THE NATURE OF THE STRUCTURE OF CELLULOSE AND ITS SIGNIFICANCE IN CHEMICAL TRANSFORMATIONS* O BY B. O. HEEZOG
In the first part of the present paper the previous X-ray spectrographic investigations* carried out with untreated and mercerised cellulose respectively, have been controlled, and new evidence obtained as to the behavior of cellulose in the magnetic and electric field. Further, the results of X-ray analysis of certain derivatives of cellulose (cellulose nitrate and cellulose acetate) are described. In the second part, a parallel is drawn between the dimensions of crystallites of cellulose and of its derivatives, and the particles found in colloidal solution, and evidence of a relation between the size of particles and the size of molecules is brought forward.
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1,
PART I
X-Ray Spectrographic Investigation of Cellulose (Carried out with W. lanché) It has been clearly shown, that cellulose, in its natural condition, consists of aggregates of crystallites as found in the bast fibres, seed hairs, and other plant2 tissue as well as in the animal body (Tunicin)3. In the bast fibres, the crystallites are often arranged with one crystal-axis in the fibre-axis, or, (at any rate those of the same layer) at a definite angle to it; in cotton, they are— with respect to this axis—twisted spiral-like, etc; it may be said with some degree of certainty that the observations made by means of double refraction, and interpreted on the basis of Nageli’s micellar hypothesis,4 *or by the theory of Wiener6 are completely confirmed by X-ray spectrography. It is also possible to use the experimental data obtained in the present investigation for a revision of the determination of the crystalline system of cellulose. Since among the bast fibres, ramie, hemp, Titaric6, broom, and mulberry-tree show the best defined interferences, and the ratio pf the distances of these on the equator are identical within the limits of experimental error, these fibres, in addition to that of cotton, were chosen as suitable obindicate the interference points jects for investigation. In Table I Ai A2 on the equator of the sphere, I, II, III, IV, etc., those on the corresponding .
*
.
.
Contribution from the Kaiser Wilhelm-Institut fiir Faserstoffchemie, Berlin-Dahlem,
Z. angew. Chem. 34, 385 (1921). 2 Ber. S3, 2162 (1920); Naturw. 12, 955 (1924). 3 Z. physiol. Chem. 141, 63 (1924). 4 Sitzungsber. Akad. Wiss. München, 9, 389 (1879). 6 Pogg. Ann., 118, 79 (1863). 6 A tropical bast fibre placed at our disposal, the exact nature of which it to ascertain. 1
was
impossible
R. O. HERZOG
458
hyperbola (layer-lines). Figure i shows the crystallographic indices of the points according to Polanyi4. The second column records the intensity of the interference spots v.str. moderate; w. weak; v.w. strong; m. very weak). very strong; st. of the observed reflection. denotes angle (Glanzwinkel). D/2 =
=
=
=
=
Table I Observed point-positions for natural cellulose using vertical incidence. (Average value) Possible
Point
Intensity
sin1
str. A2 As A4
m.
A&
m.
Ii
w.
I2
m.
I. Hi
w.
str.
II2
s.
II. II. lit nil Ills III. III. Ills IVi
s.
0/2
sin2
0,01675 0,02105 0,03210 0,03923 0,1600 0,0x296 0,08940 0,1142 0,03260 0,06480 0,07070 0,1335 0,1683 0,0604 0,0690 0,0894 0,1332 0,1455 0,1102 0,1295 0,1710
w.
v.str.
v.w.
v.w. m. m.
str. w. w. w.
str.
IV2 1V3
2
w.
error u.
0/2 ±
of io~’
3 3 3
3
9 10
17
30 -
6 7
8
40 80 20
8 10
3° 60 18 7
20
Calculations made from these values together with certain other considerations1, which are reported elsewhere, point to rhombic symmetry. We obtain from these values the quadratic equation 4 sin2
/2
=
0,03210 h2
+
0,03923 k2
+
0,02275 l2
Naturw., 12, 956 (1924). With monoclinic symmetry, a considerable splitting off of points is to be expected but does not occur. Also it is not possible to include the points A., up to A>
0,03080
II
0,0004
10° 31' 10° 33'
1
”
0,03760 0,03660
3
11
IVd
”
0,0004
2
Ills
0,02103
0,03160 0,03080
I
11
—
str. str.
