T H E PEPTIZATIOS OF PYROXYLIN* BY MILTOK L. BYRON
History Pelouzel first used the name “Pyroxylin” with reference to collulose nitrate. His product, however, and the guncotton that Schonbein2 discovered in 1846, were almost identical. At the present time England and the United States use the name “ P y r ~ x y l i n ”to~ designate those soluble cellulose nitrates used in the manufacture of collodion and lacquers, while in Germany it is used to designate the insoluble and explosive variety. In 1846, F. Domonte and Henard4 submitted guncotton to alcohol-ether and obtained incomplete solution : they claimed the soluble cellulose nitrate was identical with the xyloidine that Braconnot5 had discovered in 1883 while the insoluble cellulose nitrate contained more nitrogen. Lunge and Bebie6 working with the influence of water on the nitrating process, incidentally showed the fallacy of attempting to judge the etheralcohol solubility of cellulose nitrate from the nitrogen content as illustrated in Table I. TABLE I Percent nitrogen
Solubility in ether-alcohol 2:1
13.65 13.21 12 76 12.58 12.31
I.5 0 5 5.40 22.00
60.00 99. I 4 99.84
12.05
Percent nitrogen
11.59 10.93 9.76 9.31 8.40 6.jo
Solubility in ether-alcohol 2:1
100.02 99.82 74.22 1.1; 0.61 1 ‘ 73
According to JTorden,7 “the nitrogen content is not a trustworthy criterion of solubility, for nitrocelluloses are known t o contain 10% and I 1% nitrogen which are, or are not, soluble in amyl acetate. For example, a cotton may be nitratrd a t a comparatively lorn temperature, and after nitration is substantially cunplete so far as nitrogen content of the nitrocotton is concerned, if a sample be withdrawn, and washed to neutrality in the usual manner, while the buik of the nitrated cotton remains in the nitrating mixture, the temperature of which is considerably elevated, it is possible t o produce two nitrocottons in which the sample first withdrawn is completely soluble in alcohol* -1thisis submitted for the degree of M.Chem. a t Cornell University. l
Am.
Sci.,
(2)
5 , 142, 259 (1842).
* TVordm: “Technology of
Cellulose Esters”, 1, 111, 1570. W o r d a : “Technology of Cellulose Esters”, 1, 111, 1696. J a h r b u h der Chemie, 1, 1136 (1846-48). Worder: “Technology of Cellulose Esters”, 1 111, I j67. J. Am. :hem. SOC.,23, 528 (1901). 5 Worden “Technology of Cellulose Esters”, 1, 111, 1721.
THE PEPTIZATION O F PYROXYLIN
1117
either mixture, while the latter sample is substantially insoluble therein, both containing approximately the same nitrogen content .” In view of the fact that the temperature is considerably elevated, the nitrogen content of the second sample should be somewhat higher, and this might account for the difference in solubility. The “soluble” cellulose nitrates are distinguished from the highly nitrated guncottons by a solubility in ether-alcohol, i.e. ethyl ether and ethyl alcohol, but not in any ether or any alcohol. A. llatteoschat’ has made an extensive study of the solvent action of alcohol-ether mixtures on cellulose nitrate. In the case of the completely soluble cellulose nitrate, the proportion of ether to alcohol may be varied widely Tyithout altering the solubility figure. The solubility depends upon the cellulose nitrate, proportions of solvent, and strength of solvent used. The nitrogen which is not sample of cellulose nitrate used contained 12.95‘; completely soluble in alcohol-ether, Matteoschat’s data are given in Table 11.
