Reaction in monolayers of drying oils I - The oxidation of the maleic …rspa.royalsocietypublishing.org/content/royprsa/153/878/... · I— The Oxidation of the Maleic Anhydride

116

Reactions in Monolayers of Drying OilsI— The Oxidation of the Maleic Anhydride Compound

of p-ElaeostearinBy G. G ee and E ric K. R ideal, F.R.S., Laboratory of Colloid Science,

Cambridge

(. Received August 6, 1935)

Introduction

The mechanism by which a film of a drying oil becomes converted in the course of drying to a hard solid, and in particular the role played by oxygen, is still a matter requiring elucidation. The original hypothesis of a direct oxidation has been replaced by a process involving a primary oxidation followed by a polymerization of the oxidized body. Fahrion* and Ellist regarded the primary oxidation as the formation of hydroxy ketones which condense with the elimination of water. The work of Elm, I and especially of Morrell,§ suggests that the oxidation product which polymerizes is a peroxide, although the lability of this peroxide seems well established by the identification of dihydroxy and ketolbodies.

—CH—CH— —c= c- - C —CHOH1 1 ^ 1 - 1!

0 — 0 OH OH oStaudinger|| further considers the peroxide to be preceded by a less stable moloxide —CH—CH—, and evidence for the existence of some

\ /O6

such unstable body has been presented by Hilditch and Lea f in the oxidation of methyl oleate.

* ‘ Z. angew, Chem.,’ vol. 23, p. 722 (1910).t ‘ J. Soc. Chem. Ind.,’ vol. 44, pp. 401T, 463, 409, 486 (1925); vol. 45, p. 193T

(1926).t4 Ind. Eng. Chem.,’ vol. 23, p. 881 (1931).

§ Morrell, 4 Ind. Chem.,’ vol. 1, p. 68 (1925); 4 J. Oil. Col. Chem. Ass.,’ voi. 12, p. 183 (1929); vol. 13, p. 84 (1930); 4 J. Soc. Chem. Ind.,’ vol. 50, p. 27T (1931); vol. 52, p. I30T (1933); 44 Chemistry of Drying Oils.”

|| 4 Ber. deuts. chem. Ges.,’ vol. 58B, p. 1075 (1925).If 4 J. Chem. Soc.,’ p. 1576 (1928).

on May 31, 2018http://rspa.royalsocietypublishing.org/Downloaded from

http://rspa.royalsocietypublishing.org/

Reactions in Monolayers o f Drying Oils 117

Opinions differ as to the nature of the polymer. This is regarded by Elm as an associated compound, whereas Morrell and Staudinger regard it as a true polymeride or macromolecule. The further possibility of polymerization of the more stable oxidized forms of the glyceride is not to be overlooked, however, for Petrov and Kasterina* have shown that linseed oil oxidized with potassium chromate dries at substantially the same rate as a “ blown ” oil, although in the former case the product is mainly the dihydroxide. These reviews do not take into account the possibilities of the polymerization of the unoxidized glyceride, although it is clear that this is the main reaction in the process of “ heat bodying ” of an oil. Some workers, notably Scheifelef and Scheiber,! consider that in low-temperature drying the role of oxygen is limited to the formation of a polymerization catalyst, while Stephens§ and others|| regard • both oxidation and polymerization as chain reactions, both taking a place in the finished product. The various studies of the reaction kinetics of the oxidation process for the oil in bulk or when spread as a thin layer on various surfaces have led to somewhat conflicting results; the total quantity of oxygen absorbed appears to vary considerably, probably on account of the variation in the magnitude of the disruptive oxidation which accompanies such bulk oxidation. The observation of a period of induction at the commencement of the reaction has led most workers to regard the reaction as auto-catalytic ( e.g., Morrell), but the curvesgiven by Long^j represent a pseudo-unimolecular reaction after a short period of induction, and Coffey** obtained a similar result for linoleic acid. The cause of the induction period has been ascribed by Hilditchjf to the presence of natural anti-oxidants, for freshly distilled simple esters do not exhibit this phenomenon.

