POME RECENT WORK ON G E L S
H.FREITDLICH I - n i v e i s i f y College, London, England Received J u l y 8, 1937
In this paper the term “gel” is used in such a n a y that it includes the jelly-like structures as they are found with aluminum oxide, ferric oxide, silicon dioxide, agar, gelatin, soaps, etc. They are coherent, more or less elastic, and contain a certain excess of a liquid phase, and their particles are of strictly colloidal size. Sometimes attention will be given to closely related systems, concentrated suspensions or pastes, containing a much larger amount of solid particles visible under the microscope (diameter about 1 to l o p ) . I . Most colloid chemists are in agreement that in gels we are dealing with structures of fairly different natures. Several years ago Miss Laing and McBain (30) emphasized that transparent gels of sodium oleate are very distinct from many other gels in that they are identical as t o vapor pressure, electrical conductivity, etc., with the sols from which they have set. But so far it has not been possible to distinguish gels by a particular property. Heymann (23) found that the volume change in sol-gel transformation is very characteristic and allows an arrangement of gels into certain groups. This does not imply that volume change is t o be taken as the only criterion. Heymann used a very sensitive dilatometer immersed in a thermostat which was kept constant to 0.003”C. The capillary in which the change of volume was determined was very fine; a change of meniscus of 1 em. corresponded t o a volume change of 0.0016 cc. The volume of the sol which was turning to a gel amounted to about 80 cc. A volume change of 0.0002 per cent of the total volunie could thus be measured. Thixotropic gels, i.e., gels capable of an isothermal, reversible sol-gel transformation, being liquefied on shaking and setting spontaneously, show no change of volume. This was the case, for instance, with a concentrated sol of iron oxide (Graham) containing 9.44 per cent solid phase and a small amount of sodium chloride; the meniscus did not change its position more than a fern tenths of a millimeter in an irregular way. This behavior fully agreeq with the exceedingly small difference bctween sol 1 Presented a t the Fourteenth Colloid Symposium, held a t IIlnnespolis. Minnescta, June 10-12 1937
401 ? B E JOTRX‘AL OF PHT6ICAL C B E M I S T R T , V O L
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H. FRECKDLICH
and gel in this type of gel. The traiisforniation is probably due to a fir+t and very mild stage of coagulation (15, 41), as i t is caused by electrolytes in electrocratic (hydrophobic) bois, where no marked change in the distribution of water molecules between liquid and solid phase or in the cliemical nature of the compounds occurs. The sodium oleate sols, mentioned aborc, also shon- no change in yolume when being transformed t o a gel (24)* second type of gel, those of gelatin and agar, showed a small decrcaw in volume when the so1 which had been formed on heating wai cooled t o room temperature and allowed t o set t o a gel at this temperature. With gelatin it was 0.05 t o 0.06 cc. per 100 g. of gelatin, a value fairly independent of the p H between 3.8 and 8.8. Colloidal aqueous solutions of methylcellulose, on the other hand, shon a small increase in volume,-from 0.08 t o 0.13 cc. per 100 g. of niethylcellulose. This sol undergoes an inverse sol-gel transformation (22), i.e., the gel is formed with rising temperature, and is liquefied to a sol when the temperature decreases. The volume change was determined by letting the sol set t o a gel a t 35.ioC., or by comparing the volunie of the so1 at 34°C. with that of a gel which had been fgrmed at 55OC. and had been cooled do\m to the former temperature before it had had time t o liquefy. I n these cases a change of hydration is probably the cause of the volume change, the particles of the gel being more strongly hydrated in gelatin and agar and dehydrated in methylcellulose. Several other facts confirm this assumption. When gelatin is dissolved in water, the volume contraction is larger at low temperatures, where a gel is formed, than a t higher ones, where a sol is formed. With methylcellulose dehydration with rising temperature is made probable by the parallel marked decrease of relative viscosity, Le., of the ratio v5/qu.,qs being the viscosity of the $01, qtL that of water. I t may be concluded that the viscosity of the sol decreases more strongly with the rise of temperature than t h a t of the solvent, and that this is due t o the relative volume of the dispersed particles becoming smaller. If a gel of silicic acid iq formed on mixing a sodium silicate solution with aqueous hydrochloric acid, the volume change is also positive, but niuch larger, about ten times, than in the cases mentioned so far; i t amounts t o about 0.8 cc. per 100 g. of ;ilicoii dioxide in the first hours until gel formation has occurred. Obviously gel formation is correlated here with a further polymerization of the mono- and di-silicic acids which Willstjtter (45) has shown t o he formed originally, e.g., A \
