I N D U S T R I A L A N D ENGIXEERING CHEA!lIXTRY
June, 1924
627
T h e Chemistry of t h e Rubber Hydrocarbon’ By Harry L. Fisher THE B. F. GOODRICH CO., AKRON,OHIO
HE word “rubber” has
ing point or crystalline strucThe rubber hydrocarbon consists of many C5HS groups with a ture, and when distilled, esiraried meanings. It double bond in each group, and if we accept Harries’ latest thought, pecially in a vacuum of 0.1 may mean simply a these large molecules are joined together-aggregated-into larger mm., it decomposes, giving material that is used for rubmasses or aggregates which have a diminished chemical activity, and off highly unsaturated hydrobing out pencil marks, from that the relative size of these masses and the chemical activity may carbons such as isoprene (3.1 which use it originally obbe varied by mechanical working and by chemical reagents. The per cent), dipentene (8.8per tained its name-and in this structuralformula may be written cent), a C I ~ H Z compound ~ connection I cannot help reCHs with two double bonds and peating a n excerpt recently I two rings (4.4 per cent), and quoted from Dickens: “The (-CH~--C=CH-CHZ--), similar compounds of higher unwontc d lines on Mr. PickInside the brackets it is clear, but outside there is a little x. That molecular weight, and leaving wick’s face melted away like tells us how much is not known about the chemistry of the rubber a residue of 36.5 per cent, the lines of black lead pencil hydrocarbon! I f we can some day follow that line of connected which material is about onebefore the softening influence isoprene groups until we reach the “free” ends, we may then, like half as unsaturated as rubber of India rubber.” Rubber following the rainbow in fairy lore, hope to find a scientific as well itself. Since, as will be taken may mean the crude rubber as an industrial “pot of gold.” up later, the rubber hydrofrom the tropical forests of carbon or substances very South America or from the plantations of the Far East, or, more usually, the vulcanized much like it can be synthesized from isoprene, the question might be raised-why is so small a quantity of isoprene obtained in this product in its variety of forms. I n common parlance it may mean decomposition? This is because the decomposition temperaa n overshoe or even a game of bridge. S o w , crude rubber is a mixture of several kinds of substances, ture is high and isoprene is very sensitive to heat and polymerizes to form products such as dipentene, etc. a hydrocarbon to the extent of about 90 per cent, and the remaining 10 per cent of resins, resin acids, proteins and their deCHEMICAL REACTIONS AND DERIVATIVES composi (.ion products, cyclic sugars, inorganic salts, and moisThe chief chemical reaction of the rubber hydrocarbon is additure. 2 he chemistry of crude rubber is therefore not only the tion. I n other words, the compound is unsaturated, and a study chemistry of its chief constituent, the hydrocarbon, but also of these other substances. From the manufacturer’s standpoint of the products leads to the conclusion t h a t there is one double the chemistry of these other substances is likewise important, bond for each CjHs group. (Gladstone and HibbertSfrom meassince they have a considerable influence on the chief chemical urements of the refractive index of purified rubber in solution calreaction in the manufacturing process-namely, vulcanization. culated that there were l’/~double bonds to each CsH8 grouping, They also have much t o do with the mechanical working of rub- and therefore concluded “that the main constituent of caoutchouc ber and with the aging of rubber articles. It is, however, the is a compound which for CloHl8 has 3 pair of carbon atoms doubly hydrocarbon to which rubber owes its characteristic properties, linked.” Weber based his work on the CloHls nucleus largely beand it is the chemistry of this hydrocarbon-the rubber hydro- cause of its relation to the terpenes, and Harries also used the same carbon-which interests us here. This hydrocarbon goes by no nucleus on account of his early study of the “diozonide.” Later scientific name. Weber called it “polyprene.” Other authors work, as will be brought out, shows that the C5Hs nucleus is the have used the word “caoutchouc,” but since caoutchouc is the best working hypothesis.) It adds the halogens, the hydrogen French word for crude rubber, its use leads to ambiguity. I n halides, sulfur, the sulfur chlorides, oxygen as such and as ozone, this discussion the context will show when the word “rubber” and finally, and