December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
using high vacuum technique, may furnish additional information as t o thc true mechanism of isomerization. LITERATURE CITED
(1) Bloch, H. S., Hoffman, A. E., Oleszko, T. J., and Chenicek, J. A,, (2)
Dir.. of Petroleum Chemistry, A.C.S., New York, 1944. Bloch, H. S., Pines, H., and Schmerling, L., J . Am. Chem. SOC.,
68,153 (1946). (3) Cheniceh, J. A., Dr>er, C. Q., Sutherland, R. R., and Iverson, J O., World Petrolaum, Refinery Issue, 1944,p. 146. ( 4 ) Eveiing, B. J,., d'Ouville, E. L., Lien, A. P., and Waugh, R. C.,
presented before the Pivision of Petroleum Chemistry at the 11 I t h Meeting of the AMERICAX CHEMICAL SOCIETY, Atlantic
Citv. N. J. (5) Ipatieff; V. N., Corson, B. B., and Pines, H., J . Am. Chem. SOC., 5 8 , 9 1 9 (1936).
( 8 ) Ipatieff.
v- N.,and
2319
Schmerling, L., IND. ENG. CHEM..40,
2354 (1948). (9) Perry, S.F., Trans. Am. Inst. Chem. Engra., 42, 639 (1946). (10) Pines, H. (to Universal Oil Pioduct,s Co.) U. S. Patent 2,405,616 (Aua. 6, 1946). (11) I h i d . , 2 4 0 6 , 9 6 7 (September 3. 1946). (12) Pines, H., Kvetinskas, B., Kassel, L. S.,and Ipatieff, V. N., J . Am. Cham. S o c . , 67, 631 (1945). (13) Pines, H., and Wackher, R. C.. I h i d . , 68,595 (1946). (14) I b i d . , p. 599. (15) Ibid., p. 2518. (16) Pines, H. and Wackher, R. C., (to Universal Oil Products Co.) U . S. Patent 2,406,633 (Aug. 27, 1946). (17) Ibicl., 2,406,634. (18) Wackher, R. C. and Pines, H.. J . Am. Chem. Soc., 68, 1842 (1946).
RECEIVED December 8, 1947.
Presented before the Divkion of Petroleum Chemistry a t the 112th Meeting of t h e AMERICAN CHEMICAL SOCIETY, New York, N. Y.
Ipatieff, V. N., and Haensel, V., Ibid., 64, 520 (1942). (7) Ipatieff, V. N., and Pines, H., Ibid., 59, 56 (1937). (6)
Esters of Naturally Occurring Fatty Acids J
PHYSICAL PROPERTIES OF METHYL, PROPYL, AND ISOPROPYL ESTERS OF C, TO C,, SATURATED FATTY ACIDS CARL W. BONHORST, PAUL
M
a
ALTHOUSE, AND HOWARD 0. TRIEBOLD
Pennsylvania State College, State College, Pa.
An apparatus is described and the operating procedure is given for the determination of a complete vapor pressure curve on one or two drops of a pure liquid. The apparatus was calibrated, and the vapor pressure curve for each of the methyl, propyl, and isopropyl esters of the naturally occurring Cn to C18 saturated fatty acids was determined. Decomposition of the esters was found to be progressive above 205 C. and occurred over a wide temperature range rather than at a specific temperature. The relationships of the densities and viscosities of the esters to temperature were studied by determining these constants at 20", 37.8", 60", and 98.9" C. Relationships between the vapor pressures, densities, and viscosities indicate that the forces governing these three properties have some factor or factors in common.
T
HE value of physical data for the identification of pure
esters of the fatty acids has been proved (4,11). Among those physical constants which contribute information for the identification of a pure compound are vapor pressure, density, and viscosity. The study of the effects of changes in structure or molecular weight upon physical properties frequently yields valuable information. Further studies of these effects upon the properties of pure compounds may make possible the formulation of methods for the estimation of the composition of mixture.s, or conversely, for the preparation of mixtures having certain desired properties. The propyl and isopropyl esters should yield information of special value in the study of fats and oils, for they are the simplest esters of 3-carbon alcohols with fatty acids. The value of a thorough study of these properties with regard t o esters is enhanced considerably by the need for densities in the calculation of volumes for synthesis, and by the fact that the viscosity characteristics of esters are such as t o suggest their use as lubricants.
