THE DISPERSION OF CELLULOSE AND CELLULOSE DERIVATIVES *
BY S. E. SHEPPARD
Cellulose itself is insoluble in water acd in organic liquids, but a number of aqueous solutions are known which can peptize cellulose to a sol. Such are Schweitzer's reagent-a solution of cupric hydroxide in aqueous ammonia,- concentrated zinc chloride solution, and certain other concentrated salt solutions a t sufficiently high temperatures. It was shown first by P. von U7eimarn1that a number of hygroscopic salts, such as lithium chloride, calcium bromide, calcium and manganese thiocyanates, at certain conditions of concentration and temperature could peptize cellulose to a sol, from which a more or less hydrous cellulose separated on cooling, dilut,ing, or adding a coagulating agent. With certain salts, vie., NaI, SrI?,Ca12,CaRr?,Ca(CKS)z, Sr(CNS)z,Ba(CSS)?,the process can be carried out a t atmospheric pressure, but with others, e.g., NaC1, KCl, BaC12, autoclave conditions are necessary. Thus with concentrated NaCl solution, peptization of cellulose begins a t 70' under 8 atmospheres, while with LiX03, NaI, Ca12, SrI?, Ca(CN;S)*, Ba(CT\'S)z,Mn(CNS)?, Sr(CKS)2 in saturated solution a t the boiling point, peptization proceeds under atmospheric pressure. With many salt's which pept'ize cellulose only on prolonged heat,ing, decomposition of the cellulose (hydrolytic degradation, oxidation) occurs, giving bodies of lower molecular weight. (It is true that one does not know the molecular weight of cellulose, nor whether the expression means anything, but it is cust'omary to speak of certain more dextrin-like products from cellloseJ2 having very high copper numbers, solubility in caustic solutions, as having lower molecular weight than cellulose.) Von Weimarn also3 found that cellulose swollen by previous soaking iu the concentrated salt solution in the cold peptized more rapidly on heating the solution than the unswollen cellulose. This behavior indicates resemblance to the peptization of gelatin in water, and of certain other proteins in salt solutions, and will be referred to again. The peptization of cellulose by saline solutions was investigated rather fully by H. E. william^,^ apparently in ignorance of von Weimam's prior work. Williams found, with the thiocyanates, that, none of them peptized cellulose till the solution attained a temperature of I33'C,, or higher. That is to say, a concentration corresponding t o a certain vapor pressure reduction was necessary. This worker obtained certain empirical relations between the peptizing power, the viscosity, t,he boiling point, and the heat of solutionresults which indeed indicate that the peptizing power is directly related to the thermodynamic activity of the electrolyte. A very interesting fact, first discovered apparently by Williams, is that a concentrated solution of * Communication No. 401 from the Kodak Research Laboratories
1042
S. E. SHEPPARD
Ca(CSS)!, which peptized cellulose, would also peptize the hydrous oxides of Ca, Pb, Zn, Cd, Co, Si,Fe, and %--termed by Willianis the hydroxides. It may be noted that in this Laboratory very concentrated solutions of C‘aCL were found to peptize ferric oxide to a sol passing through filter paper, and stable at the boiling point. Williams proposed an explanation of the peptization as follows : “The hydroxyl groups of the cellulose unit link up with the salt complex in place of the water molecules, thus causing the fiber to swell considerably. The cellulose unit (? molecule) is brought by this nieans into molecular range with the water niolecules combined with the salt. By raising the temperature, the union between the salt and water molecules will weaken and tend to part from the parent molecule. The water thus freed migrates to the cellulose by which it is imbibed, causing further swelling of the fiber, which increases as the progressive hydration proceeds. The highly swollen fiber in the gelatinous condition then peptizes, and passes into colloidal solution.” This explanation is by no means clear or sufficient. If there is competition for the salt molecules between the hydroxyls of the cellulose and the water molecules, it is not elldent why the migration or transfer of water to the cellulose should take place. The dispersing action appears to be definitely connected with the strong polar field of the electrolyte solutions, and also with the lyotrope series. In some way this field decreases the association between the hydroxyls of the cellulose units, probably the primary valency chains of the micelles. It is true that Katz and l l a r k j (vide infra) were not able to find X-ray evidence for intra-crystallite swelling with the peptizing electrolytes, such as zinc chloride, and thiocyanates, as is observed in the mercerizing action of caustic alkali solutions. I t is probable, therefore, that the initial mechanism of peptization