only recently published, it adds hydrogen. The stands for the rubber hydrocarbon. best results have been obtained in solution. The addition products, like rubber itself, are amorphous substances of a high ISOLATION AND PROPERTIES molecular weight, and when soluble form colloidal solutions. It is possible to obtain the rubber hydrocarbon in what is be- Some are unstable and some oxidize in the air. Since they do not lieved to be a pure condition, by extraction of the resins with crystallize and cannot be distilled without decomposition, they acetone, by dissolving the extracted rubber in benzene, decanting are difficult to purify, but when purified as much as possible they from the insoluble material, and precipitating by pouring the show a definite chemical composition; on this account it is assolution k t o alcohol, and repeating this solution in benzene and sumed that the rubber hydrocarbon from which they are prepared precipitation with alcohol several times. When these processes is a chemical individual, or consists of complexes made up of this are carried out in the absence of air, as was recently done by Pum- chemical individual, such complexes acting like the chemical merer and Burkard,2 the analytical results on the product are individual itself. better than any previously obtained and show that it is a pure THEHALOGEN DERIVATIVES hydrocarbon of the empirical formula CjHs. It gives a colloidal The halogens all react with the rubber hydrocarbon. Chlorine solution in benzene, gasoline, chloroform, carbon bisulfide, etc., and since its molecular weight is unknown, the formula can be gives a white product in which substitution as well as addition takes place. Substitution is the chief reaction in the early written (C&H*)=, where the value of x is very high but not yet determined. I n the pure state it is much more susceptible to oxi- stages, as recently shown by McGavack4in a study of the effluent dation by the air than in its crude form. It has no constant melt- gases by a n ingenious use of flowmeters. The product appears to be a mixture of the approximate formula (CloHlsClr),.
T
1 An address delivered before the New York Section of the American Chemical Society, December 14, 1923. * Rw., 66, 3458 (1922); C. A . , 17, 898 (1923).
a 4
J. Chem. SOC. (London), 53, 679 (18881. THIS JOURNAL, 16, 961 (1923).
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Bromine acts similarly to chlorine, but in cold, very dilute solutions it gives an addition product (CrHsBrz),. The formula used to be written CloHleBrr,and therefore the compound has been known as “rubber tetrabromide.” I t s formation has been employed for some time for estimating the rubber hydrocarbon. Iodine gives various unstable products of no definite composition. HYDROHALIDES Hydrogen chloride, hydrogen bromide, and hydrogen iodide add to form definite addition products: (CjHgCl),, (C6HgBr)z,and (C~HQI),,or, as written on the CIO- basis, (CloHlsClz),, (C10H18BrZ)zrand (CloHlsI,),, whence the names “rubber dihydrochloride,” etc. Harries and his students have done considerable work on the hydrochloride, not only in the study of its formation, but also of its properties, especially the removal of hydrochloric acid with pyridine under pressure. The rubber-like product obtained, known as “alpha-iso-rubber,” had different physical properties, and the results of ozonization were also different, wherefore it is isomeric with, but not the same as, the natural hydrocarbon, the chlorine atoms evidently being removed with hydrogen atoms other than those originally added as hydrogen chloride.
SULFUR Sulfur adds with the formation of ( C ~ H S Swhich ) ~ , was isolated by Hinrichsen and Kindscher5 as a brown, insoluble powder. Hard rubber probably contains it, possibly as the chief constituent in most cases. It does not add bromine and therefore may be considered a saturated compound. Ordinary (soft) vulcanized rubber may or may not contain this rubber sulfide. The incorporation of purified rubber sulfide in crude rubber and then heating the mixture does not give a product resembling vulcanized rubber in any way. No one has ever isolated any rubber sulfide from ordinary vulcanized rubber. Soft vulcanized rubber contains most of the sulfur in a chemically combined condition, a small proportion being in simplesolution and removable by extraction with acetone. When the extracted material is treated with bromine, the amount of bromine t h a t adds is equivalent to the amount of sulfur that would be taken up to form the saturated compound (C6H8S),.
