STUDIES 15 GELATION AND FILM FORMATIOX. I1 STUDIESIN CLAYFILMS’ E. A. HAUSER
AND
D.
s. LEBE.~UZ
Deparfment of Chemzcal Engineertng, Massachusetts Instit ule of Technology, Cambridge, Massachusetts I
Received August 1, 1939
In a previous publication (5) it was demonstrated that self-supporting, coherent films could be obtained from bentonite gels3by spreading the gels on an appropriate support and drying them in this condition. The films so obtained have become generally known as Alsifilm (aluminum silicate film). Since they are entirely composed of pure clay, they offer a new and interesting condition of matter for colloidal studies. PARTICLE SIZE VS. WATER CONTENT O F DRY FILM
Instead of producing crude Alsifilm from a polydisperse gel (a gel containing particles of different qizes but all of colloidal dimensions), it was found that films produced from monodisperse gel fractions (7, 14) retained different amounts of water. These variations are shown graphically in figure 1. The results demonstrate that the water content of the films increases with decreasing size of the original particles. This would agree with previously reported data on the amount of water adsorbed on clay particles of fractions of different sizes present in a sol (6). For the determination of the total water content of a clay particle me must also consider the osmotically imbibed water. In this connection it has been found that the withdrawal of the osmotically imbibed water from clay gels is easier, the smaller the original particles (2). This is in Presented a t the Sixteenth Colloid Symposium, held a t Stanford University, California, July 6-8, 1939. Present address: Dewey & Almy Chemical Company, Cambridge, Massachusetts. a The term “bentonite” as used in this paper refers, unless specifically indicated otherwise, to the colloidal-sized particles of the swelling type of the mineral montmorillonite, which constitutes the major part of our natural bentonite deposits. All impurities and supercolloidal particles have been removed from the systems used in this work, either by sedimentation or by supercentrifuging. For the present studies a bentonite mined in Wyoming was used exclusively. I t is a sodium hydrous aluminum silicate. However, identical results have been obtained with hydrous magnesium silicates having alkali cations as counter ions attached to the complex colloidal anion. 1037
1038
E. A. HAUSER AND D. 8 . LB BEAU
agreement with the experimental fact that coherent self-supporting films can be more quickly produced from gels containing fine particles. This, however, would indicate that such films, having once been deposited to a coherent structure, should have the smallest amount of retained water. One can explain this apparent discrepancy by accepting the structure for montmorillonite as postulated by Hofmann, Endell, and Wilm (9) and by assuming one of the following theoretically conceivable possibilities aa to 6
0
0
20 40 60 80 IW APPARENT AVf. PARTICLE SIZE IN mu
FIQ. 1. Moisture content of crude Alsifilrnuersus particle size of original bentonite gel
FIQ.2. Theoretical possibilities in the formation of large clay particles. Is, growth by stacking of unit layer parcels along the c-axis of the unit crystal; Ib, growth by stacking simultaneously along a t least two axes of the unit crystal; 11, random agglomeration of unit crystals; 111, alignment of unit crystals along the aaxis or the b-axis by micellar cohesion forces; IV, originally large crystals. the difference in particle sizes (figure 2): (I, a and b) The larger particles consist of a multitude of layer parcels stacked together in the direction of at least one axis of the unit crystal. (11) The larger particles are randomly oriented agglomerates of layer parcels. (111) A series of unit layer parcels is held together in chain formation in the direction of the a- and b-axes of the unit crystal, thus forming fibrillar aggregates. (IV) The presence of particles with the original a-axis or b-axis extended.
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CLAY FILMS
In all these cases it will be more difficult to withdraw the imbibed water from the film made up originally of small particles than from the gel, because decreasing original particle size increases the capillary interstices in the film structure. This is substantiated by the fact that films produced from fractions of fine particle size show no change in water content even after prolonged storage in air, whereas films made under identical conditions from fractions of coarse particle size exhibit a noticeable drop in water content during the first days of storage. The formation of Alsi& is the result of the particles present in the gel lining up in some way and forming fibrillar interweaving crystalline aggregates. INFLUENCE OF TEMPERATURE
Further proof of the above discussion is offered by the fact that films that have been dehydrated once at diEerent temperatures ( l l O o , 180°, 250", 40O0,550°C.) will pick up the same amount of moisture if placed in
TABLE 1 Moisture content of bentonite jilma versus temperature xoIamum CONTBNT TEMPERATURE
QC.
