Cellulose Intercry stalline Structure J
N
STUDY BY HYDROLYTIC METHODS R. F. SICKERSON' AND J. -A. HABRLE2 Mellon Zmstitute, Pittsburgh 1 3 , Pa.
T U D I E S previously re111the hydrolysis of cellulosic fibers with aqueous hydroabout -1 minutes thc rate of h? droll si\ weni- to decrease ported from this laborachloric and sulfuric acids a t boiling temperatures, the perceptibly, nhcieaq the retory showed that the behavior disordered intercrystalline chain network appears t o he gain of the coriespondinq hyattached first. This inference is drawn from -harp reof urisubstituted cellulases drocelluloses ceases to fall and ductions in cupramnionium \iscosity and moisture regain during acid hydrolysis is conbegins a steady rise. These sistent with the concept of roincident w i t h the rapid accumulation of glucose. Simuleffects are assumed to inditaneously the fibers are reduced t o pow der? h?drocellulose. these as crystallites intercates the rapid decomposition R ith further hy drolg sis glucose continues to form and laced by an intercrystalline of the hvgroscopic, amormoisture regain s l o w 1y increases, but \iscosity remains cellulose chain network. The. practically constant. Attack in this stage may be chiefly phous fraction and the beginintercrystalline cellulose apning of the breakdown of on lateral crystallite surfaces. Lengths of crystallites calpears t o consist of t F o distinguishable parts, arbitrarily mesomorphous material ( 7 ) cnlated from \iscosity data range from 280 glucose units called amorphous and mesoThe lattei apparent11 hvdrofor cotton to 110 for high tenacity \iscose rayon. Sulfuric. lyzes a t a steadill- dccteasing acid is much leas artiFe than hydrochloric in hy drolyzing morphous components (7, 8). rate for about an hour. cellulose and, after neutralization, does not interfere with Thp amorphous component is Thereafter the rate remains the direct .olumetric determination of glurose. probablv the most disordered fairly constant for several part of the structure; thi. hours and is assunird to repmesomorphous, thp tranrir t w i i t hydrolysis i.)f the crystalline component. tional statp l,rt~\-ec~nmost disordi,retf arid highly ordered D a t a n-hich typify these effects for acetate grade cotton linters componrnta. arc' summarized in Table I. In addition, cuprammonium (cuam) The hydrolysis-osidation method (6) employed in these invcstigations results in estenPive dccoiiiposition of samples and so viscosity values detclrmined by the method of 1Icase (4)are yields a broad insight into hydrolytic behavior, hut it has some also presented. Estimates of degree of polynierization (D.P.) xiare calculated from viscosities with the equation of Battista major disadvantages. The saniplcs are degraded rapidly and and, accordingly. the method lacks precision in these, ( I ) . The hydrolysis data are taken from a previously reported ges of thc hydrolytic. process n.hert1 important changes Ytudy (71, Rhich contains dcstails of preparation and treatment, appeal' to or(-ur. Thcl method also affords only indirwt mtiMoisturci r e k i n values represent adsorption mates o f t h r aniounts of ccallulosf, hytlivlyzcd to glucosc~,and, at 6jPc relative. humidity. T h r ttffects of hydrolysis with hydrochloric acid-ferric chloride finally. it t,shihit.; conridvrable espc~riiiicntalerror ( 2 , 7 ) . Thun, in addition t~ this niethcd, milder conditions arid g wagt~ntare sct forth graphically in Figure 1. which is based on the cisioIi art. dt&xI,l(: for the, 5tudy of thix ribatlily attac data of Tablc I. Moisture regain and cellulose h>-drol?-zedclearly Ilinc. c~c~ilulcse. (Lshihit t h r behavior described. Viscosity, however, falls sharply with progressive h y d r o l y k , reaches a lon. valuca, and then reis papc'r describrs a direct, prc e method of drtcriiiining the coui'st' of cc~llulorebreakdon-n under much mildtar conditions niai~isroiistant for scvcxral hours despite appreciable further dethan have, h(ai,tltoforebeen utilized. Hydrol?-sis data obtained by composition ( J f the linters residue. A similar result n-as found the application of this method are givcn for a number of different tjy Staudinger and Sorkin ( I O ) , k h o investigated the effects of cellulosic materials. The variation of moisture regain arid acid hydrolysis by viecomc.tric methods and ronrludcd that, cupramnionium viscosity ivith progwasive mild hydrolysis is rhain shortening ceases where crllulose molccul(~fragments reach also shown. Hydrolysis, viscosity, and regain data for onr Imgths of from 150 to 200 glucosta units. case of t h r iiiore severe treatment are included to permit a coniparison of r r w l t s frorn the t\yo methods.