It appeared possible that some further insight into the mechanism of what is evidently a series of complex reactions might be gained by carrying * * * § **

* ‘ Masloboino Zhirovoe Delo,’ vol. 10, p. 30 (1931).t ‘ Farben. Z.,’ vol 33, p. 739 (1927); ‘ Z. angew. Chem.,’ vol. 42, p. 787 (1929).+ ‘ Z. angew. Chem.,’ vol. 40, p. 1279 (1927); ‘ Farbe u. Lack,’ vol. 33, p. 518 (1928);

vol. 34, p. 284 (1929); vol. 34, p. 477 (1929); vol. 36, p. 511 (1931).§ ‘ Ind. Eng. Chem.,’ vol. 24, p. 918 (1932).|| Eibner, ‘ Z. angew. Chem.,’ vol. 39, p. 38 (1926); ‘Chem. Umsch. Fette,’ vol. 33,

pp. 188, 201, 213 (1926); vol. 34, pp. 89, 101 (1927); vol. 35, p. 241 (1928); Miller and Claxton, ‘ Ind. Eng. Chem.,’ vol. 20, p. 43 (1928).

If ‘ Ind. Eng. Chem.,’ vol. 21, p. 950 (1929); vol. 23, p. 786 (1931); vol. 25, p. 1086 (1933).

** ‘ J. Chem. Soc.,’ pp. 1152, 1408 (1921); p. 18 (1922); ‘ J. Oil. Col. Chem. Ass.,’ vol. 6, p. 2 (1923).

t f ‘ J. Soc. Chem. Ind.,’ vol. 51, pp. 39T, 41 IT (1932).



118 G. Gee and E. K. Rideal

out the reaction in monolayers on the surface of water by the methods outlined in previous communications, especially as Long has shown that such monolayers can readily be formed with linseed oil.

Experimental

The apparatus designed to permit of simultaneous measurements of the surface pressure and phase boundary potential followed closely that described in previous communications, but was improved by substitution for the polonium source of a thin glass tube containing 1-0 mg mesothorium (attached to the air electrode) and a valve potentiometer based upon a circuit described by Compton and Haring* in place of the Lindemann electrometer. Such a system was found to possess several advantages in speed and flexibility of action over the previous design. The trough was enclosed in a metal-covered wooden thermostat, and the electrode for exploring the phase boundary potential, the glass slide for compressing the film, and the torsion head of the Langmuir trough could all be operated from outside.

The torsion wire employed gave a sensitivity of about 10° per dyne per cm, and film pressures could be measured with an accuracy of about 0 • 2 dyne per cm. The material investigated was the maleic anhydride compound of (3-elaeostearin described by Morrell. We are indebted both to Dr. Morrell and to the British Dyestuffs Corporation for preparations which gave similar results. The [i-elaeostearin preparation in the latter case was obtained from tung oil, and recrystallized from dry acetone as far as possible in an atmosphere of carbon dioxide (m.p. 60-60 • 5° C). The maleic anhydride was prepared by Mason’s method, the product being fractionated under reduced pressure and recrystallized from chloroform (m.p. 53-53 • 5° C). The condensation was carried out as described by Morrell.

It was discovered at once that a monolayer spread from petrol ether on dilute acid (N/100 HC1 or H2S04) was unstable, and underwent expansion when maintained at constant pressure, but fairly consistent force-area and phase boundary potential curves could be obtained by rapid examination. On 0*1% hydroquinone the films were found to be perfectly stable and the values obtained agreed with those obtained without the inhibitor.

The force-area and potential curves are shown in fig. 1 together with

* ‘ Trans. Electrochem. Soc.,’ vol. 62, p. 345 (1932).




the mean value of the vertical component of the effective molecular dipole moment derived with the aid of the equation

AY = 4rc« jjl.