An excellent compilation of the vapor pressures of a great number of compounds has recently been published (18). I-Iowever, the only vapor pressure data t h a t seem t o be available for the esters of the naturally occurring fatty acids are those of the methyl esters (4, I S ) . Boiling points at 30 mm. of mercury pressure and densities have been reported for the isomeric 16carbon esters (16). Densities and boiling points a t 20 mm. pressure have been reported for the esters of caproic and caprylic acids with various alcohols (9). Densities of methyl esters (2) have been reported. Viscosities of numerous low molecular weight esters ( 7 ) ,and a large number of diesters (67, and esters of nonanoic, nonenoic, oleic, and cinnamic acids (1) have appeared in the literature. However, complete studies of the relationship of the density and viscosity of the methyl, propyl, or isopropyl esters to temperature have not been reported. APPARATUS
Densities and viscosities may be determined easily aiid accurately with comparatively simple apparatus. Various methods for the determination of vapor pressures on a small quantity of pure liquid have appeared in the literature (4, 8, 1 7 ) . However, all have certain serious limitations and usually yield erroneous results when used outside those limits of temperature and pressure to which they seem t o be intrinsically adapted. Natelson and Zuckerman ( I d ) observed that when a capillary filledrby suspending a drop of liquid from the lower end is placed in an air bath heated to a certain temperature and the pressure slowly reduced, a point is reached at which the level of the liquid in the capillary will fall rapidly. They found that the pressure at this point corresponds t o the vapor pressure of the liquid at the temperature of the bath. I n an effort t o utilize these observations, a new apparatus has been designed for the determination of vapor pressures over a range of 2 t o 100 mm. and a t temperatures up to 220" C.
2380
INDUSTRIAL AND ENGINEERING CHEMISTRY
The capillary illustrated in Figure 1 eliminates the excessive evaporation and the need for very delicate manipulation associated with the use of a suspended drop. A length of capillary tubing with an inside diameter of 1.4 mm. is fused to the lower end of a second capillary of 0.15-mm. inside diameter. There must be no constriction in the smaller capillary when the fusion is completed. The larger capillary is bent upward t o form a reservoir and the open end is blown out to form a small cup which facilitates filling. A diagram of the complete apparatus is shown in Figure 2. The air bath consists of a tube 250 mm. in length and 45 mm. in diameter fitted with a bulb a t the top in which three thermometer wells are located. The auxiliary view, 2A, shows the relationship of the v-ells t o each other and to the capillary. These wells, which make possible ternperature readings a t three different levela inside the bath, also serve to increase the heat capacity of the bath and to prevent formation of convection currents. The thrce thermometer readings are equalized by the’ yariable spacing of the resistance wire (2 meters of hTo. 24 Chromel) with which the tube is wound. This gives reasonable assurance that the temperature as indicated by t,hermometer 2 is that of a fairly large section of t,he t,ube. The flow of current through the heating element is regulat,ed by two rheostats in series across the 110volt alternating current source, G. By varying the settings of the tvro rheost’ats, a series of voltagetemperature curves is obtained. Figure 1. Capillary The capillary is suspended, by Tube four strands of noncorrosive wire, from the rod sealed to the inner ground-glass joint. The use of wire provides flexibility which reduces the danger of breaking the fragile n~ellswhich cr0R.d the tybe. It, a,lso facilitates the adjustment of the level of the capillary t o compensate for differences in surface tension of different liquids. By maintaining a constant liquid level, errors due to small variations in temperature within the tube are minimized. The preheater, which serves as a Bource of heated air, also permlts the introduction of that air into the system in a manner causing the least possible disturbance of the bath temperature. Adjustment of the size of t,he capillarv air inlet eliminates the need for precise regulation of the stopcock, F . While a sudden inrush of air is essential as soon as the liquid in the capillary begins to fall, a rapid return of the system t o atmospheric pressure would result in a sharp decrease in temperature. This would require a longer period between readings. At least 1 minute should be required t o attain atmospheric pressure after stopcock F is opened. The pressure gage consists of a closed, mercury-filled tube 400 mm. long with a n inside diameter of 5 mm. This tube is set at an angle of 30” to the horizontal. The lower end opens into a vertical tube 30 mm. in diameter which is filled with mercury t o a level about 10 mm. above the upper edge of the opening. The upper (closed)
Vol. 40, No. 12
end is bent so as t o make a 30-111m. vertical portion. This seems to facilitate the (‘breaking” of the mercury from the end of the tube. The vertical portion also provides enough warning of falling pressure to make possible the calibration of the entire slanted part of the tube, thus affording maximum range with minimum size of gage. PROCEDURE
The air bath is brought to the desired temperature by adjustment of the rheostats. The rheostat in the preheater circuit is set to give a temperature about 10” C. below that of the air bath. The stopcocks are set as follows: A , D, E, and F are closed; H is open; and C is set so t h a t only the left side is connected to the vacuum through D. The capillary is charged with appioximately 2 drops of a liquid and placed in the air bath. When the reading of No. 2 thermometer indicates that the temperature is constant, D is opened and the pressure is reduced t o approximately 80 t o 100 mm. (as indicated on the conventional type manometer) above the vapor pressure of the liquid. At this time B is closed while C is turned so that both the air bath and the manometer are connected with the vacuum pump. At that point where the liquid in the capillary falls rapidly, the 3-way stopcock is turned t o its original position, thus isolating the pressure gage which may be read later. R is opened immediately to increase the pressure in the bath and t o arrest the fall of the liquid. L) is closed, isolating the syqtckiii from its vacuum source, and F is opened, permitting air to flow slowly through the preheater into the system. When the above operations have been completed the pressure gage may be rcad. The reading ,obtained is the vapor pressure of the liquid at the temperature of the bath a t the beginning of the operation. CALIBRATION
The apparatus I\-as ca1ibrat)ed by using four iiquid samples o f known purity. These samples are listed in Table I, together with the references concerning their purity. Determinations of the pressure gage readings were made at several temperatures for each of the four liquids. Each reading was plotted against the vapor pressure of the liquid used. When approximately 200 points had been plotted in this manner, the straight line, conforming to the majority of the points, was drawn. This line served as a calibration curve by which pressure gage readings were translated into millimeters of mercury. I n actual practice the slope of this line was found t o be 2.87. Accordingly, gage reading the pressure in millimeters of mercury equals 2.87
Figure 2.
Vapor Pressure Apparatus
.
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
T-ABLE 1. VAPOR PRESSURES
OF
FOURS T A N D A R D
Temperature, Compound Methylcyclohexane Mole yo impurity 0.36 f O.lSa
c.
26.5 27.2 27.2 27.5 30.6 30.8 31 .O 31 .O 39.0 39.2 39.3 39.4 62.0 66.5 75.5 78.7 80.7 81 .o 95.8 96.4 96.5 102.5 104.5 104.8 105.4
=
Tridecane Mole yo impurity 0.2 (16)
z*;
Dodecane Mole impurity 0.2
-
I
Vapor Pressure Observed, Literature, mm. min. 49.8b 49.4 51.3 50.0 51.3 50.4 52.3 52.1 60.0 57.6 60.6 59.8 61 .O 59.8 61 .O 60.7 87.7 88.1 88.5 88.1 89.0 89.8 89.5 90.3 0.85 (16) 0.8 1.04 1.1 1.7 2.6 2 .2 2.2 2.45 2.5 2.5 2.6 5.7 5.8 5.9 5.8 5.9 4.9 8.0 8.7 8.9 9.0 9.0 9.3 9.25 9.4 51.5b 51.9 53.2 52.9 65.4 65.9 66.0 65.8 67.3 67.0 68.0 68.1 69.0 69.7 79.0 78.4 80.3 80.2 80.3 80.4 83.9 84.0 84.0 84.8
161.0 161.1 161.3 178.0 179.2 179.2 186.5 186.5 187.0
Dibutyl phthalate (redistilled)
(1
128.2 129.0 134.7 134.8 135.2 135.5 135.8 139.7 140.0 140.0 141.4 141.4
LIQUIDS
1.9 2.1 2.1 4.5 4.0 4.9 6.1 6.2 6.6
2.0 (16) 2.0 2.0 4.3 4.75 4.75 6.7 6.7 . 0.8
2381
225200-
175I50-
125-
b
::1003 t
d
a: W
a
5 75+
50-
25
1
',
6M 2
I
5
I
I
20
50
I
10
IC
3.