S
TABIXI. HYDROLYSIS-0XID.ATION METHOD
This hytlrolysi~-o~idatioii method consists in digesting tht. cellulosic material in boiling 2.45 AV hydrochloric a c i d 4 . 6 JJ ferric chloi,ide aiid determiriing the extent of hJ-drolysis from thP carbon dioside evolved. T h e latter appears to originate principally from glucose and other loi\--niolecular-~~eight split products of cellulose. In practiw, h>-drolysis appears to proceed at a rapid initial rate n-hich coincides with a n equally rapid fall in moisture regain of thc recovcfirablc hydrocc~lluloses. Aftcir
F;FFECT OF SEVERE HYDROLYTIC T R E A T ~ I EOSNT ~ sEVER.Xl. (:H.~RACTERISTI(!P OF .kCETATE-GRhDE C O T T o S LISTERS Tinie Treated, Hr.
Calcd. Cellulose Adsurptiun Cunni Hydrolyzed, Regain a t Viscosity of 'Zof D r y R - t . h 1357~ R.H., 70 O.5Y0 Soln., Cp.
With boding 2.45 S HC1-0.6 .Ti FeCla except for t h e r e r y short time . intervals for which boiling 2.45 ZVHCl was used. Calculated f r o m carbon dioxide evolution ( 7 ) . h
I 2
C'alcd. BasicC D.P.
Present address, Monsanto Chemical Company, Boston 49, Mass. Present address, Crescent Heights, S e w Brighton, Pa.
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I n glucose units as found b y t h e equation of Battista ( 1 ) .
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Further support for this esplariation of muisture regain vitriation is given by adsprption and desorption data for acvtate grade linters and high tenacity viscose rayon (Table 11). Each series of progressively hydrolyzed samples was conditioned in parallel at 6 5 5 relativc humidity. The same samples were used for both adsorption and desorption measurements, drying being effected iri xwuo at rooin tempcnturc’ over phosphoric anhydride. Yisrose rayon data are included because that material is different in regain hrhavior from natural and modified celluloses. The somewhat cwatic results for the rayon samples subjected to prolonged acid treatnwnt may be attributed t o thc fact {hat theue hyclrocelliilosc+ tcincl to dry to horny transparcnt lumps.
T A B L E 11. vARIATIOS O F .$USORPTIOS A S D I)I.~JORPTIOS LIOISTURE REGAIX AT 65Tc RELATIVE HWIDITY OF APETATEGRADELINTERS ASD VISCOSERAYON WITH HYDROLYSIS
b
I
2
3 4 HOURS OF HYDROLYSIS
5
6
I
7
Figure 1.
Effects on Cotton Linters of Hydrolysis with 2.45 N HCI-0.6 31 FeC13 as Functions of Time
Percentage hydrolyzed calculated from carbon dioxide yields: regain and viscosity represent recovered hydrocelluloses.
Several conclusions may be drawn from these observations. First, the rapid initial rate of hydrolysis indicates that highly disordered or readily accessible cellulose is attacked, presumably the amorphous component. Secondly, disintegration and removal of this component coincide Tr-ith a sharp reduction i n moisture regain, n-hich suggests that in the linters this component was relativel>- highly hygroscopic. Thirdly, removal of this eomponent also coincides with a pronouncid diminution in viscwsity n-hich indicates that the highlv disordered regions occur periodically and include a large proportion of the molecules. Fourthly, reversal in trend of regain and leveling off of viscosity after the first few minutes are evidence that chain shortening ceasvs with the removal of the hygroscopic cellulose. Since decomposition of the cellulose continues and moisture regain begins t o rise, chain length remaining constant, it is reasonable t o conclutle that attack must then be principally on the lateral surfaces of crystallites. This attack ~vouldeffect a relatively rapid increase in surface area per gram of the hydrocellulose and viould perhaps explain the rising moisture regain in the case of moistureimpervious crystallites. [-$ question has been raised ‘concerning the possible effects on moisture regain data of “humic subBtance” whose formation under acidic conditions was studied by Philip, ?*Telson, and ZiiHe (9). T h a t humic material does not influence present results is probable for two reason:: (a) Its accumulation is largely, if not completely, pi,evented by the oxidizing action of ferric chloride, and ( b ) after the acid treatment the hydroeelluloses w r e mashed n-ith dilute ammonia which dissolves such material. The final hydrocelluloses were pure white (?).I Finally, hydrolysis appears to procecd at a decreasing rate for about a n hour and t h w c d t e r to continue at a fairly constant rate. If the attainment of constant rate rrpresents the slow hydrolysis of dense, homogeneous, crystalline material, then it mag be assumed that ariy mesomorphous or partially ordered cellulose has a t this stage been eliminatcd. Some evidence thitt constant rate of hydrolysis is attained in about an hour has already been given ( 7 ) ,and further evidence is presented later in this report. Howt gressive hydrolysis of the nwsoinorphoiir or transition:il cellulose has no apparent effect on osity and no unique effect on regain, it follows that this partially ordered cellulose is similar. to the highlx ordered, differing from it only in being more readily hydrolyzed.
