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Vol. 22, No. 12
Chemical Structure of Lubricating Oils' G. H. B. Davis and E. N. McAllister DEVELOPMENT A N D RESEARCH DEPARTMENT, STANDARD OIL COMPANYOF LOUISIANA. BATON ROUGE,LA.
This paper presents some preliminary data and conclusions concerning the chemical structure of lubricating oils. The work here presented has been confined to Pennsylvania oil, because this type of oil has been the most studied and also because of its relatively less complex composition. A correlation of data on paraffin and cycloparaffin hydrocarbons has indicated t h a t the relation of molecular volume to molecular weight offers a means of estimating the relative proportions of these structures in petroleum products. This relationship has been expressed in the form of the equation:
+
N = 0.358 mol. wt. 7.7 - 0.3 mol. vol. where N is the number of carbon atoms per molecule in naphthene rings. This method has been applied to a series of specially narrow cut fractions of Pennsylvania oils and a very close correlation has been obtained for the empirical formulas calculated from this molecular volume-molecular weight relationship and those from the carbon-hydrogen analysis. I t has been found t h a t the viscosity index of a Pennsylvania lubricating oil is a good indication of the relative proportions of the hydrocarbon molecules that are in ring structures or in paraffinic side chains.
. . . . . . .... UCH time has been spent by various investigators on the chemical structure of lubricating oils in an attempt to further the knowledge of the interrelation of the various properties of these oils. I n particular, Marcusson (6), Mabery ( 5 ) , Zal'kind (?), and Kyropoulos (4) have made outstanding contributions. However, the concept of the molecular structure of a lubricating oil and its
M
and that for Pennsylvania oils this lower value is in the neighborhood of C,H2,-s. The interpretations placed on these analyses are varied. For the Pennsylvania oils it is generally agreed that they are practically free from unsaturation. Mabery concludes that Pennsylvania oils are composed of naphthene nuclei with paraffin side chains, whereas Xyropoulos lists them as isoparaffins. Other types of oils, with hydrogen deficiencies of from -8 to -20, may contain olefinic or aromatic structures. Theoretical
It is thus seen that Pennsylvania lubricating oils are believed to be composed of hydrocarbons containing naphthenic (cyclic) structures with probably paraffinic or isoparaffinic side chains. T a b l e I-Characteristics of Hydrocarbons (Data from International Critical Tables, Vol. I) CARBON ATows
5
Flgure 1-Relation of Molecular Volume and Molecular Welght t o Molecular C o n s t i t u t i o n
relation to its physical properties is still indefinite. Most investigators agree that the empirical formula for lubricating oils may be represented by the series C,HZo-2 to C,,HP~--, 1 Received
September 4, 1930.
6 7 8 9 10 11 12 13 14 16 16 20 24 27 32 36 6 7 8 9 10 6 7 8 8 9 100 12 144 a
HYDROCARBON Pentane Hexane Heptane Octane Nonane Decane Undecane Dodecane Tridecane Tetradecane Pentadecane Hexadecane Eicosane Tetracosane Heptacosane Dotriacontane Hexatriacontane 3-Methylpentane 3&Dimethylpentsne Iso6ctane 4-Ethylheptane 2-Methylnonane Cyclohexane Methylcyclohexane 1,3-DimethyIcyclohexane Ethylcyclohexane 1 2 3-Trimethylcyclohexane Dekahydronaphthalene a ,u-Dicyclohexylethane Perhvdroohenanthrene ~.
MOL. WT. d i z
6"
MOL.VOL.
72 86 100 114 128 142 156 170 184 198 212 226 282 338 380 451 507 86 100 114 128 142 84 98 112 112 126 138 196 192
Conjugated ring structure.
A survey of the physical properties of pure paraffin hydrocarbons as given in the International Critical Tables has brought out an interesting correlation-viz., that when
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1930
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determine what relationship existed between the physical properties of a lubricating oil and its chemical structure. This study has been limited to Pennsylvania oils, since they were known to consist of saturated hydrocarbons, but there was some question as to whether they contained ring structures or were isoparaffins (4). A high-grade, well-refined, heavy motor oil was fractionated very carefully under high vacuum and representative cuts with average boiling points of 293" C, (560" F.), 322" C. (612' F,),344" C. (651" F.) and 359" C. (678" F.) a t 10 mm. absolute pressure were taken. Each cut was then blown with steam a t 232" C. (450" F.) until 5 per cent of the cut had distilled, in order to insure complete absence of cracked of Naphthene Hydrocarbons products. Each cut was then further fractionated by solvent extraction into four equal cuts. The yields and analyses CARBON ATOMS IN MOL. MOL. NAPHTHENE RINGS of these products are presented in Table 111. WT. VOL.
