LIQUID AMMONIA AS A LYOPHILIC DISPERSION MEDIUAI. I1 AMMONO-GELS O F CELLULOSE ACETATE BY R O B E R T T A F T AND J E S S E E. STARECK
In previous papers' the senior author has called attention to the fact that liquid ammonia is an excellent dispersion medium for the cellulose esters. Further it was pointed out that in the case of cellulose acetate, gels were formed upon standing at room temperature. This paper deals with the extent of dispersion of cellulose acetate in liquid ammonia, the chemical action of the solvent upon this substance and lastly some of the properties of two types of gels formed by this system. The cellulose acetate used in the investigation was that manufactured by the Eastman Kodak Co. which was air dried at IOOT before use. I t is probably a mixture of acetates up to the triacetate. The product now marketed by this organization as cellulose triacetate is not so readily dispersable as is the product of lower acetylation. The ammonia used was in some cases drawn directly from a cylinder of anhydrous liquid ammonia, the usual precautions being taken to prevent absorption of water. Later, when It became necessary to study the effect of water upon the gelatinization of these systems, the ammonia used was dried over sodium and distilled under slight excess pressure in a closed train into the tube containing the cellulose acetate, which was cooled by an external bath of the same liquid. I n most cases observations were made with the samples under examination sealed in tubes of pyrex in order to continue the observations at higher temperatures than the boiling point of liquid ammonia (-3 3.5 "C).
Dispersability Apparently there is no limit to the dispersability of cellulose acetate In this liquid. Ammonosols ranging from very dilute ones up to 60 grams of cellulose acetate in IOO cc. of the dispersion medium have been prepared.* Higher concentrations can be obtained but the concentrations above thls limit are difficult to estimate and the systems are so gummy and plastic that they are exceedingly difficult to work. In order to obtain these gels of higher concentration the following conditions must be observed: (a) low temperature must be maintained; it is necessary in order to secure dispersion, that the 1 Taft: J. Phys. Chem., 34, 2792 (1930); Trans. Kansas Acad. Sci. 32, j8,(!929). Clancy, U. S. Patents 1,544,809 and 1,5++,812( I ~ s ) has , covered the dispersablllty of cellulose nitrates and acetates with patents but no information is given upon the properties of such 8 stems &B herein described; Fenton and Berry (Proc. Camb. Phil. Soc 20, 16 (1920)) were d e first t o point out that cellulose acetate disperses in liquid ammoh'a. * In order to save words we have referred to our concentrations in terms of per cent. This is a misnomer but a 60% system means one containing 60 grams of cellulose acetate per IOO cc. of dispersion medium,etc.
LIQUID AMXONIA A S LYOPHILIC DISPERSION MEDIUM
579
temperature of the system be in the neighborhood of boiling ammonia (-33.5"C); (b) considerable mechanical agitation must be employed in order to secure the necessary circulation of the liquid and to secure as large a surface of contact as possible between the two phases. The latter was secured by stirring with a glass rod while the acetate was being added. I n order to obtain the highest concentrations of the sol mentioned above it was necessary t o evaporate a sol containing 30 grams of acetate per I O O cc. of solvent to one-half of its original volume. Sols containing 17~ or less of cellulose acetate were clear, colorless, mobile liquids. The viscosity was increased considerably over that of the pure disTO
60
-1 L1
so
40
* 30 0
a
20
10
0
10
16 10 conoentratioa in per cent
25
30
35
FIG.I Time of gelation as a function of concentration of cellulose acetate. Irreversible Gels.
persion medium and was a function of time, particularly at room temperature. When the sealed tubes were allowed to warm up to room temperature, the viscosity rapidly decreased at' first but after a few hours began to increase again. This increase in viscosity, as will be shown, is due to a change in the composition of the dispersed phase. On the whole it may be said that the viscosities of these systems are affected by concentration, temperature, and age as is the case with many other lyophilic systems. Sols of higher concentration were very viscous, translucent, but of light brown color. Heat Irreversible Gels As was stated above, the ammonosols when allowed to warm to room temperature (ca z 5 T ) decreased in viscosity at first, but after several hours the viscosity increased, the system eventually setting to a gel. The gels thus formed were white opaque masses and did not redisperse when recooled to the temperature of boiling ammonia, i.e. they were heat irreversible. In one case the gel which had formed a t room temperature was recooled in boiling
38
Firm translucmt gel; marked syneresis
3'
Striated; some syneresis
24
Vrrtied crack; some syneresis
1.8
Opaque, rigid, white; "&no curve" crack; syneresis apparent
46
I2
'5.
a
20,
h
30.