Vapor Pressure Curves of the Methyl, and Isopropyl Esters of Ce-Cle Naturally Occurring Saturated Fatty Acids
N.B.S. standard sample 218.
b Taken from curve obtained by plotting values obtained by Rossini (14)
VAPOR PRESSURES
To verify the accuracy of the calibration curve and t o estimate the minimum number of points necessary to establish a reliable vapor pressure curve, a second series of vapor pressures was determined on each of the four standard liquids. The data of the authors and those taken from the literature are listed in Table I.
Vapor pressure curves were determined for the methyl, propyl, and-isopropyl esters of the naturally occurring Ce-Cls saturated fatty acids which were used in a previous study t o obtain refractive index data (3, 10). Most of the vapor pressure curves were characterized by 30 t o 50 points with a minimum of 3 determinations for a 10' rise in temperature. In the case of those
TABLE 11. BOILINGPOINTS OF METHYL,PROPYL, AND ISOPROPYL ESTERS AT VARIOUS PRESSURES Esters Methyl caproate Isopropyl caproate Propyl caproate Methyl caprylate Isopropyl caprylate Propyl oaprylate
2
4
5
6
16.8 34.1 42.9 51.5 65.0 70.5
27.5 44.9 54.2 62.5 76.7 82.3
31.0 48.5 57.7 66.0 80.4 86.0
34.1 51.5 60.9 68.9 84.1 90.0
Methyl caprate Isopropyl caprate Propyl caprate
78.7 90.0 96.8
90.8 102.5 109.7
93.4 106.8 t14.2
98.1 110.5 117.8
Methyl laurate Isopropyl laurate Propyl laurate
102.8 117.4 123.7
116.0 131.3 138.1
120.7 135.8 143.0
124.4 139.8 147.0
Pressure, Millimeter6 of Hg 10 20 40 Temperature, O C. 38.8 42.8 55.6 69.6 56.5 60.4 73.7 88.7 65.9 70.2 84.2 98.9 73.8 77.8 90.9 105.2 89.3 93.8 123.4 108.2 94.1 100.0 114.1 129.8 103.3 108.0 137.8 122.3 116.5 121.1 137.0 153.6 123.7 128.5 161.2 144.3 130.4 135.0 151.0 167.8 145.8. 151.0 167.4 185.0 153.3 156.8 175.8 194.0
Methyl myristate Isopropyl myristate Propyl myristate
127.1 140.2 147.0
141.3 154.4 161.7
140.3 159.4 166.8
150.3 163.6 171.0
156.6 170.4 177.6
161.8 175.6 182.8
178.6 192.6 200.2
Methyl palmitate Isopropyl palmitate Propyl palmitate
148.5 160.0 166.0
163.2 175.4 181.6
168.4 180.6 187.0
172.6 184.8 191.4
179.2 191.6 198.2
184.8 197.1 203.6
202.2
(1
Methyl stearate Isopropyl stearate Propyl stearate
169.4 181.6 186.8
184.6 197.5 200.8
189.4 203.0 206.0
193.6 20z.O
19a8.8
206.6
a
a
Decomposition zone.
...
.. .. ..
a
...
...
-
50
60
80
100
74.2 93.4 103.8 110.0 128.6 135.0
85.3 104.4 115.3 119.1 140.1 147.0
90.7 109.4 120.3 125.6 146.1 152.6
154.8 172.0 179.6
160.6. 177.6 185.6
173.9 191.4 200.2
78.2 97.7 108.2 114.1 133.0 139.5 147.2 164.4 171.6 178.8 196.0 205.4
18!.4
192. ...
192.8
202.8
208.2
a
...
8
...
... ...
... ...
142.8 159.4 166.6
... ...
... ... ...
...
a
...
. .
.
, . .
.
I
...
...
...
... ... .
I
.
... ... ... ...
...
,..
... ..
. .
,..
, . .
..,
...