molecular volume is plotted against molecular weight a straight line is obtained for the various paraffin hydrocarbons. This includes both normal and isoparaffins. When the data on naphthene hydrocarbons are plotted in like manner it is found that cyclohexane and its homologs fall in another line parallel to the paraffin line. Likewise, dicyclohexyl compounds fall in another parallel line offset by the same amount (Figure I). This suggests that the molecular volume-molecular weight relationship is a straight line for a given homologous series including the isomers in that series. Data in Table I and Figure 1 illustrate this point. Table 11-Constants HYDROCARBON
70 84 84 98 98 98 112 112 112 112 126 126 1,2,3-Trimethylcyclohexane 112 Cyclo6ctane 126 Isopropylcyclobexane 126 N-Propylcyclohexane Isobutylcyclohexane 140 1,2,3,6-Tetramethylcyclohexane 140 Ethylcycloheptane 126 m-Methylisopropylcyclohexane 140 ~-Methylisoprop ylcyclohexane 140 o-Methylisoprop ylcyclohexane 140 2-Cyclohexyl-2-methylhutane 154 Dimethyl-1,3,5-isobutylcyclohexane 168 1-Methyl-2-isoamylcyclohexane 168 Decahydronaphthalene 138 Perhvdrouhenanthrene 192
92.6 111.0 107.2 129.2 131.0 119.8 142.7 145.8 143.3 144.3 158.5 158.1 133.6 158.8 159.5 173.5 171.1 154.8 174.1 173.9 171.9 187.0 203.0 206.0 156.0 206.4
Cyclopentane Meth ylcyclopentane Cyclohexane 1,l-Dimethylcyclopentane N-Propylcyclobutane Cycloheptane 1,l-Dimethylcyclohexane 1-Methyl-2-ethylcyclopentane Ethylcyclohexane Propylcyclopentane 1,1,3-Trimethylcyclohexane
5.0 4.5 5.6 4.0 3.5 6.8 5.0 4.1 4.8 4.5 5.3 5.4 7.7 5.2 5.0 5.8 6.5 6.4 5.6 5.7 6.3 6.7 6.9
5 5 6 5
4 7 6 5 6 5 6 6 8 6
6 6 6 7 6 6 6 6 6 6 10 14
6.0
10.3 14.5
The fact that the parallel lines are spaced equidistant suggested the possibility of continuing the series for compounds containing more than two cyclohexyl rings. This extrapolation is represented by the dotted lines in Figure 1. The relationship shown in this figure may be readily expressed in the form of an equation-viz.: N = 0.358 mol. wt. 4- 7.7
- 0.3 mol. vol. 0
where N is the number of carbon atoms per molecule in naphthene rings. This equation is applicable to hydrocarbons containing naphthene rings other than cyclohexane, as illustrated in Table 11, in which it is shown that the calculated number of carbon atoms in the rings check within one of the actual number. Experimental
With the relationship developed above a study has been made of various cuts of a Pennsylvania motor oil in order to of Solvent
Table 111-Characteristics c u t No.
1
Av. boiling point (10mm.H g ) (
I .. 1
I I
3
Orig.
293' C.
.. io0
Figure 2-Relation
$ $?gins1A. P. I. Gravity
.
Ultimate analysis: Hydrogen, % Carbon, %
..
..
13.6 12.3 13.7 13.7 86.4 87.7 86.3 86.3
Doubtful average molecular weight.
.. ..
Viscosity Index
The analyses for gravity, viscosity, flash, and pour were the usual inspections as prescribed by the A.S.T.M. The viscosity index was determined according to the method of Dean and Davis (1). The molecular weights were determined by the cryoscopic method ( d ) , using cyclohexane as a solvent. The carbon and hydrogen determinations were made by the usual organic combustion method ( 3 ) . The Extracts of Pennsylvania Motor Oil
I I
4 322' C.
Orig. 1 2 3 4Orig. 1 2 22.5 22.5 22.522.5 22.5 22.5 5:40 1.22 1.221.22 1.22 4:oo 0.90 0.90 27.8 30.222.0 30.0 33.4 34.7 29.822.828.6 Viscosit;at 100" F. (Saybolt) 870 213 440 207 175 224 269 558 309 Viscosityat210°F.(Saybolt) 85.3 47.454.6 47.446.451.0 51.060.052.8 101 93 58 103 120 125 101 62 96 Viscosity index Flash, F. 460 420 435 425 425 450 445 450 460 Pour, F. 45 40 30 40 50 45 45 35 45 Mol. weight . 398 365 384 395 425 425 395 4045 Mol. vol. . . 449 396 438 461 499 484 431 456a Extract No.
of Molecular Weight, Molecular Volume, and
5
I
6
3440 c.