Vol. 16, No. 6
since in the investigations only single experiments seem to have been carried out, these formulas have not yet been completely substantiated. It has, been demonstrated that peroxides are formed during the reaction and therefore the reaction is one of autoxidation. It is much faster with purified rubber, as already mentioned, than with crude rubber. It is interesting to note t h a t when air is passed into a benzene solution of rubber the viscosity of the solution drops considerably. Similar changes have possibly often been misinterpreted, especially when solutions have been heated in the presence of air, since they have been ascribed t o depolymerization of the rubber complex, whereas they seem to be almost entirely, if not entirely, due to oxidation. If air is excluded the change in viscosity is reversible.2 Recently, excellent results have been obtained in this field by Pummerer and Burkard,2 who found that the amount of oxygen gas absorbed by the rubber hydrocarbon in very dilute solutions a t room temperature agrees very well with the formula (C5HsO),. The result was the same with or without platinum black, and the absorption was complete in 40 to 50 hours. The products were not isolated. They also found that perbenzoic acid, C&COOOH, reacted normally toward the rubber hydrocarbon, one mol being used for each C5Hs nucleus. The course of this reaction was followed by titration of the reagent and verified by the isolation of the rubber oxide formed. It is a white, tough substance, much less elastic than rubber, and insoluble in all the ordinary solvents. Their analysis is in excellent agreement with the empirical formula (C6H&),, which fact shows very well t h a t the fundamental nucleus in rubber is C6Hs. The rubber hydrocarbon is fairly stable toward potassium permanganate, and the use of this reagent in Baeyer’s simple test for the double bond is not practicable. By prolonged action, oxidation does take place, but no definite products have been isolated. Rubber t h a t has been heavily masticated or milled is more easily attacked by permanganate than the unmasticated rubber.9 Hydrogen peroxide also slowly oxidizes rubber.
OZONE AND THE STRUCTURE O F THE RUBBERHYDROCARBON The most notable work on the addition products of the rubber hydrocarbon and the most important in the chemistry of rubber is that of Harries on the action d ozone. Like other unsaturated hydrocarbons, rubber forms an ozonide. It is a glassy solid, SULFUR MONOCHLORIDE melting a t about 50” C., is very explosive, and is saturated Sulfur monochloride in small proportions a t the ordinary toward bromine. The yield is approximately quantitative, and temperature vulcanizes rubber. It is used in the so-called the elementary analysis corresponds definitely with the ratio C6H803, or one mol of ozone to each CsH8 group. Molecular “acid,” “vapor” cure, or “cold vulcanization” processes. When an excess of the theoretical amount is used, there is obtained, weight determinations by the freezing point method in glacial according to Weber, (CsH&Cl), or, as expressed by him,6CIOHIGSZ-acetic acid were close to the calculated value for (C6&.0& or ClZ; whereas, according to H i n r i c h ~ e n ,the ~ , ~product is (ClOH16)Z- CloHl&& Harries therefore concluded that the fundamental SzClz or (CIOHI&C~)~, and Hinrichsen’s work has been verified by nucleus in the rubber hydrocarbon must be C10H16. In a previous series of classic researches he had shown that the position of Bernstein.8 The sulfur chloride compoundmay betheend product of vulcanization with this reagent, although it has never been the double bond in olefin hydrocarbons could be determined by the study of the decomposition products formed by the action of isolated from such vulcanized rubber or from rubber gels prepared with the same reagent. When heated with an alcoholic solution water on the ozonides, and by using the same method on the rubof sodium hydroxide, all the chlorine is removed along with the ber ozonide he obtained levulinic aldehyde, CHBCOCHZCHZCHO, equivalent amount of hydrogen and there remains a substance levulinic acid, CH&OCHzCHzCOOH, and a very small amount of levulinic aldehyde peroxide, besides the usual by-product, with the approximate formula (C10H16)& or ( C ~ O H I ~ S ) ~ . hydrogen peroxide. No other aldehydic or ketonic compounds OXYGEN were found, although the experiments were repeated many times. CHs Oxygen, when pure or as in the air, attacks rubber both in and I out of solution, especially in the presence of light, and slowly changes it to resin-like substances. Some of these products have been studied and formulas such as C10H160 and CloHleOa HzCI CH2 I have been assigned to them. Since the analytical results reported I I are not so close t o those required by the theory as is desirable, and HC%/CH2 6 Be?., 46, 1291 (1913). I e, “The Chemistry of India Rubber,” p. 97. CHa 7 Kolloid-2.. 6 , 202 (1910). 8
I b i d . , 11, 185 (1912).
