STUDIES OK GELATION .4ND FILM FORRIATION OF COLLOIDAL CLAYS. I' E. A. HAUSEK. A X D D. S. LE BEAU* Department o j Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetls Received July 1, 1338
The mechanism which leads to the formation of gels is still a matter of considerable controversy. A critical review of the literature leaves the impression that the lack of a satisfactory and general explanation for this so common and important phenomenon lies not so much in its admitted complexity but in the fact that so far sufficient at,tention has not been paid to the gelation of simple systems, Le., systems whose components are well defined and which permit actual observation of the changes which the system undergoes in its transition from sol t o gel. Most of the theories are based on assumptions only, or are derivcd by indirect deductions. For instance, the" gelation of a gelatin sol upon cooling, or of rubber solutions in the presence of sulfur and accelerators, etc, is assumed t o be the result of a felting together of long-chain molecules (8) and the formation of bridges and cross linkages between them. The solvent is held mechanically in the network formed or part of it may be immobilized by solvation. The formation of silica gels is considered also today as the result of a chain-like arrangement of neighboring silicic acid molecules by condensation (splitting off of mater) (6). These assumptions have been so generally accepted that even the gelation of systems with microscopically or ultramicroscopically discernible particles has been considered t o be caused primarily by a chain-like or pearl-string-like aggregation of these particles (10). I n the case of soap gels we frequently find a string-like aggregation of the highly disperse soap micelles on cooling the soap stock. However, such an arrangement cannot be esPentia1 for the gelation of soap, since gels can also be obtained without any SUCHalignment and felting. Although it cannot be denied that gelation is facilitated in many cases where the disperse phase consists of organic molecules joined, by primary 1 Presented a t the Fifteenth Colloid Symposium, held a t Cambridge, Massachusetts, June 9-11, 1938. Present address. Dewey & Almy Chemical Co., Cambridge, 3lassachusetts 961
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valences, in chains of high molecular weight, or where condensation leads to chain-like aggregates, we have ample evidence that such a structure i s not essential for an explanation of all types of gelation. Extremely dilute and monodisperse colloidal clay sols3 are excellent examples to disprove this general theory. As a result of previous work pertaining to the production of monodisperse fractions of colloidal clays ranging in apparent average particle diameter from 14 to 180 mfi (5), it was found that the concentration of clay necessary to obtain a gel (system exhibiting yield point) decreases with decreasing particle size, and that gels with extremely low concentration ( < 0.1 per cent) of the disperse phase could be produced with the smallest particle size fractions. Very careful systematic ultramicroscopic studies of freshly prepared gels of concentrations not exceeding 0.1 per cent did not permit, contrary to previous assumptions, the detection of any ultramicroscopically visible specific alignment or grouping of the disperse phase. Results reported previously where grouping of particles had been observed were caused by excessive addition of electrolyte. However, there might remain the possibility that invisible particles form the “missing links.” Therefore a carefully prepared fraction of medium particle size, Le., one from which all smaller particles had been carefully removed by repeated supercentrifugal fractionation, was selected for the present investigation to guarantee the absence of smaller, possibly ubdetectable particles. If such a fraction was studied in a slit-ultramicroscope (using an air-tight chamber), the particles were present in vivid Brownian motion. However, contrary t o older observations carried out with iinfractionated, or insufficiently fractionated, sols, only faint twinkling could be observed. This indicates that the particles in the selected fraction and concentration used are not excessively anisometric. This confirms the prevailing concept of the shape of the individual clay particle as deduced from x-ray diffraction analysis. It is assumed that such a particle consists of layers of silicon and aluminum (the latter being replaceable by magnesium) bonded together by oxygen. Whereas the length and width of these sheets do not vary with the water content, the thickness shows very pronounced changes (1). There exists ample evidence that the difference in observed apparent particle sizes is due primarily to a different number of such layer units stacked up on each other and to a far smaller degree to any difference in the length or width of the atom sheets themselves. Upon adding electrolyte t o such a dilute fractionated sol, one observes a decrease in displacement of the particles due to Brownian motion, without being able to detect, any alignment, grouping, or any specific structure. Upon reaching a certain concentration of electrolyte in the system, Browns The term “colloidal clay” as used in this paper refers t o montmorillonite, if not otherwise specifically statpd.
