Characterization of Celluloses
by Hydrolvsis d
J
J
W. E. ROSEVEARE Rayon Technical Division, E. I . dlc Pont de Nemours & Co., Richmond, Vu.
c
ELLULOSES have been characterized in terms of the per-
centage of hydrolytically reactive material ($f 4, 17,18,90) and in terms of the leveling-off or limiting degree of polymerization (9, 6,14,19,21,26).The fast reacting part was assumed first to be due to solution of the amorphous material and the slow reaction to solution of the crystalline material. Now it is recognized that the reactive fraction on hydrolysis is less than the original amorphous material becauae of crystallization on hydrolysis (3,10, 12, Id, 96, 27). The leveling-off degree of polyzherization has been interpreted as a measure of the periodic length or the length of a chain passing through one crystalline and one amorphous region (93). After it has leveled off to an apparently limiting value, the degree of polymerization has been interpreted by others in terms of the crystallite length obtained 19, BY). The present paper presents some on hydrolysis (2,14,16, additional information on the behavior of a variety of celluloses during hydrolysis and gives applications of hydrolysis t o characterizing changes produced by various concentrations of caustic. HYDROLYSIS PROCEDURES
The glucose formed on prolonged hydrolysis is recognized to polymerize into black, insoluble humic substances (20). I n order to reduce errors arising from this effect, a hydrolysis cell was designed which permits fresh acid to pass slowly over the Earn&! during hydrolysis, thereby removing the soluble reaction products as they are formed. The use of this cell increaaed greatly the reproducibility of the degrees of hydrolysis and gave E whiter hydrolysis product.
found to result when air was present. A 0.25-mm. capillary about 7 inches long was attached to the capillary of the cell, m shown in the figure. The remaining acid was poured into the top of the large tube and a constant pressure of about 40 cm. of water was applied to the top of the large tube in order to reduce the variation in rate of flow while the level of the acid dropped in the large tube. The 0.25-mm. capillary tube was cut off a t the length necessar to give a flow of acid of about 125 ml. during the period of hydro?& At the end of the hydroIysis the fine capillary was replaced by a tube with a’sto per, and suction was ap lied to draw all the acid t o the filter flast. The sample was wasged with four 20-ml. portions of water. The filtrate and wash water were diluted to 500 ml. and the h drolyzed cellulose in an aliquot portion was determined by oxi&tion with dichromate (28). This method for determining the amount of cellulose dissolved was more reproducible than determinations of loss in weight, which were made on some samples. The degrees of polymerization of a tire-cord rayon after hydrolysis were found to be the pame whether the hydrolyses were carried out in the above extraction cell or in test tubes. Therefore all reported determinations of degrees of poIymerization were made on samples hydrolyzed in loosely stoppered test tubes immersed in a thermostat. The intrinsic viscosities of the residues were determined in 0.5 M cupriethylene-diamine a t 25 C. and the degrees of polymerization were calculated as 190.5 timeb the intrinsic viscosity, which gives the same values as those obtained by multiplying the cuprammonium intrinsic viscosity at 260 (6). O
KINETICS OF HYDROLYSIS O F THE REACTIVE FRACTION
The amounts of water-soluble material formed during various The hydrolysis cell, shown in Figure 1, was made from a medium-porosity, sintered-glass filter funnel by sealing a ~~-DIUI. times of hydrolysis of several celluloses are given in Table I. tube to the top and a 1-mm. capillary to the bottom. The capillary was bent to form a siphon for drawing the solution from the bottom of the cell to a flask outside the thermostat. TABLE I. BRHAVIOR OF VARIOUS CBLLULOSES IN 2.5 N SULFURIC ACID S T I R R E D THERMOSTAT
BATH,
PRESSURE OF 40 CM. WATER HM.
