STARCH STUDIES Preparation and Properties of Starch Triesters’ JAMES W. MULLEN 112 AND EUGENE PACSU Princeton University, Princeton, N. J.
A
pared by superheating with POINT which cannot be The various methods of preparing starch glycerol, is highly degraded. overemphasized in any esters have been treated from a critical Starch should not be dried study of starch is the point of view, and a simple method of preat high temperatures, and even extreme susceptibility of this paring starch triesters in quantitative dehydration at low temperacarbohydrate to degradation. yields is suggested. The properties of the tures must be regarded with Chemists speak glibly of suspicion. starch and starch derivatives, acetates, propionates, and butyrates of With these facts in mind it but more often than not the five species of starch prepared by this is possible to examine critically material with which they are method are described. the various methods of predealing has undergone alter-4complete investigation of the viscosities paring starch esters. These ation since leaving its native of the pyridine “solutions” of these esters methods are summarized in state. Frequently the isolaTable I. tion methods cause degradahas been made. It is shown that the At the outset one may distion. It will be well to see properties of starch and starch esters, as card all the methods which what treatment is harmful they vary from species to species, may be treat starch with the acid, to starch and what treatcorrelated to a similar variation in molecuacid anhydride, or acid chlom e n t i s n o t . Meyer (38) lar weights. ride, either alone or in the recently stressed these conpresence of catalysts such as siderations. From the viscosity studies certain deducmineral acids, acid salts (e. g., Agents which may be harnitions are made as to the shape of the starch zinc chloride), or similar hyless t o glucosidic linkages molecule. It is concluded that starch drolytic splitting agents. Withare apt to do damage to starch forms a “clump” or a rough spheroid in out doubt these methods are entirely out of proportion to three-dimensional space. degradative, especially when their activity. Consider, for carried out at high temperaexamde, a molecule of 1000 tures. g l u c b s i d i c linkages. Any The easy solubility and low viscosity of the products preagent that splits one of these linkages so that two molepared by these methods testify to their low molecular weight. cules of five hundred glucose units are formed is actually More convincing evidence of degrad ltion by such methods is hydrolyzing only 0.1 per cent of the total linkages; howgiven by Sutra (60). He slowly heated starch to 70” C. with ever, the molecular weight of the starch is reduced by half. acetic anhydride. The temperature was finally raised t o The most satisfactory method of keeping check on these deg95” C. From the reaction mixture both alpha- and betaradation processes is through viscosity measurements. The octaacetyl maltose were obtained. hydrolysis just described would cause but a slight increase in This eliminates practically all of the procedures described reducing power or alkali number-in fact, an increase almost in the literature and leaves only those methods involving pyribelow the sensitivity of the analytical procedure involved. dine as catalyst. That the molecular weight of starch is kept On the other hand, the ratio vaP/C, which is directly propora t a level approximating its original value when a pyridine tional to the molecular weight, would be cut in half. treatment is used finds abundant support. Hess, Friese, and A popular treatment in starch chemistry is to disperse the Smith (I$), who were among the first to employ successfully starch by autoclaving a t 120” C. Meyer and co-workers (33) the pyridine-acetic anhydride method, found that their tripointed out the large decrease in viscosity and even increase acetate was “completely” insoluble in all customary solvents. in copper number when starch is heated in water of pH 6 for Hassid and Dore (16) prepared the acetate of canna starch 2 hours at 126” C. with acetic anhydride by two methods: (a) with pyridine as Acids and acid salts are extremely harmful to starch. catalyst and (b) with Barnett’s catalyst, sulfur dioxideReferences in the literature to pretreatment methods with chlorine (1). The former triacetate was difficultly soluble in various acidic reagents whereby starch is made “peptizable” chloroform and acetone and, by the viscosity method, had a are nothing more than a degradation into easily soluble fragmolecular weight of 22,500. On saponification, insoluble ments of lower molecular weight. starch was regenerated. The latter triacetate was readily Alkalies lead to degradation only in the presence of oxygen; soluble in these solvents, and by the same method had a when oxygen is excluded, even concentrated alkalies seem to molecular weight of 8700. On saponification a soluble starch have no destructive action (34, 66). was regenerated. Hassid and Dore seem to have erred on the It is well known that the so-called Zulkowski starch, preabsolute value of their molecular weights by use of the constant ( K , = lo+), but the relative values are sufficient for 1 T h e first paper of this aeries appeared in July (86). our proof. Higginbotham and Richardson also made a criti1 Permanmt address, 4909 Cary Street Road, Riohmond, Va. ~
1209
1210
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
cal survey of these two methods of preparing starch acetate (20). Their results check exactly with those of Hassid and Dore.