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ian motion ceases entirely. The individual particles may occasionally still show some rotation, but even so they remain fixed in their position (figure 1). At this stage the viscosity of the system has increased to a point where air bubbles do not rise any more. However, the slightest mechanical disturbance will immediately result in a revival of Brownian motion, which again comes to a standstill as soon as the outside influence stops (a truly thixotropic gel). Even if we assume that not all particles are visible when the system comes to a standstill-some might lie with their thinnest axis parallel to the direction of illumination-it seems highly improbable that all the optical voids should be filled with such invisible particles; moreover, such a configuration would have t o be detectable by changing the azimuth. However, this is not the case. It is theoretically impossible to determine the actual distance between reflection disks in an ultramicroscope. The only experimental statement which can be made is that the individual visible reflection disks are separated and that when a very narrow slit is used the probability that particles which do not happen to be in focus join the sharp reflection disks is negligible. The disks might be caused by a group of primary particles, which ia to be doubted. But even then such hypothetical groups would be separated from each other and exhibit no coherent structure, as postulated, for example, by the mechanical theory (7). This theory assumes that the individual particles actually touch each other in random arrangement, forming a skeleton comparable to a house of cards. If only a slight excess of electrolyte is added, one observes the formation of ultra flock^"^ separated by narrow channels (figure 2). Although such a system will generally still be considered as a gel, and mill not yet exhibit immediate syneresis, it is unquestionably present in a state of incipient coagulation. Further addition of electrolyte results in the formation of microscopic-and, finally, macroscopic-flocks. A gel-like system will persist as long as the flocks are sufficiently loose in structure, of microscopic size, and coherent. However, the above experiment demonstrates that true gelation is present prior to any sign of flocculation or agglomeration. It is beyond the scope of this paper to discuss these experimental facts from a theoretical angle, yet it might be pointed out that we consider the formation of these gels to be caused by the particles taking up equilibrium positions in relation t o each other. Since the overall attractions between The term “ultraflock” should denote flocculation, which is visible only in the ultramicroscope. j For the present discussion it is irrelevant if one considers only the disperse phase responsible for the development of these forces, or, in accordance with Wo. Ostwald (9), attaches a t least equal importance t o the composition of the dispersion medium and its ionic arrangement. A detailed theoretical discussion of these pertinent problems is in preparation.
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identical particles of equal size and shape in a given suspension must be considered as constant for every time differential and of sufficient magnitude in the colloidal range to cause permanent cohesion of the solid phase, if not counteracted by a repulsion force, it is the latter which becomes of predominant importance. Repulsion is composed of various factors, as, for example, the electric charge of the particle, the composition and nature of the ion arrangement in the dispersion medium, lyosorption or solvation, etc. Equilibrium will be reached as soon as the overall attraction is compensated by overall repulsion. That such a system exhibits a yield point finds its simple explanation in the fact that work must be done to move a particle out of its equilibrium position, Le., to overcome the repelling force of the neighboring particles as well as their attraction, because of which they try to hold the particle in place. This theory is a t variance with recent deductions based on the additive character of the London-van der Waals forces (4). The present theory considers repulsion as predominant, whcreas the latter stresses the importance of attraction forces. That dilute gels of hydrated clays like montmorillonite are not the result of any preferred structure or orientation is also demonstrated by a complett lack of alignment and the absence of tactoids or the like. On the contrary, the sols show orientation in polarized light only when in motion, but neither the sol a t rest nor the gel is doubly refractive. On the basis of such a concept gelation must become more pronounced, the smaller the particles, because a system of a given volume can be considered the more volume-dispersed the more particles are available. Maximum gelation will exist if the particles are of uniform size and uniformly spaced (a system of random orientation but great regularity of distribution). Still more striking is the evidence offered by the changes such gels undergo upon further drying. For example, if a sol is first converted into a gel by evaporation and the gel is spread on a n appropriate support or dpposited thereon directly from the sol state by centrifugal action, and then allowed t o dry out completely, we observe with the aid of vertical dark-field illumination that the particles when forced closer together line up or actually snap into their new location, forming long, interlacing filaments or fiber bundles. The presence of separate reflection disks, as obFIG.1. Gel prepared from montmorillonite. Exposure time = 30 sec. Note the sharpness of the reflection disks. X 500. FIE.2. Ultraflocks formed upon the addition of a slight excess of electrolyte. X 500
FIE 3. X-ray of a film formed by drying the gel. Direction of x-rays vertical t o the plane of the film. FIE.4. X-ray of a film formed by drying the gel. Direction of x-rays parallel to the plane of the film.
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served in the sol and gel stage, gives way to continuous thread-like aggregations, which upon continued evaporation seemingly grow into larger crystals of highly anisometric shape. These filaments interweave and tie together by tridimensional cross-linkages, resulting in a coherent network or structure. Ajter complete desiccation a n absolutely self-supporting film i s obtained, which can be easily removed f r o m its support as a coherent sheet. Such film production can be made absolutely continuous, starting with an appropriate sol by using the right type of filming machine. Another conceivable possibility is the production of fine threads by a combination spinning-drying process. Similar results have also been obtained with other clay minerals, vanadium pentoxide sols, etc. Depending on the particle size fractions used, films of varying degree of brittleness and flexibility are obtainable, the latter increasing with decreasing particle size. X-ray diagrams of such films,6 taken vertical and parallel to its plane, reveal a typical Debye-Scherrer montmorillonite pattern in the former case, and a clear fiber pattern in the latter. This simply proves that all the particles have arranged themselves with the same crystallographic axis, parallel t o the support (3) (figures 3 and 4). The formed crystal threads are slightly doubly refractive. The possibility of producing extremely thin, coherent, and self-supporting films without any binder and of high purity of the material offers a new method of studying the infrared absorption spectra of montmorillonite and other filming substances. Since these films can be produced from Na+, Ca++, or H+. or any other adsorbed cation, further investigation should prove of great interest as to lattice configuration and especially water adsorption and absorption. So far distinct absorption bands have been observed a t a reciprocal wave length of 3700 cm.-' due to free hydroxyl groups and between 3600-3200 cm.-l due to associated hydroxyl groups or adsorbed water. Whereas increasing temperature of drying the films reduces the second absorption band, the first remains practically unchanged' (2). Films prepared from sodium bentonite swell and finally go into solution, if placed in water, whereas films of hydrogen bentonite resist very markedly and only show limited swelling. If sodium bentonite films are heated to white heat, they become water-resistant and decidedly strodger, the flexibility decreases, and they closely resemble mica in appearance and properties. If the films are subjected t o high pressure they become transparent and much stronger. The translucency of the original film increases 4 The x-ray diagrams were kindly taken by Professor B. E. Warren of the Department of Physics, Massachusetts Institute of Technology. 7 Dr. Bowling Barnes of the American Cyanamid Co. was so kind as to determine the infrared absorption spectra.