, R V TUSE: , 0.05 H n
Hydrolysis time, hours Tire-cord rayon 7 dissolved Fiber G rayon %%issolved Sulfate wood p h p %dissolved Sulfite wood puip 2, % dissolved Cotton-linters pulp, 70 dissolved
1.5 9.55 5.92 3.41
3 13.20 8.00 5.25
4 4.75 5 . 5 8 15.20 16.30 17.4 19.90 9.14 9 . 8 7 10.50 12.15 6.20 6.86 7.33 8.70
2.45
3.87
4.56
5.00
6.38
6.32
1.97
3.22
3.79
4.11
4.40
5.29
The apparent zero-time percentage of reactive material, a, for each cellulose was calculated from the writer’s formula ($2’)for a first-order reaction
Figure 1. Extraction Hydrolysis Cell
A sample of 1.5 grams was packed in on top sf the sintered-glass disk and large glass beads were placed on top of the sample to prevent any of it from floating. Suction was applied at the top of the large tube to draw acid from a beaker through the capillar tube until the sample was covered. Acid recently boi!ed wit{ reflux condenser was used to avoid variable result8 which were
where xl, 2 2 , and xa are the percentages of material lost by hydrolysis when the time interval between XI and xz equals the time interval between $2 and xa. Table I1 gives the values of the a’s calculated from the above formula. Figure 2 shows a plot of log a / ( a x) against time using the values of the x’s from Table I and the a’s from Table 11. The values of a were calculated from the values of x for the timee
-
168
INDUSTRIAL A N D ENGINEERING CHEMISTRY
January 1952
TABLE 11.
SPECIFIC REACTIONRATESAND AMOUNTSOF
RAPIDLY HYDROLYZABLE CELLULOSE 6, a,% ZO) 5% a -!-% 7% 24 5.8 30 Tire-oord rayon 0.20 15 3.0 18 Fiber G rayon 0.20 11 1.0 12 8ultate wood pulp 0.20 8 0.7 9 Sulfite wood pulp 2 0.21 Sulfite wood pulp 3 0.21 8 0.7 9 Cotton linters 0.18 7 0.6 8
5%
2*29z~3,,
30 18 12 8 8 7
169
determined by extrapolating the amount of slowIy reacting material back to the beginning of the reaction, By trial it was found that 2.29 times the amounts hydrolyzing in 3 hours was .a convenient empirical measure of the amount of hydrolytically reactive material. CHANGE IN REACTIVE FRACTION ON ALKALI TREATMENT
Several t pes of cellulose were steeped in aqueous solutions of sodium h $oxide of various concentrations at 25' C. After this the sampres were sucked dry on a Buchner funnel, washed with either boiling water or 10% sulfuric acid, washed with cold water, and dried. The reactive fractions taken as 2.29 times the amounts of material dissolved during a %hour hydrolysisim 2 5 AT sulfuric acid at 96O C. are shown in Figure 3. The Fiber G rayon is little changed by treatment with alkalfi of any concentration. Sodium hydroxide a t low concentrations has little effect on the other two celluloses, but at higher coneen-
0 TIRE--
x
fIAYOk
C I B U Q RAIo*
0 RAIOICQRADL SULIITC PULP @RAYOWORDOE SULFATE C U U COTTOY LINTEM
HOURS IN ZSN
He%
trationa the hydrolytically reactive fractions of the wood pulp m d tire-cdrd rayon are increased and decreased, respectively, toward the value for the Fiber G rayon. The rapid change in the rayon occurs a t a concentration of about 6% while the fir& rapid o w e in the wood pulp occurs a t a concentration of about 10%. 'Although the data in the rangeof 15 to 30% were not very reproducible, the second upward swing of the curve for the wood pulp at about 26% appears to be real, because other samples of native cellulose give similarly shaped curves. ?'he removal of the more concentrated alkali solution from the tire-cord rayon with 10% sulfuric acid gives a product with a much higher hydrolytic reactivity than that obtained by washing the caustic out with boiling water. The curves of Figure 3 would be somewhat
AT 96.0.
