temperature until used. To obtain a known quantity of hydrogen iodide,

the gas was taken in the calibrated tube (T) at a known pressure and

temperature and then transferred to the sample.


Irradiation of Samples

Irradiation of samples was performed by placing them, in the

same irradiation geometry, directly adjacent to the source capsule.

Reproducible geometry was assured by using a special aluminum rack.

Post-Addition of Iodine to Irradiated Samples

In several experiments, iodine was to be added to the sample

after irradiation. The procedure used was the following: the irradi-

ation vessel was fitted with a break-seal and connected to the vacuum

line. The required sample was transferred to the vessel as described

earlier and sealed off. After irradiating the sample, the other side

of the break-seal was connected to the vacuum line and evacuated. A

known amount of iodine in the same solvent was degassed. The break-

seal was broken by a small magnetic hammer and the degassed iodine

solution transferred to the irradiated sample kept frozen










throughout the experiment. The sample was then melted, thoroughly

shaken, refrozen and kept frozen until used.


Determination of Hydrogen Iodide


Hydrogen iodide formed during irradiation was determined by

first opening the frozen sample in distilled water and thoroughly

shaking it. The aqueous layer was separated and any iodine present in

it was removed by washing several times with cyclohexane. The aqueous

portion was then slightly heated to remove any cyclohexane until a

clear solution was obtained. It was then diluted to a known volume

and the concentration of HI determined spectrophotometrically by

Beckman DU spectrophotometer at a wave length of 226 m4 and using

E = 13,000.













SECTION III


EXPERIMENTAL RESULTS

In this study the current practice of expressing radiation

yields as G values has been adopted. G value is defined as the number

of molecules of a substance produced or consumed per 100 ev deposited

in the system. Yields are based on the ferrous sulfate dosimeter2

[G(Fe ) = 15.6]. Since concentration and time had to be used for

theoretical calculations as iteration parameters on a digital computer,

it became necessary to use other convenient units. It was chosen to

state concentrations as micromoles per 4 ml sample and to use time in

minutes which is proportional to radiation dose.

The dosimetry on the source was originally done on January 1,

1961. The energy absorption rate in the Fricke dosimeter was on that

date found to be 0.931 x 1018 ev/ml min. The same value has been used

in the present work and all periods of irradiation have been corrected

to a common date January 1, 1964. Taking into account the decay of

the source, the dose rate was found to.be 0.63 x 1018 ev/ml min on

January 1, 1964. Dose rates were corrected for the differing values

of 4, the linear absorption coefficient, for the various solutions.

For pure methyl iodide and cyclohexane, values of 1.950 and 0.780

respectively have been used for 4(sample)/4(dosimeter). For the inter-

mediate solutions, relative absorption values were obtained by assuming










an interpolation linear in volume fraction. To analyse 12 spectrophoto-

metrically, the position of X and the corresponding extinction
max
20
coefficients, E, as determined by Croft and Hanrahan20 have been used

(Table i).


Effect of Additives on the Radiolysis of
Pure Cyclohexane


With added iodine


Measurementsof iodine concentration vs dose were made during

the radiolysis of several cyclohexane solutions. Some of the data

were taken by W. C. Blasky of this laboratory (see footnotes,

Figures 2 and 3). The results of the experiments in which iodine,

varying from 0.3 x 10 M to 2.0 x 10 M, was initially added are

shown by circles in Figure 2 (smooth lines are theoretical; see below).

All of the curves are concave upwards, with the solutions having higher

initial iodine concentration showing the most curvature.25 The initial

rate of iodine uptake in such experiments has been reported29 to be
-6
independent of initial iodine concentration over the range 5 x 10 M

to 5 x 10 M. The results in Figure 2 also confirm the above fact,

which is basic to the use of iodine for measuring free radical yields.

An average rate of iodine uptake of 3.08 molecules/100 ev has been

found. It is to be noted that the value obtained for the initial G

value of iodine uptake depends somewhat on the procedure used to

interpret the concentration vs dose graphs, since the graphs are non-

linear. To use the initial concentration dose increments is disadvan-

tageous, since the first few points often show the most scatter. The
















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with added 12, as
smooth curves are
right, are 0.31 x
1.16 x 10-3 M and


Radiolysis Time, min.


Iodine consumption in the radiolysis of pure cyclohexane,
a function of radiation time. Circles are experimental;
computed. Initial 12 concentrations, reading left to
10- M (W.C.Blasky), 0.61 x l0-3 M (W.C.Blasky),
1.71 x 10-3 M.


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procedure adopted in this dissertation for such calculations is the

following: the net change of concentration (or optical density) from

time zero to each successive radiolysis time t was calculated. A graph

of A (concentration)/t vs t was plotted and the desired initial rate

was found by extrapolating this graph to zero time.


With added hydrogen iodide

The results of the experiments in which hydrogen iodide,

varying from 2 x 10 M to 9 x 103 M, was initially added are shown

by circles in Figure 3. In such experiments, alkyl radicals abstract

H atoms from HI and release iodine atoms which later form 12. Thus,

iodine is initially produced rather than consumed. As the radiolysis

proceeds, the iodine enters into competition with HI for free radicals

so that the net rate of iodine production decreases to zero at the

maximum of the curves and then becomes negative. If the experiment is

continued to a sufficiently high dose, all the iodine and hydrogen

iodide are finally consumed. The graphs in Figure 3 show that the

positions of the iodine maxima and final "end points'" of the experi-

ments increase with increase in initial HI concentration. The initial

G value of iodine production is independent of initial HI concentration.

The values of G(12) ranged from 2.8 to 3.0 and a value of 2.96, based

on several consistent experiments, was used in the theoretical calcu-

lations.

In order to be consistent with notation used in connection with

the radiolysis of cyclohexane-methyl iodide solutions described later,

the initial rate of iodine disappearance in pure cyclohexane will be










referred to as the "Minimum Rate", and the initial rate of iodine pro-

duction with added HI as the "Maximum Rate". The term "Normal Rate"

which is also used in connection with cyclohexane-methyl iodide

solutions is zero for pure cyclohexane.


With both iodine and hydrogen iodide added

The results of similar experiments in which both 12 and HI

were initially added are shown by circles in Figure 4. In these

experiments the initial iodine concentration was 1.53 x 10 M in all

cases but the HI concentration varied from 1.08 x 10-3 M to 3.43 x 10- M.


Evidence of hydrogen iodide production in the radiolysis
of cyclohexane-iodine solutions

If the production of hydrogen iodide during the radiolysis of

cyclohexane with added iodine is considered to be responsible for the

curvature of iodine vs dose graphs, then additives which prevent back-

reaction of HI should linearize the curves. Such experiments with

added barium oxide or water have been done previously. 0',4,30 It was

found by these workers that the addition of BaO and H20 cause the

iodine disappearance to be much less curved than in the case of a

similar experiment with only iodine added.


