THE VISCOSITY OF CELLULOSE I N PHOSPHORIC ACID SOLUTIONS1 ALFRED J. STAMM
AND
WILBY E. COHENz
Forest Products Laboratory,a Forest Service, U.S.Department of Agriculture, Madison, Wisconsin Received J u l y 1 , 1968 INTRODUCTION
Perhaps the best physical means of characterizing cellulose for industrial use is the determination of the viscosity of its solutions. Unfortunately, there are no true physical solvents for cellulose itself, although there are such solvents for cellulose derivatives. Cuprammonium solution perhaps comes the closest to being a true physical solvent, and it is for this reason that it is almost exclusively used in viscosity studies. Cuprammonium solution, however, is subject to several variations which may affect its solvent power and the nature of the cellulose dispersion: (1) the total copper ammonia complex concentration, (.e) the relative proportions of copper and ammonia, (3) the portion of the copper (10) which exists as colloidal copper hydroxide, (4) the amount of sugar added to the solvent to stabilize the system, (5) the extent to which the ammonia is oxidized to nitrites in making the solvent, and ( 6 ) the extent to which the solvent is exposed to air in the preparation and study of the cellulose solutions. Because of trhese possible variations, it is not surprising that considerable difference of opinion has been expressed a t recent chemical meetings as to the nature of the dispersion of cellulose in cuprammonium solvent. Farr (5) contends that the cellulose is not molecularly dispersed as many believe, but is broken down into elliptical particles similar to those which she obtained by treating cellulose with various acids (6, 7). I n this case the particles, which do not deviate appreciably from spherical, would have but a slight effect upon the viscosity, as she contends. Farr believes that it is the solution of the pectic film of the cotton, rather than the cellulose itself, which is largely responsible for the viscosity (5). Presented a t the Fifteenth Colloid Symposium, held a t Cambridge, Massachusetts, June 9-11, 1938. Commonwealth Fellow, 1935-37, Division of Forest Products, Council for Scientific and Industrial Research, Melbourne, Australia. Maintained a t Madison, Wisconsin, in cooperation with the University of Wisconsin. 921
922
ALFRED J. STAhlM AND K I L B Y E. COHEN
Staudinger (12, 14), on the other hand, believes that cellulose in cuprammonium solvent is dispersed in the form of long macromolecular chains similar t o many other polymeric materials, including cellulose derivatives in organic solvents. Staudinger’s views are supported by a number of differentinvestigators (8). It seemed highly probable to the authors that both Farr and Staudinger might be right, the difference resting on differences in the cuprammonium solvent used by each of the investigators. It thus seemed desirable to try to find a solvent for cellulose that was not subject t o the foregoing variations. A preliminary survey indicated that concentrated phosphoric acid was the most promising solvent. It is true that this is a hydrolytic solvent, but hydrolysis takes place slowly enough to make possible viscosity measurements on the solutions after the cellulose has been in solution for different intervals of time. Extrapolation of the resulting relationship between viscosity and time to zero time gives the viscosity of the undegraded cellulose. PREPARATION O F SOLUTIONS
A simple means of preparing cellulose solutions of known concentration in phosphoric acid was first sought. It was soon learned that undegraded cellulose cannot be completely dispersed in concentrated phosphoric acid without the use of some water. Apparently the cellulose must be swollen in water and the concentration of the acid built up gradually to get complete solution, This is in agreement with the findings of Ekenstam (3), who has recently claimed that cellulose forms an oxonium compound (C6H1,0S.2H20.H3P04)n, with two molecules of water and one of acid per glucose anhydride group, which is soluble in concentrated phosphoric acid, The procedure finally adopted for preparing the solutions of known concentration was as follows: Accurately weigh 10 to 30 mg. of air-dry cellulose of known moisture content into a small agate mortar. Add 1 cc. of water and gently macerate with an agate pestle. Add phosphoric acid (85 to 100 per cent) drop by drop, working with the pestle between additions. When the swollen cellulose is almost completely dissolved, the acid may be added more rapidly. Wash the cellulose solution into a weighed beaker with concentrated phosphoric acid. Add phosphoric acid until the desired concentration of cellulose is obtained and weigh. Filter the solution through a No. 3 Jena fritted glass filter to remove any trace of foreign matter that might clog the viscometer. Remove air from the solution with a vacuum pump a t room temperature. The preparation of the solution from the time the cellulose begins to dissolve to the time of complete removal of air generally takes from 20 to 30 min. Solutions prepared in this way, even when the filtering step was omitted, mere found to be practically void of microscopically visible particles, in-
VISCOSITY O F CELLULOSE
923
dicating that the cellulose is not dispersed as Farr elliptical particles (5, 6, 7). VISCOSITY MEASUREMENTS