Figure 2. Hydrolysis of the Reactive Fraction of Various Celldosee after the completion of crystallization on hydrolysis, as indicated by the attainment of a constant or minimum regain, but before a eignificant amount of the slowly reacting material had reacted. Qince the a's were determined from the x's a t 1.6, 4.75, and 8 hours of hydrolysis, the agreement with a fmt-order reaction is measured by the closeness of the points a t other times to a straight line. Exact agreement with the first-order reaction behavior is not to be expected because of the heterogeneous nature of the reaction. However, since Philipp, Nelson, and M e (BO) found the reaction t o be first order for the solution of the slowly reacting crystalline fraction of cellulose, it is not surprising that the solution of the crystallized reactive fraction is also approximately fist order. Because of the higher rate of hydrolysis before completion of crystallization, the straight lines do not go through the origin. The values of the sds calculated from the intercepts are measures of the amounts of material dissolving in excess ol that which would have dissolved if cry% tallkation had been complete at zero time. These values are given in the third column of Table 11. The specific reaction rates, k, obtained from the slopes of the straight lines are given in the first column of Table I. The specific reaction rates for all six celluloses are nearly the same and nearly constant during the time in which a large part of the hydrolytically reactive fraction is dissolving. This suggests that the nature of the reactive fraction duringhydrolysis in 2.6 N sulfuric acid a t 96" C. is the same for all types of cellulose, even though the amount of reactive material present originally varies severalfold. This is quite different from the relative hydrolysis rates for the slowly reacting fractions, which Philipp, Nelson, and Ziifle (BO) found varied as much as sixfold for different celluloses. The total original Bmount of the reactive material is (a so). These values given in the fourth column of Table I are intermediate between the values reported by various observers (4, 18, $0) for the percentages of accessible cellulose as
+
m U M HYDROXIDE CONCENTRATION ( x )
Figure 3. Effects of Sodium Hydmxide on Cellulose as Indicated by Change in Amount of Hydrolytically Reactive Material different if all the cellulose disuohed in the steeping caustic had been recovered with the sample. In the 9% sodium hydroxide the tire-cord rayon dissolved to the extent of 8.5%. This diesolved material when coagulated and regenerated with Sulfuric acid and air-dried, showed on hydrolysis 33% reactive material. A weight average of this 33% reactive material and the 21% reactive material for the undissolved yarn is 22% reactive mterial if all the yarn had been recovered. Therefore, neglecting: the amount dissolved has little effect on the shape of the CurVBLy of Figure 3. These results suggest that the most stable form op regenerated cellulose has a hydrolytic accessibility between 16 and 20% and that tire-cord rayon, with a high accessibility, ie metastable and goes over to the more stable form on swelling and deswelling.
170
INDUSTRIAL A N D ENGINEERING CHEMISTRY EFFECT O F CONDITIONS ON HYDROLYSIS BEHAVIOR
For four types of cellulose the amount of material hydrolyzed at 96" C. plotted against time for the 0.25 N sulfuric acid hydrolysis could be superimposed within experimental error on similar curves for the 2.5 N acid hydrolysis. It follows from this that the amount of hydrolytically reactive material is the same for both acid concentrations.
D SULFITE
WOOD PULP I
I
YERCERILED Moo PULP 3
FIBER Q RAYON
m
IS
HOURS IN 2.5 N H em AT
f3
96 *C.
Table I V summarizw the hydrolytic characterization of a variety of celluloses in t e r m of the percentage of reactive material, the degree of polymerization after 6 hours' hydrolysis in 2.5 N sulfuric acid a t 96' C., and the percentage drop in degree of polymerization between 6 and 24 hgurs' hydrolysis. The percentage of reactive material is taken as'2.29 times the percentage of cellulose dissolved in 3 hours by 2.5 N sulfuric acid a t 96" C. The three characterization constants of Table IV do not correlate with each other and this indicates that they measure different properties of the cellulose. The sulfate pulps tested are more like linters in having a low value for the drop in chain length during the 6- t o 24-hour period. On the other hand, the percentage of reactive material is not similar for the sulfate pulps and cotton linters. Since intrinsic viscosity is determined by the weight rather than number average of the degree of polymerization, cutting of chains near their centers produces a greater drop in viscosity than the same number of cuts near the ends of the chains. This suggests that the higher rate of drop in degree of polymerization for the wood pulp during the 6- to %-hour period of hydrolysis in which the last two fifths of the reactive fraction is dissolving, is due to a greater amount of lateral relative to end attack of the chains in the wood pulps. This effectis to be expected because of the lower lateral order of the crystallites of the wood pulps relative to that of cotton linters.