STAINLESS STEEL REACTION VESSEL, SHOWING ELECTRIC HEATERSAND THERMOSTATIC CONTROLS DOWTHERM
Pretreatment of starch is necessary. I n one set of experiments untreated starch granules of several species were kept in contact with 4 moles of acetic anhydride in excess pyridine a t 20" C. for 5 months with occasional shaking. At the end of this period no noticeable reaction had occurred. The starch must first be gelatinized and then the granules burst. The danger of dispersing starch in water, especially a t high temperature, and the evidence for the harmless effect of alkali on starch were mentioned above. Consequently, the practice in this laboratory has been to gelatinize the starch in aqueous pyridine. This process was treated in the previous paper of this series (36). Maximum dispersion takes place in 30 per cent pyridine solutions; however, for esterification, peptization in 60 per cent (azeotropic) pyridine is both sufficient for reaction and more economical of pyridine on account of the easy recovery of the azeotrope. Also, aqueous pyridine solutions containing more than 60 per cent pyridine do not gelatinize the starch. On these facts is based the present procedure for the esterification of starch. The ratio of starch to liquid reactants is determined only by the mechanical characteristics of the apparatus. A concentration of 10 per cent starch is about the maximum that can be handled in glass laboratory apparatus, while 50 per cent pastes could probably be treated in apparatus built along the lines of a Banbury mixer. I n either case the natural starch grains are treated (undried) with the azeotropic pyridine-water mixture. By the time the temperature reaches 8&90" C., sufficient gelatinization has taken place. The calculated amount of 100 per cent pyridine is added to replace the azeotrope, and the latter is removed by fractional distillation at 93" C. The temperature reaches 115" C. as the last trace of azeotrope is removed. At this point 3-3.5 moles of the proper acylating reagent or mixed acylating reagents are run into the reaction vessel. With regard to decomposition and production of colored side products, it is not of any advantage to run the reaction a t lower temperatures. At 100-115" C. the reaction is 95 per cent complete in 5 to 20 minutes, depending upon the molecular weight of the reagent (5 minutes for the acetate, 20 minutes for the butyrate) as shown by the formation of a clear, transparent, light straw-colored jelly. A total reaction time of one hour is more than sufficient t o produce a quantitative yield of triester with most acylating agents. The ester is recovered by dumping the jelly into an equal volume of water while
Vol. 34, No. 10
stirring vigorously. The ester is obtained as a white, flaky precipitate, which can be easily filtered, washed, and dried. This method is limited to acid anhydrides and acid chlorides that show no interaction with pyridine. Of the acid chlorides investigated, only acetyl chloride and benzoyl chloride could be used. Propionyi chloride, benzyl sulfonyl chloride, succinyl chloride, and phthalyl chloride reacted with pyridine, even a t 0" C., to give highly colored, insoluble complexes. If it becomes desirable to use acyl chlorides, the reaction may be run in the presence of an inert diluent such as benzene or toluene. Among the acid anhydrides used, maleic anhydride alone reacted with pyridine. This it did almost explosively. Successfully prepared by means of the anhydrides were the following starch triesters: the acetates, propionates, and butyrates of potato, tapioca, wheat, corn, and rice starches. The acetate of cornstarch amylose, separated according t o a previously published method (38) was produced, as were the caproate, phthalate, and succinate of cornstarch. Partial esterification of cornstarch occurred with the maleic anhydride adduct of both methyl abietate and diethylene glycol diabietate. The tribenzoyl ester of cornstarch was prepared through the acid chloride. I n the only