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 12
erratic. Evidentlj- these erratic data result from the formation ol a mixture of volatile and nonvolatile decomposition products. Table 111d u n onstrstes t,he effect of maintaining a constant high temperat,ure for an extended period of time. The values expressed in this table indicate that decomposition is progressivc and occurs over a wide temperature rangc rather than a t a spccific tcmpcrnt,urc. Although this range may vary with the individual substance, it seems t o have its initial point at. temperatures as low as 200" C . , and in most instances is not complete when temperatures as high as 250" C. are used. Further work is needed to determine the magnitude of the dcconiposition process a t various tcmperatures. It is evident, hoivcver, t h a t an est,er will change in composition if heated at temperatures above 205" C. for an extendcd period of time. The amount of inipurit,y in t,he fractions collected i n the course of a fractional distillation is depcntlent on the nature of the volatile decornpositioii productz and their solubility in the ester at, the teinperaturc at. which t'hev are collected.
>-
t cn
Z n
W
DEN SlTIES
I
20 Figiire 4.
1
1
I
40 50
30
60
I
I
\
1
70 80
TEMP E RAT UR E,'c.
90 99
Densitp-Temperature .Relationships of the Methyl, Propyl, and Isopropyl Esters of C 8 - c ~ Naturally Occurring Fatty Acids
esters in which decomposition occurred a t low pressures, 20 points were considered to be a reasonable minimum in order to eatablibh a reliable vapor pressure curve. A summary of the boiling points a t various pressures obtained from the established curves (Figure 3) is shown in Table 11. When vapor pressures arc determined in the manner described by the authors, there are no visible signs of decomposition even a t temperatures above 250 O C. However, a t temperatures slightly higher than 205" C., the vapor pressurc values become
TABLE ITI.
I'APOR
PRESSURES O F S E V E R U , E'3TERS I N
T e m p .,
c.
Isopropyl myristate M e t h y l rnyristat: Propyl s t e a r a t e Propyl s t e a r a t e
208 211 215 240
-
0
36.4 67.0 8.0 21.7
10
20
Table I11 summarizes the results obtained in the determination of t,hc densitr-temperat,ure relat,ionships of the esters. These data are plotted in Figure 4. Of the properties studied thus far, denTities are unique in that the diffeierices betn een series of esters grouped according t o the alcohol from a hich they are prepared are greater than that bcta een members of such a series. On the other hand, the Ad / A?' values for isomeric niethyl and propyl esters are virtuallv thc same, Rith slightly higher values in each cabe for the isopropvl esters. There is a mathematical relationship betv ecn the molrculat weight and the slopes of the denGtytemperature curves of these esters. Tlic empirical equation is:
THEIRDEC'OMPOSITIOS'
T i m e , AIiniites 30 40 50 60 70 Pressure, Alillimeters of Hg-
3 3 . 7 34 2 3 4 . 0 3 3 . 5 68.2 66.8 65 8 65.0 8.3 8.2 8.2 8.15 21.4 20.8 20.1 20.4
Thcrmostatically controll(,d wvat baths were av$ilablz at tcmpcmtui of 20" 37.8 I 60 ,~ and 98.9" =t 0.03" (2. Modificd hprengl tuhcs of about 5-ml. capacity, which had been accuratcly calibrated, were filled b y suction and placed in the baths. Aft cr 30 minutcs they were brought t,o volume, rcmovcd from the ba.th, dried. and weighed. A pcriod of 5 miniitt:.; was a.llon-ed to elapse bet,w.ccll thc t,ime of removal from tlic' bath and completion of the 17-cighing. This t,iming permitted continuous and unhurried operation with Ihrcc tubes.
31.6 63.0 S.l 20.0
31.0 61.4 7.9 19.4
30.1 57 R 7.6 18.2
7
80
90
29.2 57.2 7.4 17.8
27.7 52 3 7.0 17.6
7
where D = A d / h T X loR,JI = molecular weight., a = 238, and h = 722 for the methyl and propyl esters and 739 for the isopropyl esters.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
December 1948
2383
VISCOSITIES
Viscosities were determined, using the apparatus and procedure as outlined under Method B, designation D 445-42T, of the American Society for Testing Materials (6). Table V summarizes the data obtained from the study of the viscositytemperature relationships of these esters. Figure 5 is a plot of these data on an A.S.T.M. chart (D 34143). These data gave straight lines, indicating that the empirical A.S.T.M. equation holds for esters as well as for the hydrocarbons for which it, was derived.
70-
605.0
-
4030
cn
-
2.0
O F METHYL,PROPYL AND ISOPROPYL TABLE Iv. DENSITIES~ ESTERSOF SOMENATURALLY OCCURRING: FATTYACIDS
w