2HzSi205
=
H2Si409+HzO
The large increase in volume is due to the water split off by this chemical reaction. The particles formed are polysiliric acids (38) such as
903
RECEXT WORK ON GELS
OH
OH 1
OH
OH
0
0
OH
OH
I
HO-Si-0-Si-0-Si-0-Si-OH
0
0I
HO-Si-0-Si-0-Si-0-Si-OH I
OH
I
I
OH
The volume change goes on for a very long time; even after five months there was no sign of a limit being reached. The volume increase after the gel has formed is about 1 cc. per 100 g. of silicon dioxide. Hence it is quite possible that the whole network structure of this gel is united by primary valences. I n any caqe, silicic acid gel formation is irreversible, and the colloidal units in different temporary stages of the process may have a fairly different chemical constitution. The rather extraordinary properties of silicon dioxide gel as to elasticity, irre\yersibility, etc., may be correlated with this structure, which is very different from t h a t of gels of aluminum oxide, ferric oxide, etc. The gel formation going on in maturing viscose solution is also accompanied by a similarly large increase in volume. 2 . Though we have become more thoroughly acquainted with thixotropic gels only in the course of recent years, thixotropic transformation, taken in a very general sense, is perhaps the most frequent phenomenon of this kind known. It is not confined to true gels like those of aluminum oxide, ferric oxide, vanadium pentoxide, etc., where the particles are of colloidal size (4, 41). Many concentrated suspensions of sufficiently finely powdered substances (minerals, etc.), containing a certain percentage of particles with a diameter of about 111, are liquefied on shaking and set again t o a stiff, solid paste when left to themselves (6, 7). On the other hand, setting need not go so far that a gel is formed. The “anomalous viscosity” of many concentrated sols may be reduced reversibly by shaking, a change of behavior which may be found to turn gradually into the nornial sol-gel transformation, if the concentration of the sol is increased (21). I n all caqes a structure is destroyed by mechanical treatment and forms again spontaneously. It is perhaps doubtful whether we are really dealing always with the same mechanism. I n the case of dilute sols it is most surprising that they are still able to set to a gel, aq is, for instance, the case with vanadium pentoxide sol down to a concentration of about 0.1 per cent. In concentrated suspensions, containing 70 per cent and more of solid powder, it is niore surprising that they are readily liquefied. Thixotropy is alxmys bound to a loose packing of particles, i.e., t o a certain excess in the amount of liquid (6). This is evident in gels and needs no special discuwion. I t is not equally obvious in pastes of concentrated
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H. FREUNDLICH
suspensions containing coarse particles (clays, dolnhofen slate, etc.) . Herc the degree of packing may be tested by measuring the sedimentation volume of a somewhat less concentrated suspension. On the whole, the rule holds well that suspeivions whose particles fill a large volume of sedimentation, and hence hare a tendency to be loosely packed, are markedly thixotropic. It is surprising, however, that a comparatively small deviation from the state of closest packing is sufficient to produce a certain degree of thixotropy. We have a countcrpart to thixotropy, Le., a phenomenon bound to a very close packing, in Osboriie Reynolds’ dilatancy (34, 35, 44). A behavior due t o dilatancy is readily found in wet sand, and is thus frequently observed on a nioist beach. When trodden upon or pressed the sand turns dry and hard, but it becomes moist again as soon as the pressure is released. Osborne Reynolds explained this behavior in the following way: The particles of the sand are closely packed, the amount of liquid being just sufficient to fill the free space between the particles. (With spherical particles of equal size this free space amounts to 26 per cent of