TABLE I1 Ether--ilcohol 99 5
Strength of I l c o h o l , Percent by K t . 95 90 __
__
42.3
28.7
14.2
53.9 53 . O
45.0 57.5
I : 2 I :I
34.4 52.3
2 :I
40.5
52.4
3 :I 0 : r
25.0
42.4
3.1
80
__
1.7
__
__
This shows that the weaker alcohol gives the best results provided the ether is in excess. T . Chandelon? explains this in the following conclusions, drawn from his experiments :“(I) The greater solubility of moist cellulose nitrate in a mixture of ether and alcohol does not depend on the existence of a hydrate, but simply on the dilution of the solvent by the i n t e r in the moist substance. “(2) I t is immaterial whether this water is contained in themoist cellulose nitrate. or previously added to the solvent.” G. Lunge and J. Bebie’ determined how far the mixture of ether ( 0 . 7 2 0 ) and alcohol (0.810) might be varied from the usual proportion of three to one. “.I cellulose nitrate of I I.j 4 5 nitrogen dissolved easily in ether-alcohol six t o one, only 9 j$ dissolving in nine parts ether to one of alcohol, and only 7.3‘; in twenty-seven parts of ether to one of alcohol. In the other direction, one-third of ether t o one part of alcohol acted as a complete solvent. One sixth ether t o one part of alcohol dissolved only 93%. Worden: “Technology of Cellulose Esters”, 1, 111, 2291; Chem. Abs , 8, 1669 (1914) J. Chem. SOC.,104 I, 18 (1913). 3 Z . angew. Chem., 14, 538 (1901); J. Am. Chem. SOC.,25, 568 (1901). 2
1118
hIILTON L. BYRON
From this, one sees that a decrease of the ether content has less effect on the solubility of the cellulose nitrate than that of the alcohol, and also that the more important rBle of the solution must be ascribed to the alcohol. Thus the solubility may vary in wide limits from the usual proportion of 3 : I.’’ Bancroft’ states that in a number of cases, mixed solvents will peptize a solid much better than either solvent alone; cellulose nitrate in ether-alcohol being a good example. He also says? that “cellulose nitrate swells in alcohol and not in ether, but it is not known whether this is universal nor whether the alcohol peptizes the cellulose nitrate a t higher temperatures.” Purpose The purpose of this thesis is to determine whether either ethyl alcohol or ethyl ether peptizes pyroxylin, and if so, why the mixture peptizes the pyroxyslin better than either the alcohol or the ether. Experimental
Materials The pyroxylin was macle by the E. I. duPont de Xemours and Company. It is used in making collodion and consequently the commercial name is “Collodion Cotton.” The nitrogen content of the pyroxylin was found t o be 1 2 . 1 5 , using a Dennis improved nitrometer. The absolute alcohol was made by the United States Industrial Chemical Company; using an alcoholmeter. the alcohol tested 9 9 . 8 5 a t 15Oc. The ether was made by Iiahlbaum. I t was acetone-free and had been distilled over sodium. I t was kept in a glass-stoppered bottle in contact with metallic sodium.
P r e l i m i n a r y Ex p e n nients According to the Pharma~opeias,~ pyroxylin is slowly but completely soluble in twenty-five parts of a mixture of three parts ethyl ether, one part ethyl alcohol. In making up this solution 62.5 gm. of absolute alcohol was added to I O gm. of pyroxylin, and 187.j gm. of anhydrous ether was then added. In order to determine whether the alcohol or the ether is the peptizing agent, five cubic centimeter samples of the solution were taken a t the start. To one set an excess of alcohol was added s l o ~ l yand to the other an excess of ether as shown by Table 111:
2
Bancroft: (IA4ppliedColloid Chemistry”, 167 (1921). By private communication from Professor Chamot. Korden: “Technology of Cellulose Esters”, 1, 111, 1696.
1119
THE PEPTIZATION OF PYROXYLIN
TABLE I11 Sample S o .
Volume Pyroxylin in Ether-Alcohol
I
j
cc
Volume Alcohol added
cc 2 cc 3 cc 4 cc j cc 8 cc I2 cc I
2
j CC
3 4
j CC
5 6 7 8 9
j CC
5 cc 5 cc
20
IO
j CC
I25
j CC j CC
5
CC
I j CC
cc cc
Volume of Ether added I1 I2
I3 I4 15
16
cc cc 3 cc 4 cc i cc 6 cc I
2
The addition of the alcohol had no effect on the colloidal pyroxylin except as a diluent. Sample S o . I O containing 25 parts of alcohol to one part of the colloidal solution was distilled to less than half of the original volume in order to remove all traces of ether, and the colloidal pyroxylin remained unchanged. On adding ether, however, the results were quite different. For samples S o . I I, 1 2 , 13, no change was apparent, hut sample KO.14turned cloudy. On the addition of an equal volume of ether to the original volume of the colloid. the sample began to coagulate; in saniple S o . 16 complete coagulatim n-as obtained. This would tend to prove that ether has no effect on the peptization of the pyroxylin; while alcohol should peptize it at some temperature because it swells in alcohol at room temperature. If the pyroxylin is peptized at high temperatures, the peptization is due to the dissociated alcohol, whereas, if the pyroxylin is peptized at low teniperatures, the peptization is due to associated alcohol. In either case the peptization will be due to adsorbed alcohol, the adsorbed film distintegrating the pyroxylin so that a colloidal solution results. When1 a liquid is adsorbed by a solid, it will tend to peptize it and in some cases will do so. At higher temperatures the peptizing action increases. -Ilardles2states that “although a liquid may he a non-solvent of a cellulose ester, at temperatures at and below its boiling point a t atmospheric pressure, it often happens, for example with alcohol, benzene, methyl ketone, for celluBancroft: “*Ipplied Colloid Chemistry“. 167 ( 1 9 2 1 ) . (1926); J. Soc. Chem. Ind. 42, 1 2 8 T (1926).
