SOME PROPERTIES OF CELLULOSE ESTERS OF HOMOLOGOUS FATTY ACIDS S. E. SHEPPARD
AND
P. T. NEWSOME
T h e Kodak Research Laboratories, Eastman Kodak Go., Rochesler, New I’ork Received Jtblle 14, 1034
In a previous papcr (7) attention was drawn to the utility of working with homologous series of compounds in the effort to obtain quantitative characteristics of the solubilities of certain cellulose esters. In this paper the procedure is reversed, in so far that it is the cellulose esters themselves which here form the homologous series. The data recorded here for the most part apply to the triesters of cellulose. They are only strictly comparable on the assumption that the degree of polymerization of the cellnlose remained the same through any given series, or, in other words, that the average molecular weight of the cellulose portion remained the same. In the series of compounds prepared in our own laboratories, where the same original cellulose material was used, we have reason to regard this as substantially the case, and the same may be regarded as equally probable for the material of other investigators cited. But while the comparisons in a series may be regarded as justified, it is not permissible to consider any given triester of investigator A as necessarily the same as that of investigator B. Identical in composition, they might differ considerably in average molecular weight. With this reservation made, we can proceed to collate the data showing the progressive changes produced in cellulose esters as the molecular weight of the fatty acid is increased. MEASUREMENTS OF PHYSICAL PROPERTIES
The melting points were determined by heating the sample in a glass capillary tube placed in a copper block. The densities were determined by using methyl alcohol as the displacement liquid. Methyl alcohol wets the esters, especially the higher ones, much better than water and gives a higher density value than water. The esters were dried by evacuation a t room temperature. The data are shown in table 1 and figures 1 to 4. The densities of the esters decrease regularly with increase in number of carbon atoms, corresponding to the behavior of the free acids (see table 2). The melting points of the cellulose esters pass through a minimum a t 143
.
144
S. E . SHEPPARD AND P. T . NEWSOMIT
cellulose caprate, whereas the melting points of the normal fatty acids pass through a minimum a t valeric acid, thus showing the influence of the cellulose residue. ' There is no definite evidence of an alternation in the melting points of the esters with odd and even numbers of carbon atoms as is the case with the free acids. This effect, which is more pronounced for the lower acids, is apparently minimized by the presence of the cellulose residue-large in comparison to three hydrogen atoms. Since the alternating effect is attributed to differences in crystal structure (2), the presence of the cellulose residue should lower the degree of orientation and therefore reducc the magnitude of the alternation. TABLE 1 Properlies of cellulose trieslers al 16OC. I
ESTER
NO, OF :ARBON ATOM0
PER CENT MOIBTORE REGAIN
DENBITY
MELTING POINT
cent
R.H. -
Propionate. . . . . . . . . . . . 3 Butyrate. . . . . . . . . . . . . . 4 Valerate . . . . . . . . . . . . . . 5 Caproate. . . . . . . . . . . . . . 6 Heptoate . . , . . , . , . , , . , 7 Caprylate, . . . . . . . . . . . . 8 Pelargonate, . . . . . . . . . . 9 Caprate. . . . . . . . . . . . . . . 10 Laurate. . . . . . . . . . . . . . . 12 Myristate. . . . . . . . . . . . . 14
1.618 1.377 Decomposes a t 245 1.268 239 1.178 183 1,178 160 1.110 87 97 1.081 1.058 85 06 1.032 1.026 64 1.004 80 0,991 87
--
VORK OF ADEE&IONVE. WATER
R.H.
degrees C.
0 2
CONTAC~
50 per
(Cellulose). . . . . . . . . . . . Acetate. . . . . . . . . . . . . . .