Time Hydrolyzed. Hr. 0 0 07 0 20 0.80 2.0 4.0 7.0
Linters Desorp- Adsorption, tion,
%5
7 67 6 18 6 48 6 80 7 05 7 78 8 73
4%
6.48
5,31 5 46 P.68 J 8R 6 2.5 6 40
Des.,‘
Ads. Ratio
1.18 1 16 1.15 1 .QO 1.20 1.24
I 36
Viscose Rayon Degorp- hdsorp- Des./ tion, tion, Ads C’ Fc Ratio 14 2 1.17 12. 1 9 71 8.70 1.12 9 83 1.14 8 . GX 1.15 9 90 8.60 9 72 1.12 8.68 10 2 1.15 8.90 9 96 8.75 I . 14
It is apparent that hydrolysis causes an initial sharp reduction in rcgain in both cases, but, unlike the linters, the viscose rayon fails t o eshibit a subsequent r( rsal in trend. This diffi%rc,nce ma>- arise from a relatively high perviousness of the rayon ( tallites to moisture as compared to linters crystallites ( 5 ) . small drop in desol.ptioniadsorptioii ratio Iyhich accompanic,s the initial reduction in regain suggests that, disintegration of the amorphous componcnt lon-crs slightly the hysteresis effect. Despitcl this indication of a small change in the amount of capillary ’ a result of hydrolysis, there sec’niy to be ample reason for assuming t h a t the principal cause of the large mgain diminution is the destruction of a highly liygro?copic component of thc intact structure. Free cellulosic hydrosyl groups are knoxn t o have high affinity for moisture. I t is evidvnt, therefore, that the highly hygro ic regions of the fiber must lie unusually rich in free hyclro. which \~--ould\ ) t > the caSe if these regions were highly disordered. SULFURIC ACID HYDROLYSIS
The prc.ct:ding experiments show that hydrochio1,ic. u i d ferric chloride reagent effects extremely rapid hydrolytic changes at the outset of the process. Less severe conditions of hydrolysis were obviously necessary for the investigation of this rcgion, but ion-ering the concentration of hydrochloric acid in the reagent \Todd make carbon dioside data difficult to interpret ( 6 ) . Exploratory work, however, indicated t h a t boiling 2.5 S sulfuric acid caused suitably slow hydrolysis, and this medium was selcc t c d for suhscquiliit espcrimcnt s. DIRECT DETER\IISATION OF GLUCOSE
The method of IIassid ( 3 )was found to be entirely satisfactory for the direct determination of glucose both in known glucose solutions and in sulfuric acid hytlrolyzates of cellulose. This method consists in treating a dilute glucose solution with excess alkaline ferricyanide and titrating the ferrocyanide produced by the osidat,ion of glucose with standard ceric sulfate. I n the experimerits to be described the ceric sulfate was standardized against known glucose controls run in parallel with each series of unknowns. It ~ a first a determined that refluxing with 2.5 S sulfuric acid did not inHuence the analytical results. I h o w n glucose solutions ivvrre boiled under reflux for varying times rvith 2.5 X sulfuric acid,
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1947 I
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PREP.AR.ATIOS OF CELLULOSE 1IYDROLYZ.ATES
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Alccuratcly n-eighed samples representing approximately 1.5 grams of oven-dry cellulosic material n-erc rcflurcd for varying lengths of time in about 60 nil. of 2.5 .\-sulfuric acid aiitl filtered
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Y
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and diluted t o volume. Glucosc~ as determined in triplicate on 10 ml. portions of the 1attc.r solution, and the average so found vias corrected ro unit weight of dry starting niateriitl and converted to cellulose by the factor 0.9. The air-dried liydrocelluloses rcpresenting varying times of sulfuric acid hydrolysis of a given material n c r e coiiditidhed simultaneously a t 21 O C. and 6 5 7 , rtxlative humidity to constant neight and then dried in vacuo over phosphoric anhydride a t room temperature. .liter moisture regain was so found, the hydrocclluloses Ivere used for cuprammonium viscosity dcterniinations.