* J. Phys. Chem., 30, 348
*
I120
MILTON L. BYRON
lose acetate that it becomes a solvent at higher temperatures. Anhydrous alcohol becomes a solvent for a particular sample of cellulose acetate a t 120’ C and industrial at IOOOC. Thus alcohol cannot be considered merely a diluent, since in mixtures, especially when dissociated, it must contribute greatly t o the solvent action.” I n order to determine the effect of high temperatures on the peptizat)ion of pyroxylin by the ethyl alcohol; samples weighing 0.1and 0.01gm. respectively, were placed in j cc of ethyl alcohol in pyrex tubes, the alcohol frozen in liquid air, suction applied and the tubes sealed. The tubes mere heated in an ashestos box using 2 2 0 watt lamps as the heating medium, the thermometer being placed beside the tube. The temperature was slowly raised, the tubes being removed and thoroughly shaken every 1 5 O - 2 0 ~ C. rise in temperature. Between I 55’ and 1 6 j o C. two of the tubes exploded. There had been no apparent change below I j j o C. Two other samples were heated to 14jO C.for over an hour with thorough shaking erery ten mirutes with no apparent peptization in either case. L. E. Smith,’ in YcRain’s laboratory, “sealed nitrocotton and alcohol in a glass tube and heated to 100’C. for two or three hours. but no gelatinization appeared to take place in this time. The tube was then heated at 120’ for upwards of 1 2 hours and at the end of that time most of the nitrocotton had dissolved leaving a slight residue. The solution obtained was yellow and very mobile even when IO“, of nitrocotton had gone into solution. Alcohol containing varying amounts of water also dipsolved nitrocotton, but the greater the proportion of water the greater the amount of residue, and the darker the solution.” Smith says: “It is obvious that this solvent action is not due t o unchanged alcohol alone. but partly to the products obtained hy the nitration of the alcohol by the nitrocotton. The solution smelled strongly of esters. The residue was probably nitrocotton but this has not been tested.” I n view of the fact that the alcohol and cellulose nitrate were heated a t I zoo for upwards of tv-elve hours before solution was obtained while heating a t 100’for two or three hours, or at 145’ for over one hour had no effect, it would seem that a chemical reaction had taken place. This also would be indicated by the yellow solution, the smell of esters, and by the fact that a mobile solution n-as obtained even when 10% of the cellulose nitrate had gone into solution. The cellulose nitrate might be peptized hy t1:e ethyl nitrate formed; the ethyl nitrate might polymerize the ethyl alcohol which in turn would peptize the pyroxylin or a cellulose nitrate with a lower nitrogen content and soluble in ethyl alcohol might he formed in the chemical reaction. I . S.Eiugelmass? made a study of solutions of cellulose nitrate in alcohol, in ether, and in a mixture of the two. h t room temperature neither ether nor alcohol peptized the pyroxylin. At lower temperatures, however, using carJ. Phys. Chem., 30, 348 (1926). Rec. Trav. chim., 41, 751 (1922).