@
Harries, Ann., 406, 198 (1914).
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June, 1924
629
previous based largely on the molecular weight of cauprene chloFrom these data he built up the foregoing possible structural unit 1,5-dimethyl-cyclo-octadi- ride, the polymerized product of vinyl chloride (CH2=CHCI).lZ of the rubber hydrocarbon-viz., ene-1,5. He considered t h a t this unit was polymerized into the HYDROGEN form in which rubber is known. Mention should also be made of the fact that when impure ozone was used he obtained a so-called The chemical reactions of the rubber hydrocarbon all indicated “dioxozonide,” C10H1808, and that this also on hydrolysis gave the the presence of double bonds in its structure, and yet until very same products as the “diozonide,” although in different quantirecently one of the chief reactions of such an unsaturated comties. pound, hydrogenation, had not been realized. Harries and Later Harries found that the molecular weight of the ozonide others had tried to hydrogenate it, but without success. Doubt in benzene corresponded t o that of a larger molecule ( C S H ~ O ~ ) ~ was cast upon the presence of the unsaturated linking familiar or C2~,H40016. Furthermore, ozonization of the alpha-iso-rubber to us all in olefin hydrocarbons, and one author a little over a obtained by the removal of hydrogen chloride with pyridine under year ago placed himself on record against this hypothesis. H e pressure, from the rubber hydrochloride, as given above, gave the stated: “It is conceivable that the rubber molecule itself contains following hydrolytic products : no double bonds whatever, and that these are only produced by the breaking up of the complex rubber molecule by the action of bromine or hydr,ochloric acid.” The printing of his article was scarcely begun before a complete and excellent report was being given in Leipzig by Pummerer and Burkard2 of the addition of hydrogen to the rubber hydrocarbon in the presence of platinum Acids Formic acid HCOOH black at the ordinary temperature. The amount of hydrogen Carbon dioxide co2 absorbed agrees with the theory for the addition of two atoms of Levulinic acid CHsCOCHzCHzCOOH Hydrochelidonic acid HOOCCHzCHzCOCHzCHzCOOH hydrogen for each C5H8 group. By very careful work the hydroA n acid CHaCOCHzCHzCHzCOCHzCHzCOOH rubber was isolated and analyzed, and the results completely The formation of all these compounds could not be explained on check the theory for (C5H10)2. The hydro-rubber is similar t o the cyclo-octadiene formula. Harries then (1914) adopted the rubber itself in that i t is highly elastic, but i t is almost colorless hypothesis, already put forth by Pickles in 1910,’O that the C5H8 and is soluble in ether, giving a colloidal solution. It is extremely groups were united in a large ring, and from his new data he susceptible to air oxidation, thereby being transformed into a believed that i t must contain at least 16 and most likely 20 carbon substance with the same empirical formula as natural rubber. This new rubber is not the same as natural rubber and is called atoms, or five C5H8 groups: iso-rubber H. It is soluble in ether, and, like rubber, on CHs CHa hydrogenation adds two atoms of hydrogen for each C5Hs Hz 1 H Hz Hz 1 H Hz Hz nucleus. ,c-c = c-c-c-c = c-c-c, Pummerer and Burkard give the equation for the addition of 6H9 8-CHs hydrogen as follows:
+
or, better yet, as he also expressed it: 1
i
I:
I
CHI-C=CH-CH~
L
CH,-cH=C--CH, CHB I
i
A,
When hydrochloric acid is added to such a compound and then split off again, it is readily &en how the chlorine may go odt with H atoms from different adjacent carbons and thus form double bonds in different positions from those in the original hydrocarbon.