etherification reaction attempted, benzyl chloride was added dropwise to the active starch-pyridine gel. After a small quantity of reagent had been added, the starch precipitated out as a compact rubbery mass. Some reaction must have occurred, since neither the pyridine nor the benzyl chloride alone caused the starch to assume this condition. This situation might be remedied by the use of an inert solvent. I n this paper the acetate, propionate, and butyrate of the five starch species will be discussed. The remaining esters will be considered in a later paper. The deciding economic factor in this process is the efficiency of pyridine recovery. This material is relatively expensive, and 100 per cent recovery is desirable. The pyridine-water azeotrope used to gelatinize the starch may be recovered quantitatively so that no loss occurs a t that point If the pyridine jelly of the ester is precipitated in water as described, the water solution contains both pyridine and the pyridine salt of the acid used. This solution may be limed and the pyridine freed and recovered as the azeotrope, which, in turn, may be broken by a salting out process, solvent extraction, or a ternary distillation with benzene (49). It is even more efficient to take advantage of the volatility of the pyridine salts. At one atmosphere pressure pyridine hydrochloride boils a t 218-219' C., while pyridine acetate boils undecomposed a t 139-140" C. (12). The acetate consists of 2 moles of pyridine t o 3 moles of acetic acid. Consequently the simplest procedure, allowing full recovery of the pyridine (as the salt which can then be limed), would be to dry the jelly itself in vacuo a t a very low temperature. When prepared in this fashion the ester is produced as a transparent, somewhat brittle film, which can subsequently be ground to a powder.
Apparatus and Procedure ESTERIFICATION. The starches employed in this work have already been described (36). The early experiments were carried out in ordinary laboratory lassware. The setup consisted of a one-liter three-neck flask ground-glass joints), equipped with fractionating column, mercury-sealed stirrer, and dropping funnel. On a water-free basis, 25 grams of starch were treated per run in an initial concentration of 10 per cent in the pyridine-water azeotrope. The fifteen esters t o be described were prepared in 3-pound batches in the stainless steel kettle described in the previous paper (36). A liquid-solid ratio of 3 to 1 was maintained throughout gelatinization, 1 liter of pyridine being added for each liter of azeotrope removed. 3.5 moles of the proper anhydride were used. The esters were recovered by the precipitation procedure. The yields were practically quantitative. The esters thus prepared on a relatively large scale analyzed
f
INDUSTRIAL AND ENGINEERING CHEMISTRY
October, 1942
1211
L
TABLE I. HISTORICAL DEVELOPMENT OF STARCH ESTERS Date
Investigator
Reagent
Catalyst
Esterification Complete
Product Degraded
NO No Yes No No Yes No No No No No NO No Y es Yes Yea Yes No Yes Y es Y e8 NO No Y e6 No Yes
Yes Yes Yes Yes Yes Yes" Yes Yes Yes Yes Y es Yes Yes Y es Yes5 Y es Yes Yes Y eF Nc Yek Yes Yes0 Yes Yes Y es Yes Yes Y e6 Yesa Yes" Yesa Ye8 No Yes6
ACETATES 1865 1869 1870 1883
Schuetsenberger (61J Schuetsenberger and Schuetsenberger (68) Schuetsenberger (60) Michael (96)
1901 1905 1905 1908 1909 1916 Boeseken 1921 1922 1923 1924 1927 1927 1928 1928 1929 1929 1929 1930 1932 1933 1933 1933 1934 1936 1937 1938 1938 1941 1941 1941 1941
et
al. (4)
Barnett (I) Pringsheim and Lassmann (4.8) Escales and Levy (f1) Pringsheim and Wolfsohn (4.9) Bergmann and Knehe (2) Peiser (40) Hess et al. (18) Haworth et al. (17) Tsuzuki (69) Tsusuki (84) Bri 1 and Schinle (8) I. Farbenindustrie (24) Clarke and Gillespie (7) Reich and Damansky 46) Reich and Damansky \44) Sutra (61) Stein (69) Sakurada and Inoue (48) Staudinger and Husemann (67) Dejarme (IO) Higginbotham and Richardson Whinfield and Ritchie (66) Nisida (87) Pacsu and Mullen (89) Seiberlich (69)
8.
Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetyl chloride Acetic anhydride Acetic acid Acetic anhydride Acetic anhydride Acetic anhydride Acetic acid Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic acid Acetic acid Acetic anhydride Acetyl chloride Acetic anhydride A cetjr anhydrjde Acettc anhydr!de Acetic anh dride Acetic a c d Acetic anhydride Acetic anh dride Acetic a c i z Acetic anhydride Various
acetic acid
acetic acid
acetic acid
None None None None None Sulfuric acid None Hydrochloric acid gas Zinc chloride None None None Mineral acids and acid salts Sulfur dioxide-chlorine Pyridine Twitchell acids Sulfuric acid None Sulfuric acid Pyridine Sulfur dioxide-chlorine Potassium thiocyanate Zinc chloride Pyridine Sulfur dioxide None Pyridine-Na acetate Pyridine Sulfuric or phosphoric acid Perchloric acid or thionyl chloride Pyrjdjne Pyridine Methylamine sulfate Pyridine Sulfuric acid Pyridine None Various
NO
Yes No No Yes Yes No Y es Yes No Yea
Yes No
OTHERFATTY ACIDESTBRS 1904 1923 1923 1923 I. G. Farbenindustrie ($2) 1923 Berthon ( 8 ) 1924 Gault and Ehrmann (14) 1929 I. G. Farbenindustrie (21) 1932 Lorand (80) 1936 Genin (16 1942 Mack a n d Shreve (Sf)
Formic acid Lauryl chloride Stearyl chloride Palmityl chloride Stearyl chloride Palmityl chloride Undecyl chloride Palmityl chlqride Stearyl chloride Palmityl chloride Lauryl chloride Lauryl chloride Myristyl chloride Lauryl chloride Palmityl chloride Stearyl chlorjde Stearyl chloride Propionic anhydride
None Pyridine
No No
i
Tertiary bases Tertiary bases Pyrjdjne Pyrid ine Pyridine Pyridine Sodium hydroxide Magnesium erchlorate or sulfuryPchloride]
+ propionic acid
$one None
.
P
1
b
No No No
Yes ? Yes Yes
1
?
4 i
i
i
i i
? No
Yes
i 1 r
ii
MISCBLLANEOUS EBTlRE 1895 Boetringer (6) 1904 Kldiesl~vili(28) 1905 Kldiashvili (27) 1924 I. G. Farbenindustrie (28) 1929 I. G. Farbenindustrie (21) 1932 Rudy (47) 1933 Reich and Damansky (46) 1933 Damansky (8) 1934 Hess and Eveking (18) a
Glyoxylic acid Monochloroacetic acid Dichloroacetic acid Trichloroacetic acid Linolenic acid chloride Tung oil acid.chlorid?s Phenacetic acid chloride Diphloroacetip anhydride Trichloroacetic anhydride Cinnamyl chloride Benaoyl chloride Tosyl chloride
Prepared from a starting material which was itself degraded-e.
None None None None Tertiary bases Tertiary bases Sodium hydroxide Sulfuric acid Sulfuric acid Pyridine Pyridine Pyridine
NO
No No Yes YBe
Yes Yes
r
Yes* ?
g., Zulkowski's staroh. soluble starch, etc.
between 0.5 and 1.5 per cent below their respective theoretical acyl contents (44.8 per cent acetyl, 51.8 per cent pro ionyl, and 57.2 per cent butyryl). This, however, was due s o h y to the mechanical difficulties of mixing the relatively thick doughy astes in the type of apparatus used. That this is true is shown y! the ease with which these esters could be brought up to their full acyl content, This was accomplished by making, with vigorous stirring in a Waring Blendor, a 3-5 per cent solution of the artial ester in pyridine; one mole of anhydride was added (on gasis of original starch). After standing overnight at room temperature, the ester was recovered by pouring into a large excess of vigorously stirred water. The esters so treated had their theoretical acyl content, and such esters were used in all the viscosity studies.
