THE YISCOSITY O F NITROCOTTON I N TARIOUS SOLTENTS AKD MIXTURES J. W. McBAIN, EVELYN LIARY GRANT, A S D I,. E. ShlI‘I“’ Department of Chemistry, Stanjord UniLerszty, C a l i f o r n i a Receized M a y 3, 1931
A previous communication2 (6) presented a systematic report upon the apparent viscosity of solutions of nitrocotton (12.10 per cent nitrogen) in a large number of solvents and solvent mixtures over periods of time up to one year. Many interesting and significant regularities appeared, of which three may be mentioned. (1) The best sohents give the least viscous solutions with the same nitrocotton, the viscosity being referred to that of the solvent itself in each case. (2) Mixtures are better solvents than pure liquids. ( 3 ) The apparent viscosity is largely due to a structural effectI3since it is altered by shaking, change of soli ent, previous treatment, previous precipitation and recovery of the nitrocotton, and because of its enormous magnitude. All this is additional to any effect of the length of the molecule itself, so much stressed in subsequent years by Staudinger and others. The effects quoted above cannot reasonably be ascribed to alteration in the length of the molecule itself, more especially since changes occur in both directions. Many questions demanding further study arose, some of which are dealt with in the present report. CXPERIMESTAL METHODS
The nitrocotton “1919” has been described, together with the method of drying, storage, and preparation of the solutions ( 6 ) . Units of weight, The experiments were performed by E II G except nhere othernise noted. The work was carried out a t Bristol Vniversity, England, during the years 1925-27. The following errata in this artirle should be noted: p 321, table I-,for methyl phenyl urethane (Smith) the time of standing should be 3 2 hours instead of 32; p 325, table V I 1 (last column). the slope should be 0 344 instead of 5 589 ithis is not a fair comparison as the concentration is so great); p 326, figure 3, on the figure itself “phenol urethane” should be “phenyl urethane,” in the title of the figure “phenyl methane” should be “phenyl urethane” and “ortho tolyl methane” should he “benzyl phenyl urethane”; p 330, table XI, the viscosity of pure ethyl phthalate (Nobel’s; December. 1923) should be 0 04215 instead of 0 4218; p 333, third line from the bottom. “less impure” should be “less pure ‘’ This n a s developed as a general explanation of the high viscosity of numerous colloids by J. IY \fcBain (J. Phys Chem 30, 239 (1926)). 1217
1218
J. W.
MCBAIS,
E . M. G R A S T A S D L. E. SMITH
volume, temperature, viscosity, etc. were corrected to absolute standards. Viscosity determinations a t 25°C. were exactly as a t 55”C., except that the time a t 55°C. mas 23 hours and 10 minutes before cooling to 25°C. T’iscometers were chosen of dimensions to give absolute viscosities with true liquids of the same apparent viscosity as tlie actual nitrocotton solutions. The results are usually the mean of independent duplicate experiments. VISCOSITY I N A SERIES O F ACETATES AS SOLVESTS
I n the previous communieation (7) it was indicated “that molecular weight is an important factor in a series of closely similar compounds. The lower the substance in the homologous series the better the solvent.” Also aliphatic substituents appear more favorable than aromatic. One expects the greatest differences with the lowest homologues. These rules appear to obtain with the acetates, although the lowest homologues could not be dried with calcium chloride and mere too impure. It has already been emphasized that impurities greatly affect and generally improve solvent power. Hence for comparison the solvents should be of comparable purity and dried in similar fashion. Otherwise, divergent values are obtained. For example, the solvents were dried with calcium chloride or copper sulfate. The latter, however, introduced a further impurity, presumably sulfur dioxide, which lowered the viscosity, as did sulfur dioxide, the viscosity rising again upon removal of tlie sulfur dioxide. For example, with isobutyl acetate, the viscosity 0.00643 became 0.00636 after treatment with copper sulfate; 0.00632 after copper sulfate and carbon dioxide; 0.00543 after sulfur dioxide, and 0.00630 after removing most of the sulfur dioxide. Similarly, with isoamyl acetate, 0.00793 became 0.00799 after drying with calcium chloride, and 0.00791 after drying with calcium chloride and copper sulfate. The solutions were prepared at 55°C. and measured at age 24 hours at 25OC. The data are calculated as in the previous communication, that is, the logarithm of the ratio of the viscosity of a solution to that