Determination of hydrogen iodide after post-addition
of iodine in irradiated cyclohexane

Four runs for the determination of HI after post-addition of

iodine in irradiated cyclohexane were done. The apparent G values for

HI production were.found to be 0.35, 0.37, 0.35, and 0.40 for the four

runs. In order to find out whether the hydrogen iodide so obtained is



























































































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really due to the effects of radiolysis on cyclohexane and then shaking

with iodine under vacuo or just due to equilibration of iodine with

water during extraction, two experiments were done under identical con-

ditions, but without irradiating the cyclohexane. It was observed that

HI was still formed and a G value of 0.35 was obtained. Any difference

between this G value and some of the G values for irradiated cyclohexane

experiments is probably due to experimental errors. Therefore, it is

to be concluded that HI is not formed due to reaction with radiolysis

products when iodine is added to cyclohexane after radiolysis.


Effect of Additives on the Radiolysis of
Cyclohexane-Methyl Iodide Solutions

The theoretical deductions described later in this dissertation

require a knowledge of the rates of iodine production in cyclohexane-

methyl iodide solutions under three circumstances: with no additives,

with added HI, and with added 12.


Without additives


Initial yields of iodine production in pure, degassed solutions

of methyl iodide in cyclohexane without additives were reported earlier

by Croft and Hanrahan.20 The general character of the results reported

by them is that all solutions ranging from 10 to 95 volume percent

cyclohexane produced iodine nearly linearly with dose. The G value for

iodine production decreased with increasing cyclohexane concentration.

The slopes of the graphs of iodine concentration vs dose are designated

"Normal Rates" of iodine production in the present work.










For the purpose of fitting theoretical curves to experimental

iodine production data at long radiolysis times, it became important to

obtain data on iodine production in cyclohexane-methyl iodide solutions
20
at total doses larger than those used by Croft and Hanrahan. Such

experiments were performed and their results are shown in Figure 5.

Iodine production rates obtained from the limiting slopes of these

graphs at long radiolysis times are given in parentheses in Table 2.


With added iodine


Results of iodine production in the radiolysis of the above

solutions in which iodine was present initially as a free radical

scavenger are shown by circles in Figure 6. In all the experiments,

the initial concentration of iodine was about 1.5 x 10- M. The

initial iodine concentration has been subtracted from the total

measured iodine concentration in plotting the ordinate points and the

graph shows net iodine production or consumption. All the curves of

the graph are concave upwards. In the case of solutions with 80 to

95 volume percent cyclohexane, the iodine concentration actually

decreases at first and then increases. The minimum in the 95 percent

curve occurs off the scale of the figure at 2400 minutes. In the

case of the solution with 99.9 volume percent cyclohexane, iodine is

regularly consumed until none is left after 148 minutes. The initial

slopes of these curves, whether positive or negative, are designated

as the "Minimum Rates". The positive values indicate that iodine is

initially produced and the negative values show that iodine is initially

consumed at the corresponding rates.
















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With added hydrogen iodide

Results of iodine production in the radiolysis of various

cyclohexane-methyl iodide solutions with added hydrogen iodide are

shown by circles in Figure 7. The HI concentration in all the cases

was about 6 x 10 M. All the curves of the graph are concave down-

wards. For 99.9 volume percent cyclohexane solution, only a portion

of the graph has been shown. The complete graph is given in Figure 8.

The iodine concentration first increases, becomes maximum at about

one millimolar iodine concentration after 100 minutes, and then

decreases until after 314 minutes it is totally consumed. The initial

slopes of the curves in Figure 7 are designated as the "Maximum Rates".

The G values for the normal rate of iodine production without

additives, the maximum rate with added HI, and the minimum rate with

added iodine are listed in Table 2.














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SECTION IV


DISCUSSION AND INTERPRETATION


Mechanism of the Gamma Radiolysis of Cyclohexane
and Cyclohexane-Methyl Iodide Solutions


General


A great variety of conflicting viewpoints have been expressed

on the mechanism of cyclohexane radiolysis by numerous workers who

have attempted to interpret it. However, essentially all the investi-

gators have agreed that cyclohexyl radicals, c-C6H11', are involved in

the mechanism.5,9,12,15'31-34 The role of hydrogen atoms is far more

obscure.

While discussing the isotopic composition of hydrogen produced

in the radiolysis of C D12-C6 H2 mixtures, Dyne and Jenkinson5 have

postulated the presence of "precursors or reactive intermediates" which

provide hydrogen atoms but which may or may not actually be hydrogen

atoms. During the reinvestigation of radiolysis products of pure cyclo-

hexane system, Ho and Freeman have found that cyclohexyl radicals are

involved in the production of cyclohexene, bicyclohexyl and cyclohexyl-

hexene.

To interpret the present experimental results, attempts have been

made to provide an internally consistent free radical mechanism. A

simplified mechanism, which assumes the presence of hydrogen atoms with

an effective hydrogen atom yield dependent on the total concentration

32










of scavengers present, has been adopted. At lower concentrations, it

is assumed that the hydrogen atoms are replaced by a complementary

yield of cyclohexyl radicals. At sufficiently high concentrations,

both HI and I may become involved in electron capture or energy trans-

fer processes.12,35 However, the highest concentration of scavengers
-2
used in the present experiments is about 10- M, and most of these
12
experiments were done at millimolar scavenger concentrations. It is

justified to assume that HI and 12 behave predominantly as free radical

scavengers at these concentrations. A simplified mechanism is given

below:


Reactions in spurs


-C6 H12

c-C6 H2-- 1 (c-CH 12+ + e-)

c-C6H12 ---

c-C6 H12" :

CH361
C-HI ----*-_--








CH3 I*

CH I* + CH I 3 ---

CH I* + CH I

H- + CH31


c-C6H1 *

> c-C 6H212

c-C6H o + H2

c-C H11 + H.

CH I*

--- CH I*

c-C6H12 + CH3I*

CH3 + I.


CH6 + 12

CH4 + 2HI

HI + CH3


(la)

(Ib)

(2)

(3)

(4a)

(4b)

(5)

(6)

(7a)

(7b)

(8)









Steady-state radical reactions

I. + I. 12 (9)

R- + 1 > RI + I. (10)

R- + HI RH + I (11)

H* + 12 HI + I. (12)

H* + HI > H2 + I (13)

Asterisks represent electronically excited species. Several steps,

especially 5 and 7, represent net processes rather than actual mechanisms.

This scheme allows for production of hydrogen atoms, alkyl radicals,

cyclohexene, and molecular hydrogen from cyclohexane as well as methyl

radicals, stable hydrocarbons, e.g., C2H6, and iodine from methyl

iodide. The distinction between atomic and molecular iodine indicated

in equations 6 and 7a is of minor significance, since iodine atoms com-

bine to form 12 in any event; see equation 9. However, the important

distinction between equations 6 and 7 is that 7 suggests that net

iodine is produced only if the corresponding methyl radicals are stabi-

lized as hydrocarbons. Some of the steps in the sequence are intended

to represent net processes rather than actual mechanisms. In particular,

energy redistribution between cyclohexane and methyl iodide in equation 5

could involve electron capture or charge exchange as well as transfer

of excitation energy. Furthermore, there is evidence that net iodine

production from methyl iodide (eq. 7) occurs by an ionic route. For

the purposes of quantitative treatment, it is important that, for a

fixed ratio of methyl iodide to cyclohexane in the solution, the rate

of each of the processes listed under "Reactions in spurs" is taken as

a constant, unaffected by scavengers at low concentrations. Reaction 8










is not a spur reaction in the usual sense but is included in this group

because methyl iodide, which is a good radical scavenger, was present

in much higher concentration than any other scavenger in most of the

experiments. Under these circumstances, all hydrogen atoms are con-

verted to HI by reaction 8.