TABLE LV.
CHARACTERIZATION OF CELLULOSES BY HYDROLYSIS
Figure 4. Characterization of Celluloses in Terms of the Viscosity after Hydrolysis Battista ( 8 ) has observed that an increased resistance of some rayons to hydrolysis is produced by a mild prehydrolysis for 10 days in 2.5 N hydrochloric acid a t 18' C. This behavior appeared inconsistent with some of the writer's unreported experience using sulfuric acid on a tire-cord rayon. Therefore, three types of rayon were treated with the severe hydrolysis and with the mild hydrolysis followed by the severe one exactly according t o Battista's published procedure (8). The percentages of residue with respect t o the original cellulose on dry basis obtained are given in Table 111. The textile and tire-cord rayons are commercial types. These results show that an increased loss in weight is caused by the mild prehydrolysis instead of the stabilizing effect found by Battista. This difference may be due to differences in the nature of the rayons or to differences in experimental procedure not recognized by the writer.
Vol. 44, No. I
Reactive Material, %
Sulfate pulps (Rayon grade) 1 2 Cotton linters Mercerized wood pulps
Rayom
9
12 7
2 3 6
19
Fiber G Textile Tire oord
18 28 30
Degree of Polymerieation after 6 H o p ' Hydrolysis
Drop between 6 and 2 4 Hrs., %
1.58 158 187 69 76 65 46
36
DEGRADATION CHARACTERIZATION O F CHANGES PRODUCED BY MERCERIZATION
TABLE 111. EFFECTOF MILD HYDROLYSIS ON SUBSEQUENT SEVXU~E HYDROLYSIS % Residue after 15 Min. in 2.5 N HCl at Boll Without mildo prahydrolysia
With mild' prehydrolysk 82.3 Desulfured tire rayon 83.2 80.6 Textile rayon. 81.6 80.8 Above after liquid ammonia treatment 81.3 a Mild hydrolysis, 10 days in 2.5 N hydrochloric acid at 18' C.
DEGRADATION CHARACTERIZATION O F TYPES OF CELLULOSES
The behavior of the intrinsic viscosity of the residue after various periods of hydrolysis shows up differences between pulps, between mercerized celluloses, and between rayons. The intrinsic viscosity and degree of polymerization of a variety of celluloses after hydrolysis for various lengths of time in 2.5 N sulfuric acid a t 96" C . are given in Figure 4. The degrees of polymerization of all these celluloses before hydrolysis are too hish t o be shown on this plot.