of the solvent contained therein is divided by the concentration expressed in grams of nitrocotton per 100 grams of solvent. This gives the slope of the graph of the logarithm of the relative viscosity against the concentration, which is, as a first approximation, a straight line. The results are collected in table 1. The source of solvents is indicated as follows: IS: = Kahlbaum, K = Xobel‘s, B.D.H. = British Drug Houses, E.K. = Eastman Kodak Co., AI = 1Ierck. Purest solvents obtainable from each source were redistilled. It follows from table 1 that the higher homologues under similar conditions have a value 1.75 f 0.05. This does not include the aromatic derivative, phenyl acetate, a poorer solvent as indicated by its higher slope, but it does include benzyl acetate with its interposed CH, group. Methyl, isopropyl, and butyl acetates are among the most highly sorbed of volatile
VISCOSITY
1219
OF KITROCOTTOK IN SOLVENTS AND MIXTURES
solvents, x j m , the sorption expressed in grams, exceeding unity for high relative pressures ( 5 ) . The number of molecules taken up is greatest for methyl and least for butyl acetate. TABLE 1 M e a n r2scosity slopes (log 7/70 + per c e n t ) of a p p r o x i m a t e l y 0.6 per cent ( g r a m s nitrocotton per 100 grams s o l w n t ) nitrocotton in various solvents dried a n d u n d r i e d Prepared a t 55'C.; measured a t 2 5 . 0 0 " C . ; age 24 hours M E A N VISCOSITY S L O P E S SOLVENT
Untreated
Dried with CuSOa
1.44
1.12
Methyl acetate iK). . . . . . . . . . . . . . . . . . . , , . . . . , . . Ethyl acetate (IC),,, . , . . . . . , , , . . . . , , . , . . , , , . .. . ., ,.. Ethyl acetoacetate (X)., , . . . , , . , . , . . . Propyl acetate ( K ) . . . . . . . , . . . . . . , , . . . . . . . . , . . . . . . . Isopropyl acetate.. . . . . , . . . . . , . . , . . . , . . . . . . , . , , n-Butyl acetate (K). . . , . . . . . . . . . . . , , . . . . . . . . . ... .... Isobutyl acetate (K).. . . . . . . . , . . . . , . . . . . Isoamyl acetat,e (B. D. H . ) ,, , , . . . . , . . . , . . . . . . . Benzyl acet,ate (K; hlay, 1925). . , , . . , . . . . , . . , . . Benzyl acetate (K; h,farch, 1926). , , , . . . . , . . . . . . . , . Benzyl acetate (K; November, 1926).. . . . . . . . . . . . . Phenyl acetate ( K ; May, 1925). , . , , . . . , . . . . . . . . Phenyl acetate (K; March, 1926). , , . . . , . . , . . . . . . . Phenyl acetate ( K ; November. 1926). , . . . . , , , . , . . . ,
,
1.55
,
, ,
0.778 1.06 1.09
,
,
1.16 1.16 1.24 1.66
,
,
Dried with CeClz
,
1.72 0.832 1.05 1.18 1.41*
1.79 1.82 1.94 2.35 2.20
1.92 2.34
,
1.72 1.77
* Dried with calcium chloride and then copper sulfate TABLE 2 M e a n viscosit!l slopes of approximately 0.6 peT cent nitrocotton in solvents ut 25°C. a n d 56'C. SOL\ EKT
Ethyl acetoacetate (N) Methylphenylurethan (N; February, 1925) Ethylphenylurethan (K;February, 1925) Diethyl carbonate (K)
1
1
MEAN VISCOfiITY S L O P E S
0 778 1 69
0.952 1.41 1.54 1.52
0.91 1.61 1.80 1.58
* Previous communication; different specimens of solvent.
To compare the results in table 1 for acetates too volatile to measure at 55°C. with the solvents previously studied at this temperature, the data in table 2 for solutions at both temperatures are assembled. It will be noted from the low value of the slopes that the acetates would be classed as good solvents, with the exception of phenyl acetate. For the
1220
J. W. MCBAIS, E . XI. G R A S T AKD L. E. SMITH
last three substances in table 2, the slopes are higher a t 25°C. than at 55°C. and the order in the three columns is not identical. This is partly because other samples of a different degree of purity u-ere employed for the last colunin. SOLLTBILITY A T D VISCOSITY I N ALCOHOLS
Methyl alcohol is almost a complete solvent. Ethyl alcohol is such only a t very low temperatures, the solution becoming more viscous and setting to a jelly on warming to room This reversible change cannot, of course, be ascribed to a change in the length of the molecule. Butyl alcohol is a lion-solvent and it is sorbed to a far less extent by nitrocotton, not exceeding 5 per cent by weight. The solubility and slope of nitrocottori were determined in Ilahlbaum's methyl alcohol; also in a sample of the latter which had been dried with TABLE 3 V i s c o s i t y and dissolcing p o u e r in methyl alcohol, ethyl alcohol, and i n a m i x t u r e o j equal weights, at 26.OO"C. and age t 4 hours ' SOLVENT
SITRO-
I
I
~%,=, COTTON DISBOLYED
~
,
VISCoSITY
DECRE.%SE
SLOPE
-I
Methyl alcohol (K) Methyl alcohol (IC), dried n i t h CaC1 Ethyl alcohol Methyl alcohol (K) ethyl alcohol
+
* .4t
85 2 24 1 4%54
21
0 0 2 1
620' 054 275 199
DECREASE
p e r cent
18
age 47 hours.