Because of the complexity of the situation, further approxi-

mations are required before attempting mathematical analysis. In con-

sidering the steady-state radical reactions, the distinction between

methyl and cyclohexyl radicals is ignored, and it is assumed that the

same rate constant can represent reactions of either with a scavenger.


Modifications for pure cyclohexane

Steps 4 to 8 do not appear in the mechanism for the radiolysis

of pure cyclohexane. Thus the net result of primary processes is the

production of hydrogen atoms and cyclohexyl radicals which then take

part in competitive reactions under steady-state conditions.


Modifications for cyclohexane-methyl iodide solutions


In cyclohexane-methyl iodide solutions with 0 to 95 volume

percent cyclohexane, the concentration of methyl iodide is sufficiently

high so that reaction 8 should be the predominant fate of H atoms. The

net result of primary processes is then the production of I2, HI, and

alkyl radicals which subsequently take part in competitive reactions

10 and 11 under steady-state conditions. However, in the 99.9 volume

percent cyclohexane solution, some of the H atoms escape reaction 8 and

enter into competitive reactions 12 and 13. It is to be noted that 12

and HI are also formed during the radiolysis of cyclohexane-methyl iodide

solutions in addition to that added as a scavenger.










Mathematical Analysis


General


The proposed reaction scheme can be treated by conventional

kinetics. It is assumed that 12, HI, H atoms, and alkyl radicals are

produced according to zero order kinetics by the radiation and that

12 and HI then compete for alkyl radicals and H atoms according to

equations 10 to 13. The rate constants for reactions 10 and 12 and

for reactions 11 and 13 might be expected to be quite similar. The ratios

k 1/k10 and k 13/k2 should be even more similar. During the present

calculations these ratios have been assumed to be equal, which greatly

simplifies the mathematical expressions involved.

To begin the calculations, steady-state assumption is applied

to the alkyl radical and H atom concentrations. It is necessary to

define the following quantities which represent the constant rates

of various elementary processes:


A = Rate of production of thermal alkyl radicals which

escape the spurs (steps 3, 6 and 8).

B = Rate of production of iodine in spurs (step 7a).

C = Rate of production of hydrogen iodide in spurs or by

reaction 8.

D = Net rate of production of thermal H atoms which

escape the spurs.


Then the thermal alkyl production rate is set equal to the

rate of removal by reactions 10 and 11:








A = klO[R.][I2] + k11[R.][HI]

= [R.](k10[12] + k11[HI]) (14)

and [R-] = A/(klO[12] + k11[HI]) (15)

Similarly, the thermal hydrogen atom production rate is set
equal to the rate of removal by reactions 12 and 13:

D = kl2[H-][I2] + kl3[H-][HI]

= [H.](kl2[I2] + kl3[HI]) (16)

and [H.] = D/(k12[12] + kl3[HI]) (17)

The rate of iodine production may be expressed as

d[I2]/dt = B (k10/2)([I2][R.])+ (kll/2)([HI][R.])

(k12/2)([I2][H.])+ (k13/2)([HI][H.]) (18)

The factor of 1/2 is introduced because only 1/2 mole of iodine is
consumed when a mole of radicals.react with 12 (reactions 9 and 10).
After substituting expressions for [R-] and [H*] from equations 15 and

17 in 18 and rearranging, this becomes

d[I2]/dt = B (A/2)(klO[I2] kll[HI])/(k10[I2] + kll[HI])

+ (D/2)(k12[12] k13[HI])/(k2[1 2] + kl3[HI]) (19)

= B + A/2 A[I2]/([12] + [HI]k11/kl0)

+ D/2 D[I2]/([I2] + [HI]kl3/kl2) (20)

After putting kll/k10 = k13/k12 = kHI/kI2, this simplifies to

d[I2]/dt = B + (A+D)/2 (A+D)[12]/(112] + [HI]kHI/k12) (21)









The rate of HI production may be expressed as

d[HI]/dt = C + k12[12][H.] kl3[HI][H.] k11[HI][R.] (22)


= C + [H-](k12[2] kl3[HI]) k11[HI][R.] (23)

After substituting expressions for [H-] and [R.], this becomes

d[HI]/dt = C + D(k12[12] k13[HI])/(k12[12] + kl3[HI])

A(k11[HI])/(k10[12] + k11[HI]) (24)

d[HI]/dt = C (A+D) + (A+2D)[I2]/([I2] + [HI]kHI/kl2) (25)

The above equations are derived for the most complex case which

can occur for the proposed reaction scheme. This is exemplified by

the 99.9 volume percent cyclohexane solutions, in which some of the

hydrogen atoms escape reaction with methyl iodide according to equation 8,

and enter into competitive reactions 12 and 13. The 12 and HI produc-

tion rates in this case are given by equations 21 and 25.

In the radiolysis of pure.cyclohexane and of the solutions with

5 percent or more methyl iodide, some of the terms in the rate equations

are negligible. Therefore, a simplified version of the equations can

be employed.


Modified differential equations for pure cyclohexane

Since iodine and hydrogen iodide are not produced in spurs

when the radiolysis of pure cyclohexane is done, the parameters B and

C are zero. Hence, the expressions for the rates of production of 12

and HI become

d[12]/dt = (A+D)/2 (A+D)[I2]/([12] + [HI]kHI/kI2) (26)

d[HI]/dt = (A+D) + (A+2D)[I2]/([I2] + [HI]kHI/k 2) (27)










Modified differential equations for cyclohexane-methyl
iodide solutions

For solutions having 0 to 95 volume percent cyclohexane, all

the hydrogen atoms react with methyl iodide to produce hydrogen iodide

according to equation 8. Thus, the parameter D is zero. The corre-

sponding rates become

d[I2]/dt = B + A/2 A[I2]/('[2] + [HI]kHI/k12) (28)

d[HI]/dt = C- A + A[I2]/([12] + [HI]kHI/kl2) (29)


Integration of the differential rate equations

To compare the present experimental results with the theoretical

calculations, it is necessary to solve equations 21 and 25, or 26 and

27, or 28 and 29, a pair of simultaneous, first order, non-linear

differential equations. Although a set of integrated rate expressions

was presented earlier by Hanrahan and Willard6 for a similar kinetic

problem in irradiated ethyl iodide, the results are not directly appli-

cable in the present case. The over-all stoichiometry of the radiation-

induced reaction in cyclohexane-methyl iodide solutions is somewhat

more complicated than in ethyl iodide. In addition, the analytical

integration done for ethyl iodide radiolysis kinetics requires that

the rate constant ratio for reaction of radicals -with HI and 12 respec-

tively be unity. This is not a sufficiently good approximation in the
37
present case. More recently, Perner and Schuler have presented an

indirect analytical solution for HI-I2 scavenger kinetics in irradiated

hydrocarbons. Again, it appears impossible to extend the mathematical

treatment to hydrocarbon-alkyl iodide solutions. Their analysis applies










to a kinetic scheme which is similar to the present scheme.but they did

not make any allowance for the direct participation of hydrogen atoms.