The viscosity measurements were made in a Bingham viscometer with a capillary bore of 0.0337 cm., a capillary length of 10.1 cm., and a bulb capacity between reading marks of 3.995 cc. The viscometer was securely clamped in place in a thermostatically controlled water bath held a t 25°C. f 0.002". The applied air pressure was controlled with a reducing valve followed by one to four air-escape bubblers containing sulfuric acid, which were connected in series with each other and in parallel with the line to the viscometer. The air-pressure valve was set so that a constant but not too rapid rate of bubbling occurred through the bubblers. In this way the air pressure was controlled during a single measurement to 0.01 cm. of mercury, as was indicated by a mercury manometer that was read with a cathetometer. The readings could be reproduced in subsequent measurements to 0.03 cm. of mercury. It would be practically impossible to prepare a phosphoric acid solution of exactly the same concentration as that serving as solvent for the cellulose. Fortunately, the viscosity of the solvent, which is required to calculate the specific viscosity of the cellulose, can be obtained by a simpler means (2). After completing the viscosity measurement on the solution, the solution can be heated in the viscometer to complete the hydrolysis of the cellulose to glucose and other final hydrolysis products. This avoids an extra cleaning and filling of the viscometer. Concentrations of hydrolysis products of 0.1 per cent and less will affect the viscosity of the phosphoric acid less than 0.25 per cent. If more accurate results are desired, the viscosity can be corrected for the presence of the hydrolysis products with an error of less than 10 per cent, using the Einstein viscosity equation (2). Preliminary tests indicated that when the solutions of cellulose in phosphoric acid were heated in boiling water for half an hour or more to complete the hydrolysis, the solution developed a slight straw color and the viscosity of the solution after cooling to 25°C. increased for several days. Tests were made on solutions of 1 per cent glucose in phosphoric acid prepared in the same way as the cellulose solutions. The viscosity increased as much as 5 per cent in two days after heating to 100°C. for an hour, The change that occurs is so slow that the first viscosity value obtainable after cooling is but very slightly greater than the original viscosity before heating. Apparently the heating causes a caramelization which consists of a slow polymerization to chain molecules. This polymerization requires heat to initiate the reaction but is capable of continuing after cooling. When the solutions of glucose in phosphoric acid were not heated
924
ALFRED J. BTAMM AND WILBY E. COHEN
above 60°C. no discoloration resulted, nor was an increase in viscosity obtained. The solutions of cellulose in phosphoric acid were thus heated to 50" t o 60°C. for several hours t o insure complete hydrolysis and yet t o avoid subsequent polymerization. Figure 1 shows the viscosity in terms of the applied pressure multiplied by the efflux time for phosphoric acid of different concentrations and hydrolyzed solutions of cellulose in phosphoric acid plotted against the acid Concentration for two different applied pressures on the viscometer. The curies indicate that within experimental error the hydrolyzed solutions have the same viscosity as the solvent. The measurements were made on a cotton linters alpha cellulose (99.6 per cent alpha) and a normal spruce sulfite pulp and the same after beating
*rPEcific
cafiww OF
PLWWGPIC
,qc:rO m w h i
FIG.1. Product of pressure and efflux time for phosphoric acid solutions of different specific gravity and concentration and the same containing hydrolyzed cellulose for two different applied pressures on the viscometer.
for 20 hr. in a rod mill. The concentrations used ranged from 0.000077 t o 0.00099 g. per cubic centimeter (0.00048 t o 0.0061 units of glucose anhydride per literj. The viscosity measurements were made at 30.9, 20.5, 10.3, and 3.4 em. of mercury pressure on each of the solutions in the order given, a measurement from left t o right, and from rigkt to left being made a t each pressure. The average of these two times of effiux was used in the viscosity calculations. The mean time between starting t,he first, measurement and ending the second was taken for calculating the time that, the cellulose had been in solution. The agreement between the readings taken in the iwo directions was quite good when an i n t e r d of 2 min. w a s allowed between measurements for drainage of thc ~ i s c o u sphosphoric wid.
VISCOSITY OF CELLULOSIC
925
except for the initial readings where the viscosity was changing very rapidly \.vith time. Although this short pause between measurements eliminates any difference in drainage between back and forth measurements a t a single pressure, there is an unavoidable drainage difference between measurements made under different pressures. Figure 2 gives the relationship between the product of the applied pressure and efflux time under different applied pressures plotted against the logarithm of the efflux time in seconds for completely hydrolyzed cellulose in 86.15 and 86.40 per cent phosphoric acid. When the efflux time is doubled the pressure-efflux time product is increased about 1.2 per cent.
