Viscosity of Esters of Saturated Aliphatic Acids Relation t o the Synthesis of Fine Lubricating Oils AUGUSTUSH. GILL AND FOREST P. DEXTER,JR.,Massachusetts Institute of Technology, Cambridge, Mass,
ORPOISE jaw oil has been used for a'bout a hundred years for the oiling of the finest and most delicate machinery-i. e., timepieces. Its lack of change in air, low viscosity, and high lubricating power especially adapt it for this purpose. It is, however, expensive, costing a t times 860.00 per gallon. A synthetic substitute is to be predicted. Gill and Tucker ( 2 ) found the oil to be composed of about 60 per cent isovalerin, 6 per cent tripalmitin, 4 per cent triolein, 3 per cent free isovaleric acid, and 27 per cent dodecyl isovalerate. The synthesis of the glycerides would not be easy and the palmitin and the olein could well be omitted on account of the instability of the latter. The artificial preparation of the oil resolves itself practically into that of the liquid wax, dodecyl valerate. I n fact, it is believed that a satisfactory substitute could be made solely with this latter compound, or one of a n
acids. Their viscosities are shown in Table I, which values agree with many of those in the International Critical Tables. They are charted in the semi-logarithmic Figure 1. TABLE
Methyl acetate (C?) Ethyl acetate Propyl acetate Butyl acetate -4myl acetate
6. Methyl propionate (C3) 7. Ethyl propionate 8 Prop 1 propionate 9: Butyy prop?onate 10. Amyl propionate
11. 12. 13. 14. 15.
Methyl butyrate Ethyl butyrate Propyl butyrate Butyl butyrate Amyl butyrate
16. Methyl valerate ( C I ) 17. Ethyl valerate Prop 1 valerate 19: Butyr valerate 20. Amyl valerate 18
21. 22. 23. 24. 25.
1. 2. 3. 4. 5.
57.5 77 101-102 125 148 79 99
N VISCOSITY A T 25' C. Poises 0.003503 0.004158 0.006389 0.006714 0.008528
122.4 144 164
0.005082 0.005045" 0.006104 0.007618 0.009355
102 119.9 143 165 176
0.005259 0.006127 0.008319 0.009766 0.012558
127.3 144.5 167.5 186-187 204-207
0.006227 0.007623 0.008875 0.013479 0.018153
Methyl caproate (Cs) Ethyl caproate 166.6 Propyl caproate 185.5 Butyl caproate 204.3 Amyl caproate 224 Checked several times, probably impure.
0.012864 0.017250 0.022707
Considering the different esters containing the same number of carbon atoms, their viscosities are seen to be fairly close to eqch other; it appears safe to conclude that, in aliphatic esters of noma1 carbon chain of high enough molecular weight to be practical for a lubricant, there will be interchangeability of compounds of the same molecular weight well within 0.05 centipoise. Also, in case there is a difference, or in compounds .oo 2 2 3 4 5 6 7 8 9 10 II 12 of lower molecular weight, the alcoholic methylene group NUMBER OF CARBON ATOMS gives a greater viscosity than the acidic methylene group. It would seem likely, \Then it is feasible to make synthetic equal molecular weight, and a saturated free fatty acid. ester lubricants and when the molecular weight is the factor Dodecyl valerate, C4H9COOOC12H25, which makes up more to be considered, that, as far as viscosity is concerned, it than a fourth of the oil, would seem to be necessary, but its makes little difference what the groups concerned are;' synthesis from dodecyl alcohol would be expensive since it is comparisons must be made, however, with groups of the same a chemical curiosity. The question arose as to whether this constitution-that is, normal with normal esters, is0 with isovalerate was actually necessary, or whether any similar ester esters, and hydrocarbons with hydrocarbons. Substances of of the same number of carbon atoms, e. g., C~HI~COOOC~HIT, the same molecular weight do not necessarily hare the same or any other arrangement of CI6H34 could replace it. In viscosity. other words, is the viscosity of ACOOB practically the same The subject is being still further studied. as the viscosity of BCOOA? If this is the case, a synthetic substitute is nearer and less expensive. LITERlTURE CITED The viscosities of the various esters from methyl acetate to (1) B i n g h a m , E. C., " F l u i d i t y a n d Plasticity," p . 1 3 5 et seq., amyl valerate were determined a t 25" C. by the Ostwald McGraw-Hill Book Co., 1;.Y., 1922. viscometer. This was cleaned with chromic acid, washed 2 ) Gill a n d Tucker. Oil &. Fat I d , 7, 101 (1930). with alcohol and ether, and standardized from 0" to 45" C. ((3) I n t e r n a t i o n a l Critical Tables, T o ] . V , hlcGraw-Hill Book Co., by water according to the procedure recommended by BingS . P I 1926. ham ( I ) , taking the viscosity in centipoises from the Inter& ~ l l 18, 1934. national Critical Tables ( 3 ) . The esters used were as pure as RECEITED could be obtained from the Eastman Company, or were careI The same seems to be true of the boiling points, which meane t h a t their fully prepared in the laboratory from the normal alcohols and flash and fire points would be nearly the same.