I
degrees
W 8
5.6 2.3
18.8 10.0
50
117
.1.3 0.7 0.5 0.3 0.2 0.1 0.1 0.3 0.3
4.4 3.5 1.7 0.9 0.8 0.9 1.0 1.5 1.4 1.5
78 84 90 93 100 99 100 100 101 104
87 79 72 68 59 61 59 59 58 55
-
-
The moisture regain apparently passes through a minimum ncw cellulose heptoate. The reason for this is not known. It appears that the moisture regain should continue to decrease with increase in number of carbon atoms in the side chains. The contact angle against water steadily increases with increase in length of the hydrocarbon chain and approaches that of a long chain hydrocarbon-pure paraffin having an angle of 108' and work of adhesion of 50 ergs. The method used for the determination of the contact angle, and therefore of adhesion tension or work of adhesion, was by measurement of thc height of a drop of water placed on a horizontal surface of the solid (5).
CELLULOSE ESTERS OF HOMOLOGOUS FATTY ACIDS
145
The contact angle is given by the equation h = a d s sin 0/2
-
where a =
fix, h .
IIP -
= height of drop in cm.,
T = surface tension of liquid,
g = gravity, and p = density of liquid. The drop should be a t least 1.5 em. in diameter. Glass plates were coated uniformly with the various esters, from solutions of these in chloroform. The height of the drop was determined with a spherometer. 1.700
-
1.100
-
I.000
-
.so0 .8W0
I
2
I
I 4
'
I
I
I
'
I
I
6 8 10 I2 NUMBER O F CARBON ATOMS
I
'
14
FIG.1. DENS~TY OF CELLULOSE TRIESIERS AT 25°C.
The work of adhesion is obt.ained from the contact angle e by the formula
W = T,,, (1 -
COS
6)
where T,, is the surface tension, liquid to air. It will be noticed that the values of the work of adhesion of water for the fatty acid esters of cellulose appear to fall steadily, finally approaching that of para&. On the other hand, A. H. Nietz (6) found for the fatty acids themselves a minimum value, in the neighborhood of twelve carbon atoms. Nietz discusses various molecular and thermodynamic explanations for this minimum, which are, however, unnecessary. The actual
146
8. E . SHEPPARD AND P. T. NEWSOME
reason is the fact that the lower fatty acids (up to pentadecylic) have appreciable solubility in water, and therefore appreciable lowering of the surface tension T L A . Consequently, while the contact angle 0 rises more or less steadily (there is an alternating effect not found with the cellulose esters), the surface tension is a t first nearly constant at a lowered value, then rises suddenly to be nearly equal to that of pure water above pentadecylic acid. Hence the minimum for the expression
w = T,, (1 - COS e) 250
230 210
-
J 190-
z+ 170--
0
-
E
a0 140z I30 d
ZllO 5w
90
70
-
Q
-
. BOO
'
::
I
I' li '
FIG. 2. MELTINGPOINT
A
' Ib ' Ih '
OF CELLULOSE
1'4
TRIESTERS
SOLIDIFICATION AND MOLECULAR ORIENTATION
If the surface layers of molecules of these substances were completely oriented, as in a monomolecular film (4), then in accordance with Langmuir's theory of surface structure, we should expect to find either very high wetting (for the cellulose esters) or a very low (paraffin) value. The progression of values from high to low wettability indicates a-probably kinetic-effect of the chain length (in the case of the cellulose esters, side chain length) on the surface orientation on solidification.
CELLULOSE ESTERS O F HOMOLOGOUS FATTY ACIDS
*
147
I n the case of the fatty acids themselves, Nietz found, using stearic acid, that the surface orientation on solidifying the melt depended decisively upon the medium in contact, ie., the contiguous liquid or gas phase. High humidity or liquid water produced a surface of much higher adhesion tension, showing orientation of the polar groups to the water. The increasing domination of the properties of the cellulose ester aggregate by the aliphatic side chains is well brought out in the studies of N. K. Adam (1) on the spreading of films of these substances on water,
NUMBER O F CARBON A T O M S
FIG. 3. MOISTUREREQAINOF CELLULOSE TRIESTERS AT 25°C.