ANALYTICALPROCEDURE. Analysis for acyl content was carried out as follows: 0.5-1.0 gram samples were placed in a glass-stoppered flask, covered with 10 cc. methanol, and allowed to stand for an hour. An equal volume of water was added and then 20 cc. of 1 N sodium hydroxide. This mixture should stand for at least 12 hours with occasional swirling. Back-titration is carried out with 0.5 N hydrochloric acid. VISCOSITYDETERMINATIONS. All viscosity measurements were made a t 25" C. with an Ostwald pipet immersed in a kerosene-filled thermostat maintaining a constancy of *0.02 O C. Dispersion of the ester in a proper solvent is a critical point and will be discussed later. DENSITYDETERMINATIONS,These were made in a 15-cc. pycnometer immersed in the thermostat just described.
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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Properties of Starch Acetates, Propionates, and Butyrates APPEARAWE. Regardless of species, these esters are all white powders or friable fibrous lumps; their condition depends more on the effectiveness of agitation during precipitation than on any other factor. If they are prepared by evaporating the solvent (pyridine plus pyridine salt) from the reaction jelly, they appear as transparent films. The acetate films are brittle, the butyrate films much less so.
go/
I O 0 c c.
O F ST14RCH E S ~ E R FIGURE 1. DENSITY “SOLUTIONS”
MELTINGPOINTS.These compounds melt over a broad range, the acetate a t the higher temperatures, the butyrate a t the lower. Observation of the softening and melting phenomena by the usual melting point method is worthless. Some information was obtained from pressure drop-teniperature curves taken during the molding of these esters. However, for lack of special equipment our data are meager and uncertain. We believe that the acetate softens a t 170” C. and becomes clear above 200” C.; for these two points the propionate ranges from 130’ to about 200’ C., whereas like figures for the butyrate are 90” C. and slightly less than 200” SOLUBILITY.These esters do not form true solutions, and it is with great difficulty that one even obtains good colloidal dispersions. Dispersion seems to be most difficult for potato starch, least difficult for rice. Ordinary stirring produces thick gels that may be thrown out by centrifuging. Better dispersion is obtained if a device with high energy transfer is employed. The ideal apparatus would be a colloid mill, although fair results have been obtained with a homogenizer and much better results with a Waring Blendor. Dispersion is much easier to accomplish with the acetates than with the propionates or the butyrates, in that same order. When a transparent compact plug of the molded ester is placed in a flask of acetone and left undisturbed, it exhibits gradual swelling over a period of days to a size many times its original. Toward the end of this period the interface between the gel and the solvent becomes hazy, and it is difficult to distinguish between the two phases. The initial period of thii process resembles the swelling of rubber in toluene. The esters are all insoluble in mater. The acetate is insoluble in the lower alcohols and ether, but the propionate and butyrate tend to have a slight tackiness when submitted to these solvents. Most of the other common organic solvents have a swelling effect upon these esters. The aromatic hydrocarbons, the halogenated hydrocarbons, and the tertiary nitrogen bases afford some of the better peptizing agents. Even in the best solvent a faint opalescence persists, but nothing is thrown don-n even on prolonged centrifuging,
of concentration. The results are given in Table 11 and Figure 1. TABLE 11. DENSITY OF STARCH ESTER SOLUTIONS (1 GRAMPBR 100 CC.) A T 25’ C. Solvent C8HkN
C~H~Cl~
DENSITY.The density of the dilute ester “solutions” were determined for use in the viscosity determinations. Within the accuracy obtained, the density is independent of both ester species (even cellulose) and ester type. It is a linear function
Density of Solvent 0.9780 1 588
Density of Bo111 o.os10 1.586
OPiw.iL Ror-mon.. The data for the optical rotation of the starch esters are given in Tables I11 and IV. The value4 for the acetates are fairly constant, whereas those of the propionates and butyrates are each, in turn, lower than the corresponding acetate. This is to be expected since a given weight of butyrate would contain less asymmetric system than a given weight of acetate, due to the extra weight concentrated in the side chains. The molecular rotations are approximately identical. The slight differences that do occur among the acetates lie in the order of granule size discussed in the earlier paper (56), the higher rotation going with the smaller granule size. Wheat again is somewhat inconsistent. The differences, however, are too small to be convincing. VIscosrrY. Since no quantitative data regarding the esters could be obtained from melting points or solubility, and since the optical rotation data are inconclusive, we found it necessary to make viscosiby measurements on dilute “solutions” of the esters. This is the most sensitive method available for the study of high polymers, and it enabled us to detect differences among tlie esteTs of the various species of starch ytudied.
c.