2
”
11
0,01623
10° 14' 10° 6,5'
1
11
11
3
2 =
?
—
—
1
”
—
—
1
”
—
0,0004
A2
11
—
—
0,01143
3
11
—
—
w.
2
11
—
—
6° 8,5'
—
11
—
—
3
1
11
—
0,03660
” ”
0,03345
0,0010 ” ”
0,09127
0,0008 ”
0,0020 ”
0,09720
THE STRUCTURE OF CELLULOSE
463
Assuming a unit-cell with four molecules, and rhombic symmetry, the quadratic equation is given by: 4 sin2 6/2
=
0,03080 h2
+ 0,03660 k2 + 0,02430
l2
From this quadratic equation a: b: c 8,88: 8,05: 9,88 is obtained, and the volume V of the unit-cell is seen to be 706 sq. Á.U., a value in exact agreement with that computed from the molecular weight (704). As already stated, the new point B0 does not fit in with this form, and is therefore due to the presence of another substance. Possibly it is also related to the points Ai and A2 which have disappeared, and which likewise could not be classified. The alteration of the intensities of the points B3 and B4, as compared with those of A3 and A4, (unless attributed to distension), forces one to the conclusion that the lattice planes are now otherwise occupied. For the sake of comparison, we have also examined a sample of /3-cellulose, prepared from wood cellulose by dissolving in NaOH and reprecipitating, this procedure being repeated three times. The values obtained with mercerised cellulose and with this /3-cellulose are shown in the following Table IV (w weak; str. strong). =
=
=
Table IV Mercerised cellulose
°o
8' w. 6' str.
0H
2' str.
6°
Oq
5°' 2'
3'
Oq
1' str.
w.
14°
56' w. 14' w.
w.
17°
3°'
w.
w.
Oq
25'
w.
14° 19' w. 17°
/3-cellulose
6°
Oq
By comparing the dimensions of the unit-cells of the untreated and mercerised cellulose, it appears that the distance along the direction of the fibre has diminished from 10,22 to 9,88 Á.U. This means a contraction of 3% along the direction of the fibre. An even higher value has actually been observed during mercerisation. The horizontal dimensions, however, increase from 8,60 to 8,88 and from 7,78 to 8,05 A.U. That the points of mercerised cellulose, as well as the interferences of untreated cellulose do in fact fit in with a rhombic form of the same kind, enhances the probability of the accuracy of this form. If the dimensions of the crystallites of the untreated cellulose are calculated according to Scherrer’s1 equation from the breadth of suitable interference spots, measured between points at which the intensity has decreased to half of its maximum value, two values, viz., 117 and 66 A.U. are obtained as maxima. Since marked differences in the intensity of the interference spots do not exist, we may conclude that the third value is substantially the same. 1
Zsigmondy: “Kolloidchemie”, Appendix (1920).
R. O. HERZOG
464
Behavior of Natural and
Artificial Cellulose Fibres in the Magnetic1 and Electric Field.
Bast fibres proved to be diamagnetic, but bundles of fibres arranged parallel behave like irregular crystals.