FIG.2. Product of pressure and efllux time versus the logarithm of the efflux time for hydrolyzed cellulose solutions i n two different concentrations of phosphoric acid.
The maximum time of efflux variation between the first measurement and the final hydrolyzed value for the most concentrated solution of cellulose used was 75 per cent. This would result in a viscosity error of 0.9 per cent, due to the drainage factor. In the case of solutions containing less than 0.0003 g . of cellulose per gram of acid, the error was less than 0.25 per cent. As the viscosity values of chief interest are those for the more dilute cellulose solutions, no attempt was made t o make a drainage correction. The specific viscosity v,,(the increase in relative viscosity caused by the solute) per unit concentration in grams per cubic centimeter is plotted against the time the cellulose has been in solution in figures 3 and 4 for
926
ALFRED J. STAMM AND WILBY E. COHEN
o
zoo
400
600 BOO 1000 TIME IN JOLUVON (MINUT€S)
izoa
1400
FIG.3. Specific viscosity per unit concentration for a 0.000557 g. per cubic ,centimeter solution of unbeaten pulp in an 88.4 per cent phosphoric acid solution versus time in solution for different applied pressures on the viscometer.
FIG.4. Specific viscosity per unit concentration for a 0.0000979 g. per cubic centimeter solution of unbeaten pulp in an 89.4 per cent solution of phosphoric acid versu8 time in solution for different applied pressures on the viscometer.
unbeaten pulp solutions containing 0.000557 and 0.0000979 g. of pulp per cubic centimeter, respectively. The specific viscosity per unit concentration, qep/c,increases with a decrease in the applied pressure used in making
VISCOSITY O F CELLULOSE
927
the viscosity measurements for the higher q a p / c values. The divergence of the q s p / c values obtained a t different applied pressures increases with an increase in the concentration of cellulose in solution. The apparent reason for this is that in concentrations sufficiently great for interference between the elongated cellulose molecules the interference is decreased with increasing velocity of shear, owing to the increased orientation pf the molecular chains. This will be considered further in the next section. MOLECULAR WEIGHT AND LIMITING CONCENTRATION O F CELLULOSE
The apparent molecular weights of the pulp in different concentrations, determined under different applied pressures and for different lengths of time that the cellulose has been in solution, were calculated from the data of figures 3 and 4 and the corresponding data for other concentrations of the unbeaten pulp, the beaten pulp, and the cotton linters alpha cellulose, using the relationship of Staudinger (12) :
in which M is the molecular weight of the cellulose, qsp is its specific viscosity, 0.162 is a factor t o transpose the concentration c in grams per cubic centimeter t o glucose anhydride units per liter, and K , is the Staudinger proportionality constant. The constant K , used in the following calculaat 25°C. This was obtained by a linear extrapolation tions is 11.8 X a t 2OoC. and K , = 14.7 of Ekenstam's (3) values, K , = 12.4 X X a t 0°C. Staudinger (13) gives a value for K , of 18.0 X at 20"C., which is larger. Further work on the molecular weight of cellulose in phosphoric acid solutions by other methods will have to be done before a more exact value for K , can be obtained. Until then the actual molecular weight values can be considered only as approximate. The relative molecular weight values should be considerably more accurate, however, over the molecular weight range considered. The apparent molecular weights of the undegraded celluloses (zero time in solution) for the different concentrations and different applied pressures are shown in figure 5 for the pulp and in figure 6 for the alpha cellulose. The apparent molecular weight varies but slightly with concentration over the range studied for the higher applied pressures on the viscometer, but increases appreciably with concentration for the lower pressures. The molecular weights of the beaten and the unbeaten pulp are identical within experimental error. Over the range studied (86 to 92 per cent) the concentration of phosphoric acid has no effect. The practical limiting concentration above which molecular interference appreciably affects the viscosity and the calculated molecular weight is thus dependent not only upon the molecular weight and the concentration of the cellulose solution,
928
ALFRED J. STAMM AND WILBT E . COHEK
FIG.5. Apparent molecular weight of beaten and unbeaten pulp versus the concentration of the pulp in phosphoric acid solution for different applied pressures on the viscometer.
FIG.6. Apparent molecular weight of cotton linters alpha cellulose versus the concentration of the cellulose in phosphoric acid solution for different applied pressures on the viscometer.