HARRIES’ ALPHA-HYDRO-RUBBER Harries“ has recently removed only the chlorine from rubber hydrochloride by means of zinc dust in ethylene chloride solution in the presence of hydrogen chloride. The “alpha-hydro-rubber” obtained in this way approximates the empirical formula (C~oHls). and seems to be a rubber “dihydride.” The molecular weight determinations require (C&ts)s or C ~ O H , ~It. should be stated, however, that the figures on which these calculations were made are based on the depression of the freezing point amounting to only 0.001 O C., which is less than the usual error of such determinations. The conclusions must therefore be taken with reserve. But Harries nevertheless enlarges his 20-carbon ring formula to 32 carbons for the rubber hydrocarbon, thus bringing it in line with Ostromuislenski’s contention of about five years J . Chcm. SOC.(London), 97, 1085 (1910). WSJ.VertiYentZ. Siemeizs-Konzevn, 1, 2 Heft, 87 (1921): C. A . , 16, 3232 (1922). 10
11
(C5H8)z XHZ= (CSHIO)~ where x: is the number of isoprene residues in the rubber molecule. Moreover, a n equally long open chain must take up one mol more of hydrogen-i. e., x 1 mol, or
+
(CsH8)z 4- (% f 1)Hz = CssHios + z It is readily seen that the number of mols of hydrogen consumed in the case of the open chain is l/x times greater than the corresponding number of isoprene groups of the ring system. Now, the authors are certain that a difference of as much as 10 cc. in excess could and would have been detected in their apparatus. If the difference were as great as this, it would correspond to a value of 20 for x , where the volume absorbed was about 200 cc., as it was in several of their best experiments. The actual excess was never greater than 2 cc. They therefore feel that they have proved that the rubber molecule must contain a ring system or an extremely long chain in which x > 20. Staudinger and Fritschi,l 8 working independently of Pummerer and Burkard, reported a t almost the same time that they had also hydrogenated rubber. They also used a catalyst but no solvent, and found i t necessary t o use high pressures (93 atmospheres) and high temperatures (270” C.). Their product is a colorless, transparent, tough but inelastic mass, very stable toward chemical reagents, and is soluble in the rubber solvents and, like rubber, insoluble in alcohoL and acetone. Its solutions are colloidal and in benzene showed no depression of the freezing point. On account of the method used, it may consist of the hydrogenated products of the heat decomposition of rubber rather than a simple hydro-rubber. 1% J . Russ. Phys.-Chcm. Soc., 47, 1932 (1915);48, 1132 (1916);abstracted, respectively, in J. Chem. Soc. ( L o n d o n ) , 110, 274 (1916). C. A , , 10, 1948 (1916);and J. Chem. SOC.( L o n d o n ) , 112,404 (19171,C.A , , 11, 1767 (1917). 13 Helvefica Chzm A d a , 6, 785 (1922); J . Chem. SOC. (London), 122, 1043 (1922); J . SOL.Chem. Ind., 41,S68A (1922).
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much less reactive than the original substances, but that t o a certain extent they still show the reactions of the monomolecular compounds.
OTHERDERIVATIVES Chromyl chloride14 (CrOzCln) and n i t r o s ~ b e n z e n ealso ~ ~ form addition products with rubber. Nitrogen trioxide (NgOa) gives a yellow-green product, whose composition is close to that of a true nitrosite (CIoHIBN2Oa)s. It is insoluble in the ordinary solvents. By longer treatment with the gas there is produced a soluble compound, generally called rubber nitrosite C, of the molecular formula (CioHlsNa0P)z or CzoHaoNeOlr. It is a yellow solid soluble in alcohol, acetone, and dilute alkalies, and, like the “tetrabromide,” has often been used for the estimation of the rubber hydrocarbon. Concentrated nitric acid reacts vigorously with rubber, once the reaction has been started by heating or by adding a little fuming acid, with the production of a compound similar to the nitrosite but having a molecular formula approximating CioHlzNzOe. Concentrated sulfuric acid slowly oxidizes rubber, but in solution it rapidly forms a brownish precipitate which is insoluble in the ordinary solvents and, according to Kirchhof, appears to be an isomerized rubber which is less unsaturated than rubber itself. Selenium oxychloride dissolves rubber and reacts with it, forming a substance containing selenium and chlorine of indefinite composition and insoluble in ordinary solvents.