I
3590 c.
3 43rig. 1 2 3 4Orig. 1 2 3 4 22.5 22.5 22.522.5 22.5 22.5 22.5 22.5 22.5 22.5 0.90 0.90 3:io 0.70 0.700.700.70 i:50 0.340.34 0.34 0.34 31.3 34.3 28.6 22.826.5 31.0 33.3 27.5 20.327.6 31.0 32.3 267 215 586 1235 726 440 393 984 3300 855 634 650 51.7 49.6 58.0 86.073.5 64.062.0 89.0 140.6 85 4 76.880.6 107 120 97 70 91 111 118 98 62 101 114 124 460 470 515 510 510 505 510 550 535 535 550 570 45 60 50 45 50 55 60 55 40 50 60 65 413 445 480 453 455 500 505 551 452 536 612 597 477 522 543 494 508 572 589 619 484 603 703 690
13.4 12.4 13.2 14.0 86.6 87.6 86.8 86.0
.. ..
13.3 12.9 13.4 13.8 13.3 12.8 12.213.2 86.7 8 7 . 1 86.686.2 86.7I 87.2 87.886.8
.. .. .. ..
1.VDUSTRIAL A S D ENGILVEERING CHEMISTRY
1328
t,
m m
-
N
I
carbon-hydrogen results are presented on the basis of 100 parts of hydrocarbon, since the actual analyses varied from 99.5 to 100.3. By usiflg the data presented in Table I11 and the formula derived previously, it is possible to calculate the average number of carbon atoms in naphthene ring structures for each of the oils given in the table. These results are presented in Table IV. Here the empirical formula for each sample is also c a l c u l a t e d from the number of c a r b o n atoms in naphthene rings by assuming an average of 6 c a r b o n atoms per ring and a deficiency of 2 hydrogen atoms per ring. The empirical formula calculated in this manner is found to correspond very closely to that calculated from the c a r b o n-h y d r o gen analyses. The widest discrepancies are in the first extracts of each f r a c t i o n . These show a greater deficiency in hydrogen determined by c a r b on-h y d r o gen analysis than those determined by the molecular volumemolecular weight rel a t i o n s h i p . This difference is probably due t o the concentration of such unsaturated or aromatic compounds as might be present in the first e x t r a c t . The close correlation of the empirical f o r m u l a s in the other cases indicates that the assumption that 6 carbon atoms was t h e a v e r a g e number in a naphthene ring was probably correct.
1~01.22, KO. 12
Correlation of Viscosity Index and Chemical Structure
It has previously been suggested that viscosity index, which represents the change in viscosity of the oil with temperature, may be used as a guide to the relative composition of the oil, and that a definite relation must therefore exist between the viscosity index and the physical properties of the oil. From the data presented in Table I11 a plot has been drawn showing the molecular volume-molecular weight relationship for lubricating oils of various viscosity indices (Figure 2). This plot shows that this relationship is a straight line for the oils of different viscosity indices, However, the lines tend to converge a t low molecular weights. This suggested the idea that the viscosity index was a function of the percentage of the carbon atoms in the molecules that were in naphthene rings. The relationship found is shown in Figure 3. The correlation is good.
Figure 3-Relation of Viscosity Index t o Per Cent of Carbon Atoms i n Naphthene Rings i n Aromatic Free Lubricating Oils
Since this entire study was made on Pennsylvania oils, the results are only applicable in the higher viscosity-index ranges (75 to 125). The lower viscosity index oils obviously must contain some olefin or aromatic compounds, whereas this study is based primarily on saturated oils. The extrapolation, such as is made in Figure 3, is of benefit as an indication, but will require substantiation before it can be accepted. Conclusions (1) Pennsylvania-type oils (saturated hydrocarbons) are composed essentially of a grouping of paraffin chains with naphthene rings.