by these electrolyte solutions (salts) differs somewhat from the swelling by caustic alkalies, and the peptization by cuprammonium solution. The nature of the regenerated cellulose from these sols requires further investigation, since we have observed in this Laboratory that cellulose regenerated from Ca(CNS)2solution gives a water adsorption value corresponding to that of “hydrat,e” cellulose prepared by mercerization and washing, or by regeneration from cuprammonium solution, from xanthate or acetate.6 A further examination by X-rays of these regenerated celluloses is necessary, therefore, to ascertain whether, on solution, the “hydrate” cellulose is formed. The same examination is required of the celluloses regenerated from concentrated solutions of phosphoric and sulfuric acids. If cellulose is treated with an ice-cold seventy per cent solution of H2S04,it dissolves to a clear, slightly yellowish ~ o l u t i o n . ~It may be reprecipitated by dilution with cold water, and washed free from acid by water and alcohol. The alcohol dry material gives a milky dispersion on digestion with water. It is not known whether this cellulose corresponds to “hydrate” cellulose in its X-ray diagram and moisture adsorption, but these matters are under investigation. I t is known That the parchmentizing of cellulose with strong nitric acid iKnecht)Sdevelops the “hydrate” cellulose form, so that it may be suspected
THE DISPERSION O F CELLCLOSE
1043
that these regenerated cclluloses also contain “hydrate cellulose.” The “solutions” in the concentrated aqueous acids certainly have strong affinities with those in the strong saline solutions, but, a t the same time, a considerable amount of molecular degradation of cellulose to cellulose-dextrins and to sugars is taking place, the cellulose dextrins being present chiefly as dextrin ester^.^ I t is very possible that these dextrins and dextrin esters play the part of “protective colloids” in regard to the residual peptized cellulose. The uncertain point at present is whether in these mineral acid solutions we have peptized ”active cellulose,” peptized “hydrocellulose,” or peptized “hydrate cellulose,” the relative stability being in any case effected by an external layer of dextrin acid ester molecules, the acid ester group homologating the particles to the solution, just as the stearate radicle of silver stearate homologates the surface of colloid silver to a hydrocarbon. d generalized statement of the facts discussed is presentedin the following classification of cellulose sols and gels, based on the two fundamental forms of cellulose which seem to be established at present on the bases of X-ray spectroscopy and water adsorption.1° I t will be seen from this that it is doubtful at the present time whether we have any examples of dispersions of cellulose to sols in which the cellulose is present in the “native” form, which, according to X-ray findings, corresponds to a slightly deformed, energy-richer lattice unit. Apart from the subsidiary uncertainty as to whether the saline dispersions contain cellulose in the “native” or the “hydrate” form, the most important question undecided at present is the following: Is the “hydrate” form of cellulose, understood to be a lattice modification, L e . , a multi-molecular aggregate form, produced solely by a limited intra-crystallite swelling process? That is, do the swollen crystallites persist in all the types of these solutions, being merely reaggregated but in disoriented form, on evaporation or coagulation? Or is it possible that the dispersion may proceed still further, to the point of disaggregation of the swollen crystallites into molecular fibrils, so that the hydrate celluloses in the gel forms are produced by a process of recrystallization? I t appears that R. Herzog” and H. Mark1* endorse the former conception, although Herzog admits the possibility of some degree of recrystallization from solutions. There exists, however, certain evidence for the alternative conception, which implies that’ ‘imacromolecular” colloids in general, and the cellulose family in particular, can be dispersed to states equivalent to “true solution.” I t is indeed evident that the peptization or dispersion of such bodies in solvating, ie., hydrating, solvents might proceed to disruption of the cryst,allites, by sufficient reduction of the intermolecular association. I n the case of cellulose and its derivatives, this would produce molecular, or near-molecular dispersions of the primary valence chains constituting the crystallite. I t is not probable that in such dispersions the solute units would reach quite the degree of kinetic independence characterizing solutions of low molecular substances. I t is much more probable that such macromolecules, particularly of the cellulose family, exercise an important degree of mutual attraction
S.