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VISCOSE R A I O U MERCERIZED COTTON PURIFIED WOOD PULP '175'COTTON LINTERS ACETATE GRADE LINTERS UUMERCERIZEO COTTON
4
5
6
HYDROLYSIS DATA
7
0
HOURS I H BObLING 2 5 N H.50.
Figure 2. Effect of Time in Boiling 2.5 '1 Sulfuric icicl on Cellulose Hydrolyzed for Various Celluloiic Iraterials
neutralized with 5 S sodium hj-droxide, and then tested by the Ilassid method. The data from a t>-pical experiment are presented in Table 111. Seither the acid treatment for periods up t o 12 hours nor the suhscquc-nt ueutra1iza:ion alters the glucose estimates. AFTER TRE-~TJIEST OF 0 . 2 7 ~ T.~BI,E 111. GLKCOSEOBHERYED SOLL-TIOS WITH BOILISG 2 . 5 *V H,SOI ASU ~-ETTR.ILIZ.ITIOS
Hours boiled Glucose found, 5; of taken
0 100
Onset of boiling 98
2
4
6
103
103
103
1
2 99
Similar check tests ~ v e r eniadc on cellobiost, since it is probably a product of cellulose hydrolysis. Aliquots were removed from a known cellobiose solution after varying times of reflux with 2.5 S sulfuric acid, neutralized n-it11 5 S sodium hydroxide, made u p t o suitable volumes, atid assayed for glucose. The apparent yields are shown in Table IV. 'The data indicate that at least 15 minutes' hydrolysis under these conditions is necessary t o convert cellobiose to glucose. However, the error froni this source in cellulose hydrolyzates would not be appreciable, especially \There the hydrolysis extended over relativel>- long periods.
Ctkllulose liydrol>-zcd-tinic data obtained as ticscribed for scveral typical cellulosic material* are prescntecl graphically in Figure 2 . These curves indicate that, the three unmodified cc3lluloses-that is, unmcrccriz:cd lint cotton, acetate-grade lintcrs, and "175" lintcrs (175 seconds viscosity, Ilercules similar in behavior during the first half hour but thc,reaftcLr b c ~ o m e divergent, Purified wood pulp appears initially t o hydrol?-ze like the ummodified celluloses and Inter t o aimulato mercerized cotton. Since the niercerizt,d cotton represents ~ o m eof the unmcrcerized cotton which liad b c ~ ntreated with 2jcC sodium hydroside in the abscnce of ieiifion (81,it can readillbe seen that mercerization increases the extent of hydrolysis a t all stages of the process. The relativc,ly rapid breakdown of high tenacity viscose rayon over the wliole range confirms prcvious results ( 5 , 8). I n general, the c u r v ~ sindicate that all the materials may represent molecular ordc'ring ranging from highly disordered and readily accessible to well ordered and lw? readily hydrolyzed fractions. The absence of transitional irregularities in the curves suggests that all degrees of ordering may coexist in the intact substances. The variation of moisture regain and cupraminonium viscosity with h i e of sulfuric acid hydrolysis is shon-n for a number of cpllulosic materials hy the d a t a in Table I-. Thc effects of such
TABLE IV. APPAREST GLL-COSEIS 0 . 2 7 CELLOBIOSE AFTER TREATMEST KITH BOILISG2.5 S H,S04 A S D SEL-TRALIZATIOS Hours boiled 0 0.083 0.25 0 50 1.0 2.0 4.0 Apparent glucose, % of theorericala 83 93 97 100 101 100 97 0 * 3 332 X 0.2 = 0 . 2 1 0 5 , glucose concentration a t c o m p l e t e hydrolysis
Thr~sc~ experimcnts indicate that sulfuric acid hj-drolyhis and for ~ I U C O S Ccan , be applied in cellulose invcstigntions. A supsequent secrion of this paper shows that sulfuric acid and liydroch arid as used in t h w e experimc~nts and appear to diffpr only i n the have similar hvdrolytic e rate at whirh the effects prodpced. Therefore, it may be assumed that the rapid changes which occur in the early stages of li>-drol>-sisn i t h hj-drochloric acid-ferric chloride reagent can be cxxamined more closely by use of sulfuric acid hydrolyjis and volumetric glucose anal)