I N D U S T R I A L A N D ENGINEERING CIiEhfISTRY
December, 1930
(2) For this type of oil the number of carbon atoms in naphthene rings may he determined from the molecular weiEht and densitv. (3) viscosity index may he used as an indication of the chemical structure of the oil and for that reason should serve as a basis for the correlation of the various physical properties of lubricating oils. Y
I
1329
Literature Cited ~aem. ~&rei.Ei - ~ . ,~~6 , 8 1 ,8(1929) ( 2 ) Findlay, "Practical Physical Chemistry," Longmans. 1925 (8) Fisher, "Laboratory Msnual of Organic Chemirlry." Wiley, 1924. (4) Kyropoulos, 2. phyiik. Chem., 144, 22 (1929). (j) Mabcry. IND. 1233 (19~3). ( 6 ) ~ a r c u n r o n11fiii. , .M~rrri=ipruiunaromr,40,308 (19221. (7) Znl'kind, Pcirolevrn Shale . I(Ruisio), Ill, 154 (1922): J . Insf. Pibo[rum Tach., 9. l57A (19221.
(I! D~~~ and ~
c,,~~.,
N e w l y Discovered Microscopic Structural Units of Wood Fibers' Geo. J. Ritter and R. M. Seborg U. S.Fonasr PXODUCTS LABORATOXY, MAUISON,Wrs
R
ESULTS published by Ritter (1) regarding the dissection of wood fibrils by chemical means showed that the smallest structural 1mit.s of the vood fiber which had been isolated at that time were fusiform bodies. They are short, spindle-shaped units which, whcn arranged parallel to one anot,her with an overlapping of the pointed ends, form the fibrils. The fibrils are long, slender, filament-like structures, arranged to form the fibers. Careful microscopical examination of the fusiform bodies after various treatments suggested that they are composed of still smaller structural units. A t the bime of the meeting of the American Association for the Advancement of Science, Des Moines, Iowa, December, 1929, sufficient progress had been made in the study to warrant tlie following prediction (3) :
Optical Properties l'reviously described structural cellulose units ( 2 ) sliowcd between crossed Xicol prisms an inmasing degree of sharpness in the angles of minimurn and maximum luminosity with decreasing size of unit. I n other words, the parallelism of the crystalline structure which exhibits the effect in polarized light was greater in the smaller units. Such a condition would be expected from the fact that t,here is less opportunity for variation in the parallelism of crystalline arrangement in the smaller than in tlie larger units which are composites oi the small ones. It was found, however, that between crossed Nicol prisms the isolated spherical units were uniformly luminous in all positions, indicating
I t is quite probable that still smaller microscopic building units in the fusiform bodies will be discovered in the near future, and that the source of thc optical properties described in t h e preceding paragraphs will then be found in the newly discovered units.
Recently, separation of the fusiform bodies into smaller units was accomplished a t the Forest Products Laboratory, but suitable photomicrographs of the results were obtained only after much experimcntation aiddifficulty. Materials The materials used in this study consisted of two batches of mixtures of fibrils and fusiform bodies vhich were prepared from delignified white spruce fibers. One batch was prepared by the sulfuric acid method; the other, by chlorination in the sunlight. Procedure a n d Results Fleure I-Spherical
Small samples of the mixtures containing fibrils and fusiform bodies were treated on glass slides wit11 phosphoric acid (78 per cent) a t approximately 75" C. When slight pressure was applied to the cover glass over the specimen, the fusiformbodies separated into smaller units. The newly discovered units (Figure 1) are spherical in form when detached from the mother unit. They are approximately 0.45 micron, or 4500 A. in diameter, which is nine times the length of the cellulose micelle rcrcaled by the x-ray diffraction pattern. Since those units h a ~ cnot been observed in the fusiform bodies, their original fonn is unknown, but from observed optical properties it would seem that tlie shape is other than spherical. 1 Received September 20, 1930. Presented belore the Division oi Cellulose Chemistry a t the 80th Meefine of the American Cheoilcal Society, Cincinnati, Ohio, September 8 t o 12, 1930
Units.
700 X
a random arrangement of tlie crystalline structurc. Such a disarrangement in tlic internal stmcture of the new units would result during the deformation of an angular body to a spherical one. This conclusion is confirmcd by the fact that it would be impossible to build up a solid structure with spherical units. The resu1t.s lierein described suggest a future study in which the spherical units may be isolated under minimum swelling condit,ioiis, so as t,o reduce the possibility of deforming them. If the s8mc units can be obtained by such n method, which the writers believe is possible, their optical properties sliould be siioilar to those of tlic fusiform bodies. Literature Cited (I! ~ i t t e i IN". , RNO. C H E W . . a i , 289 ( 1 ~ 2 9 ) ?a! R i t t r i . 3 . P'o"?stry, 28. 533 :l9:3'>!