Mercerization of linters and wood pulps produces a large change in the intrinsic viscosity after hydrolysis. This suggests that the change in periodic length, as measured approximately by the viscosity after hydrolysis, could be used to characterize the changes in native celluloses produced by treatments with caustic. Two pulps were steeped in various concentrations of sodium hydroxide for 30 minutes and washed with water. The intrinsic viscosity in 0.5 M cupriethylene-diamine solution after hydrolysis in 1% sulfuric acid for 24 hours a t 96" C. was determined for each sample. (This work was done before standardizing on the &hour hydrolysis in 2.5 N acid for characterizing celluloses. For empirical characterization, either set of conditions is satisfactory.) The results of these tests given in Figure 5 show that the treatment of pulps with caustic solutions below a concentration of about 9% has almost no effect on the viscosity after hydrolysis. There is a higher concentration range in which the viscosity is decreased rapidly with increasing caustic concentration. At still higher concentrations, the viscosity after hydrolysis again be
January 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
comes practically independent of the concentration of the steeping solution. A comparison of the different curves show that modification of the structure of the wood pulp occurs at a tower concentration than that required to modify cotton linters at the same temperature. These comparisons show also that modification occurs at a lower caustic concentration a t lower temperatures. The shapes of these curves are in agreement with the x-ray observations of Simon and Saner (84) and moisture regain observations of Mitchell (16): They found that at a given temperature there is a range of concentration of caustic in which only partial mercerization occurs. I n addition to producing a new crystalline form, as indicated by x-rays, the above results indicate that mercerization changes the original network of crystallites t o a new one with a much shorter periodicity. The increased reactive fraction and increased moisture regain on mercerization indicate a lower degree of crystallinity after mercerization. The swelling of the.crystalline aa well as amorphous portions in the mercerizing caustic t o give largely an oriented gel and the subsequent formation of new crystallites on deswelling is a reasonable explanation of this phenomenon. The very low lateral order of celluloses swollen in mercerizing camtic, as indicated by x-rays, and the formation of a new type of crystalline lattice on regenerating, supports the view that the original crystal structure is disrupted by swelling on mercerizing. More direct evidence for the formation of new smaller crystallites is the observation by Heyn (11) that the lateral periodicityaverage lateral distance from center of one crystallite t o the center of the next one-as determined by small-angle scattering of x-rays, is reduced 35% on mercerizing cotton. A different interpretation has been given by Jorgensen (14). He interprets the decrease in limit degree of polymerization with increasing concentration of caustic used in treating the cellulose before hydrolysis as being associated with changes in intercrystalline regions rather than in the crystallites. INHIBITlON OF CRYSTALLIZATION ON HYDROLYSIS
Partial substitution of the hydroxyls of cellulose by other groups can greatly inhibit its crystallization (1). This suggests that the increasing crystallization on hydrolysis could be largely eliminated by partially oxidizing the amorphous regions of the cellulose before hydrolysis. In order to test this mechanism of inhibition of crystallization by oxidation, the amorphous regions of samples of a rayon-grade cotton linters and a viscose rayon were oxidized in 0.8 M potassium dichromate (soluble at 50" C. but not at room temperature) in 0.25 N sulfuric acid for 18 hours at 50" C., and then hydrolyzed for 6 hours in 2.5 N sulfuric acid a t 96" C. and washed with water, The loss in weight was determined only in the case of the rayon but the intrinsic viscosities of the residues of both samples were determined in 0.5 M cupriethylene-diamine at 25' c. Davidson (7) has concluded from x-ray evidence that the amorphous regions of cellulose can be rather highly oxidized by dichromate in dilute sulfuric acid with little destructive effect on the crystalline regions. The combined oxidation and hydrolysis treatment caused the rayon to lose 67% of its weight compared to an 18% loss on hydrolysis alone. This loss in weight is difficult to reproduce, because the residue after oxidation and hydrolysis is gelatinous and difficult to filter. In addition, some of the crystallites appear to pass through the
TABLE V. EFFECTOF OXIDATIONON DEGBEE OF POLYMERIZATION APTER HYDROLYSIS Degree ot Pol meriaatiqn &br 6 H? Hydrolyw
Cotton linters
Degrqe of. Polymerization after Oxidation and 6 Hrs.' Hydrolyeis , I49
Visaose rayon
80
40
190
171
filter, as indicated by the turbidity of the filtrate which was discarded. Because of these difficulties this is not a good method of determining the amount of amorphous material originally present. However, the 67% loss in weight agrees better witd the 68 t o 86% accessible material in rayons found by deuterium oxide exchange (8)and with the 60%'amorphous material found by x-rays ( 9 ) for most rayons than with the 33% reactive portion found by hydrolysis alone. The degree of polymerization drop of cotton linters and a tire-cord rayon after oxidation and hydrolysis and after hydrolysis alone are given in Table V. Varying the degree of oxidation of both the cotton linters and the rayon from 0.2 atom of oxygen per glucose to 0.5 produced no measurable difference in the degree of polymerization after subsequent hydrolysis.