In the present investigation parallel-arranged fibres were formed by attaching them to thin plates by means of a solution of gum arabic and then, after hardening, making them plane parallel by strong pressure. From these plates, thin circular disks 8 mm in diameter were punched by of a very accurately worked punch, the centres of the disks being simultaneously pierced by a needle, inserted centrally into the punch, so that the plates could be easily attached to the lower end of a straight glass filament about 10 cm in length and 0,5 mm in thickness. To the upper end of the glassfilament, which was suspended on a thin cocoon filament, 30 cm in length, small mirrors of silver-plated cover-glass (about 1X4 mm) were cemented, by means of which the deflections and vibrations in all positions of adjustment could be optically observed. The suspension itself was found to be quite satisfactory. The disks under investigation had a mean weight of about 2 o mg. They may be regarded as monaxial crystal-plates. They are also to be considered as rather homogeneous, since the embedding medium, gum arabic, was found to have, in the solid state, a refractive index nD 1.51 as compared with a mean value nD for bast and artificial fibres. 1.55 In the magnetic, as well as in the uniform electric field, the orientation of the fibres was parallel to the lines of force. The difference of the magnetic susceptibilities (x—x') was computed according to Graetz2; t0 and t are the times of vibration without and within the field, respectively. The field of force was 3,25.10s Gauss. The electric measurements were carried out in an homogeneous alternating field of 220 V and 50 cycles, with the pole-plates at a distance of 1,2 cm. Ea and Ei are the dielectric constants, parallel and perpendicular to the optical axis, respectively. Table V gives a survey of our experimental results, the optical measurements being also listed. nDl and nD, denote the refractive indices with the fibre orientated parallel and perpendicular to the plane of polarisation, means
=
=
respectively. Table V illustrates the parallelism between the various values characteristic of anisotropy and are in remarkable agreement with the results of our X-ray investigations3. The turning moment in the electrical field is so strong that this method may possibly prove of practical value for technical purposes, such as, for example, a method of control in artificial silk manufacture. See J. Pauksch: Akad. Wise. Wien, Math.-Naturwiss. Kl., 115, I, 553 (1906). “Handbuch der Elektrizitát und des Magnetismus”, 4, 829. 3 Kolloid-Z., 35, 201 (1924). 1
2
THE STRUCTURE OF CELLULOSE
465
Table V Magnetic measurements mean error
(X ramie cuprammonium silk viscose I viscose II viscose film
(32% stretched)
V /
(x
±
—
mean error
x').I06
%
0,581
0,0110
0,066
1,9
0,224 0,213 0,090
0,0115 0,0093 0,0069
0,025 0,024 0,010
4,4
0,002
0,0077
0,007
12,5
0,039
0,0083
0,004
21
5,1
7,7
viscose film
(20% stretched)
Dielectric measurements
/'JL-.L'I
V /
X t*
ramie cuprammonium silk viscose I viscose II viscose film (32%, stretched) viscose film (20% stretched)
mean error d=
0,120 0,063 0,062 0,047
0,004 0,003 0,002 0,002
0,027
0,003
0,019
0,002
mean
E2
—
error
Ei
% 10
4,84 2,54 2,50 1,90
10
4,7 2,9
4,9
3,o
0,77
Optical measurements1 nDi
ramie cuprammonium silk viscose I viscose II viscose film
(32% stretched)
1,5879
i,5595 i,556o i,5442
nDi
Double refrac-
1,5331 1,5249 1,5221 1,5273
—
—
—
—
tion
0,0548 0,0346 0,0339 0,0169 0,0115
viscose film
(20% stretched)
1 The specific double refraction of the stretched films was determined by Babinet compensator, that of the artificial silk by Becke’s method.
0,0064 means
of the
R. O. HERZOG
466
Rdntgenspectrographic Investigation of Cellulose Nitrate and Cellulose Acetate. (iCarried out with Th. Nickl.)