and in those of J. J. Trillat (9) on their x-ray spectroscopy. I n figure 5 is reproduced Adam’s data for the spreading of a number of fatty acid esters of cellulose. There is observable an increased “molecular” area covered as the length of the fatty acid chain increases (48 sq. A.U. for triacetate, ca. 78 sq. A.U. for tristearate). Adam concludes that “the hexose ring, lying $at on the surface of the water, occupies an area greater than that of the three eighteen carbon chains [also presumed to be flat on the water?]; and that lateral
148
8. E. SHEPPARD AND P. T. NEWSOME #
compression of the film allows some rearrangement of the hexose rings next the water, which form the base of the film' the superstructure of which no doubt consists of the long chains arranged at a fairly steep angle to
' 6 ' L
60:
I
d ' Ib'
I
1;
la'
NUMBER O F C A R B O N ATOMS FIG.
4. WORK
OF
ADHEsION OF CELLULOSE
TRIESTERS AQAINST
WATER
TABLE 2 Densities and melting points of normal fatty acids ACID
1
DENSITY AT
20°C.
I
MELTING POINT
degrees C.
Formic ........................................... Acetic ............................................ Propionic. .............................. Butyric. .......................................... Valeric ............................................ Caproic ........................................... Heptylic, ......................................... Caprylic, ......................................... Pelargonic ........................................ Capric ...........................................
...................................
Myristic.. . . . . . . . . . .. ... Stearic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.220 1.049 0.992 0.959 0,942 0.929 0.922 0.910 0.907 0.895 0.883 0.858 0.847
8.4 16.6 -22.0 -7.9 -34.5 -9.5 -10.0 16.0 12.0 31 .O 48.0 58.0 69.3
the surface.1 The lateral adhesion between the long 18-carbon chains is not sufficient to counteract the tendency of tmhehexose rings to lie flat." 1
Our italics.
149
.CELLULOSE ESTERS O F HOMOLOGOUS FATTY ACIDS
These conclusions do not seem to us to be entirely tenable in the light either of general x-ray evidence on the structure of the cellulose macromolecule, or more specifically of Trillat’s (9) results with the same series of fatty acid esters of cellulose whose properties were described here. This
FIG.5. FATTY ACID ESTERSO F C E L L U L O S E NOT 1 . Tristearate 2. .Tripropionate on sodium hydroxide 3. Dilaurate 4. 2) Butyrate on sodium hydroxide 5. Triacetate on sodium hydroxide N. K. Adam
FULLYSPRE.4D
ON W A T E R
6. 2+ Butyrate
7. Distearate 8. Tripropionate 9. Triacetate
TABLE 3 Tensile strength and total elongation of cellulose ester sheets CELLULOSE ESTER
TENSILE STRENGT.
kg. per
Acetate.. . . . . . . . . . . . . . . Propionate . . . . . . . . Butyrate ........................................ Valerate . . . . . . . . . . .......... Caproate ........................................ Pelargonate ......................... Laurate ............................. Stearate. ........................................ Naphthenate .....................................
TOTAL ELONQATION I N P E R CENT
mnc.i
9-12 6-7 5-6 4-5 2.5 3.5-4 .O 0.8-1 .o 0.5
15-25 10-15 8- 10( 1 ) 18-25 60 20-30 100-130
0.3
110
140
investigator found that the Debye-Schemer transmission diagrams showed a structure greatly modified by the fixation of the long aliphatic chains. I n brief, there were reproduced the diagrams of the fatty acids, or their metal salts, superposed on a (fading) cellulosic pattern. While the lower
150
6. E. SHEPPARD AND P. T. NEWSOME
members (triacetate, tripropionate, tributyrate) have a definitely crystalline structure, the higher members show structure more and more mesomorphic. w.12 I€
2
4
6 8 IO I2 14 NUMBER O F CARBON ATOMS
I 0
I L
FIG. 6 . TENSILE STRENGTH O F CELLULOSE ESTERSHEETS Hagedorn and Moeller I€
E
‘IO
ri
12
-
-
0
y : -
20
40
60
80
100
I20
140
q o ELONGATION FIG.7. TENSILE STRENGTH A N D ELONGATION OF DIFFERENT CELLULOS~ ESTERSHEETS Hagedorn and Moeller
Using the rotating crystal method for thin films of the esters compressed on glass sheets, it was found that in the degree that the side chain length
CELLULOSE ESTERS OF HOMOLOGOUS FATTY ACIDS
.