They assumed that hydrogen atoms attack the substrate and are converted

to alkyl radicals. In order to make the present scheme applicable not

only to the radiolysis of pure hydrocarbons but also to more complicated

systems of hydrocarbon-alkyl iodide solutions, the problem has been

attacked by numerical integration. A published program for the

second-order Runge-Kutta method of solving simultaneous differential

equations is applicable with slight modifications in the present case

(see appendix). The integration has been done on the IBM 709 computer

of the University of Florida Computing Center.


Assignment of Parameters


General

Before the equations for the rate expressions can be solved by

the computer, it is necessary to provide values for the quantities A,

B, C, and D, and for the ratio of rate constants kHI/k 2. Some informa-

tion about these parameters is obtained from the limiting forms of

equation 21. When the concentration of hydrogen iodide greatly exceeds

that of iodine, the equation reduces to the form

(d[l2]/dt)Max = B + (A+D)/2 (30)

and when the concentration of iodine greatly exceeds that of hydrogen

iodide, the equation becomes


(d[2]/dt)Min = B (A+D)/2


(31)










The subscripts "Max" and "Min" have been used because the first case

refers to the initial, maximum value of the'rate of iodine production

with added HI, whereas the second refers to the initial, minimum value

of the rate of iodine production or disappearance with added iodine.

Equations 30 and 31 yield


B = (Max. Rate + Min. Rate)/2 (32)


(A+D) = (Max. Rate Min. Rate) (33)

In order to substitute the value of (A+2D) in equation 25, it is necessary

to establish the value of D. Some evidence on its value is given by

the experiments of Meshitsuka and Burton4 in which the initial value

of the HI yield in cyclohexane with added iodine was found to be 2.1

molecules/100 ev. This can be taken as an upper limit for the hydrogen

atom yield applicable to solutions with scavenger concentrations of
-2
about 2 x 10-2 M or greater. At sufficiently low scavenger concen-
-4
trations of about 10 M or less, the scavengeable hydrogen yield is

effectively zero because the hydrogen atoms react with the hydrocarbon

substrate to form H2 and are replaced by a corresponding yield of alkyl

radicals:17,18,40

H* + c-C6H12 H2 + c-C6 H (34)


At higher concentrations, iodine and other scavengers can compete with

reaction 34. It has been reported by Schulerl2 that, using iodine as

a scavenger, the reduction in the corresponding hydrogen yield occurs
-3 -2
mainly in the concentration range from 10 to 10- M. According to
-3
him a concentration of 3 x 10 M iodine decreases the hydrogen produc-

tion by 50 percent of the ultimately observed reduction. Hence, for the










purposes of the present calculations, it can be assumed that the

effective G value of scavengeable hydrogen atoms varies from zero at
-4 -2
10 M 1I or HI to a maximum of about 2 at ca 2 x 10 M scavenger

concentration.

The quantity C could be found experimentally if HI yields were

measured in the radiolysis of cyclohexane-methyl iodide solutions with

additives. However, the HI yield is difficult to measure accurately.

By an indirect method, using equations 21, 30, and 31, C can be found

as follows: the rate of iodine production expressed by equation 21

is just the Normal Rate without additives. Hence, subtracting equation

31 from equation 21 one obtains

Norm. Rate Min. Rate = (A+D) (A+D)[I2]/([12]+ [HI]kHI/k2)

and subtracting equation 21 from equation 30 gives

Max. Rate Norm. Rate = (A+D)[I2]/([I2] + [HI]kHl/k) 2

Therefore,

Norm. Rate Min. Rate ([12] + [HI]kHI/kl2)

Max. Rate Norm. Rate [12]

[HI]kHI

'[I2]k
2

and ([HI]/[]) () Norm. Rate Min. Rate
a2Norm H Max. Rate Norm. Rate

The subscript "Norm" has been used because the ratio of HI and 12 is for

the radiolysis experiment without additives.









Since HI and 12 are produced at a constant rate

d[HI]/dt t HI]
2]/dt Norm 21/ Norm

(d[HI HI] d[12
dt IdtNororm
Norm Norm Norm


Addition of equations 21 and 25

d[I2] l[HI]
t Norm d t Norm


Substitution

C = 1 I]
[2 ] N


= C + B (A+D)/2 + D[I2]/([12]


+ [HI]kHI/kI2)

of equations 31 and 36 in 37 and rearrangement yi


[2) (\d dt d[It
orm dt Norm Norm \ Min


(37)


elds


D[I2]/([2] + [HI]kHI/kl2)

or C = H- ) (Norm. Rate) + (Norm. Rate Min. Rate)
D[2Norm
D[I 2]/([ 12]+[HI]kHI /k l2)


(38)


For pure cyclohexane


The parameters needed for substitution in equations 26 and 27

for the radiolysis of pure cyclohexane with additives are A, D, and

kHI/k12 only. As already mentioned, the effective G value of scavenge-
-4
able H atoms may be considered to vary from zero at 10 M 12 or HI
-22
to a maximum of about 2 at 2 x 10-2 M scavenger concentration. However,

since sufficiently detailed data on effective hydrogen atom yields at

intermediate concentrations are not available, the hydrogen atom yields


(36)










in the present calculations have been obtained by using the D factor

in equations 26 and 27 as an adjustable parameter for curve fitting.

The sum (A + D) can be calculated from equation 33. Hence, the radical

yield A can be obtained by difference. The calculated effect of varying

the hydrogen atom yield in a typical experiment on the radiation-induced

uptake of iodine is shown in Figure 9. It is evident from the graph

that increasing G(H.) increases the end point of the experiment, i.e.,

the dose for complete removal of iodine. Figure 10 shows that

increasing G(H.) causes a similar increase in the end point for the

radiolysis experiment on pure cyclohexane with added HI. In Figures 9

and 10, the ratio of the rate constants kHI/kI2 was taken as 0.71,

which is the value giving the best fit of experimental data.

Although there is experimental evidence suggesting that kHI/k12

36
in non-polar solvents is of the order of unity, there were insufficient

data to assign an accurate value for the present experiments. There-

fore, the ratio kHI/k 2 has also been treated as an adjustable para-

meter in the calculations. The effect of changing this ratio is shown

in Figures 11 and 12. Figure 11 shows the calculated results for the

radiolysis experiments on pure cyclohexane with added iodine. It is

evident that changing the ratio kHI/kI2 modifies the curvature of a

graph of iodine concentration vs radiolysis time, but does not affect

the end point unless the ratio is zero. If the ratio is zero, the

products at the end point would include HI as well as alkyl iodides.

As long as kHI/k12 has a finite value all the iodine, whether present

as the chemical intermediate HI or as iodine, must ultimately appear

as alkyl iodides. For all finite values of the ratio kHI/k 2, the end
HI 2'3




























































































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point of the experiment is the same but the curvature increases as the

ratio decreases. However, the initial slope of all the curves remains

the same and approaches that for which the ratio is zero.

The calculated effect of varying the ratio kHI /k2 for an

experiment on the radiolysis of pure cyclohexane with added HI is shown

in Figure 12. Again in this case, variation of the value of the ratio

does not affect the end point of the radiolysis. Increasing the ratio

increases the maximum of the graph which is the maximum concentration

of iodine produced in the experiment and also increases the radiolysis

time corresponding to the maximum.