Properties of Ester “Solutions”
Vol. 34, No. 10
K ~ S L Iof ~ ~Viscosity S Studies In order that the viscosity studies be of any value, it i s first necessary that the ester be properly dispersed in a good solvent. Higginbothnm and Richardson (20) reported that their farina starch acetate, prepared with pyridine catalyst, was not “completely soluble a t room temperature and under suitable conditions highly swollen particles could be seen on
b
I
r t i m e of beating in minutes
in tetrachloroethane C .d
I
400b
v
,-in
I
pyrtdtne
r
200
2
4
6
8
IO
12
14
No. passes thru viscometer FIQURE 2. EFFECT OF BE.4TING (COLLOIDINQ) ON APPROXIMATELY ONE P E R CENT SOLUTIONS OF POTATO STARCH
ACETATE
October, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
1213
g./IOO c c. FIGURE 3. RELATIVE VISCOSITY OF PYRIDINE SOLUTIONS
amount of foreign matter centrifuged out, and the dispersive the walls of the vessel. These particles passed through a Jena No. 3 filter, but a considerable amount of acetate was action continued for another 30 minutes. The viscosity was determined for each ester in approxiretained by filter paper. The times of flow of the solution (0.25 per cent by weight in tetrachloroethane a t 25” C.) dimately 1, 0.5, and 0.25 per cent pyridine solutions, the true minished with successive passages through the viscometer, but concentration being determined by drying a weighed sample of the dispersed solution 48 hours a t ll_OoC. The results returned on keeping overnight to a value close to the original, are given as relative viscosities in Figure 3. These data are again with a diminution in successive passages,” With the starch acetates prepared here, the drop in viscosity was also also given for the various esters in 1.0 per cent solution observed with successive passages. However, consistent (taken from curves) in Tables 111, IV, and V. results on the thixotropic effect described by Higginbotham The relative viscosity of the acetates of various species decrease in the order of decreasing granule size (order listed in and Richardson could not be obtained. To avoid these difficulties, a study was made of the effects of solvent and of time of dispersive beating in a Waring Blendor. The results are summarized in Figure 2. TABLE 111. PROPERTIES OF STARCH ACET.4TES Pvridine is obviously a better “solve& for the starch esters than tetrabIolecular SizeL [%] Mol. wt. Mol. wt. chloroethane since the dispersion time flow in the viscometer is less than half that required for a similar conditionin tetrachloroethane. It must be noted that the dispersion time for constant flow is greater for the propionates and butyrates, in that order. I n the examples given below, dispersion was carried out for 30 minutes, a slight
f:gt:$ ;:2;
!:$
X::!
Wheat Corn Rice
+151.0° 220.5 C 1 . 5 6 . 5 ~ 224.3 +157.9 210.4
2.54 2.57 2.24
2.90
CaHrN
- .28,20 ...
3,24 3,65 0.887 1 . 0 0
0
b
2s5,0 76.7.
2.82 2.53
1.20 1.18 0.98 0.99 0.92
..
38.0 36.0 30.5 31,5 27.5
..
..
One gram per 100 cc. CaHaN. From visoosities of the acetates in CbHrN st 25O C., using K m = 0.7 X 10 - 4 .
acetate
starch
543.000 515,000 436,000 450,000 393,000
305,000 290,000 245,000 253.000 221,000
,...
.... ....
....
INDUSTRIAL AND ENGINEERING CHEMISTRY
1214
TABLEIV. PROPERTIES OF POTATO STARCH ESTERS Ester Acetate Propionate Butyrate Acetate in
Rotation in CsHsN Mo!. [a]? rotation +158.0: 45,500 +143.2 47,200 +122.5O 45,500
.. ..