calcium chloride; also in dry ethyl alcohol and in a 50-50 mixture by weight of undried methyl alcohol and dry ethyl alcohol. I n all cases nitrocotton solutions were prepared in the ordinary manner as for measuremelit a t 25"C., but before the viscosity was taken any undissolved fibers were filtered off. Then a known weight of the filtrate was evaporated to constant weight. From this was calculated the percentage of the 0.5 g. of nitrocotton which had dissolved in the 100 grams of solvent. The nitrocotton was, of course, not uniform, and this affords a sensitive test of progressive dissolving power without measuring a true solubility in the ordinary sense. The data are given in table 3. I t will be seen from table 3 that, as was expected and in accordance with C. W. Hutchinson in our laboratory finds t h a t ether-alcohol ( 7 5 : 2 5 ) containing 5 per cent nitrocotton a t room temperature forms an opaque gel on heating in a sealed tube t o 90°C. The phenomenon is completely reversible, the gel quickly reverting t o a fluid sol a t room temperature.
VISCOSITY O F NITROCOTTON IN SOLVESTS AND MIXTURES
our rules, drying5diminishes the solvent power of methyl alcohol, as showTn both by the increased viscosity and the small proportion of nitrocotton dissolved. On the other hand, the mixture of the two alcohols yields a viscosity as well as a proportion of nitrocotton dissolved which is one-fifth less than that of the arithmetic mean of the two solvents separately. HIGH M E L T I S G SOLVESTS IS MIXTCRES WITH ALCOHOL A S D BEXZESE
Since seven possible solvents are solid a t 55"C.,the attempt was made to study them a t 55OC. in admixture with ethyl alcohol or benzene. Table 4 presents the results with alcohol. Equal parts by n-eight of benzene with phenylurethan or diethyl carbonate did not dissolve the nitrocotton. This was likewise the case with one part of phenylacetanilide and three parts of alcohol, and one part of cr-pyridylurethanfiwith five parts of alcohol. Pome TABLE 4 V i s c o s i t y slopes of solidtons of nitrocotton in substances 10 which a n equal weight of ethyl alcohol has been added Temperature, 55°C.; time, 24 hours SOLFESI
1
~____
Form-o-tolLidide ( S ) Phrnylurethan ( X ) Methylphen>lurethan ( N : Fehruarg , 1925) Diet111 1 (arhonate ( X ) Ethylpheiiylur~thaii(X; Fehruar!, 1925) Ethyl phthalate (E K )
1
'
1
SLOPE
1 058 1 340
1 611
1
___
,
0 870 1 04 0 89
I
1 27
PERCEXT
18 22 37 29 39 29
substances previously measured in purp condition are included for coniparison. I t has already been pointed out that a small addition, even of a reputed non-solvent, lowers the viscosity and improves the solvent power of any solvent for nitrocotton. Thus at 55OC. even the extreme non-solvent hexane, added in one or two parts per hundred, lowers the actual viscosity slope of the solvent methylphenylurethan by 3 and 6 per cent.' Other examples are given in table 4. This is likewise shown in figure 1 for the effect of the semisolvent, ethyl aIcohol, upon methylphenylurethan over For solvents previously studied a t 55'C.. drying a solvent over phosphorus pentoxide for 14 days a t room temperature raised the slope for benzylphenylurethan (N) 8 per rent, leaving t h a t for methylphenylurethan ( N ; February, 1925), ethylphenylurethan (9; November, 1926), and diethyl carbonate ( N ) unaffected. a-Pyridylurethan is insoluble in benzene. 'I An opposite effect is probably produced upon acetone.
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J . TV, MCBAIS, E . M . GRAiXT A S D L. E. SMITH
the whole possible range. Beyond 85 per cent alcohol, progressively less of the nitrocotton dissolves, and the true slope cannot be measured, although it must become very high (cf. table 3). For comparison with the mixtures in table 4 the following results were obtained with similar mixtures of high melting substances : diethyldiphenylurea (N), 0.677; phenyl oxaniic ester (N), 1.136; p-toluenesulfonanilide (N), 1.110; diphenylurethan (N) (42 g. in 58 g. of alcohol), 0.849.