E. SHEPPARD
I045
THE DISPERSION OF CELLULOSE
in solution, This could give rise to swarms of long solvated molecules having more or less parallel orientation in the swarms, the latter having, therefore, a greater degree of kinetic independence than the constituent molecules. The following diagram will make this clearer.* I n state A, the crystallites pass by intra-crystallite solvation to the “hydrate” cellulose form, but retain a three dimensional ordering of the primary valence units. It is assumed that a t a certain stage of solvation they pass over into state B, in which primary units of nearly the same length still have parallel orientation, but freedom to roll within the swarm, which is easily deformable in one dimension. I n state C, a residual degree of local parallel ordering remains between primary units of any unequal lengths, it A
0
C
FIG.I
being assumed that, insofar as statistical conditions permit sufficient parallel ordering of equal length chains, there be formed swarms or droplets of the state B, provided also the thermodynamic conditions admit. In state C, deformation of the swarms is possible in two dimensions, the size-frequency of the swarms depending upon thermal and mechanical (shearing stress) conditions. Such swarms or droplets would not be truly two-dimensional, and would have no stable magnitude in any but one dimension, whereas the swarms in B ordering would have limit magnitudes in two dimensions or perpendicular directions. State C implies intermediate degeneration to complete disorder, or kinetic incoherence. We have obtained evidence in this Laboratory with organic solutions of the cellulose esters, from the measurements of thin films on mercury,13 that a t sufficient dilution the macromolecules are oriented on the mercury at a thickness of the order of magnitude of the thickness of the chains themselves, a result which indicates that dispersion has proceeded as far as state C. Katz14 entirely independently obtained results in very good concordance with ours for the thickness of thin films of cellulose esters spread on water. It
* These stages are very possibly related to the smectic and nematic mesophases in liquids of Friedel ( A n n . d e Phys. 18 273 (1922).
I 046
S. E. SHEPPARD
shoula o mentioned also that there is evidence from the viscosity and plaaticityI5 of cellulose ester solutions that by adequate peptization in specific binary solvent mixtures, completely fluid, structureless solutions of cellulose esters may be produced. Here again it appears that dispersion has been carried t’o state B, if not to state C. Further information in regard to these solution states may be expected from experiments at present in progress under the writer’s direction on the birefringence of cellulose ester solutions on being sheared, and its correlation with viscosity and plasticity. We have demonstrated already strong birefringence in these solutions on shearing, showing the presence of asymmetrical particles, and agreeing with our results on the production of optical anisotropy in cellulose ester films by tension.16 Quite recently R. Herzogl’ has published data on the characterization of colloidal solutions by the polarization state of the Tyndall beam. With technical cellulose acetate in inethyl acetate solution, at 0.08 per cent concentrations, the intensity of the Tyndall beam was almost zero, and residual depolarization was due to the solvent itself. The final values of the depolarization at various higher concentrations were the same, for the same concentration, whether proceeding by dilution of a stronger or concentration of a weaker solution, indicating a completely reversible aggregation disgregation process. Ilerzog remarks: “There is present a colloidal solution of the second type,” by which he means a hetero- or poly-disperse colloid, as a mobile equilibrium of particles of different degrees of aggregation. It is evident that this could refer equally to a reversible aggregation of crystallites, of state -1,among themselves, or to reversible equilibria between states I,B, and C. There is little evidence that the limiting solutions, a t sufficient dilution, are dispersions of the crystallites of the original fibers, and, as has been pointed out, some evidence that they are dispersions of the macromolecules, or at least of nematic swarms of these. The dependence of the Tyndall phenomenon, and of the polarization state of the light upon shearing stress may give further information on this. I t should be noted that the dilution at which Herzog found the Tyndall effect to disappear is of the same order as that at which we find “monomolecular” layers are produced. I n sum, it appears that in these dispersions and solutions, we have solvated or lyophile colloidal solutions, tending very definitely toward true solutions-henee termed by yon \Veimarn18 “solutoid.” We can, following this author, provisionally divide cellulose dipersions according to Table 11. Table I1 indicates that, besides the lyophile sols so far considered, there are possible suspensions and dispersoids of “native” cellulose, or lyophobe sols. Mechanical Dispersion of Cellulose * I n conjunction with L. W.Eberlin, I have found that by application of wet grinding in pebble or ball mills, cellulose, in the form of cotton fibers or wood pulp, can be dispersed in suitable organic liquids to very finely divided * T h e procedure described is t h e subject of a I?. S. patent application by the writer and L. W.Eberlin.