x
" 3
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TIME
HYOROLIZED - uoum
3
4
Figure 3. Effect of II?drol>sis with Roiling 2.5 S Sulfuric i c i d on Cuprammoniuin \lscosity and 3Ioisture Regain of Cotton Linters
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 39, No. 11
chain shortening will rcacli an ultimate value for other cellulosic materials in a similar time. FurTime therniore, this ultimate visc isitv Hydrolyzed, should indicate the a! Hr. length of the crystaliitc?. Vi.cosity data and crystitllitc lcngth t d m a t e s derived from thcme valucxs by mean:: of Battista's equation are prvsentcd in Table High T e n a r i t y Saponified Egyptian 1-11 for a variety of cdlulosic Linen Cellulose Acetate Cotton (RIalaki) materials. The data shon. that 0 6.91 6.55 10 3 2.07 0.88 21 7 crystallites are probnbly longest 0.2 5.80 1.41 2.88 1.95 0 5 6.33 1.82 8 45 1.40 a.19 1.8G in unmodified crllulom. that a At 657, R . F . ? 21° C . mercerization causes crystallite b 0.5% solution i n cuam. shortening, and t h a t the shortest crystallites are in regenerated celluloses. The C I J stallites in hydrolysis a t the outset of the process are illustrated in greater higlilv oriented, inponifitd cellulose acetatr appmently arc somedetail in Figure 3. The data and curves reveal that both regain n hat longer than those in the viscosr ravon, as might be expected and vixosity fall sharply a t first and then begin t o I( Viscosity appears to reach a l o v ultimate value in half and thereafter t o remain constant. The regain, hoir-evc,r, after reaching a minimum value in approximately half a n hour, apparently begins a gradual rise. The absence of such a r i v in the A v . Crystallite T.iscosity of L e n g t h Calcd. case of viscose rayon has been noted and has been discu 0.5yGSoln , f r o m Visrosiry. briefly. Material Cuam-Centipoise D.P. T h e hydrolysis curves in Figure 2 s h o v t h a t no major chnnge Egyptian cotton (Malaki) 1.86 2x3 Linen 1.82 270 in rate of breakdon-n occurs in the vicinitv of half an hour. SimiVnmercerized cotton 1.76 2.53 duetate-grade cotton linters 1.75 218 larly, undrr the more drastic hydrolysis-oxidation conditions alS o . 173 linters 1 74 211 ready mentioned, the attainment of constant viscosity and the RIercerized cotton 1,x 179 High tenacity saponified cellulose acetate 1.40 132 inversion of the regain trend are not reflected in the cellulose High tenacity viscose rayon 1.34 110 hydrolyzpd relation. Simultaneous values of cellulose hydrolyzed by sulfuric acid in half a n hour and of moisture regain reduction in the same period are recorded for corresponding samHYDROLYSIS BY SULFURIC AVD HYDROCHLORIC ACIDS ples in Table VI. These data are taken from Figure 2 and Table I t n a s mentioned that the hvdrol\ t i ? process appeals t o be V. A correlation between these variables is clearly indicatpd, the same for sulfuric and hydrochloric acids, even though sulalthough the number of samples is not large enough to establish furic acid is much less active than hydrochloric of equivalent a valid relation. concentration. The similarity in action of the two acids is
TABLE 1'.
~ A R I A T I O SO F ADSORPTIOS 1LEGAIN .4ND CC.411 \'ISCoSITY O F SEJ-ERAL CELLLT20SIC hfATERI.4Ls WITH HYDROLYSIS BY B O I L I S G 2 . 5 ATH$Oi High Tenacity S o . 175 Linters Unmercerized Cotton RIercerized Cotton - Tiarose Rayon Regain=, Yiscosity h , RepainQ, 1-iscosityb, Regaina, Yiscosityb, RegainU. Tiscositj b , 7c CP. % CP. 70 CP. "c CP.