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with derreasing particle size and depends to some rxtent on thr raw material used (aluminum silicate, magnesium silicate, iron content, etc.). The structures which have been so far observed with dark-field microscopy are so niimrrous and complex that it has been impossible &$ yet to draw definite conclusions as to their significance for the formation of the film, However, it seems clear that high temperatures and pressure caiisr a drastic rhange in the hanir crystal structurr of the film (fignrra 5 , 6, and 7). The authors wish to have it understood that thr dinrussion of thr formation of self-supporting rlay films is to he considrrrd only as a brief sum-
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mary ni prrliminary observations. A niorc drtailrd rrport will t w publislrrd a t a later date. The ability of inorganic partirlrs to align and gror together to form the discussed networks or strurturrs rxrliidrs thr prrexistence of a preferred arrangement in the grl. ISvrn thr slightest agglomrration in the gel, resulting in vacuolar, prrvents thr formation of continuous films. Furthermore, this phrnomrnon arrms to offrr a new insight into the possibility of crystal growth from rolloidal dispersions instead of from true solutions and might he of sperial valor for minerological and geologiral considrrationn. The work is hring rontinned. In conrlusion we should like to mrntion n prrulinr phrnomenon which
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was observed in the very finest fractions of bentonite. Besides the particles exhibiting regular Brownian motion, there exist particles which trayel through the field of vision in a straight-line motion, like shooting stars, and then suddenly disappear. All tests* for living organisms (bacteria) were absolutely negative. At present the only plausible explanation for this peculiar phenomenon seems to be the possibility of point disintegration of submicrons, Le., crystals, resulting in a recoil type of motion.g Further work is in progress. SUiMMART
The most commonly accepted theories of gelation are discussed in connection with ultramicroscopic studies of the gelation of extremely dilute colloidal clay sols. A new concept for the transition from sol to gel is offcred. The formation of self-supporting coherent pure colloidal clay films of high flexibility has been observed, and the arrangement of the individual partirles in chain aggregates is demonstrated. The possibility of crystal growth from colloidal dispersions and not from true solutions is considered. X-ray and infrared absorption diagrams are discussed. Attention is drawn to a peculiar rocket-like movement of particles found only in the finest colloidal clay fractions. REFERENCES
W.: Proc. Roy. Inst. Great Brit. 30, 39 (1938). (1) BRAGG, (2) BUSWELL, A. AI., KREBS,KARL,AND RODEBUSH, W. H.: J. Am. Chem. SOC.69, 2603 (1937). (3) CLARK,G. L., GRIM,R. E., AND BRADLEY, W. F.: Z. Krist. 96,322 (1937). (4) HAMAKER, c . : Symposium on Hydrophobic Colloids, p. 16, Utrecht (1937). ( 5 ) HAVSER, E. A., AND REED,C. E.: J. Phys. Chem. 40,1169 (1936); 41,911 (1937). (6) HERD,CHARLES B.: Sigma Xi Quarterly 26, 28 (1938). (7)LEWIS,W. K., SQUIRES,I,., AND THOMPSON, W. I.: Trans. Am. Inst. Mining Met. Engrs. 118, 1 (1936). (8) See, for example, XIEYER,K. H., ANDMARK, H.: Der Aufbau der hochpolymeren H. : Die hochmoleorganischen Naturstoffe, Leipzig (1930) ; STAUDINGER, AND HERkularen organischen Verbindungen, Berlin (1932) ; GERNGROSS MANS: in Bechhold’s Einfuhrung in die Lehre von den Kolloiden, p. 145, Dresden (1934); HAWSER, E. A , A Y D BROWN,J . R.: Ind. Eng. Chem., in press. (9) OSTWALD, Wo.: J Phys. Cbem. (Kovember, 1938). (10) USHER,FRANCIS L : Proc Roy. Soc. (London) Al25, 143 (1929). These tests were kindly carried out by Dr. AT. W. Jennison of the Department of Biology, Massachusetts Institute of Technology. This suggestion was originally made by Dr. 31. J. Buerger of the Department of Geology, Massachusetts Institute of Technology.