The information provided by these curves serves to interpret the effect of compression of the film on the molecular orientation. Several structures have been proposed for the maleic anhydride compound of (3-elaeostearin. Our results are most readily interpreted on the structure given by Morrell, which can be represented as a cis-glyceride, fig. 2.

420 32

380 24

340 S 16

200 280 Area (AVmol)

Fig. 1—F — AV — jx — A curves of maleic anhydride p-elaeostearin on N/100h 2s o 4.

The effective width of the tilted double ring system is found to be about 6-5 A. Hence if these are just in close contact at A, the point on the compression curve where the pressure becomes sensible is :

For the area as far as the doublebonds ................................ 19*5 X \ (10 + 3 x 6*5) = 287 • 5 A2

For the terminal C4H9 chains.. 5 X 19-5 = 97-5 A-

Total ..................................................................... 385 A2

which is very close to the limiting area observed experimentally, viz., 380 A2.




On compression of the chains the molecule may be regarded as passingthrough the following stages:

“ I

R

/\/

1Area calc. — 385 A2 Area obs. ~ 380 A2 F obs. = 0-2 dynes/cm g obs. = 25-4 x 10“19 e.s.u.

IIArea calc. = 290 A Area obs. = 300 A F obs. = 2-9 dynes/cm g. obs. = 24-6 x 10-19 e.s.u.

Ill IVThis position does not corre- Area at final collapse

spond with a sharply defined point Area calc. = 110 A2 (ring system) on the curve. Area at F = 12 is 80 A2

Area extrapolated to F = 0 is 120 A2

(x obs. — 8-2 x 10-19 e.s.u.

During the final stage the double bond leaves the surface and a marked fall in the reaction velocity occurs.

The polar portions of the molecule consist of the glyceride molecule in R, the anhydride ring system Q, and the double bonds in Q and P. The value of [x obtained for a typical glyceride such as tripalmitin is [x = 10-0 x 10~19 e.s.u., while that for the ring system can be evaluated from the known value for an etheric oxygen of (x = 2-5 X 10~19 e.s.u., and of a ketonic oxygen jx — 3-0 x 10"19 e.s.u. In the anhydride ring the ketonic oxygen atoms form an angle of approximately

CH—CH

O O O




18° with the horizontal. Hence the contribution to the total vertical component of the dipole system due to the three rings in the glyceride molecule is

[a = 3 x {2-5 + 2 x 3 sin 18°} 10“19 = 13 x 10~19 e.s.u.

Each double bond in the system contributes some 0-3 x 10 19 e.s.u. (e.g., jjl of myristic acid 1-8 x 10-19, oleic acid 2 1 x 10~19 e.s.u.). Hence the six double bonds in the molecule contribute 1 - 8 x 10~19 e.s.u.

19-5A.

19-5 At

We thus obtain for the total value of the vertical components of the effective electric moments per molecule of the trigylceride the following values:

(j. x 10-19 e.s.u.The glyceride group................. 10 0Ring system ............................ 13-0Double bonds ........................ 1-8

Total, fi. = 24-8 x 10~19 e.s.u.

a value to be compared with that experimentally determined of 25-4 x 10~19 e.s.u.




On submergence of the glyceride (position III) the value of the moment falls to about 15 X 10“19 e.s.u. and finally, on compression to the erect state the moment might be expected to fall still further, a limiting value of [x = 8-2 x 10“19 being actually obtained.

It may be concluded that both the limiting areas and phase boundary potentials support the structural formula proposed by Morrell for this compound, and further support is obtained from the changes in these molecular properties on compression of the film.

ju at F- 7

at F=8

A at F-10

Time (min.)Fig. 3—Rate of oxidation on N /100 H2S 0 4.