shon n by the expeiiments which are now described. Samples of acetate-grade cotton linters from the batch used TABLE VI. MOISTURE REGAIN REDUCTIOS AND PERCEUTAGE OF for the preceding experiments were treated for varying times CELLULOSE HYDROLYZED IN 30 MIXUTES BY BOILITG 2.5 S HzSOd with boiling 2.5 N sulfuric acid as outlined in the section on prepaAlerUnmerration of cellulose hydrolyzates. T h e insoluble residues of hvdroViscose cerized S o . 175 cerized cellulose so obtained were carefully mashed and dried. ThereMaterial Rayon Cotton Linters Cotton after samples of the hydrocelluloses representing varying amounts Cellulose hydrolyzed, % 5.70 3.25 2.00 1.80 of sulfuric acid pretreatment Lvere oven-dried for 2 hours a t Regain difference, % 3.28 1.91 1.14 1.07 105' C., weighed accurately, and refluxed for a n additional half hour with 2.5 Y sulfuric acid. Hydrolyzates were separated from these digests and employed for glucose determinations in accordance with the above procedure. Similarly, solid hydroAgain, the attainment of constant viscositj- suggests t h a t cellulose residues representing acetate-grade linters which had cellulose chain shortening ceases just as it did in the more drastic been treated with boiling hydrochloric acid-ferric chloride reagent hydrolysis with hydrochloric acid-ferric chloride reagent. The for varying times (Table I) were dried, weighed, and hydrolyzed for an additional half hour in 2.5 S sulfuric acid. Glucose was reversal in trend of moisture regain also indicates the elimination also determined in these hydrolyzates and calculated t o cellulose. of a hygroscopic fraction of cellulose and the development of The cellulose-hydrolyzed data obtained from these t n o series absorptive effects which may be caused at least in part by inof hydrocelluloses are presented graphically in Figure 4. The creasing surface per gram. Thus the parts of cellulose molecules curves represent reaction rate variations with time. I n other betneen crlstallites in the chain direction appear to be both words, the curves show the relative amounts of cellulose hydrohighly hvgroscopic and readily hvdrolyzed. After the removal lyzed in comparable half-hour increinents after the hours of preof this intercrystalline cellulose, hydrolytic attack continues tieatnient nhich appear as abscissas. It is apparent that 2.5 practically unabated but at right angle- t o the chain directionthat is, on the lateral surfaces of crystallites. The assuniption N hydrochloric acid, as the hydrolytically active constituent of then appears to be justified that the crystallites have someivhat hydrochloric acid-ferric chloride reagent, hydrolyzes the linters spongy lateral surfaces and t h a t the ordering of molecules inat a rapidly decreasing rate which reaches a low and constant value in about an hour. A similar conclusion regarding the creases n ith depth. I n other words, mcsoiiiorphous or partially effects of hvdrochloric acid was reached in a study by the difordered cellulose may occur mainly on the lateral crystallite ferential method ( 7 ) . The figure also s h o w t h a t 2.5 Y sulfuric surfaces. Since celluloses as different from each other as unmercerized acid decreases the rate much less rapidly than does hydrochloric of approximately equivalent normality. Kevertheless, the two cotton and high tenacity viscose rayon reach constant viscosity in half an hour of sulfuric acid hydrolysis, it is probable that processes appear to be essentially the same. This fact becomes
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1947
rCI 5
1.4
n I I
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manifent ~ l i c ~the r i time scale fOr t l w sulfuric acid curve is shortened hy R factor of I / $ : the hydi~ocliloricand sulfuric acid curves then practically coincide. I n nt1it.r Ivordq, 2.5 sulfuric acid requires 6 hours to attain the rate reached by 2.45 S hydrochloric acid in 1 hour. T h e ratio of hydrolytic activities of the t x o acids on an equivalent normality basis appears, therefore, t o lie about 1:R. The same activity ratio is indicated by the effects on moisture regain produced by the two acids (Figure 5 ) . The moisture regain data were ohtained 1)y the procedure given. Corresponding minima in regain arc produced hy 2.45 S hydrochloric acid in approsimatrlj- 4-5 minutes and by 2.5 S sulfuric in about 30 minutes. Theae rcsults also suggcqt that the 1:ydrolytic activities of sulfuric and h>-drochloricacids are in the ratio of ahout 1:6. Finally, a comparison of the cuprammonium viscosity data for unmodified cotton celluloses in Tables I and V demonstrates that the constant low value produced by 2.45 .V hydrochloric acid in 4 minutes agrees favorably with Chat produced by 2.5 S sulfuric in 30 minutes. DISCUSSION OF RESULTS