929
VISCOSITY OF CELLULOSE
but also on the velocity of shear under which the measurements are made. The theoretical curve in figure 5 for the apparent molecular weights at the limiting value of zero applied pressure was obtained by plotting the apparent molecular weight against the applied pressure on the viscometer for different concentrations and extrapolating to zero pressure. The curves for the different applied pressures in figures 5 and 6 converge a t a moisture content below 3 x 10-5 g. of cellulose per cubic centimeter. This limiting concentration can be more accurately estimated from figure 7, in which the limiting molecular weight values for the partially hydrolyzed pulps (corresponding to the points on curves of the type of figures 3 and 4 above which 1.0
c
\,
I
l \ I
I
I
I
I
I
I
I
I
1
I
I
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s
the ? s p / c values for different applied pressures diverge) are plotted against the pulp concentration. From this figure the limiting concentration appears t o be less than 2 X 10V g. of cellulose per cubic centimeter for a molecular weight of 100,000. This limiting concentration is practically equal to the theoretical value for packed spheres, calculated upon the assumption that the limiting volume of uninterfered motion of the elongated molecules is a sphere with the long axis of the molecule equal t o the diameter. The pulp with a molecular weight M = 100,000 on a phosphoric acid-free and water-free basis, and M I = 183,000 on a hydrated and phosphated )asis (4),has a degree of polymerization of 618, and a length, I , of 3180 A. (5.15 A. per glucose anhydride unit). The limiting concentration C on this basis is given by the equation
930
SLFRED J. STAMM AND WILBY E. COHEN
in which A is the number of molecules in a gram molecule, This gives a value of 1.8 X g. per cubic centimeter for the limiting concentration. d limiting concentration of the same order of magnitude is obtained on the basis of the limiting volume of uninterfered motion of elliptical molecules increasing, according to Onsager (9), as the square of the ratio of the major t o the minor axis. This gives a limiting volume 1.5 times that obtained on the limiting sphere of action basis (11) and a limiting concentration 33 per cent less or 1.2 X 10V g. per cubic centimeter. Both of these values are within the range of accuracy of the experimental value. The velocity of efflux under 1he different applied pressures was calculated from the ~iscometerdimensioils and the theoretical time of efflux for the cellulose in solution for a n infiiiitesinial length of time. These data indicate that over 1he concentration and applied pressure range used molecular weight values can be calculated with a n accuracy of 5 t o 10 per cent whenever the efflux velocity exceeds 10 t o 12 cm. per second. hlolecular weight values can thus be determined with a fair degree of accuracy a t concentrations appreciably greater than the limiting concentration for velocities of shear approaching zero by increasing the velocity of shear sufficiently t o reduce molecular interference as a result of molecular orientation. Because of this Staudinger (12) has been able t o determine molecular weights of cellulose in concentrations of several tenths of a per cent. HYDROLYSIS CONSTANT OB CELLULOSE
The viscosity measurements of cellulose dissolved in phosphoric acid furnish the data for calculating not only the molecular weight of cellulose but also the hydrolysis constant of cellulose. Ekenstam (4) has derived the following equation for calculating the hydrolysis constant :
where t is the time, JM the initial molecular weight, Mt the average molecular weight after time t , and m is the final molecular weight. When M and M t are large compared t o m this equation can be simplified to
The molecular weight of the final hydrolysis product m dissolved in phosphoric acid is 240, according t o Ekenstam (4). Table 1 gives the hydrolysis constant when f is expressed in minutes for the unbeaten pulp, beaten pulp, and cotton linters alpha $ellulose calculated from equation 4,using
TABLE 1 Hydrolysis constants of cellulose dissolved 'I concentrated phos ,oric acid at 36°C. KYDROLYSIS CONSTANT0
CONCENTRATION
-
MATERIAL
Celluloso
TIXE I N lOLUTION
IOLECULAE
WEIQHT
IJPO
x
c rams per cc. 0.000557
0.0000979
0.000206
3.000178
0.000989
0.000311