09 0
8
I
I
10
I8
I
w
SODIUM HYDROXIDE CONCENTRATION
CX)
Figure 5. Effects of Sodium Hydroxide on Cellulose as Indicated by Vifrcosity after Hydrolysis
The much lower values of the de'gree of polymerization and its independence of degree of oxidation as well a8 the greater loss in weight obtained on hydrolysis after oxidation support the hypothesis that chemical substitution is inhibiting crystallization during hydrolysis so that the amorphous regions hydrolyze rapidly to soluble materials. According to this hypothesis, the degree of polymerization after oxidation and hydrolysis is the length of the original crystallites of the cellulose. SUMMARY
The specific reaction rates for the sblution of the &active fraction on hydrolysis are practically the same for a variety of native and regenerated celluloses. The kinetics of the solution of the reactive fraction on hydrolysis gives the same accessibilities as have been obtained by extrapolations of the slow reacting fraction back to zero time. Aqueous alkali treatments of native and regenerated celluloses bring their hydrolytic accessibilities very much closer together in the region of 9 to 12% sodium hydroxide and to less extent for neighboring concentrations. A mild hydrolysis of three rayons in 5 N hydrochloric acid at 18" C. did not stabilize them with respect t o loss in weight on subsequent severe hydrolysis at high temperatures. Rayon-grade cotton linters and sulfate wood pulps, as well as rayons, show more of a leveling-off of degree of polymerization on hydrolysis than do several rayon-grade sulfite HIPS. The degree of polymerization after hydrolysis is used to characterize the changes produced in cotton linters and a wood pulp by treatment with various concentrations of caustic. Crystallization of amorphous material on hydrolysis is inhibited by it8 preoxidation, so that all of it may be removed on subsequent hydrolysis.
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
172
LITERATURE CITED
(1) Baker, W.O.,IND.ENG.CHEM., 37,246 (1945). (2) Battista, 0.A.,Ibid., 42,502 (1950). (3) Brenner, F. C.,Frilette, V. J., and Mark, H., J . Am. Chem. Sor., 70, 877 (1948). (4) Conrad, C. C., and Scroggie, A. G., IND.ENG.CHEM.,37, 592 (1945). (5) Conrad, C.M.,Tripp, V. W., and Mares, T., paper presented a t the 117th Meeting of the AMERICAN CHEMICAL SOCIETY, Detroit. Mich. ( 6 ) Davidion,-G. F., J . Teztile Insl., 32, T132 (1941).
(7) Ibid., 34, T87 (1943). (8) . , Frilette. V. J., Hade. J.. and Mark, H., J. Am. Chem. SOC.,7 0 , 1107 (1948). (9) Hermans, P. H., and Weidinger, A., J. Polymer Sci., 4, 135 (1949). 10) Ibid., 4, 317 (1949). 11) Heyn, A. N. J., Testile Research J . , 19, 163 (1949). 12) Howsmon, J. A.,Ibid., 19, 152 (1949). 13) Ingersoll, H.G., J . Applied Phys., 17, 924 (1946). 14) Jorgensen, L.,Acta Chem. Scund., 4, 185 (1950).
or
ressures
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Vol. 44, No. 1
(15) Mitchell, R. L.,IND.ENQ.CHEM., 43, 1786 (1951). (16) Morehead, F. F.,Teztile Research J., 20, 549 (1950). ENG.CHEM.,33, 1022 (1941); 34, 86 (17) Niskeraon, R. F., IND. (1942): 34.1480 (1942). (18) Niokerson, R: F., and Haberle, J. A., Ibid., 37, 115 (1945);38, 299 11946). --, (19) Ibid.i39, 1507 (1947). (20) Philipp, H.J., Nelson, M. L., and Ziifle, H. M., Tertile Research J., 17, 585 (1947). (21)Reeves, R.E., Schwartz, W. M., and Giddens. J. E., J. Am. Chem. SOC.,68, 1383 (1946). (22) Roseveare, W. E., Ibid., 53, 1651 (1931). (23) Roseveare, W.E.,Waller, R. C., and Wilson, J. N., Tertile Research J . , 18,114 (1948). (24) Sisson, W. W., and Saner, W. R., J. P h y s . Chem., 45,717(1941) (25) Staudinger, H., and Sorkin, M., Bw., 70B, 1565 (1937). (26) Waller, R. C., Bass, K. C., and Roseveare, W. E., IND.ENG. CHEM.,40,138 (1948). (27) Ward, K.,Jr., Textile Research J., 20,363 (1950). (28) Windeck-Schultze, K., and Peiper, I., Melliand Textilber., 29 (1948).