4.5
35.
3.5
Striated; oblique cracks; marked syneresis of brown liquid; gel contracted.
LIQUID AMMONIA AS LYOPHILIC DISPERSION MEDIUM
581
The appearance of the gels is also recorded photographically in Fig. 2 , the gels here depicted being approximately two weeks old. The time of gelation as a function of temperature is shown graphically in Fig. 3., the temperatures of observation being the boiling temperature of ammonia, the gel the system was ice point and room temperature. I n the case of the kept at -33.5OC for two weeks, the sol becoming slightly cloudy in that time but gelation did not occur. On removing it from the bath and allowing it to warm to room temperature, gelation occurred in eight hours or about one-
see
91
PO 19 16
17 16
13 14
1s
3
12
9
11
i
8 * o v H
e 7 6
5 4
I
a
PIpr.%YTI,
C1~flgr.6..
FIG.3 Time of gelation as a function of temperature. Irreversible Gels.
third of the normal time. h s will be shown, gel formation occurs as a result of chemical action, and consequently it was assumed that two-thirds of the reaction had taken place and it is upon this basis that we have plotted the point a t -33.5"C. I n the case of the roYc gel, a gel formed at the boiling point of ammonia within a few hours but this proved to be of the heatreversible type described later. On warming, it melted and set to the irreversible type in about the usual time. This seems unus, for the more concentrated system would be expected to set more quickly, but apparently the reversible gel structure prevented the reaction between disperse phase and dispersion medium from proceeding so rapidly, i.e. the disperse phase is not in as intimate contact in the gel state as when in the sol form, for in the gel state most of the dispersion medium would be present in the gel pores.
ROBERT TAFT AND JESSE E. STARECK
582
I n the more dilute gels considerable difficulty was at first. experienced in obtaining concordant data on the time of setting for a gel of a given concentration. This was finally traced to the presence of small but variable quantities of water present in the dispersion medium. This led us to investigate the effect of water upon the dispersability of the acetate and upon the properties of the gels thus formed. Known quantities of water were added to ammonia distilled from sodium. Under these conditions results were obtained which were reproducible for dilute sols but for those more concentrated than o.gC;: dispersion did not take place quickly enough to obtain a uniform system by simply shaking the tube containing the materials under examination. The data recorded in Table I1 and graphically in Fig. 4 shows the effect of varying quantities of waterupon the structure of the gel and the time of gelation. Fig. j shows photographically the results at a concentration of 0.I:; cellulose acetate. The facts to be noted in connection with the effect of water are: (a) dispersion of small quantities of cellulose acetate can take place in the presence of large quantities of water;3 (b) precipitation and not gel formation takes
T ~ L 11 E Gels formed in the Presence of Water 0 jrc Cellulose Acetate Amount of H 2 0
None I c';
Time of Setting
38 hours 32
2 c/c
27
u
20
L
3 /C
Structure
Coarse gel, gradually settling Slightly grainy gel Fine gel Fine opalescent gel
0 Zc; Cellulose Acetate 46 hours Gelatinous precipitate 36 Coarse, granular gel 28 22
Fine gel Fine, Opalescent gel
0 irC Cellulose Acetate 54 hours Flocculent precipitate 42
26 21
IS
opalescent gel Precipitate Gelatinous precipitate Grainy, coarse gel
0 O j c c Cellulose Acetate 4 I hours 41
35 25
'7 9 z -6
Precipitate Precipitate Opalescent, soft gel Precipitate Precipitate Gelatinous precipitate Precipitate
3 The range of dispersahility is not great in the presence of large quantities of water, however. Thus a system containing 2j5: water and 0 . 1 gm. of the acetate t o 100 cc. of dispersion medium showed no appreciable dispersion.