-
THE DISPERSIOS O F CELLULOSE
I047
TABLE I1 Cellulose Disaersions
1
J
P Dispersions
Solutions (Hydrate Cellulose) LY
I
(Native Cellulose)
T
i i
Smectic and nematic solutions. suspensions. A small fraction of this, increasing with time of grinding, becomes colloidally dispersed. Since beating cellulose in water hydrates the cellulose, with definite displacement toward the “hydrate” form of cellulose, chemically inert organic liquids were tried. When the same ratios of charge of liquid .cellulose:balls, the same mill-radius and r.p.m., were used, it was soon observed that different organic liquids gave very different rates of dispersion of the cellulose. This is brought out in the following table: TABLE
Liquid
Acetone
Methyl alcohol Ethyl alcohol Ethyl acetate Turpentine Acetic acid Benzole Gasoline Carbon tetrachloride
Mill
6 gallon jar
111 Charge
grams in 1 5 liters
500
Time for Reduction to I M Mesh ~ Powder
65 hours
11
72
13
90
’’ ’’
72
j 1
90 90
’’ ’’
11 11 11 11 11 11
-
11 11
,l
These figures are very approximate in regard to the time to produce a given degree of subdivision; they serve to show the broad differences between liquids in the grinding of cellulose. This difference was traced principally to t,he friction between the liquid and the cellulose, a relation confirmed by measurements of the static friction using a glass slider on a cellulose surface lubricated with the liquid in question.lg The higher the boundary lubrication, in Hardy’s sense, the less effective was the liquid, or, conversely, the higher the friction coefficient, the shorter the time of grinding to given subdivision. Incidentally, these results indicate that in wet grinding in ball mills the disintegration is not effected by impact, as probably chiefly occurs in dry grinding, but by shearing of the layer of oriented liquid molecules adhering to the solid particles.
1048
S. E. SHEPPARD
I n view of the fact that chemical action between these liquids and cellulose seems precluded (except possibly to a minute extent in the case of acetic acid) it is interesting to note that the viscosity of the solutions in cuprammonium (Schweitzer’s) reagent fell p a r i passu with increasing subdivision.
TABLE IV NO. I
Original Viscosity
Cellulose Sample
Linters X K
220
c.p.
Treatment
Viscosity after Treatment
Ground in ethyl acetate 48 hrs.
75
C.P.
2
220 ”
Methyl alcohol 44 hrs.
5 0 . 3 C.P.
3
220 ”
Acetone 7 0 hrs.
36.2
”
4
220
.’
Acetic acid 48 hrs.
16.3
”
5
220 ”
Turpentine 48 hrs.
I8
”
6
7 8
HzB Linters S o .
I
3;oo
”
4800 ’‘ s800
”
j
llethyl alcohol 96 hrs.
36j
’’
Acetone 96 hrs. Acetone I j o hrs.
200
”
I7
It will be seen also that the higher the viscosity of the cellulose initially, the longer the time required to disintegrate to a given degree of dispersions. At the same time, it was observed that the copper number was increased by grinding in these liquids. It was suspected first that this might be due to secondary autoxidation, increased by fine subdivision. On carrying out both grinding and drying in an atmosphere of COS, little or no reduction of the increase of copper number was observed. It has been observed by C. Staud and H. LeB. Grayz0 that samples of the same cellulose gave larger copper numbers as the material (paper pulp) was cut up into smaller pieces. The considerable increase in copper n u m b e r which we have found on milling cellulose is no doubt partly explained by sorptive effects caused by increased specific surface. Another part, however, is probably due to the mechanical production of new reactive end-groups, where the fresh surfaces are produced by the shearing of the cellulose aggregates. Dispersed Cellulose and Hydrocellulose The fine dispersions of cellulose produced as described, ciz., by mechanical disintegration in inert organic liquids, behaved in practically all respects
I049
THE DISPERSIOF OF CELLULOSE
identically with the so-called “hydro-cellulose” produced by the action of more or less diluted mineral acids upon native cellulose. The modified properties include : a. Diminished cuprammonium viscosity. b. Increased “solubility” in I O per cent KOH. c. Increased reactivity on esterification. d. Increased adsorption of basic dyes. The production of “hydrocellulose” by treatment of cellulose with mineral acids is not attended by obvious increase of dispersity, in that the fibers may remain apparently unaltered. It is well known, however, that such treatment results in “tendering” of the fiber. It has been shown by Farrow and Neale,*l
$
L
WCWUPIC LCIP YIVRDCHLOSIC &CLD
s enowuomc m u 4
6
wLenumc .,
ACID 5 pm CLHT WLTLS
A L L TESTS L Y A U U I
\I(
0OTTLL
NOUPS
IO 0
FIG.2