According to Hermann Ambronn1 stretched cellulose nitrate films are doubly refractive, while X-ray investigations indicate an amorphous structure. In agreement with Hans Ambronn2 it was only found possible to detect a crystalline structure in the case of a nitrocellulose prepared under conditions in which the fibrous structure was maintained and which had not been subjected to any process of solution. The same result was also found with cellulose acetate.3
The X-ray investigation of nitrated and of acetylated hemp fibre points to the conclusion, that these two derivatives of cellulose most probably crystallize in the rhombic system. The dimensions of the unit-cells are approximately as follows: (b) cellulose acetate (a) cellulose nitrate a 10,1 A.U. 9,2 A.U. b 8,6 A.U. 7,1 A.U. c io,8 A.U. 9,8 A.U. Hence v 935 cubic A.U. and v 63 6 cubic A.U. for (a) and (b) respectively It is evident, that the values differ but little from, each other and from, those found for untreated cellulose, (see p. 460). This is obviously the fundamental condition for the possibility of a topochemical reaction. Fibres obtained from a denitrated cellulose nitrate and from a saponified cellulose acetate, both nitrate and acetate originally prepared from untreated cellulose, yield the diagram of the untreated cellulose, while the corresponding fibres obtained from mercerised fibres, instead of untreated cotton, show the diagram of mercerised cellulose (cellulose hydrate). In spite of this remarkable result, decisive conclusions cannot as yet be drawn as to whether the change produced by mercerisation is to be interpreted from a ‘chemical’ or ‘physical’ point of view. The lengths of the edge of the crystallites of cellulose nitrate appear to Ije 100 and 58; of cellulose acetate in and 61 Á.U., that is about the same value as found for cellulose. =
=
=
=
=
PART II Effect of Previous Treatment on the Size of Particles of Dissolved Cellulose Nitrate (Carried out with D. Kriiger). It has long been recognized and was pointed out in Part I, of the present communication, that the nitration of cellulose fibres can easily be carried out in such a way that the fibrous structure remains unchanged. From various experiments it seemed probable that these and other chemical transformations 1 Kolloid-Z., 18, 99, 222 (1916); 20, 173 (1917): Z. Mikrosk., 32, 43 (1915); Nachr. Gottingen ges. math.-phys. Kl. (1919). 2 Dissertation. Jena (1914). 3 Ber., 57, 329, 750 (1924); E Mohring: Wiss. und Ind., 2, 70 (1923).
THE STRUCTURE
OF CELLULOSE
467
of cellulose actually take place within the structural unit of each crystallite, partly without alteration in the mutual arrangement of the crystallites. It has been shown in Part I that in spite of chemical interaction, neither the dimensions of the unit-cell nor those of the crystallites are markedly changed. It is of equal importance to show that in colloidal dispersions the size of particles is identical with that of the original crystallites. It follows from, this that primarily, the process of dispersion consists only in a separation of the crystallites. A large number of celluloses of different origin and pre-treatment have been carefully nitrated, washed, dried, etc., and then dissolved in acetone or other solvents, the influence of the “process of dissolving” being studied separately. The determination of the size of the particles in colloidal solution was carried out by free diffusion into the solvent. The diameters of the particles, which may be approximately regarded as spheres, were calculated on the one hand from the formula given by Einstein1; on the other hand from the semi-empirical relation M\/D mole7. ( cular weight; D coefficient of diffusion in aqueous solution at 20°C)2, the observed D being corrected for 2o°C and converted to the value for aqueous solutions by means of the ratio of the viscosities of the solvent actually used to that of pure water. The calculation of the diameter of the particles is based on the assumption of spherical particles and accomplished by the customary procedure from Avogadro’s number N. In Table VI, D is the coefficient of diffusion determined at i3°C, 2ß and 2p2 the diameters computed according to Einstein’s law and from the square root relation, respectively. The table shows that with products of different origin, the size of particles is approximately of the same order, a conclusion based on the dimensions of the crystallites in the fibre. =
=
=
Further it follows that a short grinding of cellulose in the beating-engine brings about a considerable reduction in the size of particles in the solution of cellulose nitrate. The treatment of cellulose with reagents such as strong acid or alkali, results in a decreased size of particles in the solution of cellulose nitrate. The size of particles, as found in the case of viscose silk after nitration, corresponds approximately to the reduction in size, which takes place during the “dreripening” in the manufacture of viscose silk, i.e. the mercerisation process. The values of zpi found are about 2 to 3 times those of 2p2. If the 2 p2-value of the dissolved cellulose nitrate from hemp is compared with those calculated from the breadth of the interference blackening of hemp fibre and nitrated 1
2
Ann. Phys., (4) 19, 303 (1906). L. W. Oeholm: Medd. Vetenskaps. Nobel-Institut, 2, Nr. 23 (1912).