For cyclohexane-methyl iodide solutions

For solutions with 0 to 95 volume percent cyclohexane, the

parameters needed for substitution in equations 28 and 29 are A, B,

C, and kHI/k 2. Since the thermal hydrogen atom yield D is zero in

these solutions, the radical yield A is given by equation 33 as

A = Max. Rate Min. Rate (39)

B = (Max. Rate + Min. Rate)/2 (32)

C = ([HI]/[I])Norm (Norm. Rate) + (Norm. Rate Min. Rate) (40)

The different rates, namely Maximum, Minimum and Normal are the

rates found experimentally by taking the initial slopes of the graphs.

The value of ([HI]/[I2])Norm is found by expression 35. (Difficulties

caused by fall-off of the Normal Rates are discussed below in the

section entitled "Comparison with Experiments".) Substitution of the

values of various rates for a particular solution in equations 32, 39,










and 40 yields values of A, B, and C for the corresponding solution..

For very dilute solutions of methyl iodide in cyclohexane, B is given

directly and the sum of A and D is established by equation 33.

Resolution of A and D is discussed below.

Due to lack of experimental data on the ratios kHI/kI2 for the

series of solutions studied, kHI/k12 was treated as an adjustable

parameter in the calculations. The computed effect of varying the

ratio kHI/kI2 for the various solutions with added 12 is shown in

Figures 13 to 19. For the entire range of cyclohexane-methyl iodide

solutions, it is found that increasing the rate constant ratio increases

the iodine concentration achieved at a given radiolysis time. This

effect becomes more pronounced as the percent cyclohexane in the

solution is increased. However, the initial slope of the graphs is the

same in all cases and approaches that for which kHI/ki2 is zero.

Similar effects of varying the rate constant ratio are observed for

experiments with added HI and are shown in Figures 20 to 26. The

effect of varying the ratio in the case of 99.9 percent solutions is

quite similar to that observed for pure cyclohexane (see above).

For solutions from 0 to about 95 percent cyclohexane, that is,

for all cases in which there is a non-zero Normal Rate of iodine

production, the arbitrary assignment of the single parameter kHI/ki2

should suffice to fit the equations to experimental data. Under these

circumstances A and B come directly from experimental data, and C can

be calculated if kHI/kI2 is fixed. It has been found that adjusting

only kHI/kI2 gave a good fit of experimental data for solutions with

20 to 90 percent cyclohexane. However, this procedure gave a rather












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poor fit for 95 percent solution. It was found that the difficulty was

due to the parameter C for HI production. Results of calculations

using C as an adjustable parameter are shown in Figures 27 and 28. It

can be seen that the results for experiments with added 12 are very

sensitive to this parameter. The value predicted from equations 35

and 40 is 0.033 micromoles/min; the best fit of experimental data was

obtained when C was adjusted to 0.029. Since the calculation of C

from equations 35 and 40 is somewhat indirect and requires several

mathematical manipulations of the experimental data, the difference

between 0.029 and 0.033 is within experimental error. Hence, the

scheme can be considered to fit directly to.all solutions through 95

percent cyclohexane.

However, determination of the various parameters for solutions

with more than 95 percent cyclohexane involves some difficulties. For

these solutions, the normal rate of iodine production is zero. This

indicates that the yield of .radicals exceeds that of iodine, but does

not tell by how much. For the 0 to 95 percent concentration range,

four parameters A, B, C, and kHI/kI2 are needed and three experimental

data, Maximum, Minimum, and Normal Rates of iodine production are

available. Hence, only one parameter needs to be adjusted arbitrarily.

Above 95 percent cyclohexane, only two useful pieces of data, Maximum

and Minimum Rates of iodine production, are available and only two of

the four parameters, A and B are fixed. Hence, C as well as kHI/kI2

must be used as adjustable parameters. However, the situation is somewhat

simplified because kHI/kl2 was found to have the same value of 0.71

for 90 to 95 percent solutions and for pure cyclohexane, and it can









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67




6






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0 10 20 30 40 50
Radiolysis Time, min.

Figure 28. Role of HI production from spurs as an adjustable
parameter: 12 production in the radiolysis of cyclohexane-methyl iodide
solution, for 95 volume percent cyclohexane with added HI (6.1 x 10- M),
as a function of radiation time. G(HI) values, reading downwards, are
0.85 and 0.0.




68




reasonably be assumed to have the same value between 95 and 100 per-

cent cyclohexane solutions.

Another complication develops in solutions approaching pure

cyclohexane, exemplified by the 99.9 percent case. For such solutions,

it can no longer be assumed that the effective yield of thermal H atoms

is zero. For these solutions, B is determined from equation 32 and

kHI/kI2 can be taken as 0.71. The sum of A and D is determined from

equation 33, and C is undetermined. Hence, it is necessary to vary two

quantities, C and D, to fit the equations to experimental data. After

several computer calculations, it has been concluded that the sum of

C and D was rather well defined as 0.041 0.003 micromoles/min.

Considerably more variation is possible in the individual values. An

adequate fit of the experimental data could be obtained with values of

D in the range of 0.012 to 0.024 with C adjusted to maintain the sum

of C and D as constant. The effects of varying D for experiments with

added 12 and HI are shown in Figures 29 and 30 respectively. Similar

effects of varying C are shown in Figures 31 and 32. The combination

C = 0.023 and D = 0.020 was used to calculate the curves for 99.9 per-

cent solution in Figures 6 to 8. This set of values, with a rather

high H atom yield and low HI yield, gave a better fit at longer radiol-

ysis times than the reverse combination. However, this is probably

somewhat fortuitous. A lower H atom yield (G 0.5) and higher HI

yield (G 0.9) is more consistent with other work and the trends in

the present data. The major difficulty for the 99.9 percent solution

is probably due to the depletion of CH I during radiolysis; its initial
-2
concentration is only about 1.6 x 10- M. Complete consumption of HI

(and 12) as shown in Figure 8 required five hours of radiolysis at an





























































































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absorbed dose rate of about 0.5 megarad/hr, which would consume 25

percent of the methyl iodide present if G(- CH3I) 2.


Comparison with Experiments


Pure cyclohexane

Since varying the H atom yield affects the end point but the

ratio kHI/kI2 does not, it is fairly easy to adjust both of these

quantities to give the best fit of the experimental data. The results

of such calculations are shown by smooth curves in Figures 2 and 3;

the circles represent experimental data. Since the radiolysis tem-

perature (25 20C) was constant for all experiments and the solvent

was essentially pure cyclohexane, the ratio kHI/k 2 should be the

same in all cases.

Because of the competition between solvent cyclohexane and

added scavenger for H atoms, the D factor is expected to be dependent

on initial scavenger concentration. Since the scavenger concentra-

tion during each experiment decreases as the experiment progresses,

it is possible to account for scavenging of hydrogen atoms only semi-

quantitatively. However, the alkyl iodides produced during radiolysis

are also good hydrogen atom scavengers, so the change in total scavenger

concentration is never more than a factor of two.'39 The effective

hydrogen atom yields used in the calculations for Figures 2 and 3 are

listed in Table 3. It will be noted that a greater concentration of

HI than that of 12 is required to achieve a given value of G(H.),

implying that 12 is a somewhat better hydrogen atom scavenger than HI.
























o \D -

i-i -< i i i-


0 0 0 0




o4 m 0 C
Cuj (n ~ -:j


+


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c- CO -4

0 0 -1 -1


I


l-l ry l >-l (n
0 0 0 0






0 0 r
OO N


z
0
E-4




U
z
0
u



z






S0
z
0
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-o a
4-l



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0) *4


HO4

4-j

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04-




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As a further test of the proposed kinetic scheme, a series of

experiments, with both HI and 12 added initially, have been performed.