and we have confirmed their results, that the fiber strength diminishes progressively with decrease in cuprammonium viscosity. It appears justifiable therefore to speak of a “latent peptization” or of an increase in latent dispersity having been effected. This agrees with the fact, to be noted specifically shortly, that the combination of acidic treatment with mechanical shearing results in a great acceleration of the actual disintegration of cellulose to fine powders and suspensions. The property expressing the modification of the cellulose produced by acidic treatment which can be most readily measured is the cuprammonium viscosity. It is preferable to express this by its reciprocal, the Juidity, and since the scale of values is very extended, by the logarithm (common or Briggs’) of the fluidity. A general comparison of the actions of sulfuric hydrochloric, and phosphoric acids in acetic acid solution is shown in Fig. 2 . The data give the values of log 4 for four hours’ treatment a t room tem-
io50
S. E. SHEPPARD
perature-z5’ C.-of cellulose (cotton linters) shaken with 1 2 : I liquid to solid for four hours. In each case, the initial value log d, = 1.0refers not t o the original viscosity of untreated cellulose, but to the cellulose after four hours’ treatment in (glacial) acetic acid alone. Comparison of the action of the mineral acids is preferably on a basis of molar equivalents (cJ”. Fig. 3 ) but this does not affect the main features. It appears that sulfuric acid is much more effective than hydrochloric acid, and that hydrochloric acid is more effective than phosphoric. Certain singularities which appear require comment. There is a marked juctuation in the activity of low concentrations of the mineral acids-up to one per cent,
,
6uuLPye5c LClD *cID
7. *“m.-osIc
5 P*OW“OILIC LLL
ACiD
TELTI SUMEN
/* m T L C
1 *OURS
FIG.3
maxima and minima appearing on the curve. These are greatly reduced by the additions of 5 and I O per cent water to the mixture, and appear t o be connected with the presence of the small amounts of water in the cellulose and the acetic acid. I t is possible that a mixture having a very small amount of water has a much greater effect than either anhydrous or more aqueous solutions. Electrometric measurements of the “activity” of these super acid solutions, by the method of Conant, point in this direction, but the phenomenon is being studied more closely. Otherwise, the action of the mineral acids is lessened as the amount of water is increased. This, and the relative efficiencies of the mineral acids would indicate that we are dealing with the activity of super acid solution. The result showing small amounts of phosphoric acid to have a rplatively negative effect may indicate that phosphoric acid at these concentrations inhibits somewhat the action of acetic acid itself. Comparison of the action of sulfuric acid at different concentrations for different times up to four hours is shown in Fig. 4 and Fig. 5 .
THE DISPERSION O F CELLCLOSE
1051
I t will be seen here that the mechanical disintegration and the chemical ctttack superpose in accelerating the disintegration of the cellulose. Afore work both on the ultra-microscopic and the X-ray side is required before the character of these effects can be defined with precision. At present, it appears that there is a loosening of the cohesion between the cellulose crystallites of the fiber, which possibly passes over into an actual disorientation and shortens the primary valency chains composing them. As the concentration of the mineral acids is increased, it is known that bodies of lower molecular weight, sugars and sugar derivatives, are formed from the cellulose.
TIME aF TREATP~CNT IN HOURS
FIG.4
In any case, the peptization of cellulose fibers in these acid solutions shows certain resemblances t o the etching and corrosion of metals and their oxides. In both cases we are dealing with topochemical reactions of the space lattices of solids. The influence of small amounts of “impurities” in metals in affecting their rates of corrosion is very probably paralleled with cellulose fibers by the “non-uniformity” of the material, which depends upon local differences in the amounts and distributions of non-cellulose materials. Nature of Hydrocellulose Formation Although agreeing in a broad way in the properties mentioned, the degraded or “hydro-celluloses” produced by: a. action of mineral acids in glacial acetic b. action of mineral acids in water c. action of inert organic liquids in pebble mill show certain differences in the relative exaltation of certain factors. Even when brought to the same level of mechanical subdivision, there exist fine
S. E. SHEPPARD
1052
Ef?CLNT SULPWJUIC -0
32-
10
1.0
IO 0
3028-
26
$ff[ 14-8 1210-
OB-
0 604-
az-
;;
;; ; 4 ; ;;
In case (a), we have a process of acetolysis, probablyof the C -0 -C or oxygen bridges, this process being catalyzed by the mineral acidsin superacid solutions. The relative activities of the mineral acids observed are in agreement with this. The process probably involves a small fixation of acetyl on end groups of the residual (broken) chains, but not necessarily
-
(HZ W.)
anyI true n case acetylation. (b), we have a process of hydrolysis, again probably of the - C - 0 - C - or oxygen bridges. This process is also catalyzed by mineral acids, but being in aqueous solution, their activities are much less, and the