R. O. HERZOG
468
Table VI Size of particles of cellulose
nitrate prepared from various celluloses.
Material
cotton
(as
”
nitrate) 11
Solvent
D
acetone
0,035
300
0,023 0,039 0,042 0,043 0,045 0,053
340
0,075
104
0,043
182
11
66
88
11
4i
11
68
methylethylkeytone
hemp ramie flax sulphite-cellulose viscose silk
hemp, ground 25 min. in the beating-engine mercerised cotton, hrs. 3-4 mercerised cotton, hrs. 70
cotton, treated with 50% H2SO4 2 hrs. at room temperature with treated cotton, at 20 hrs. H2SO4 50% room temperature
2Pi*
11
91
11
11
1)
11
11
”
11
11
11
11
11
11
11
11
11
11
11
11
hemp cellulose, the agreement is strikingly good,
0
'o
00
VI
’’
200
11
186
19
182
11
174 148
0,039
200
0,059
132
as
lu.) \
1
>
94
j
11 11
11
11
74 66 66
64 56
44
52
is shown by the following
table: Dimensions of crystallites of hemp fibre: 1x7 and 66 A.U. Dimensions of crystallites of cellulose nitrate from hemp (solid state): and 58 Á.U.
100
Diameter of colloidal particles of nitrated hemp fibre (dissolved)
74
Á.U.
This result shows, as stated above, that the chemcial transformation is accomplished within the complex of atoms united in the crystallite, without the latter undergoing disintegration to single (C6HioOB) groups, and is thus in complete agreement with the results of X-ray analysis. It is thus possible, from the dimensions of colloidal particles as found in solutions of cellulose nitrate to determine the size of the original crystallites of cellulose. In order to make the diameters of particles of cellulose nitrate and the length of crystallites of cellulose directly comparable with each other, the values of 2pi and 2p2 quoted above are to be divided by 2· Mi, M2 and V,, V2 denoting the molecular 1
=
weight and the molecular volume of cellulose nitrate and of cellulose, respectively.
THE STRUCTURE OF CELLULOSE
469
Summary The results of the Rontgenspectrographic investigation of cellulose are given, and the observed values compared with those calculated from the quadratic equation for rhombic symmetry, assuming the unit-cell to consist of four (C6Hiq05) groups. It is shown that rhombic symmetry and the adopted size of unit-cell are in better agreement with the experimental data than any other assumption. 2. The quadratic equation for mercerised cellulose is given. 3. The dimensions of crystallites of natural cellulose fibres have been determined. 4. It is shown that the behavior of bast fibres in both the magnetic and the electric field runs completely parallel to the results of optical measurements and X-ray investigations. 5. The X-ray investigation of cellulose nitrate and cellulose acetate indicates that the dimensions of these derivatives differ but little from those of the original untreated cellulose. It has been pointed out that this fact is a necessary condition for assuming a topochemical reaction. 6. Cellulose fibres obtained by the denitration of a cellulose nitrate and from the hydrolysis of a cellulose acetate (both of them being prepared from an untreated cellulose) give the diagram of the latter. On the other hand, cellulose regenerated from esters prepared from a mercerised cotton give the diagram of cellulose hydrate. 7. The dimensions of the crystallites of nitrated and acetylated hemp cellulose are given, the esterification being carried out as carefully as possible, and under conditions in which the fibrous structure is maintained. 8. The size of particles in colloidal solutions of cellulose nitrate as determined by the diffusion method corresponds exactly to the dimension of crystallites, computed from the X-ray diagram. 9. The size of particles of cellulose of various origin and pre-treatment is given. 1.