FIG.5 Effect o i Water Content on Gel Structure. Celluiore acetate content, ".,%. Tubes left to right h w e wster content of: ".Of$$
2'jb
5%
,of,;,
Tube on extreme right is pure ainmon$a. 'The Reis are not 80 opaque 88 these photogrsphs would indicmte
584
mmnr 'rmr
A N D JEBL;E E. 8T.mBcK
place in t,he absence of water; (e) gels form more rapidly and arc of firmer sttructure at an optimuni value of water concentration; (d) very minute concentration of the xeetate csn produce gels. In connection with the last point it can be asid that the lower limit of concentration of gel formation is hi, least, a6 law as 0.05 per cent (rather 0.05 per 100 cc. of ammonia). This is probably the lowest concentmtion a t which Relation has been recorded. Weiser' produced aqua-gels containing o.og per cent chromic oxide. Kruyt5 stat,es the1 :%naqueous system containing o.iqY, agm will gel a t room temperature. As far as the writers are aware these are the lowest ooncentrntions previously recorded for gel formation in other systems. One striking peculiarit,y of these gels was observed, particularly in gels containing 7.5% of cellulose acetate. This wa8 the development of cracks of very regular form, which we have termed "sine curve'' breaks. These are shown photogap6ically in Fig. 6 ; they were not restricbed to gels of this particular concentration but characterized gels from 5 to IO%, appearing in from one to two days after gelation, depending upon the previous history of the sample. The "wave length" of these curves was found to he R function of the bore of the tube as shown in the illustration. These breaks are undoubtedly caused by the gel's contraction which begins shortly after gelation, but sinee the exterior surface of the gel adheres to the glass wall a strain within the gel is produced. As contraction proceeds, the stress produced Flo. 6 finally becomes sufficient lo rupture the structure. Since t,he structure is thinner near the walls of the tube it will tear there first as resistance to strain is proport,ional to cram sectional area, but also, iw the membrane becomes thinner, it will stretch more for a given load, or stress; consequently the stress is relieved on this side before the break reaches the edge. Rut the strem on the ot,her side must alao be relieved since it is now greatest and a break is then produced in this region. The original production of strain is charactenzed by an absorption of all of the liquid of syneresis, giving the surface of the gcl a dry appearance. Af1.t~the completion of !,he break, however, synerrsis bakes place a second time. We havc observed the produotion of somewhat similar breaks in silica aqua-gcls but never so regular a6 here shown. In fact we have noticed many points of similarity between these ammono-gels of cellulose and aqua-gels of silica. I n external appearance they are very much alike; they are both heat ' J. Phys. Chem., 26, 4.3, ( 1 9 2 2 ) . ~"Colloids,"r% (1922).
\
LIQUID AMMONIA AS LYOPHILIC DISPERSION MEDIUM
585
irreversible; further the time of gelation is a somewhat similar function in both cases.6 Freshly prepared gels of higher concentration vibrate also when struck, as do silica gels. Evidence of Chemical Change in Heat-Irreversible Gels A number of tubes containing gels were recooled in liquid ammonia, opened and the gels removed for examination. The 5 per cent gels were quite rigid and brittle, but shrank rapidly and increased in toughness as the ammonia evaporated. A gel four inches by one-half inch diminished in size to one inch by one-eighth inch, leaving a yellow transparent residue, which did not swell or disperse again when placed in liquid ammonia. Further this horn-like structure was not only inert toward ammonia but also to all liquids which disperse cellulose acetate. Hydrochloric acid had no effect upon it and concentrated sulfuric acid only softened it slightly in a few hours; the original cellulose acetate is rapidly attacked by these acids. These gel residues are dispersed, however, by Schweitzer’s reagent. Apparently the material comprising the gel structure is cellulose, i.e. the dispersion medium reacts with the cellulose acetate converting it to cellulose. I n the absence of any water in the dispersion medium, acetamide would likely be formed in addition to cellulose. In the presence of water ammonium acetate would be the most probable second material. T o test this point out a gel was prepared and allowed to synerize. The tube was then cooled, opened and the synerized liquid removed. This liquid was then added to water (it produced no residue) and tested qualitatively by adding amyl and ethyl alcohols to separate tubes containing the above liquid in dilute sulfuric acid. The characteristic odors of banana oil and ethyl acetate were easily detected. I t will be recalled that for dilute concentrations of cellulose acetate, the best gels were formed in the presence of water. This would seem to indicate that traces of water are necessary for gel formation. That it is not solely a case of reaction is evident from a consideration of the data of Table 11. If it be assumed that the reaction [CIZHI,O,(COOCH~),], 6 H z 0 = [ClzHz,012], 6CH3COOH takes place, the mass of cellulose acetate is roughly five times that of the water involved. From Table 11, 0.05 gram of cellulose acetate requires 1% of water to produce the best gel, whereas 0.5 gram of cellulose acetate requires z to water to produce the best gel, i.e. relatively more water is required in the case of the dilute gels. I t would appear that the gel structure is dependent upon adsorbed water to secure the firmest gel.