The sulfuric acid hydrolysis data presented in Figure 3 indicate t h a t neither regain nor viscosity change in direction so abruptly as might be expected from the hvdrolysis-osidation data in Figure 1. For this reason it is rather difficult t o determine precisely the moment or moments a t nhich chain shortening ceases and destruction of highly hygroscopic cellulose is complete. In the cases of both the slon. and rapid hydrolysis, moreover, the rate of cellulose decomposition is still relatively fast during these changes. I n v i m of these facts and of the previously given criticisms of the hydrolysis-oxidation method, it is not surprising that the amounts of cellulose hydrolyzed in 30 minutes by sulfuric acid differ considerably from the estimates of amorphous cellulose found by the hydrolysis-oxidation method (7, 8). The data obtained by sulfuric acid hydrolysis and volumetric determination of glucose, and hence of cellulose, appear to be reliable. I n addition t o the proof already given, it was found for several different materials t h a t the sum of cellulose hydrolyzed to glucose and hydrocellulose recovered did not vary more than l%, from the Tveight, of sample taken. It appears, therefore, t'hat the amounts of amorphous or highly disordered cellulose may be closer t o the 30-minute sulfuric values than t o the 4minute hydrolysis-oxidat ion values. T h e inclusion of viscosity measurements in these experiments permitted a clarification of the structure which previously could only be surmised from regain alone. I n fact the viscosity behavior is n-orthy of further emphasis. The curves in Figure 3 indicate that only 2, 3.5, and 6 5 ,respectively, of cotton, mercerized cotton, and viscose rayon are hydrolyzed as the viscosities of these mateiials arr .reduci.d to relatively constant values.
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This relation suggests that the selective chemical treatment of this small part of the structures, as by etherification or esterification, {vould effectivelv stabilize them against degradation, such as by microorgariisms. In other ivords fiber strength and structural continuit!- are highly dependent on this small percentage. I t is now clear that the full appraisal of celluloses bv hydrolysis requires a t Itlast tn-o sets of conditions, one relatively mild and one sufficiently severe t o estend the breakdown well beyond the point at nhich the rate appears t o become constant. For cxho\l--s that constant rate of hydrolysis is not. uric acid in 8 hours; from this it n1s.y be assumed that the mesomorphous cellulose is not conipletely hydrolyzed in that time. The more Revere condition, on the other hand, causes too rapid hydrolysis at the outset but vields a better over-all concerntion of behavior.
7.4
"
w 7.21
~
.
5 7.0 a.
"0
20
40
60
80
TIME HYDROLYZED - MIUUTES
100
0
Figure 5 . Aloisture Regain a t 65yo Relative IIumidity of Linters Hydrocelluloses Recovered after Varying Times of Digestion with Hot 2.5 A- Sulfuric i c i d and 2.45 SHC1-0.6 11.3 FeC13
At least one further problem is prmented by these results.
If, as the viscosity data suggest, hydrolytic attack soon ceases on the end5 of crystallites and continues a t a decreasing rate on the sides of crystalliter, extrapolation of breakdown curves as a means of estimating total noncrystalline cellulose may not be permissible. The reason is that extrapolation assumes concurrent attack on different parts of crystallites, of which one part is eliminated b y projection t o zero time. Since concurrent processes may not be going on, the total noncrystalline cellulose may be represented by the total cellulose hydrolyzed during the time required t o attain constant rate. SUMMARY
The effects of boiling 2.5 S hydrochloric acid-0.6 JI ferric chloride solution on cotton linters are described in terms of cuprammonium viscosity, moisture regain, and percentage of cellulose hydrolyzed. Rapid initial breakdoll-n of cellulose occurs, and viscosity and regain fall sharply. K i t h further hydrolysis, cellulose breakdon-n continues and regain increases, but viscosit)- remains practically constant. Less severe conditions represented by boiling 2.5 S sulfuric acid were used in the investigation of the rapid initial phase of the process. Cellulose decompoeition was determined by a volumetric method for glucose. Viecosity and regain effects were obtained for several different cellulosic materials. Results indicate that the t w o methods supplcnir~iitr a r h other and permit
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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adeauate coverage of estensive sample breakdown. Under the. conditions emploJ-ed, sulfuric acid appears to be only about one sisth as active in hydrolyzing cellulose as is hydrochloric acid of equivalent concencration. The data suggest that cellulose molecule sections which intcrlink the crystallites in the chain direction are attacked first. This part of the structure, highly hygroscopic and probably quite disorganized, represents relatively m a l l percentages uf the intact materials. Hydrolytic attack thereaftcr appcars t o bo limitcd to lateral crystallite surface.;. Crystallite length estimaks derived from from 280 glucose units for cotton to 110 for rayon. Mercerization apparently causes a lite length. I
Vol. 39, No. 11
descrilxd in this series of paners. and the kind assistance of L. Bass, G. D. Beal, K. A. Hamor, and W. H. Child of ilon Iiistitutc in the preparation of manuscripts. I
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ACKIVOWLEDGMENT
made by the Satiorial Cotton Council for the invcstigiitions
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Mean Molecular Weights of Asphalts and Their Constituents GEORGE IT. ECICERT AND BRUCE WEETMAS The Texus Gonipany, Beacon, S. Y .