In fig. 3 are presented a series of curves showing the increase of area at several surface pressures on N /100 H 2S04 solution at 287° K. It is evident that the reaction velocity begins to fall off markedly at high pressures. The reactive groups in the molecule are the double bonds in each chain remote from the glyceride group, and the interpretation already given of the force area curve is consistent with the view that this bond is being removed from the surface at high pressures, the reaction velocity thus falling as in the compression of films of oleic acid examined by




Hughes and Rideal.* The reaction thus apparently involves the double bond, and that the oxidation of this glyceride is indeed accompanied by expansion of the film was shown by spreading the film on 0-001% solution of KM n04. On this substrate the areas measured initially were somewhat higher than those of the unoxidized material, but fell rapidly and continuously practically to zero. It is clear that the primary oxidation is almost instantaneous but is followed by a disruptive oxidation leading to soluble products.

Further evidence that the reaction on dilute acids is an oxidation process is provided by the fact that like films of a- and (3-elaeostearic acids the reaction is completely stopped by the presence of 0-1% hydroquinone in the substrate; whilst the reaction can be accelerated by the salts of the heavy metals such as cobalt sulphate which are used as oxidation catalysts.

The reaction at constant pressure is accompanied not only by an increase in area but also by a rise in the vertical component of the effective dipole moment. This quantity is calculated from the measured values of the phase boundary potential, and the compression by the Helmholtz equation AV = Aizn\i. The variation of g. with time is also given in fig. 3.

Owing to the fact that the variation in the area as oxidation proceeds is greater than the change in the phase boundary potential, the former has generally been used to calculate the velocity and order of the reaction, although experimentally it was found that both methods of computation gave the same values. The reactions have been followed at constant pressure to preserve identical molecular configuration during the run.

Some typical results of plotting dA/dt against the area are shown in fig. 4.

It is clear that the reaction velocity is extremely sensitive to variations in pressure. At high pressures, we note that the curves are unimolecular over the greater part of their course with deviations both at the commencement and end of the reaction, the causes of which will be discussed later.

The temperature coefficient of the reaction was determined by carrying out runs at 275-5° K and 287° K. Some of the data obtained are given in Table I, yielding a value for the energy of activation of the reaction of E = 6500 cals.

With another sample of material the values obtained at F = 8 were: at 275° K, k — 0-49 min-1, at 303° K, 1-5 min-1, giving a value again of E = 6500 cals. In fig. 5 are plotted the mean values of the

* ‘ Proc. Roy. Soc.,’ A, vol. 140, p. 253 (1933).




velocity constant in reciprocal minutes reduced in all cases to that at 275° K as a function of the two-dimensional pressure F.

T able I k in min 1

F dynes/cm At 275 • 5° K At 287° K k 287/k 275 • 58 0-34 0-53 1-67 0-67 1-04 1-6

Fig. 4 Velocity constants for oxidation stage, N/100 H2S 0 4, 290-5° K.

In addition to this change in the velocity constant as a function of F another effect of pressure is evident; the total increase in area at high pressures is greater than at low, and similar differences are found in thevalues of the dipole moment. Typical values are shown in Tables II and III.

It should be noted that the areas and moments given in Table II refer to the pressure of the experiment: an area of 130 A2 at F = 10 is approximately equivalent to 160 A2 at F = 6.

It must be concluded that the reaction product varies with the reaction pressure, a high pressure giving a product of high area and low moment.




F (dynes/cm)Fig. 5—Effect of pressure on oxidation velocity constant, N/100 H 2S 0 4, 275° K.

Table II—Final A reas (A ^ ) and Moments ( y. ^ . ) Obtained by Extrapolation of the Linear Portion of and (a)Curves

Oxidation pressureA qc in A2 at oc x 1019 at

-—F in dynes/cm 275-5° K 287° K 275-5° K 287° K

to 119 130 10-59 119-5 128 10-55 10-58 122 124-5 11-47 119-5 124-5 12-06 127-5 124-5 12-6

Table III—Film Characteristics after Oxidation at D ifferent

Pressures

F in dynes/cmA in A2 after oxidation at [x x 1019 after oxidation at

F = 5 F = 10 iiPL, F = 105 157 173 14-15 12-86 142 157 13-25 11-87 132 146 12-5 11-28 129 140 11-8 10-859 110 133-5 10-55 10-45

10 106 128-5 10-15 10-15




This variability clearly suggests that the primary oxidation product is not stable but can undergo a subsequent reaction or reactions, the extent of which depends on the pressure. Provided such reaction is rapid compared with the primary oxidation, the reaction would remain pseudo- unimolecular except that the initial velocity would be lower than that required by the unimolecular law. Inspection of the curves indicates that this behaviour is actually observed.