the total volume. Spring (40) proved this to be true for a fine sand with spherical particles.) When the particles are displaced by some force from outside, the volume occupied by the interstices increases and the amount of liquid is no longer sufficient to fill the free space; the mass turns hard and dry. On releasing the pressure the particles return to their original state of close packing, the liquid is able to fill the free space, and the whole mass is moist again. It was shown experimentally (6) that concentrated aqueous suspensions having a small volume of sedimentation, hence being closely packed (quartz, fluorspar, >Ionax glass, porcelain, etc.), are dilatant. A certain independence of the particles of each other-a lack of any tendency to adhere to each other and to form clusters-seems to be essential for close packing. Dilatancy may not be confused with another phenomenon which is similar, but only when being compared superficially. Some thixotropic sols and suspensions of coarser particles set more rapidly to a solid system when nioved gently, whereas they are liquefied by intense shaking (8, 20a, 26). A suspension of finely powdered gypsum in water is solidified in a few seconds when rolled betm-een the palms of the hand, whereas it needs about ten minutes to set spontaneously. A thixotropic, old sol of vanadium pentoxide, containing a suitable amount of acid or a lithium salt, turns t o a gel in one minute if the test tube is tapped on the table; it takes many hours to solidify spontaneously. This phenomenon has been called rheopexy. Both in rheopexy and in dilatancy movement favors solidification. But in rheopexy the final state is that of a gel or solid paste, though the structure of a gel or paste formed by rheopectic setting is probably different from that of those which hare formed spontaneously.
RECEXT WORK O X GELS
905
In dilatancy the solid state produced by the movement is unstable; t h e bystem, left to itself, returns to a more liquid or even distinctly liquid state The following experiment makes this evident: If the dilatant mass of finely powdered quartz or starch in water is picked up with a spatula, the hard cake sticking to the latter runs down as a treacly liquid as soon as the movement has stopped. Kheopexy is probably due to a certain coagulation of the particles as soon as they have been made to approach each other by the movement of the liquid. That colloidal solutions may be coagulated by being stirred or shaken is a well-known fact (9, 11, 13). A non-spherical shape of the particles seems to be an important factor for producing rheopexy. Furthermore, with vanadium pentoxide sol a certain special structure of secondary particles, according to recent experiments, appears to be essential. This is why only a fen- electrolytes, acids and lithium salts, are able to produce the phenomenon. The importance of coagulation in rheopexy becomes clear, when the Qimple mechanical treatment is replaced by ultrasonics. Ultrasonic waves of high energy are known to liquefy thixotropic gels (3, 14, 16). This was shown to be due to the strong destructive effect caused by the collapse of cavities which are produced when the liquid is unduly stretched in the expansion phase of the sound waves. Thixotropic pastes with coarser particles (gypsum, etc.) are also liquefied by ultrasonics of high energy. Several pastes of this kind, being both thixotropic and rheopectic (for instance, pastes of gypsum and of Solnhofen slate), may, hoxever, also be solidified rapidly if exposed to ultrasonics of smaller energy (1, 26). This effect is so pronounced that it may easily mask the liquefying effect mentioned above. When the test tube containing the liquefied suspension is removed from the interior of the oil fountain from which the sound waves pass into the test tube, the system unavoidably traverses a region of weaker action of the ultrasonics, and there solidification occurs quickly. This solidification effect has nothing to do with the action of cavitation, because it also occurs under conditions-under hydrostatic pressure and in vacrro-where no cavitation takes place. It is correlated with the coagulating action of ultrasonics on emulsions, suspensions, smokes, etc., which is caused by the phenomenon of Kundt’s dust figures, i.e., by the accumulation of particles of suitable size in the nodes or antinodes of stationary sound waves (39). Not much can be said yet on the theoryof thixotropy and related phenomena. Obviously we are dealing with an interaction of forces of attraction and repulsion. Hence the particles which are moving in Brownian movement in a certain excess of liquid, thus forming a fluid system, may be macle to settle in certain distances, thus producing a structure and a more or less solid system. In many thixotropic gels and pastes repulsion