Cotton linters alpha cellulose. . . , . , , , . . , , ,
0.000142 D .0000766
Average 4 u e a 4 to 4 hr. X 10'
hours
39.3
0 2 3 4 6 8 10 12 16 20 24
105,500 72,000 65 100 59 600 50,600 43,200 37,400 33,200 26,900 22,600 19,200
8.83 7.84 7.30 6.85 6.83 6.90 6.89 6.93 6.96 7.10
6.97
0 2 3 4 6 8 10 12 16 20 24
100,000 70,000 63,000 57,600 48,700 41,150 35,650 31,550 25 100 21,400 18,500
8.58 7.83 7.36 7.03 7.15 7.22 7.23 7.46 7.35 7.35
7.27
0 24
102,700 19,200
6.89
0 24
102,700 18,800
7.23
0 24
101,300 17,400
7.18
0 24
109,600 21,550
6.77
0 24
102,700 18,900
7.07
0
24
116,500 23,850
5.67
0 24
113,700 23,850
5.79
0
113,000
24
23,300
39.4
18.4
16.1
12.3 86.4
Beaten p u l p . . . . . . , , , , , 0.000188
106
ET cen
Unbeaten pulp
0.000293
ndividua values
36.1 36.3
36.4 16.3
931
5.58
932
ALFRED J. STAMM AND WILBY E. COHEN
Ekenstam's value for m. The individual hydrolysis constant values calculated for various intervals of time, from 0 to 24 hr., that the cellulose is in solution, are given for the two concentrations of unbeaten pulp shown in figures 3 and 4,together with the average value for 4 hr. through 24 hr. I n the case of the other concentrations of unbeaten pulp, beaten pulp, and cotton linters alpha cellulose only the average values are given. The individual values for these solutions are similar to the individual values given in that they are constant within experimental error after the first 4 t o 6 hr. Ekenstam (2, 4) obtained similar high initial values for some of his cellulose samples. The beaten and unbeaten pulps have the same hydrolysis constant within experimental error. The hydrolysis constants of the pulps are larger than for the cotton linters alpha cellulose, as would be expected, because they undoubtedly contain more readily hydrolyzable carbohydrates. SUMMARY
Cellulose can be completely dissolved in concentrated phosphoric acid to form dilute solutions void of microscopically visible particles when the cellulose is first swollen in water and the acid concentration built up gradually. Viscosity measurements on these solutions after different times in solution and under different applied pressures were made. The viscosity of the solvent was shown to be equal t o the viscosity of the dilute cellulose solutions after completely hydrolyzing for 1 hr. at 50" to 60°C. Hydrolysis a t 100°C. initiates a slow polymerization of glucose dissolved in phosphoric acid which continues for days. The limiting concentration of cellulose above which molecular interference occurs when the viscosity measurements are made under a low rate of shear is about 2 X low5g. per cubic centimeter. This is practically equal to the concentration in which sphercs described about the long axis of the molecules are packed. When the viscosity is determined under higher rates of shear, higher concentrations can be used in molecular weight determinations because of the molecular orientation. The molecular weights of a highly beaten pulp and of the unbeaten pulp calculated from these data are both 100,000 on a water- and acid-free basis. The corresponding value for a cotton linters alpha cellulose is 113,000. Hydrolysis constants for cellulose in phosphoric acid calculated from these data are quite constant after the first 4 hr. in solution, The average hydrolysis constants for the pulp and for the cotton linters alpha cellulose are 7.05 X and 5.68 X IO-', respectively, after the first 4 hr. REFEREKCES
EIXSTEIS, A , : Kolloid-Z. 27, 137 (1920). (2) EKEUSTAX, A : Svensk Kern. Tid. 46, 157-67 (1934). (3) EICENST\\I, h.:Eer. 69B, 549-52 (1936). (1)
VISCOSITY O F CELLULOSE
(4) (5) (6) (7)
933
EKESSTAX, A , : Ber. 69B, 553-6 (1936). FARR,W.K.: Textile Research 7(2), 65-9 (1936). FARR,R.K . : J. Phys. Chem. 41,987-91 (1937). FARR,W .K., AND ECHERSON, S. 13 : Contrib. Boyce Thompson Inst. 6,189-203 309-313 (1934). (8) KRAEMER, E. O., AND LANSIXG, K. D.: J. Phys. Chem. 39, 153-68 (1935). (9) ONSAGER,L.: Phys. Rev. 40, 1028 (1932). (10) STAMV,A. J.: J. Phys. Chem. 36, 659-60 (1931). (11) STAMX, -4.J . : U. S. Dept. Agr. hlisc. Pub. S o . 240 (1936). (12) STAUDINGER, H. : Der Aufbau der hochmolekularen organischen Verbindungen Ksutschuk und Cellulose im Sinne der Kekuleschen Strukturlehre. J. Springer, Berlin (1932). (13) STAUDISGER, H., AND DAUMILLER, G.: Ber. 70B, 2508-13 (1937). (14) STAUDIXGER, H., AXD RITZENTHALER, B. : Ber. 68B,1225-33 (1935).