+
+
Diffusion Experiments The ammono-gels thus described make suitable systems for diffusion experiments. We have not tried a great number of these but have carried out a few experiments with liquid ammonia solutions of sulfur in the hope that ‘For example, Holmes (J. Phys. Chem., 22, 510 (1918)), showed that the time of setting of a silica aqua-gel was greatly increased at 0°C and decreased with rising temperature on curves similar in form to those shown in Fig.3.
j86
ROBERT TAFT A S D JESSE E. STARECK
some information on the colloidity of these systems could be obtained. An inverted Y tube with the ends of the lateral branches sealed was made, so that the cellulose acetate sol could be placed in one branch and the sulfur solution in the other. The tube was then sealed, brought to room temperature and the gel allowed to set. The sulfur solution was then added to the top of the gel by tipping. Diffusion took place rapidly at the rate of about two inches in twenty-four hours. This rapid diffusion might indicate that a considerable proportion, if not all, of the sulfur was present in true solution. I t was found necessary to carry these diffusion experiments out in the dark as the sulfur solutions were reversibly light sensitive, turning pale yellow in the light and deep blue in the dark. In fact the reversals on one specimen were carried out a number of times without any apparent effect on the sulfur. Heat-Reversible Gels
As was stated previously, when the I O per cent sol of cellulose acetate was placed in an ammonia bath for several days a gel structure appeared. This led to the investigation of the possibility of gel formation of a heat-reversible type upon cooling the ammonia below its boiling point. Reversible gels of cellulose acetate have been prepared in benzyl alcohol by Mardies' and Poole,* consequently one might expect an analogous system in liquid ammonia. Samples of j and I O per cent cellulose acetate were prepared and cooled in a bath of gasoline to which had been added solid carbon dioxide with the result that a t -45' C the I O per cent sol changed to a beautiful opalescent gel which was quite rigid and brittle with an elasticity similar to that of gelatin gels. The color was also less intense for a given concentration than it was in the irreversible type. The j per cent sol remained fluid until a temperature of -60°C was reached, when a similar opalescent gel appeared. Thus it will be seen that the higher the concentration of the gel, the higher its temperature of gelation-a characteristic of heat-reversible gels such as agar or gelatin in water and cellulose acetate in benzyl alcohol. On warming u p the bath gradually, it was found that the gels would not melt at the same temperature at which they jelled, but a temperature of about ten degrees higher was necessary to bring about this change. This is similar to what Poole8found in studying cellulose acetate in benzyl alcohol and is generally characteristic of heat-reversible gels. The above observations were made on samples sealed in pyrex tubes which were afterwards allowed to warm up to room temperature. The sols become more fluid at first but later set to the heat-irreversible type of gel previously described. The characteristic sine curve break already noted for gels of these concentrations formed in about twenty-four hours after gelation with the usual syneresis following. 'Trans. Faraday SOL, 18, 318 (1922). sTrans. Faraday SOC.,22, 140 (1926).
LIQUID AMMONIA AS LYOPHILIC DISPERSION MEDIUM
587
summary I. Cellulose acetate will disperse in liquid ammonia apparently without limit. Sols as concentrated as 60 per cent have been obtained. 2. Ammono-sols of cellulose acetate present the unique case of forming two types of gels, one type being heat reversible; the second heat-irreversible. 3. The heat-irreversible gels, apparently consist of a structure of cellulose which has adsorbed water. 4. Heat-reversible gels may be obtained by cooling the ammono-sols to low temperatures. The setting points of these gels was found to be lower than their melting points, a phenomenon which is of common occurrence in reversible gels.
Uniuersity of Kansas,
Laurence.