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H. FREUKDLICH
may be mainly due to electrical forces, as they are instrumental in causing the stability of aqueous colloidal solutions. But we know of thixotropic systems of organic substances in insulating liquids ( 3 i ) , for cxaniple, of mercaptobenzothiazole in benzene, toluene, etc. Hence solvation (and hydration) may also be essential in keeping the particles apart. The nature of the forces of attraction is even more enigmatical. It is obvious that in most thixotropic systems strong chemical forces can not be responsible. Weak forces, like those of van der Waals, appear from the first t o be much more likely. But the following fact causes some trouble: There are quite a number of cases known where it has to be assumed that the particles in a thixotropic gel are fairly wide apart (20), the average distance between them being up to 0.111. Perhaps the modern conception of van der Waals forces, as developed by Idondon, will allox this difficiilty to be overcome (17, 2’7). 3. Hydration comes into play in sol-gel transformation, which has mainly interested us so far. But i t is only one feature among others, and not so decisive as in sonie other phenomena shown by gels. Here swelling must be particularly mentioned. Quantitative values arc obtained by determining the swelling pressure. It has been determined x i t h an apparatus of the following type (12): -1disc of the gel, exposed to :t measured pressure, is separated from the liquid by a porous membrane; it lies on the bottom of a cylindrical vessel of dried white china clay. The liquid is imbibed by the gel and the ensuing change in volume is measured under different pressures. Swelling pressures have so far not been dctermined successfully on a large scale, except for the swelling of rubber in organic liquids. Gelatin could he investigated only in pure water. In salt solutions it was strongly peptized; it passed through the pores of the membrane in too large an aniount in the rather long time which is needed t o reach equilibrium. Isinglass was found to be much tougher and more resistant to peptization than gelatin ( 5 ) . A number of electrolytes could be investigated, and a not too narrow range of pH. I n agreement with previous, more qualitative, work the experiiiients proved swelling t o be a reversible, but not a simple, phenomenon (25, 29). The influence of the p H niay be attributed to a Dolman equilibrium (31, 36, 46). The amphoteric protein combining n i t h acid or alkali forms dissociated salts, whose ions are strongly bound to t h e qtructure of the protein gel; hence water is taken up by the gel until an ccluilibriuni betv een the ions in the gel and in the liquid outside is established. A minimum in the absorption of water and in swelling niay be expected a t thc isoelectric point of the protein; this has indeed been found in several cases, alcjo with isinglass (5). Its isoelectric point, iq 5.9. In figure 1 the ordinate is
907
RECENT WORK O N GELS
log y , where y is the concentration of the gel, Le., the aniourit of isinglass in 1000 cc. of isinglass and liquid, the abscissa is the pH. the swelling pressure always being 3060 g. per square centimeter. .it a p H of about 6.5, close to the isoelectric point, there is a maximum in y, hence a minimuni in swelling. Salts in aqueous solutions deviating only slightly from neutrality influence swelling to about the same degree as the changes in pH in figure 1; but this effect can not be correlated with a change in pH. One example niay bc sufficient to prove this assertion. During swelling in a 0.2 11.’ solution of sodium sulfate the pH of the external solution (and of the gel) \+as found to be 8.5. If swelling depended only on a change in pH, according to figure 1, we would have expected increased swelling; the concentration of the gel could have changed from 612 (in the distilled water used the pH was found to be 6.0) t o 520 at pH = 8.5,-swelling pressure 3060 g. per square centimeter. Instead of that, the concentra-
I 2 Sd---
2
3
4
5
PH 6
7
8
9
fo
FIG.1. Yariation of swelling with pH