THE PEPTIZATIOX O F PYROXTLIN
I121
bon dioxide snow and ether as the cooling medium, he obtained a colloid in ether which analyzed 13.7 j nitrogen: and one in alcohol, which analyzed 14.02 nitrogen, while the original pyroxylin only analyzed I I .9 nitrogen. According to these results, Kugelmass must have worked with a mixture of cellulose nitrates, some of high and some of low nitrogen content. The more highly nitrated cotton v a s peptized, leaving a residue of cellulose nitrate of low nitrogen content. Kugelniass found that the pyroxylin-ether system formed suspensoids which gave a blue opalescence; with the pyroxylin-alcohol, he observed a series of distinctly separate layers. With regard to the peptization and protection of colhilose nitrate in ether, he says: “The dissolved eelhilose nitrate reduces the superficial surface tension of the same cellulose nitrate and, as x result, it is strongly adsorbed. The effect of the adsorption is to produce a more advanced reduction in the superficial tension, thereby diminishing the tendency of the particles of cellulose nitrate to unite.’’ It h i s already been shon-n that the addition of ether will precipitate cellulose nitrate froni colloidal solution in ether-alcohol. I n attempting to confirm the observations made by Kugelniass. the usual ratio of one part pyroxylin to twenty-five parts of solvent was used. -Anhydrous ether will not peptize pyroxylin at room temperature. To determine whether the ether would peptize the pyroxylin at low temperatures or not. 0.288 gm. of pyrosylin and I O cc of anhydrous other were cooled in carbon dioxide snow and ether. The pyroxylin reniained unaffected even after alternate shakicg and cooling for several hours. I n order to be certain that the temperature was low enough. the mixture was cooled in liquid air with no apparent result. I,. E. Smithi tested ether to see if it exerted a solvent action on cellulose nitrate a t 10v teniperatures. X sample of ether nhich was in contact v i t h cellulose nitrate for eighteen months, showed that it had dissolvcd 0.004. He could not confirm the results and observations of Ilupelmass that ether at lory temperatures dissolves cellulose nitrate forming suspensoids showing a illiie opalescence, and a series of distir,ctly separate layers in alcohol. The results of our experiments agree with Smith‘s results and this tends t o corroborate the statement that Kugelmass worked with a mixture of cellulose nitrates. I n determining the effect of lower temperatures on the peptization of pyroxylin by absolute alcohol alone, the same proportions of one part pyroxylin t o twenty-five parts of solvent were used. 0.31; gm. of pyroxylin and I O cc. ( 7 . 9 2 j gni.) of absolute alcohol were cooled in carbon dioxide without apparent change. This solution, being 4 7 cellulose nitrate, was obviously too concentrated, and I O cc. more of the absolute alcohol was added, giving a 2Yc solution. On cooling with carbon dioxide snow and ether, and after considerable shaking the alcohol began to peptize the pyroxylin. In order to hasten the J. Phys. Chem., 30, 348 (1926).
I122
MILTOX L. BYRO?;
action I O cc. more of the absolute alcohol mas added, giving 1.33% cellulose nitrate. After standing at the low temperature for some time, the alcohol completely peptized the pyroxylin. On warming to room temperature, the sol became viscous. By increasing the temperature to 3 j oC. for some time. the sol set to a flowing jelly. Another sol containing 1C; pyroxylin completely peptized by alcohol alone, set to a stiff jelly at room temperature. On cooling, the jelly formed a so1 and, on warming. went back t o a jelly again, showing that this change is reversible. L. E. Smith1 carried out the following experiments on the gelatinizing power of alcohol. “It was found that the nitro cotton containing 1 2 . 1 7 nitrogen was soluble at room temperature t o the extent of 0.35C; in 9 5 7 alcohol. Experiments were devised t o find the solubility of the sa:ne nitro cotton at low temperatures in anhydrous alcohol, The alcohol was thoroughly dried and then distilled into tubes already containing the nitro cotton. Solutions of various concentrations were prepared. The tube containing the smallest quantity of nitro cotton m s first cooled in a mixture of solid carbon dioxide and ether: and the nitro cotton formed a solid compact inass which could not be broken up by shaking. The other tubes were well shaken before cooling to prevent this froin happening. On cooling, the second tube containing approximately 0 . 8 ~nitro ~ cotton the liquid became more viscous and dissoh-ed the nitro cotton, forming a viscous sol nhich on warming to room temperature set to a flowing jelly or gel. The change froin sol t o gel was reversible, for, on cooling the gel3 it reverted t o the sol form. Two other solutions showed the same phenomenon. The gels became more turbid as the concentration of the nitro cotton was increased. The solution containing 55 nitro cotton was a stiff and a very turbid gel. These gels are stable at room temperature and even after a period of 18months still show the reverse change on cooling. They show the phenomenon of syneresis to a marked degree. It is impossible to measure the actual solubility of nitro cotton under such conditions.” It is apparent then from the results of our experiments and those of L. E. Smith? that, despite the statement made by K u g e l m a s ~ anhydrous ,~ ether, free from alcohol and acetone, has no apparent peptizing effect on pyroxylin. Absolute alcohol will cause pyroxylin to swell but will not peptize it a t room temperatures or a t higher temperatures. From the experiments of Ramsay and J. Shields4 we know that ethyl alcohol is polymerized as liquid, the degree of polymerization increasing Lvith falling temperature, T h e n a sample of pyroxylin in alcohol is cooled for some time in carbon dioxide snow and ether shaking every few minutes, it is not completelj peptized immediately, as the low temperature does not polymerize all of the alcohol J. Phys. Chem., 30, 347 (1926).