POLYMERIZATION, AGGREGATION, AND AGGLOMERATION I n a very recent article by Harries,” and possibly his last printed publication (this great organic and rubber chemist passed away six weeks ago), he discusses this very matter of the relation of physical state and chemical activity. He writes: There is still much uncertainty about the peculiar colloidal Physical chemists of the present time usually designate these changes as “polymerization” and “depolymerization.” Now the term “polymerization” has long been used in organic chemistry to designate a very definite process in which two or more molecules of a substance of low molecular weight condense by exchange of atomic unions, forming a compound of higher molecular weight-e. g., isoprene to rubber. I do not believe it is advisable to use the same expressions for different phenomena in closely related fields, since the practice is likely to lead to misunderstandings. I n the changes ol state referred to, we have to do with phenomena in which only the degree of dispersion is changed. The disperse phases undergo aggregation (a term used by Zsigmondy) or disaggregation ***. The difference between polymerization and aggregation is that aggregates can be converted to simpler disperse phases by peptization, while polymers cannot be made to undergo a true depolymerization by peptization.
changes of state.
SYNTHESIS
He was evidently led t o these conclusions by his recent work on shellac.Is It is known that one form of shellac is insoluble in alcohol and also only give5 about 3 per cent of aleuritic acid by hydrolysis with 5 N potassium hydroxide. He found that this alcohol-insoluble shellac could be changed into the alcohol-soluble form by the “peptizing action of an organic acid” like acetic or formic, and that this form could then be completely hydrolyzed by alkali to give the usual 30 per cent of aleuritic acid. This disperse phase can be changed back into the unreactive aggregated phase (coagulated) by treatment with ether containing a little hydrochloric acid. “This,” he says, “is probably the first authentic proof that a change in degree of dispersion of complex organic structures can give rise to a totally different reactivity.” And, further, concerning the coarse dispersion, “several disperse phases form by mutual absorption an aggregate in which they are so arranged t h a t from a purely mechanical point of view they do not present toward external chemical forces a sufficient point of attack.” These ideas led Harries to believe that herein was the reason that rubber had not been hydrogenated. He therefore tried t o hydrogenate rubber that had been thoroughly plasticized or masti-
I n spite of the fact that the constitution of the rubber hydrocarbon is still unsettled, attempts to synthesize it from isoprene ‘3%
I
CH2=C-CH=CHz, seem to have been successful. It is, to be sure, impossible at present to tell whether the synthetic product is exactly the same as that isolated from crude rubber, since there are no physical constants available for proper comparison. Harries caused the polymerization of isoprene to take place in the presence of glacial acetic acid, by heating the mixture a t 100’ C. for eight days. The product shows the same solubilities as the natural hydrocarbon, is vulcanized with sulfur chloride, forms hydrohalides, a “tetrabromide,” a nitrosite, and an ozonide whose decomposition products are the same as those from natural rubber. This type of polymer is styled “normal.” “Abnormal” rubber is obtained when the polymerization is effected in the presence of metallic sodium. It answers similarly, although not so well, to the general tests, and the decomposition products of the ozonide show that the linking is irregular. Staudinger‘ohas indicated how these differences can happen in the polymerization as follows: CHa
Synthetic rubber:
Ita ozonide:
-cH2-cH2-cH= CHs
Decomposition products:-CH-CH-C=O
I
CHs
I
OH
I
CHS
CHI
I
I
I
1
I
I
o-o-o
0
I
OH
Spence and Galletly, J . Am. Chem. Soc., 33, 190 (1911). Allesandri, Alii accad. Lincei, 84, I, 82 (1915); C.A . , 9, 2240 (1915). 16.Rer., 58, 1083 (1920).