110 180 250 400 550
76 m#*
47
27 m(l*
psr
w rnf
pr
5.1 2.7 1.9
5.0 2.8 1.9
5.0 2.7 1.9
1.3 0.8
1.4 0.8
1.4 0.8
air (40 per cent moisture content) irrespective of their original particle size. The results are recorded in table 1. Such films, if brought in contact with water, will swell visibly, eventually be reconverted to a gel, and finally peptize to the sol condition. Films that have been heated up to 700°C. and 800°C. will no longer pick up water either from the surrounding atmosphere or if immersed in water. These facts substantiate previous observations that the colloidal properties of the bentonitic clay particle remain practically unaltered up to about 700OC. At higher temperatures a complete collapse of the lattice prevents osmotic imbibition. An interesting, but so far not yet fully explainable phenomenon is the fact that films deposited from electrodialyzed or hydrogen bentonite pick up nearly twice as much water as regular crude Alsifilm, inasmuch as they
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E. A. HAUSER AND D. 8. LE BEAU
have not been subjected to temperaturh above 250OC. The results are recorded in table 2. The most probable reason for this phenomenon is that an irregular agglomeration of the clay particles takes place during their transformation into hydrogen bentonite. The agglomerates would again tend to include larger quantities of water. The tendency to agglomerate and form clusters has been verified by ultramicroscopic observations.
-
TABLE 2 Moisture content o j hydrogen bentonite film versus temperature Apparent original particle size = 47mr TEMPERATURE
MOISTURE CONTENT
'C.
prr cent
25 110 250
10.9 6.5 2.3 1.3 0.8
400
550
FIG.3. Moisture pick-up of Alsifilm in water made from hydrogen bentonite after having been heated to different temperatures.
It has long been known that hydrogen bentonite, if once fully desiccated, can no longer be redispersed in water. This phenomenon can be demonstrated nicely with hydrogen bentonite films. Figure 3 shows the water pick-up of such films, which had been heated to various temperatures for 18 hr., then placed in water a t 25°C. for 48 hr., and air-dried, in comparison with the pick-up by regular films. Even the films that have been heated to temperatures below 180OC. do not redisperse, but they are too fragile t o be handled as coherent sheets.
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Presunably what happens during progressive dehydration is that two unit parcels are forced so close to each other that the forces acting between them become predominant. The relatively low temperatures needed to bring about such a change could be explained by the fact that the hydrogen ion is the least hydrated ion. INFLUENCE O F ELECTROLYTES ON THE CRUDE FILM
Most interesting of all reactions is the influence of different electrolytes on the properties of Alsifilm. The well-known fact that swellable sodium montmorillonite exhibits a large base-exchange capacity in comparison with the non-swelling types (fullers' earth) or with kaolinites led t o experiments to determine whether such base exchange could also be accomplished if the clay were present in film form. For this purpose crude Alsifilm was immersed in saturated solutions of different electrolytes.
YOIBTURE COWI'ENT
RLY
Original film. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Film treated with potassium hydroxide. . . . . Film heated to 110°C. after treatment.. . . . . . After immersion in water. . . . . . . . . . . . . . . . . .
76 m p *
41 mp*
27 rnr*
par cent
par m t
par cent
5.4 4.4 0.3 7.2
5.5
6.1 4.7 0.6 7.8
4.7 0.6 7.3
* Apparent original particle size. The use of saturated solutions, or a t least highly concentrated ones, is necessary in this case, as otherwise the film would peptize prematurely. This means that base exchange must take place in the presence of a concentration of cations such that neutralization of the particle charge at the same time is guaranteed. Alsifilm immersed for 12 hr. (at 25OC.) in a saturated solution of potassium hydroxide showed upon analysis complete replacement of the sodium originally present. The moisture content dropped from 5.4 per cent in the original film to 4.4 per cent after treatment and to only 0.3 per cent after heating the treated film to 110'C. for 18 hr., Le., its moisture content was far below that of the original dry clay. Even when a film so treated had been immersed in water for 2 days, it had picked up only 7.2 per cent. The film no longer swelled and remained absolutely coherent. Films made from originally finer dispersions resulted, as was to be expected, in slightly higher figures for moisture content, but the values were still of the same order of magnitude. The results are given in table 3.