* J. Phys. Chem., 30, 347 3
(1926). Rec. Trav. chim., 41, 751 (1922). Phil. Trans., 184 h,647-673 (1893).
THE PEPTIZATION O F PYROXYLIN
1123
and swelling precedes peptization. The best results are obtained by cooling, shaking as the alcohol warms up to room temperature, and repeating two or three times. Alcohol polymerizes as the temperature is lowered, and it 1Tould seem that the lower the temperature, the greater the peptization. However, if liquid air is substituted for carbon dioxide snow and ether, the alcohol freezes, but the peptization is not enhanced. The reason why warming to room temperature gives better results is not as yet clear; but it is probable that the same results would be obtained if the alcohol could be vigorously shaken at the lower temperature. Akxordingto L. E. Smith’ there are very few instances of a gel formed from a sol on heating and re-forming the sol on cooling. Szegvari’ thought he had discovered the first one; namely, nitrocotton dissolved in a mixture of amyl acetate and benzine (not benzene). He said that at a lower temperature the amyl acetate combined with nitrocellulose rather than the benzine. whereas at higher temperatures a compound was formed between amyl acetate and benzine to the exclusion of the former combination. In the case of absolute alcohol and pyroyylin, the alcohol polymerizes when the temperature is lowered; it is then adsorbed by the pyroxylin. The p j roxylin is completely peptized at this temperature ; then, as the temperature is raised, the sol slo~vlychanges to a gel, due to the depolymerization of the alcohol. i. e. the finely divided pyroxylin is suqpended throughout the sol or gel. \Then pyroxylin is peptized in ether-alcohol mixtures, the ether apparently polymerizes the alcohol, TT hich then peptizes the pyroxylin. The conclusions from X. Matteoschat ’s3 n-ork are summarized as follows : “(I) The difference in the solubility at different volume relations of the same compocents of the solvent mixture within relatively small limits of I : I and 3 : I and also by the use of different concentrations of alcohol in the same ratio mixtures is considerable. The mixture I : I is only favorable when alcohol of the highest con(2) centration is employed : with technical alcohol better results are obtained with a z : I and 3 : I mixture. ( 3 ) Adding the alcohol and allowing it to thoroughly saturate the cellulose nitrate, before adding the ether, gives a much better result than the direct use of ether-alcohol. (4) The results of Natteoschat agree with the opinion of Schwalbe as well as Lunge and Bebie, i. e. decrease in alcohol decreases the solubility of nitrocellulose in ether-alcohol, in so far as highly concentrated alcohol is concerned. Tet the increase of alcohol above the I : I ratio does not increase the solubility, but lowers it.” Applying the theory of polymerization : ( I ) An excess of ether polymerizes more of the alcohol, and also acts as a diluent. J. Phys. Chem., 30, 349 (1926).
* Iiolloid-Z , 34. 34 (1924).
Korden: “Technology of Cellulose Esters”, 1, 111, 2292.
1124
MILTON L. BYRON
TYater tends to depolymerize the alcohol and requires more ether to (2) counteract this tendency. ( 3 ) K h e n the alcohol is added first, the ether polymerizes the alcohol already on the pyroxylin and gives complete peptization in a shorter time. (4) Decreasing the alcohol should have the same effect that increasing the ether has, provided the solution was not already saturated with cellulose nitrate, in which case the solubility would decrease. Pyroxylin is peptized very readily by acetone and by methyl alcohol at room temperature. It is not peptized by a mixture of alcohol and benzene either in hot water or in carbon dixoide snow and ether. Cellulose acetate is not affected by ether-alcohol mixtures. Absolute alcohol does not peptize it either in carbon dioxide snow or in watcr bath. It is readily peptized. however, by acetone. A mixture of absolute alcohol and tetrachloroethane will peptize cellulose acetate after long heating in a water bath. On cooling the cellulose acetate precipitates, on warming it is again peptized, showing the change is reversible. Cellulose acetate gives a jelly in tetrachloroethane after heating in a water bath for two or three days. If the heating js continued for twice this period, a very viscous mass is obtained.
Conclusions &Asa result of these experiments, the following conclusions are drawn : ( I ) Pyroxylin is not peptized by anhydrous ether at any temperature. (2) It i? peptized by absolute alcohol a t low temperatures. (3) The peptization is due to the adsorption of polymerized alcohol. (4) The alcohol is polymerized in ether-alcohol mixtures by the ether. The author wishes t o take this opportunity to thank Professor IT. D. Bancroft, who suggested this problem, and under whose direction the work, was carried out. Cornell Crlirerszty.