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RECEIVBD October 18, 1950.
istillation Some
J TRQY A. SCOTT, JR., DUlPilCAN MACMILLAN, AND EUGENE W. MELVIN Northern Regional Research Laboratory, Peoriu, Ill.
T
HE usefulness of distillation methods as applied to the Cia
fatty acid methyl esters has been principally in the preparation of samples for analysis (10). Although it is not difficult to separate saturated acid esters from one another, only a slight enrichment can be brought about by the fractional distillation of mixtures of the CISunsaturated esters. With Bonhorst, Althouse, and Triebold (3) having reported decomposition a t temperatures higher than 200" C., any hope of separation by distillation would be a t low temperatures and pressures. Thus, it was decided to determine the requirements for a still that was capable of fractionating the C1sunsaturated acid esters, and to extend the range of vapor pressure measurements to as low as 0.1 mm. of merCWY.
The boiling points of the methyl esters of some of the fatty acids, a t pressures down to 1 or 2 mm. of mercury, have been determined by Bonhorst, Althouse, and Triebold (Q),Norris and Terry (ii), and Althouse and Triebold (1). Bonhorst et al. (8) measured vapqr pressure by observing the pressure a t which the level of a liquid in a capillary fell. Norris and Terry (li), using a simple type of ebulliometer, reported boiling points to 0.5" C. Althouse and Triebold (i), using the dynamic method of Ramsay and Young, reported boiling pointa to 1O C. The three papers disagree on boiling points at certain pressures by as much as 4 or 5 degrees. All three papers show straight-line relationships between log p and the reciprocal of the absolute temperature.
ture a t which the plateau is found is taken as the correct boiling point. This procedure was followed in the work reported in this paper. Figure 1 represents the modification of the Hickman tensimeter used in this work. During the evolution of this instrument, both external and internal heaters were used. It was found that the combination of an internal heater of Nichrome wire and the large surface resulting from the widely flared bottom gave the smoothest boiling and the longest plateau in the plot of boiling temperature us. wattage. About 40 ml. of liquid are required to operate the tensimeter. The iron-constantan thermocouple used was calibrated a t 122 C. with the National Bureau of Standards benzoic acid apparatus (id),and a t the ice point. A copper-constantan thermocouple was not satisfactory because of heat loss through the copper wire. The electromotive force of the thermocouple was measured with a Leeds and Northrup potentiometer No. 8662. A wattmeter was used to measure the input to the tensimeter (8). Pressures were measured with a McLeod gage and an oil manometer; these were protected from possible contamination by a trap surrounded by a slush of ethanol and solid carbon dioxide. Methyl esters of fatty acids are not easy materials to work with and Hickman's apparatus was adopted after trying without success another type of ebulliometer, the gas saturation method, and the dew point method. Hickman's original design was better suited to pressures less than 0.1 mm. of mercury and the present authors' modification was more satisfactory a t higher pressures.
APPARATUS
I n the present study a tensimeter similar to one described by
Hickman, Hecker, and Embree (8) was used. These authors operated their tensimeter by applying an increasing wattage to the heater and making a plot of boiling temperature us. wattage. I n this plot, a plateau occurs which represents constant boiling temperature over a fairly wide range of wattage. The tempera-
PREPARATION OF MATERIAL
The methyl esters of the even-numbered fatty acids from caprylic through palmitic were obtained by careful fractionation of the esters made from coconut oil in a Podbielniak column with a 4-foot length of continuous helical packing. In the fractional distillation the products were collected in small portions; transition