From the bulk properties of drying oils it seems likely that the primary peroxide is capable of undergoing at least two types of reaction, isomerization and polymerization, so that we may represent the total reaction schematically as follows:

Polymer/>

X -* XO'2

xo2where X is the initial unoxidized material, XO'2 the unstable primary peroxide, and X 0 2 the isomer.*

We might anticipate that the rate of polymerization should be pressure sensitive but that the rate of isomerization might be independent of the pressure. The observed variation of the oxidation product with the conditions of reaction is thus understandable. Further, at low pressures the rate of polymerization may fall to values commensuiate with the rate of oxidation and the reaction will no longer be pseudo-unimolecular; this behaviour is already evident at pressures as low as F = 6 dynes/cm. Finally, if we carry out the oxidation at low enough pressures, the polymerization should become negligible and it will be possible to produce only the isomer. The film, however, is no longer homogeneous when expanded to such a high degree and the pressure is too small to measure; but the reaction can be studied by compression of the film at intervals to measure either the area or the potential under a standard pressure (F = 8 dynes/cm was actually employed). As soon as the film, consisting of a mixture of XO'2 and XOa, is compressed the XO'2 commences to polymerize, necessitating rapid measurement and forming a fresh film for each determination. At these large areas the oxidation proceeds extremely rapidly so it is possible to construct a curve giving the rate of conversion of the unstable peroxide XO'2 into the stable isomer X 0 2. The results are shown in fig. 6.

[Note added in proof, October 15,1935—An alternative possibility is that the peroxide may loose oxygen with the formation of a monoxide XO. This reaction has been shown to occur in bulk (Morrell, private communication); the present experiments do not permit of a decision between the two possibilities.]



Two series of measurements are given for readings at 295° K and 303° K. The experimental points at 295° K obey a unimolecular law accurately; the results at the higher temperature, while not so reproducible, are well


303°K

Time (min.) — >Fig. 6—Rate of isomerization (X O / -> X 0 2) N/100 H2S 04.

Fig. 7—F-A curves of X 0 2 and XO,' on N/100 H,SO

represented by the curve drawn. The velocity constants are Ar295 = O'089 min-1, A303 = 0-21 min-1, and the calculated energy of activation is E = 19,000 cal/mol. In fig. 7 are shown the force area curves for the



128 Reactions in Monolayers o f Drying Oils

stable isomer of the unstable peroxide. The force area curves for the latter, on account of its reactivity, are extremely difficult to determine, but a few points which appear to be reliable are plotted in the same figure. Comparison of this curve with the one for the unoxidized material shows that there is no appreciable change in area involved in the primary oxidation. Further examination of the properties of the stable isomer of the peroxide revealed the fact that it could slowly undergo a reaction like the unstable peroxide to form apparently a similar polymer, the kinetics of which will be discussed in the next section.

We may conclude that the mechanism of “ drying ” of the maleic anhydride compound of p-elaeostearin in the form of a monolayer on dilute acid may be represented by the following series of changes,

Polymer

X —- XO'a^l*.

XO,

and we have already established the fact that the reactions denoted by k\ and k 3 are unimolecular with energies of activation of 6500 cal/'mol and 19,000 cal/mol respectively.



Documents

Reaction in monolayers of drying oils I - The oxidation of the maleic …rspa.royalsocietypublishing.org/content/royprsa/153/878/... · I— The Oxidation of the Maleic Anhydride