-3
Iodine concentration in all the .cases was 1.53 x 103 M but HI concen-

tration varied from 1.08 x 10-3 to 3.43 x 103 M. The experimental

results and the corresponding theoretical curves are shown in Figure 4.

As in the-previous cases, the value of kHI/kI2 was taken as 0.71 and

G(H-) as 1.50 was used for all the calculations illustrated in Figure 4.

The present kinetic analysis can be compared with the recent

37
work of Perner and Schuler by considering Figures 9 to 12. Starting

with essentially the same mechanism, they presented a simplified kinetic

scheme for radiolysis experiments for pure hydrocarbons with added HI.

They integrated the rate equations analytically, although indirectly.

Their analysis allows the variation of the parameter kHI/k 2 but does

not provide for direct allowance for reaction of H atoms with 12 pro-

ducing additional HI. However, the effect of the participation of H

atoms is clearly evident in their results as shown by deviations in

experimental data compared with predictions of their kinetic scheme.

Their analysis would give a curve identical to that for D = 0 in

Figure 10. The error in ignoring the H a'tom effect is rather large for

concentrations as great as 2 x 10-3 M (Figure 10). Perner and Schuler

worked mainly at much lower HI concentrations which minimized such

deviations. They did not treat the experiments with added 12. For such

experiments, the approximation that G(H-) = 0 gives merely a straight

line (Figures9 and 11). Their analysis of the effect of varying kHI/kI2

is qualitatively similar to that given here. However, their resulting

graphs differ slightly because of the allowance for the participation of

H atoms in the present work.










Cyclohexane-methyl iodide solutions


Input data for a computation on a given cyclohexane-methyl

iodide solution are Maximum, Minimum, and Normal Rates of iodine pro-

duction. An initial concentration of 12 or HI is given to the computer,

matching an actual experiment. The ratio kHI/kI2 is used as an adjust-

able parameter. The results of such calculations are shown by smooth

curves in Figures 6 to 8; circles represent experimental data. It can

be seen that the theoretical curves fit the experimental points quite

satisfactorily. From these curve fittings, different kHI/kl2 values

have been obtained for the various solutions. These values are listed

in Table 4. Initial 12 concentrations in the solutions were 1.4 x 103

to 1.7 x 10-3 M and the initial HI concentration was 6.1 x 103 M in

the experiments shown in Figure 7.

A difficulty which was encountered in fitting theoretical curves

to iodine production and consumption data requires comment. Some of the

iodine concentration vs dose graphs for radiolysis of HI or 12 in cyclo-

hexane-methyl iodide solutions cover several hours of radiolysis. As
20
mentioned above, Croft and Hanrahan2 reported that the rates of iodine

production in cyclohexane-methyl iodide solutions without additives

were nearly linear at low doses. For the longer radiolysis times used

in the present work, changes in the Normal Rates, as shown in Figure 5,

become significant. The decrease in G(12) from its initial value to

the value obtained after several hours of radiolysis was of the order

of 0.15 to 0.25 molecules/100 ev (Table 2). This is a relatively small

effect considering that total G values of 12 production in spurs were

in the range of 1.5 to 3.5 (Table 4). This fall-off cannot be due to






















CO O L0 r-4 -4 -4 -
ON ON 01 kp >-- -- L-- c-


S C 0 u \0 IO -

or- 0'0 0 0

















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SL o LC 4 -:1- CO L\ -=O :



0 0





W *-4 0 -4 0) N C -4 M0
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--f0--4 c t\ 0 CO 0 0 L 0 000


s- :j
o o *O

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0
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a,
00
u 0
c 3
uo
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-4


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vr: ~4:


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u














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78




HI-I2 competition kinetics as described above. Equations 21 and 25

unambiguously predict constant rates of formation of 12 and HI for the

radiolysis with no additives, provided only that the yields of primary

processes (A, B, C, and D) are constant. It is probable that HI and 12

are actually formed at constant rates but react with olefin produced

during extended periods of radiolysis.

Although the fall-off in Normal Rates was not considered deci-

sive in interpreting the role of HI-I2 competition kinetics in cyclo-

hexane-methyl iodide solutions, it did pose problems in several respects.

If the true initial values of the Normal Rates of Iodine production are

used in equations 21 and 25, then the equations will predict rates for

solutions with added HI and 12 which will approach these values at large

doses. In the actual radiolyzed solutions, however, lower rates are

found at large doses, as in the case of radiolysis with no additives.

As a result, the predicted yields from the integrated equations could

not match the experimental results. It was possible to avoid this

difficulty by utilizing the large-dose values of the Normal Rates in

the equations, rather than the initial values.

Although use of the long-term Normal Rates permitted more

satisfactory fitting of curves to experimental data, it could be con-

tended that the initial values are more significant in establishing

true yields of primary processes. After careful consideration, it was

decided, nevertheless, that the long-term values of the rates should be

used in preparing Tables 4 and 5 to avoid inconsistencies with other

parts of this dissertation. It can easily be verified that the G

values for the thermal-free radical yields and for the yields of 12

from spurs are not affected (see equations 32 and 39). However, the
















CO0 *
M -4
01 11

*U 3


o >0

'. >
U u 0


ON O 0 N \O -4 N \o
c 0 o O 00 (1J -O
J --I -- O OO


S- -- 0 -i n --
o:n 0n \ f 0= uO0 0 0 0 0
Omm OO O O O

0 Lo\ L\ LfN OD Lr\ Lr\ LC \LO \o




-a


01
-4 0OL 0 Lil _:t Lt' L- -=-t j co -:-
-c OZ-- C\J 1o LN MC' -4 -4 t- -
S4- (..

E-i ro .,-
0 - 0 o o.o Lo c-;t Ci

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*.- 01 CO

















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m r u Ci C\ Ci LO L NO O
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Ci 3 00


Csi O-400000
01 o -i j m c^ r -


(O)

0

-4 0 r-4
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>


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yields of HI from spurs as given in Tables 4 and 5 are under-estimated

by 0.1 to 0.25 molecules/100 ev for solutions with 20 to 90 volume

percent cyclohexane, and the total yields of scavenger equivalents are

under-estimated by about 0.2 to 0.5 molecules/100 ev.


Concentration Dependence of Primary Yields
in Cyclohexane-Methyl Iodide Solutions

For the entire range of solutions of cyclohexane-methyl iodide,

the present calculations provided data for the yield.of thermal alkyl

radicals, 12, and HI from spurs. These data are listed in Table 4. As

the cyclohexane concentration is increased, G(I2) decreases and G(HI)

increases fairly smoothly. However, G(R') does not show a clear trend

as a function of composition of mixtures; it is approximately constant

at 5.5 0.5. The fluctuations which are observed are probably due to

the fact that this is a composite quantity which includes the yields of

both cyclohexyl and methyl radicals. The net yields of each of them

and their sum are determined by numerous reactions, some competitive

and others additive, resulting in an approximately constant combined

yield.