0
IO
20
30
40
50
60
70
80
90
100 E t O H
COMPOSITION OF SOLVENT IN PER CENT BY WEIGHT
FIQ.1. VISCOSITYSLOPESO F 0 5 PER CEST SOLGTIOSS OF NITROCOTTON IS THE SOLVENT METHYLPHENYLURETHAN, T O W H I C H T H E S E M I S O L V E X T E T H Y L ALCOHOL IS ADDEDIN INCREASING AMOUNTS, LOWERING THE VISCOSITYO F THE MIXTURE
Temperature, 55.00"C.; time, 24 hours
With benzene (K) mixtures containing equal parts by weight, the slopes for the mixtures were 0.953 and 1.66 for forni-o-toluidide (K)and methylphenylurethan (N; February, 1925), respectively, being a decrease of 15 per cent for the former but an increase of 18 per cent for the latter. The slopes for 50 per cent mixtures of benzene (K) with diethyldiphenylurea (hi), phenylacetanilide (N), and diphenylurethan (N) were 1.47, 1.45, and 2.10, respectively. It follows from the data adduced that all are to be classed as good solvents except a-pyridylurethan and phenylacetanilide. These tlyo are
VISCOSITY
OF SITROCOTTON
IS SOLTEXTS A N D MIXTURES
1223
too insoluble even in alcohol and benzene to be given a comparable test. Further, it is to be remembered that the specimens of the high-melting solvents were in most cases not as pure as the standard solvents used for comparison in table 4. Lastly, the relative order is certainly affected 11y such large additions of alcohol or benzene. Contrary to expectation, the data indicate that diphenylurethan is potentially a better solvent than phenylurethan. Besides these and the numerous striking examples of the lowering of viscosity slopes in mixtures given on pp. 325 to 328 of the previous conimunication (B), three more may be cited; namely, that in table 3, and the 50 per cent mixtures of n- and isobutyl acetates (IC),and of benzyl n ith phenyl acetate (10. The slopes for the last two mixtures at 25.00"C. were 1.384 and 2.009, being 13 and 10 per cent less than the means for the respective pure solvents. Further, 50 per cent mixtures of formanilide and form-otoluidide yielded a slope of 0.985 a t 1 day and of 0.074 at 80 days, being 7 per cent less than the mean of the values for the two solvents at 1 day, 1.02 and 1.085, respectively. C. R. Hutchinson in the Bristol laboratory found that the addition of a few per cent of the following impurities did not cause the residue of nitrocotton particles left undissolved by specially purified formanilide (X) to dissolve : aniline (except in certain cases upon prolonged shaking), formic acid, ether, alcohol, 5 per cent ether-alcohol, forniamide, benzene, toluene, tartaric acid, citric acid. On the other hand, 2 or niore per cent of acetone was successful. Guncotton and another sample of nitrocotton dissolved in the same incomplete manner. Aniline and formic acid by themselves do not affect the appearance of nitrocotton. PROLONGED P R E H E A T I S G O F SOLVEIiTS
Ethyl phthalate has given us much trouble throughout the many years in which we have studied solutions of nitrocotton therein. (See figure 4, page 335, of reference 6.) The first saniple obtained was an excellent solvent with one of the lowest viscosity slopes. Subsequent much purer specimens have given high slopes. Since in prolonged aging experiments at 55°C. it might be surmised that the solvent itself is changing, different specimens of two solvents \!-ere maintained a t the very much higher temperature of 130°C. for 7 days in sealed tubes previously evacuated with a mercury pump with a trap containing solid carbon dioxide and ether or liquid air. TThere necessary the liquid itself was surrounded by solid carbon dioxide and ether. o-Tolylurethan was partially converted to a form with a melting point higher than 130°C. as compared with the original melting point of 45°C. The results given in table 5 v-ere obtained with 0.5 per cent solutions subsequently prepared, aged, and measured at 55.00"C. Each solution was in a separate sealed tube.
1224
J. W. MCBAIK, E. h l . GRANT A S D L. E. SMITH
Viscosity is distinctly lowered by the previous drastic treatment of the triacetin. The effect is slight with phenoxyacetic ester. With ethyl phthalate a distinct increase results as shown in table 6. A graph of all these slopes against the logarithm of time yields approximately straight lines, indicating that the viscosity of the solution would fall TABLE 5 Results oblained in preheated solvents s'
ISCOSITY SLOPES
SOL7 E N T
42 d a i s
14 dais
Triacetin ( S ) * Triacetin (S)t Triacetin ( K ) * Triacetin (K): Phenoxyacetic ester (N)" Phenoxyaretic ester (N)T
'
1 1 1 1 1 1
225 023 230 025 451 319
1
1
84 days
168 days
I -~
~~
0 0 0 0 1 0
890
0 626 0 324 0 508
511 874 759 038 976
0 770 0 744
1
0 373 0 226 0 362
0.174 0.124
0 519 0 3886
0.283 0.293
* Untreated original
t 5
Aged seven days at 130°C Evacuated a t 3 X lo-' mm. ; aged seven days a t 130'C. One hundred and ninetten days.
7 ISCO3ITP SLOPES
~
E. K., untreated., . . . . . . . . . . . . . . . 1 664 E. I