151
increased, there appear reflexions of planes separated by intervals varying with the number of carbon atoms, planes parallel to the surface of the glass. “This indicates that the aliphatic chains, when their length is sufficient, orient themselves parallel to the direction of pressure, perpendicular to the plane of the support.” The aliphatic chains are placed perpendicularly, or nearly so, to the direction of the cellulose chain, that is, to the plane of the glucosan (or pyranose) ring. Further, there will be an alternation of direction of the two secondary substituents and the one primary. In this way it is easy to see the mutual orientations as attractions of the cellulosic chains submerged and lost as the aliphatic chains increase in length. But also, this structure is difficultly compatible with that of Adam for the spreading on water. For the pyranose rings to lie flat on the water, there would have to be, not only a superstructure of hydrocarbon chains, but also an identical substructure extending down into the water. That does not seem very probable. Moreover, if this took place, the area in compression should remain substantially the same, independent of the lengt,h of the side chains. For these reasqns it seems more probable that the main valence chain must lie either edgewise or very much tilted on the water surface. This progressive submergence of the cellulose skeleton by an adipose carcass is again brought out in the figures for elasticity and mechanical strength. The tensile strength and total elongation a t the breaking point of cellulose triesters are shown in table 3, and figures 6 and 7 as determined by Hagedorn and Moelier (3). The tensile strength decreases with increase in the number of carbon atoms in the side chain and the total elongation increases. SOLUBILITY
The solubility of the higher fatty acid esters of cellul’ose has been discussed already by G. S. Whitby (10) at a previous symposium. He concluded that they were less polar than cellulose acetate, because of the longer hydrocarbon residues. But this does not seem a necessary conclusion, since according to Smyth (8) the polar moment need not change with length of chain. We do not propose to discuss this subject further here, beyond saying that the solubilities of the series of homologous triesters of cellulose do not support a theory of solubility based solely, or even mainly, on polarity and polar moment. SUMMARY
A study has been made of the physical properties of a series of triesters of homologous fatty acids, from the acetate to the stearate. It shows the
152
6. E. SHEPPARD AND P. T. NEWSOME
“cellulose” character being progressively submerged as the length of the side chain is increased. The structure of the solids is interpreted from xray data and from spreading and wetting experiments. ‘ REFERENCES (1) ADAM,N. K.: Trans. Faraday SOC.29,90 (1933). (2) GARNER,W. E., AND RANDALL, F. C.: J. Chem. SOC.126, 881 (1924). (3) HAGEDORN, M., AND MOELLER,P. : Verofftlich. wiss. Zentr-Labor. photogr., Abt. Agfa (I. G. Farbenindustrie AG), Vol. 1, p. 150 (1930). ULMANN, M.: Azetylzellulose-Folien und Filme, p. 6. W. IZnapp, Halle (1932). (4) LANGMUIR, I.: Chem. Met. Eng. 16, 468 (1916). (5) LANGMUIR, I.: Trans. Faraday SOC.16, 62 (1920). (6) NIETZ,A. H. : J. Phys. Chem. 32,620 (1928). (7) SHEPPARD, S. E., AND SWEET, S. S.: J. Phys. Chem. 36,819 (1932). (8) SMYTH,C. P.: Dielectric Constant and Molecular Structure, p. 96. The Chemical Catalog Co., Inc., New York (1931). (9) TRILLAT, J. J.: Compt. rend. 197, 1616 (1933). (10) WHITBY,G. S.: Colloid Symposium Monograph 4,203 (1926).