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E. A. HAUSER AND D. S. LE BEAU
With lead acetate we again obtain complete exchange of the sodium and the small amount of potassium present in the natural material. In this case the water content of 5.4 per cent of the original film dropped to 2.4 per cent after treatment and to 0.5 per cent after having been heated to 110'C. for 18 hr. After immersion in water for 2 days the treated and heated films showed a moisture content of 8.8 per cent. Again, no signs of swelling and disintegration were observed, the films being absolutely resistant to the influence of water for many months. Results obtained with fractions of finer particle size again lie slightly higher. The results are recorded in table 4. The treatment of crude Alsifilm with other electrolytes, as,for example, silver, resulted in complete base exchange, but these films were not waterresistant (for an explanation, see below). It is interesting to note that small amounts of magnesium could always be detected in the treating bath (prepared from C.P. chemicals and conTABLE 4 Moisture content of films treated with lead acetate Y O I W B E CONTENT
n L M
Original 6lm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Film treated with lead acetate. . . . . . . . . . . . . Film heated to 1lO'C. after treatment, . . . . . After immersion in water.. . . . . . . . . . . . . . . . . .
70 m#*
47 m p *
27 mp'
par cent
par cent
p wnt
5.4 2.4 0.5 8.7
5.5 2.8 0.7 8.9
6.1 2.9 0.8 8.9
ductivity water) after treatment of crude films. This indicates that measurable quantities of magnesium have been removed during treatment. Since the presence of magnesium as counter ion is highly improbable, however, its presence in the lattice being an accepted possibility (11, 12), this fact seems to substantiate the findings of Kelley, Dore, Brown, and Jenny (10, l l ) , who were able to remove from bentonite, ground for a prolonged time, increasing quantities of magnesium and potassium. An x-ray analysis of the ground material showed a partial destruction of the crystal lattice. I n the present case, also, a marked change in structure was found by x-ray and microscopic analysis. Another fact of interest is the amount of cation retained by treated films deposited from fractions of different particle sizes. It was found in the case of lead acetate that the films produced from a particle size range of 67 to 85 mp retained 12.13 per cent lead, those made up of particles ranging from 44 to 50 mp retained M.18 per cent, and those of particles
CLAY FILMS
1043
between 24 and 30 mp retained 15.16per cent lead. A film deposited from hydrogen bentonite of the finest fraction retained even 20.7 per cent of lead. This fact is in contradiction to the data given by E. A, Hauser and C. E. Reed (S),who found that the total base-exchange capacity of bentonite in dispersion was practically independent of particle size. KO fully satisfactory explanation for this discrepancy can yet be offered. However, the loose network structure of crude bentonite films, as demonstrated by ultramicroscopic studies (5), might offer an answer. The fabric-like structure is denser when depositing a film from fractions of fine particle size than from fractions of coarse particle size. This makes increasing mechanical inclusion highly probable. STRUCTURE OF FILMS
The reactivity of the crude film finds its explanation in an extensive x-ray study, to be published in detail in the near future. Diffraction patterns obtained by x-ray radiation perpendicular and parallel to the plane of the film have demonstrated that the unit crystals lie with their c-axis perpendicular or at a slight angle to the plane of the film, so that the individual lattice layers must lie parallel or at a slight angle thereto. Reverting to the four theoretical possibilities as to the difference in particle size, we are now in a better position to consider their probability. The fact that crude films when immersed in water swell predominantly in regard to their thickness would conform to all possibilities. Possibility IV can be eliminated, as large particles would have to be detectable by microscopic observation and sedimentation analysis. Possibility I1 seems highly improbable, because the presence of unoriented agglomerates would call for unoriented growth upon desiccation; moreover their presence would be detectable. This would be contrary to the observed film structure. Possibility I11 must be considered aa likely, especially if one accepts the recent theories of gelation and alignment, as only such an assumption would explain the loose coherence of particles surrounded by ionic atmospheres of identical charge. On the basis of all the reactions possibility I seems to furnish the most plausible explanation of the formation of the film structure. From the two theoretical possibilities schematically shown in parts Ia and Ib of figure 2, the stacking of unit crystals according to I b seems the most probable configuration. This arrangement is the only one that is able to explain the phenomenon observed when a thin strip of the film is hammered in its middle with the edge of a ruler. The film bends on both free ends, owing to displacement of the stacks against each other when impacted. If the film is then turned over and the procedure repeated, it will flatten out first and then bend upward in the opposite direction as before.
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E. A. HAUSER AND D. 8. LE BEAU
A film that has been previously treated with electrolytes and been made water-resistant thereby will no longer show this phenomenon. This is in full agreement with the explanation offered for water resistance aa schematically demonstrated in figure 4b.