I
I
I + C-CH-CHrC=O I1
OH 0 Succinic Acid Levulinic Aldehyde
I
o-o-o
CHa
CHs
0 Acetonyl Acetone
I
o-o-o
I + C-CHB-CH~ -CIII + O=C-CHP-CHFC=O II I
It should be added that neither type of synthetic rubber vuicanizes with sulfur as well as the natural rubber. Staudinger believes that it is not necessary to assume a ring for the rubber molecule, it being sufficient simply to formulate free valences a t the ends of a long chain. He says: If we take hundreds of molecules of formaldehyde, we would have, in the unpolymerized state, twice that many hundred reactive atoms. If we assumed that these hundreds of molecules polymerized to a paraformaldehyde molecule, then we would have only two unsaturated atoms, and therefore the reactivity is several hundred times less. This is confirmed by the fact that polymerization products of high molecular weight are 16
CHs
I
= C-CHl-CH2-CH~C-CH1-CHl-
-CH-CH~-CH-C-CHZ-CH-C-CH-CH~-CHCH~-CH~-
o-o-o
14
CHs
CHa
~-CIIz-CK2-~=CH-CHl-cH2-cH CHa
I
Vol. 16, No. 6
CH3
I
OH
I + C-CHz-CH/I 0
cated on the mill, and was successful. “Probably,” he states, “the mechanical plasticizing disturbs or breaks up the mutual absorption of two or three phases,” giving “a final effect similar to the peptizing effect of an organic acid.” He used platinum black as his catalyst, and applied for a patent in April, 1921, seventeen months before Pummerer and Burkard published their work referred to above. Harries, evidently recalling his bitter experience in his fight for priority in the synthesis of rubber, remarks “but I expressly state that I do not wish to use this fact as the basis for any claim of scientific priority over Pummerer.” It will be recalled t h a t Harries’ idea of the vulcanization of rubber was a change from the so-called metastable form into the If Bn.,56, 1048 (1923). 11 Ibid.. 56,
3833 (1922).
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June, 1024
stable form, and t h a t the chemical combination of sulfur was a secondary effect.’Q He later states t h a t this change is a change be of a higher order, since it is in aggregation, but t h a t it more difficult t o peptize it”-that is, to regenerate the natural rubber from it. ‘‘For this type of disperse Systems the term 19 “Uotersuchungen fiber die Naturlichen and Rtinstlichen Rautschukarten,” p. 105.
‘agglomerates’ might be used.”
631 Then he adds:
It seems to me that Ostromuislenski’s interpretation, that the essence of vulcanization consists in oxidation, is incorrect. probably his discovery of vulcanization without sulfur by using dinitrobenzene depends on an agglomeration similar to that which Occurs with Sulfur. It is remarkable that no one has noticed the relation between my work and Ostromuislenski’s.
The Chemistry of Milk and Dairy Products Viewed from a Colloidal Standpoint’ By Leroy S. Palmer DIVISION OF AGRICULTURAL BIOCREMISTRY, UNIVSRSITY O F MINNESOTA, ST. PAUL, MINN.