CzHzCL
+158.0° (1 One gram er 100 cc. CsHaIi. 6 Esters in &HEN a t 25’ C.
Viscosity a t 25O C.5 ,, n, seooentionds poises ma 290.0 3 . 4 3 3.87 232.0 2 . 7 4 3 . 0 8 167.3 1.99 2.28
..
...
Effect of Acyl Groupsb on K,
cp20
2LL;ive
of Calod.
543,000 706,000 620,000
38.0 35.0 32.0
..
..
the highest viscosity types commercially available. Its acetyl content was approximately 40 per cent. It is of more value for comparative studies to obtain the value of (vaP/C’) as C’-+ 0, for a t zero concentration the disturbing interaction between solute
molecules and solvent, as well as between other solute molecules, are supposedly a t a minimum. Concentration C’ represents base moles per liter-e. g., for the starch acetate, grams per liter divided by 288. The extrapolation to zero concentration is shown in Figure 4; data are included for the comparison between potato starch acetate and cellulose acetate, potato starch acetate in two solvents, potato starch with different acyl groups, and the acetates of various starch species. Since considerable data have accumulated in the literature employing Kraemer’s intrinsic viscosity (88)’it is advisable to make the present comparisons in those terms. This is done in Figure 5, where the limit as C -+ 0 is found for the ratio In vrar/C. Concentration C is in grams per 100 cc. The various parts of the diagram have the same meanings they had in Figure 4. The curves show excellent linearity.
....
Table 111), wheat alone being anomalous. This is in keeping with the behavior of wheat starch as discussed in the previous paper (36). Increasing the weight of the acyl group decreases the viscosity, This, however, is to be expected since the viscosity of a solution depends more upon the number of polymeric chains present than upon the weight of the individual chain; i. e., viscosity is more influenced by the chain length of the polymer than by its molecular weight. The sample of cellulose acetate with which the potato starch acetate is compared was characterized by being one of
chain K m ,t o1engtE kee 1880 0 . 7 0 x 10-6 0 . 54 75 X x 10-4
Vol. 34, No. 10
.......
TABLEV. EFFECT OF SOLVEST ON VISCOSITY OF POTATO STARCH AT 25” C. (1 GRAM PER 100 Cc.) ACETATE Solvent CrHsN CzHzCla
70
q , Seconds
n, Centipoises
0.887 1.588
290 410
3.43 4.05
3.87 4.57
Discussion of Viscosity Results The intercepts in both Figures 4 and 5 are proportional to a constant multiplied by molecular weight. Because of the more ready availability of a suitable constant, Staudinger (55) will he followed in the present discussion. According to his relationship,
20+, 0
I
,
I
0.010
I
I
0.020
I
~~
I
I
0.030
1
1
1
Base moles/ liter FIGURE4. EXTRAPOLATION AT ZERO CONCENTRATION
where K , is a constant and M is the molecular weight. Strictly speaking, K , varies with solvent (Figure 4b). Since the same acetate in different solvents exhibits different intercepts, a different constant has to be applied in each case to obtain equal molecular weights. However, as a first approximation it is satisfactory to use K , = 0.7 X determined by Staudinger and Husemann (58) in acetone on a series of degraded starch acetates of known molecular weight. Using this constant, the order of magnitude will be appropriate and the differences betnyeen species will be correct, with the reservations to be made later. The absolute magnitude of the molecular weights of the acetates alone suffers in accuracy. The molecular weight calculations are summarized in Table 111. I n both this and the previous paper (86‘) various properties (i. e., consistency of pastes, temperature of gelatinization, heat of gelatinization, ease of dispersion in solvents, etc.) have been related to the granule size of the various starch species. It is now possible to relate these same properties (also including granule size) to the average molecular weight of the starches of different species; the small granules occur in conjunction with the starches of low molecular weight. However, there are several possible variations of this hypothesis which must be discussed. Up to this point starch has been treated as a single com-
INDUSTRIAL AND ENGINEERING CHEMISTRY
October, 1942
tetrachloroethane
1.2 acetate
C
proptonate
-
- -
--
butyrate
potato tapioca
-
-
corn wheat