It can be seen that the net, observed yield of iodine (col. 2,

Table 5) is much less than that produced from spurs (col. 3, Table 4).

The difference, of course, is due to reactions of cyclohexyl and methyl

radicals with 12. There is also a thermal process producing 12, namely

the reaction of radicals with HI, but this is a relatively small effect.

The same situation occurs in the radiolysis of pure alkyl iodides; it

has been shown previously 36 that in such systems the observed 12

yield is the sum of the excess of iodine production over alkyl radical










production in spurs plus the contribution due to the reaction of thermal-

free radicals with HI.

A discussion of trends in primary yields in the radiolysis of

cyclohexane-methyl iodide solutions was presented several years ago by
20
Croft and Hanrahan. Their remarks were of necessity hypothetical,

since detailed yield data were not available at that time. They inves-

tigated the postulate that the various intermediates, reacting in a

simple free radical scheme similar to that used here, might be formed

in yields determined directly by the electron percent of the parent

compound in the mixture. A graph was presented showing the concentra-

tion dependence of total radical yields and total iodine yields pre-

dicted by such a model. The graph predicts that net iodine production

should fall to zero for solutions richer in cyclohexane than about 30

electron percent. Since their experimental results showed that net

iodine production persists until methyl iodide is diluted with about

93 electron percent cyclohexane, they concluded that there was evidence

of substantial sensitized decomposition of the methyl iodide. This

conclusion has been confirmed during the course of the present work.

Taking into account the role of HI, G values for total scavenger equiv-

alents for the entire range of solutions have been calculated and' are

plotted as a function of electron percent cyclohexane; see Figure 33.

It will be noted that the yields of HI and 12 from methyl iodide in

the mixture are markedly greater than would be predicted by an "ideal

solution" law. There is only a small gradual dimunition in the total

scavenger yields up to about 80 electron percent solutions; the yields

fall sharply in solutions richer in cyclohexane than 80 electron percent:
















0

0





co







0
-4

-O X
0 0





.%
*,
o a









\O cA
Q
cE




X O~C
0 0O
i 00
o 0 0



o o.
0 m




0 0 o




0
0 OS

O 0
O)


0Q z Q)
-o c





,E o
b0 EQ)



O




o
cO






0
0 0






O ~0
SCU





(C




b. 0 0
*H e o
san3e 0, Q)I
cJ E<
0 O





83




Although the figure indicates that there should be no net iodine pro-

duction for solutions with more than 85 electron percent cyclohexane,

difference between this value and the experimentally observed value is

probably due to small errors in determining the various parameters.

In considering the relationship of the various HI rates given

in Tables 4 and 5, it should be recalled that the HI which is produced

from spurs has two possible fates. Most of it reacts with alkyl radi-

cals giving a complementary yield of iodine atoms. Therefore, the rate

at which HI plays the role of a scavenger for R. is given by (Norm.

Rate Min. Rate), where Norm. and Min. Rates are the rates of iodine

production without additives and with 12 added respectively. These

values are listed in column 3, Table 5. The remaining HI appears as

net HI accumulating in the solutions; see column 4, Table 5. The sum

of these figures is the total rate of HI production, given in column 4,

Table 4.

Since, on reaction with alkyl radicals, HI furnishes a hydrogen

atom to the radical and releases an iodine atom, the activity of HI is

equal to two scavenger equivalents. Hence, the total scavenger activity

in equivalents is the I- produced from spurs plus twice the HI that

reacts with alkyl radicals. Column 5 shows the total scavenger activity

in equivalents. Column 7 shows the scavenger equivalents which are in

excess of the free radicals produced, and is obtained by subtracting

column 6 from column 5. The figures given are just the sum of the 12

yield, which is measurable, and the HI yield, which is inferred. The

negative values for total scavenger yields for 95 and 99.9 volume percent

cyclohexane solutions simply indicate that there is no net HI or 12 pre-

sent in the irradiated solutions over this concentration range.









21
It has been proposed by Gillis, Williams and Hamill1 that the

production of iodine in the radiolysis of pure CH I is due to the ion-

molecule mechanism

CH + + CH I ---3 (CH3)21 + I. (41)


The product ion, upon neutralization, gives a net yield of C2H6 and 12.

Croft and Hanrahan suggested that the efficient production of iodine

in dilute solutions in cyclohexane is due to the same mechanism and

that it is able to occur with considerable efficiency because of charge

transfer from cyclohexane:

C6H12 + CH3I -- 6H12 + CH I (42)

The present calculations confirm these observations. The postulate

that charge exchange process 42 occurs appears necessary to account

for net iodine production in solutions in which methyl iodide is the

minor constituent. However, the possibility that electron capture
10
also occurs, as suggested by Forrestal and Hamill, cannot be excluded.

It is likely that electron capture and charge exchange both take place

in solutions with more than 90 electron percent cyclohexane.


Conclusion

The kinetic analysis given here accounts for competitive

reactions of radicals with 12 and HI in the radiolysis of pure cyclo-

hexane and cyclohexane-methyl iodide solutions over the whole concen-

tration range. A simplified model, involving an internally consistent

free radical mechanism, has been provided. The experimental quantities

are the initial rates of iodine production under three conditions:










with added 12, with added HI, and with no additives. The rate constant

ratio kHI /k2 has been used as an adjustable parameter in all cases.

For very dilute solutions of methyl iodide in cyclohexane, H atom yields

and HI production in spurs are also treated as adjustable parameters.

This scheme successfully approximates the complete shape of the

experimental curves by utilizing the three experimental rates, namely

Normal Rate with no additives, Minimum Rate with added 12, and Maximum

Rate with added HI. However, the necessity of using large-dose Normal

Rates has already been discussed. Analysis of experimental data

according to this scheme provides values for the rates of production of

thermal radicals, I and HI from spurs. Trends in these yields corrobo-

rate the conclusion that there is extensive energy or charge transfer

from cyclohexane to methyl iodide in the mixtures studied. This con-

clusion had been inferred earlier from data on iodine production
20
alone.

The deviations of the theoretical curves from the experimental

points are most pronounced towards the end of the experiments in all

cases. This is perhaps expected, since there are a number of further

complicating reactions intentionally ignored in the simplified scheme

given. These include reactions of H atoms with alkyl iodide products

in the case of pure cyclohexane, and reactions of HI, 12, and possibly

radicals with unsaturated radiolysis products in both pure cyclohexane

and the mixtures. It is possible that corrections for some further

reactions such as these could be incorporated in the kinetic scheme

and subsequent computer analysis. However, the scheme as given is

already rather complicated, and these further refinements would tend










to obscure its value as an aid in visualizing the kinetic behavior of

the system.

The type of kinetic scheme given here for HI-I2 competition

kinetics during radiolysis was first suggested several years ago for

pure alkyl iodides.2136 The present work shows that this approach is

useful also in pure cyclohexane with HI or 12 added as a scavenger, as

well as for the intermediate situation in which cyclohexane-methyl

iodide solutions are irradiated. During the course of the present

work, a parallel interpretation of the kinetics in pure hydrocarbons,

using an indirect analytical integration, was developed by Perner and

Schuler of the Mellon Institute. It has been scheduled for publication

simultaneously with the portion of the present work dealing with pure

cyclohexane.37h3

The present kinetic analysis obviously can be extended to the

radiolysis of other hydrocarbon-alkyl iodide solutions. The original

scheme was successfully applied to the photolysis of alkyl iodides by
h4
Luebbe and Willard, and it would be of interest to investigate

extension of the present analysis to the photolysis of hydrocarbon-

alkyl iodide mixtures.