T h e molecular weight method based o n the Tiscosit) of dilute solutions was investigated for its applicability to asphalts and their constituents. The two equationi proposed by Staudinger for calculating molecular w eights from Iiscosities of solutions w ere considered. The constants in the equations were etaluated bj using cryoscopic molecular weights for t h e oil and resin fractions from asphalts. The molecular weights of t h e oils and resins ranged from 370 to 900. Viscosities of benzene solutions were determined a t 7 i b F. i n a capillary t j p e tiscometer. RIodifica-
tions of Staudinger's equations are not required for theoil arid resin fractions of asphalts 011 the basis of the present ~ o r k>lean . molecular weights of 700 to 1800 were obtained 1,) the ~ i s c o s i t )inethod for asphaltenes from asphalts of different source5. Both equations proposed b) Staudinper were used to calculate the molecular weights of asphaltenes, and neither appears preferable. Reliable independent data for the molecular weights of asphaltenes are not atailable to define conclusiYe1) t h e constants i n the original and modified forms of Qtaudinger's equations.
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thc valucs obtaincd with benzene too high and ntt rihutcd incunresults t o ,separation of thc benzene solution into t \ w a t the freezing point. Hillnian and Barnc'tt (4)determined lar w i g h t s of asphalts by the cryosropic method, using both berizcne and n a p h t h a h e , antl obtained valiws for asphaltcncs from iioncraclred residua of 2300 and 1660 n i t h the two solw u t s , respoctiwly. They concluded that, the average molmxlar w i g h t o i ill-dcfined mistures, such H S asphaltenes, cannot he accepted as absolutc, sirice th(x solutions of these fraetioni proliabl?; dopitrt apprc'ciably f r o m the ideal solution Ian-. Grader ( 3 , t gives incorrect rcsults even d naplithalene as a solvent. molecular weights of asphaltencs by the Langmuir monomolecular filiii method and reportcd the following values:
URIKG studies in this laboratory on the properties of charac-
teristic components isolated from asphalts-namely, asphaltenes, resins, paraffin oils, naphthene oils, and waxes (fS),it has been of interest t o obtain the mean molecular neights of th(,se fract,ions. There is general agreement in the literature regarding the ordcr of magnitude of the mean molecular weights of t h r oil and resin type fractions. However, no such agreement exists for thr asphaltenes, probably because of their l o ~ vsolubility, eomplcx molecular structure, tendency to form aggregates, colloidal nat u w , and the fact t h a t their solutions may depart appreciably from the ideal solution Ian-s. I n view of the confusion esisting i n the literature, some attention has been given t o possible mcthods for clarification in regard t o the mean molecular w i g h t s of asphaltenes. Sakhanov and Vassiliev (14, reported molecular weights of 5000 t o 6000 for asphaltenes determined by the cryoscopic method using benzene. Strietcr (16) used bcnzcne for the cryoscopic molecular weight determination of asphalts and reported the following values: Trinidad bitumen 1131.8, Bermudez bitumen 620.3, gilsonite bitumen 4251.5. Katz ( 7 ) employed the cryoscopic met,hod,with camphor and brnzene, t o obtain the molrcular weights of asphaltenes. He obtained values of 4300 t o 5600 with benzene and 2219 t o 5160 with camphor solutions. He considered
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Source of .\;phnltenes M e x i c a n , stearn-refined Alexican, blown Venezuelan. steam-refined Tenezuelan, b l o w n
M e a n l l o l e c u l a r Weight 80,000 80,000 110,000 140,000
Sivmison ( 1 : ) carried out diffusion experiments in which the h u n d a r y wts follon-td by light absorption, and claimed t h a t the rcijults suggest molccular w i g h t values for asphnltcnes at least as high as those obtained by Pfeiffer and Saal. Kirby ( 8 )used an isotonic or equal vapor pressure method ( I O ) m d obtained molecular weights of 1510 for gilsonite (select,) a r d 715 for a Nexican as--