-
b = 9.02 A
n !
n l " 1
-
I
@ -ALUMINUM
6 -HYDROXYL 0-OXYGEN 0 -POTASSIUM
-
0 SILICON
b FIQ.4. (a) Schematic drawing of the structure of mica (muscovite) projected a t a plane normal to the a-axis (from W. L,Bragg). (b) Probable bonding of unit layer parcels by large cations in water-resistant Alsifilm.
A combination of possibilities I and I11 is equally conceivable and would also be in accord with the facts so far discussed. The fact that films treated with certain electrolytes lose their ability to swell appreciably or to redisperse completely, whereas the treatment with other electrolytes results only in base exchange, is of importance. A careful study of the cations causing water resistance and those which do
CLAY FILMS
1045
not has so far shown that only those cations which form extremely soluble salts and whose apparent diameters are of about the same order of magnitude as the spacing between opposite oxygen atoms in the hexagonal silicon-oxygen sheets of the clay lattice (1) will result in water-resistant films, whereas smaller cations, for example, silver, will remain free moving in the space available, although both presumably are attached to the lattice by residual valencies; such an arrangement would bond unit parcels in a way similar to that known in mica (figure 4a). Since it has been previously demonstrated (8) that certain organic ammonium compounds transform bentonite sols into thixotropic gels even if added in extremely low concentrations, crude Alsifilm was also treated with several such compounds. I n accordance with the previous statement it was found that those compounds, such as dibenzyldimethylammonium hydroxide or cetylpyridinium bromide, which in water dissociate into a large cationic complex and a small anion, produced non-swelling, waterresistant films, whereas films treated with compounds yielding comparatively small cationic complexes resulted only in base exchange. X-ray studies, details of which are being published elsewhere, of films so treated have revealed a pronounced increase in the intensity of the diffraction from the (001) plane. These results are in full agreement with x-ray studies on the base exchange of bentonite sols with large, substituted ammonium ions recently reported by Gieseking (3). Microscopic studies of the films that had been heated to different temperatures correlate the changes in moisture content and colloidal structure discussed above. This is best illustrated by photomicrographs taken of normal films and of films that had been heated t o B O " , 250", 400",550", and 800°C. Up to a temperature of 250°C. a noticeable increase in definition of the structure is clearly visible, which is caused by increasing reduction in hydration. Above a temperature of 400°C. the microphotograph distinctly demonstrates a breaking up of the original structure into finer aggregates, which finally defy ultramicroscopic resolvability. In the presence of electrolytes, particularly lead acetate, these changes become noticeable a t lower temperatures. However, this might be due partly to the activity of the electrolyte itself (figures 5 and 6). With the purpose of obtaining a better insight into the actual mechanism of film formation, clay sols were treated with solutions of inorganic electrolytes and organic ammonium compounds and the sols so treated were then used for the production of films. I t was found that systems which by ultramicroscopic investigation showed incipient flocculation and correspondingly a minimum in viscosity formed films which could not be considered water-resistant. If flocculation were carried into the visible range, frequently resulting in the formation of jelly-like systems, it was impossible to produce coherent films, only individual water-resistant flakes being
CLAY Fll'MS
1047
iniglrt iw wiisidcnd: SufFkimt ncw facts h a ~ cbcwn r e p o r i d to iiiukii t t i r layw iattirc struetow of clay minerals, as advanced by Paiiiirrg (I:$) :urd (;riincr (4) and &mdcd for crrtain niontinorillonitrs hy IIoErn:uin, I~ndrll,and Wilm (9), tlrc most acccptabk. In the case of swcllahle laycr la1ticr structures h a h g largr numlms of exehangeablc cations as couiitcr ions, ito has Im?n demonstrated t.liat ihr rrystal units dcposi?, with thrir latticc planes parallcl io thc planr of tlrc film. Microscopic ohserrations of film formation correlatcLd with x-ray analysis seem
I:L(~.
5. Alsifiim treated witlr lead avetatc and liealcd t o difiererit teinperat,uren. (a) 25"C., (b) IWC., (c) IIWC., (rl) RWC.
to iridioati: that it is thi: c-axis that takm on a prrpendicular positioii. I h i s incairs that, i n the formation of t,hr thrt*ads thc parf.i('lrs of the sol prohilhly liirc 111) with their p l a i i ~surfaers facing cactr other (figun: 2).
r .
Insofar m the apac~ingI x t w e n the layer pareels is not appnrt.iably r