BIBLIOGRAPHY


1. C. S. Schoepfle and C. H. Fellows, J. Ind. Eng. Chem., 23,
1396 (1931).

2. J. L. Magee and M. Burton, J. Am. Chem. Soc., 73, 523 (1951).

3. J. P. Manion and M. Burton, J. Phys. Chem., 56, 560 (1952).

4. A. C. Nixon and R. E. Thorpe, J. Chem. Phys., 28, 10004 (1958).

5. H. A. Dewhurst, J. Phys. Chem., 63, 813 (1959).

6. G. R. Freeman, J. Chem. Phys., 31, 71 (1960).

7. G. R. Freeman, Can. J. Chem., 38, 1043 (1960).

8. P. J. Dyne and J. W. Fletcher, ibid., 38, 851 (1960).

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(1961).
11. R. R. Williams and W. H. Hamill, Radiation Research, 1, 158 (1954).

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13. M. Burton, J. Chang, S. Lipsky and M. P. Reddy, Radiation
Research, 8, 203 (1958).

14. G. Meshitsuka and M. Burton, ibid., 10, 499(1959).

15. P. J. Dyne and W. M. Jenkinson, Can. J. Chem., 38, 539 (1960);
39, 2163 (1961).

16. P. J. Horner and A. J. Swallow, J. Phys. Chem., 6L, 953 (1961).

17. C. E. McCauley and R. H. Schuler, J. Am. Chem. Soc., 79, 4008
(1957).
18. R. H. Schuler, J. Phys. Chem., 60, 381 (1956).

19. R. W. Fessenden and R. H. Schuler, J. Am. Chem. Soc., 79, 273
(1957).
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87










21. H. A. Gillis, R. R. Williams and W. H. Hamill, J. Am. Chem. Soc.,
83, 17 (1961).

22. S. Z. Toma and W. H. Hamill, ibid., 86, 1478 (1964).

23. J. Roberts and W. H. Hamill, J. Phys. Chem., 6, 2446 (1963).

24. W. V. Dusen and W. H. Hamill, J. Am. Chem. Soc., 84, 3648
(1962).

25. B. M. Hughes and R. J. Hanrahan, J. Phys. Chem., 69, 2707 (1965).

26. R. J. Hanrahan, J. Chem. Ed., 41, 623 (1964).

27. R. J. Hanrahan, J. Appl. Radiation Isotopes, 13, 254 (1962).

28. R. J. Hanrahan, J. Chem. Ed., 40, 69 (1963).

29. R. H. Schuler, J. Phys. Chem., 62, 37 (1958).

30. G. Westmoreland, T. S. Croft, W. C. Blasky and R. J. Hanrahan,
Paper No. 119, Division of Physical Chemistry, American
Chemical Society National Meeting, September, 1961.

31. T. D. Nevitt and L. P. Remsberg, J. Phys. Chem., 64, 969 (1960).

32. P. J. Dyne, ibid., 66, 767 (1962).

33. J. W. Falconer and M. Burton, ibid., 67, 1743 (1963).

34. S. K. Ho and G. R. Freeman, ibid., 68, 2189 (1964).

35. J. R. Nash and W. H. Hamill, ibid., 66, 1097 (1962).

36. R. J. Hanrahan and J. E. Willard, J. Am. Chem. Soc., 79, 2434 (1957).

37. D. Perner and R. H. Schuler, J. Phys. Chem., 70, 0000 (1966).

38. J. M. McCormick and M. G. Salvodori, "Numerical Methods in Fortran,"
Prentice-Hall, Inc., Englewood Cliffs, N. J., 1964, pp.
253-255.

39. D. Perner and R. H. Schuler, J. Phys. Chem., 70, 317 (1966).

40. T. J. Hardwick, ibid., 66, 291, 2246 (1962).'

41. B. Smaller and M. S. Matheson, J. Chem. Phys., 28, 1169 (1958).

42. A. O. Allen, "The Radiation Chemistry of Water and Aqueous Solu-
tions," D. Van Nostrand Company, Inc., Princeton, N. J.,
1961, pp. 20-23.




89



43. I. Mani and R. J. Hanrahan, J. Phys. Chem., 70, 0000 (1966).

44. R. H. Luebbe, Jr. and J. E. Willard, J. Am. Chem. Soc., 81,
761 (1959).












APPENDIX


RADIOLYSIS KINETICS OF HYDROCARBON AND ALKYL IODIDE SOLUTION

C MAIN PROGRAM

15 READ INPUT TAPE 5,1,DT,XLAST,TIME,BII,AHI

1 FORMAT(F4.2,5X,F6.2,4X,F4.2,4X,F1O.5,5X,F1O.5)

READ INPUT TAPE 5,2,AMAX,AMIN,ANORM,RATIO

2 FORMAT(F10.5,5X,F10.5,5X,F1O.5,5X,F10.5)

READ INPUT TAPE 5,3,CONC

3 FORMAT(F4.1)

WRITE OUTPUT TAPE 6,4,CONC

4 FORMAT(F5.1,1X,35H PERCENT HYDROCARBON + ALKYL IODIDE)

WRITE OUTPUT TAPE 6,5,AMAX,AMIN,ANORM,RATIO

5 FORMAT(7H AMAX =,1X,F10.5,2X,6HAMIN =,1X,F10.5,2X,7HANORM =,1X,F1O.5

2X,7HRATIO =,1X,F10.5)

A=AMAX-AMIN

B=(AMAX+AMIN)*0.5

RNORM=(ANORM-AMIN)/((AMAX-ANORM)*RATIO)

C=(RNORM*ANORM)+(ANORM-AMIN)

WRITE OUTPUT TAPE 6,6,A,B,C

6 FORMAT(4H A =,1X,F10.5,5X,3HB =,1X,F10.5,5X,3HC =,1X,F1O.5)

WRITE OUTPUT TAPE 6,7

7 FORMAT(3X,5H TIME,6X,6HIODINE,6X,15HHYDROGEN IODIDE)

30 IF(MODF(TIME,2.0))12,10,12











10 WRITE OUPUT TAPE 6,8,TIME,BII,AHI

8 FORMAT(2X,F6.2,3X,F10.5,5X,F1O.5)

12 DELIIO=DT*P(A,B,RATIO,BII,AHI)

DELHIO=DT*Q(A,C,RATIO,BII,AHI)

DELIIT=DT*P(A,B,RATIO,BII+DELIIO/2,,AHI+DELHIO/2.)

DELHIT=DT*Q(A,C,RATIO,BII+DELIIO/2.,AHI+DELHIO/2.)

TIME=TIME+DT

BII=BII+DELIIT

AHI=AHI+DELHIT

IF(TIME-XLAST)30,30,20

20 GO TO 15

END


C P SUBPROGRAM

FUNCTION P(A,B,R,Y,Z)

P=B+A/2.-A*Y/(Y+R*Z)

RETURN

END


C Q SUBPROGRAM

FUNCTION Q(A,C,R,Y,Z)

Q=C-A+A*Y/(Y+R*Z)

RETURN

END














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