HE microstructure of milk and its products offers a new and fascinating field for research. Many of the important
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problems in dairy chemistry, both those of theoretical interest and those of practical importance, are colloidal problems. I n fact, there is scarcely any phase of the chemistry of milk and its various products to which one can turn without being confronted with colloidal phenomena. Milk not only represents a complex colloidal system, but in addition is one of the few examples of anatural biological emulsion. It is true. that the dispersion of fat in milk is in the microscopic, not the ultra-microscopic, realm. Nevertheless such a close relation exists between permanent microscopic emulsions and colloidal particles that no consideration of the colloidal problems of milk and its products can be complete without reference to a few of the many interesting phenomena in emulsion chemistry which the field of dairy technology affords. The literature regarding the colloid problems in dairy chemistry has recently been reviewed in some detail by Clayton.ll* The subject, however, is of sufficient importance to warrant a recapitulation of some of its more important features, as well as to point out some of the recent advances in the field. MILK
STRucTuRE-Structurally, milk consists essentially of a microscopic dispersion of fat in an aqueous plasma containing molecularly dispersed lactose and certain mineral salts, and colloidally dispersed proteins, of which calcium caseinate and lactalbumin predominate, as well as a colloidally dispersed calcium phosphate,2 CazH*(PO&. The important colloids of milk, therefore, are calcium caseinate, lactalbumin, and neutral dicalcium phosphate. It is not likely that the colloidal lactoglobulin and alcohol-soluble protein play important roles in the physical-chemical structure of normal cow’s milk, except in the case of colostrum, which is rich in globulin. The casein content of normal cow’s milk varies between 2.25 and 2.75 per cent, that of lactalbumin between 0.5 and 0.7 per cent, while the calcium phosphate comprises slightly less than 0.2 per cent of the milk. Inasmuch as pure isoelectric casein will disperse in pure water to the extent of only 0.01 per cent, it has been assumed by some that the high percentage of casein in milk is due to the protective action of the lactalbumin. This assumption fails to take into consideration the fact shown by Van Slyke and Boswortft2 that casein exists in milk as calcium caseinate, a compound capable of dispersing in water to form highly concentrated colloidal solutions. Van Slyke and Bosworth believe that this compound is tetracalcium caseinate, but inasmuch as Received November 17, 1923. Published with the approval of the Director as Paper No. 395, Journal Series, Minnesota Agricultural Experiment Station. Presented in abstract at the World’s Dairy Congress, Syracuse, N . Y.,October 9, 1923. * Wumbers in text refer t o bibliography at end of article,
this particular caseinate appears to be found only a t pH values of 8 to 10 (neutrality as indicated by phenolphthalein), it seems likely that the calcium caseinate occurring in milk a t its natural pH of 6.5 is a mixture of the basic tetracalcium caseinate with a more acid calcium caseinate.s The calcium caseinate and calcium phosphate compounds that are colloidally dispersed in milk may be readily and. completely filtered out of milk by ultra-filters of relatively large porosity, thus indicating that their degree of dispersion is relatively coarse. There is also every reason to believe that the calcium caseinate is highly hydrated although no experiments have so far been reported to show the amount of water that is bound t o this colloid in milk. Although the casein molecule is undoubtedly a very large one, it is not likely, as Robertson that even the largest possible casein molecule has pointed has a sufficient diameter to scatter the light particles and cause the opalescence of casein solutions, particularly the solutions of the calcium caseinates. The aggregation of the calcium caseinate molecules which is thus indicated as existing in milk does not, however, preclude the possibility that they are also more or less highly hydrated. It seems to have been definitely shown that the colloidally dispersed lactalbumin of cow’s milk exists uncombined with base.6 That it is highly hydrated is indicated by its almost water-clear solution in the milk serum which has been freed from casein, as well as by the ease with which the greater part of it may be dehydrated and coagulated by heat. That this protein in milk is in very highly dispersed state-much more so, in fact, than the other two important colloids of the plasma-is shown by unpublished experiments of the writer, who finds that i t is necessary to construct an ultra-filter of very fine porosity to prevent the dialysis of appreciable quantities of heat-coagulable protein from cow’s milk. The factors determining the stability of the three principal colloids of milk have not been thoroughly studied. No doubt each of the proteins contributes to the stability of the colloidal calcium phosphate, but there is no evidence as to which exerts the greater effect, because the gold number of neither lactalbumin nor calcium caseinate has been determined. It is true that Zsigmondy has assigned a gold number of 0.01 to casein, but this is for an ammonium caseinate sol, which does not exist in cow’s milk. No important technical problems involving the stability of the lactalbumin sol have yet arisen. However, the stability of both the calcium caseinates and calcium phosphate is a matter of considerable importance in certain operations such as the sterilization of evaporated milk. Evidence is rapidly accumulating6 t o show that the character of the mineral salts of milk and the chemical relations between these and the calcium caseinates of milk are of considerable importance in determining the stability of the colloidal casein compounds. The idea that the lactalbumin of the milk plays the all-important role in this