tion in the sulfate solution was found to be 865, i.e., swelling had decreased. Hence we have a transport of liquid in and out of the gel, not depending on t h e pH. The simple conception of a Donnan equilibrium mentioned does not cover all facts concerned with the influence of foreign substances on .welling (25, 29). When comparing the action of different neutral salts, the ions are found to be arranged according to the lyotropic or Hofmeister series: Li’ < S a - < K- and SO;- < F- < C1- < NO, < I- < CKS-. These are considered to agree with the order of their hydration (25, 29, 33). Generally thP term “bound” water has been favored for the water whose amount in the gel is changed by foreign substances independently of changes in pH. The importance of a “binding” of water in colloidal and biological phenomena has been strongly emphasized (16a: cf. also 19), and the clifficulties which are encountered concerning “bound” or “immobilized” water hare also been frequently discussed (cf. 2a, 28). Perhap. the following conception may prove valuable in explaining the
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H. FRECSDLICH
influence of neutral electrolytes and non-electrolytes in swelling. It has been shown (18, 42, 43) that at constant temperature a gradient of concentration of one substance may cause an unequal distribution of another substance, not only of ions, which was originally uniformly distributed. A unifornily distributed electrolyte (sodium chloride) or a non-electrolyte (sugar) could be made to diffuse, and thus to become unequally concentrated in a column of a solution by a concentration gradient of a nonelectrolyte (alcohol). (This phenomenon is in some way similar to Soret’s phenomenon, where, owing to a temperature gradient, a substance in an originally uniform solution is distributed unequally.) In these cases of anomalous diffusion, a movement of both solute and solvent has to be taken into account. The force to which the effect is due is the affinity between solute and solvent active in causing solubility, etc. A gel whose striicture maintains a gradient of concentration between the micelles and the inedium of dispersion at any time would particularly favor an exchange of substance, owing to this kind of anomalous diffusion, i.c., to this very general Donnan effect. This assumption has the advantage that it does not make such a strict distinction between the swelling of a protein like isinglav in pure water and in solutions of neutral salts and non-electrolytes on the one hand, and in solutions of acids and alkalis on the other. It u-as not very satisfactory that a fundamental difference had to be assumed as to swelling in these two groups, whereas actually the laws governing swelling pressure are always practically the same. This does not imply that the beha\ior of gels, when swelling in acids and alkalis or in neutral solutions, is identical in all points. There are distinct differences as to appearance and to r x tension or contraction, when comparing, for instance, the swelling of collagen fibers in acids or alkalis or in neutral salt solutions (25). But, I believe, they may be explained by the fact that we are dealing with the swelling of different substances, namely, of protein salts on the onp hand, and of more or less neutral proteins on the other. It is readily understood why swelling is such a complex phenomenon, because of the additional way in which it depends on the concentration of the foreign substance. The distribution of water between gel and medium depends actually on all substances present, and their number will generally be more than three, if there is one foreign substance besides protein and water, because the protein will probably react and form a compouhd. It was mentioned above that the hydration correlated to the sol-gel transformation of gelatin does not change appreciably with a change in pH. This need not be in contradiction to the marked influence of the pH upon the swelling of proteins such as gelatin or isinglass. In sol-gel transformation we are dealing with dilute gels; in swelling with
RECEK'T
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909
concentrated gels. I n swelling the uptake of water, the properties of t'he micelles, etc., may distinctly depend on the transition of neutral protein into protein salt. I n the dilute gel, capable of turning to a sol and vice versa, these differences may hare become quite irrelevant as to the distribution of mater, whereas some other factor causing hydration may still be sufficiently active t o produce the observed change in volume. REFEREXCES (1) BURGER, F. J., ASD S ~ L L X EKR. :, Trans. Faraday Soc. 32, 1598 (1936). (2) DONNAS:Kolloid-Z. 61, 24 (1930). (2a) EIRICH, F., A N D MARK,H . : Ergeb. exakt. Naturw. 16, 1 (1936). H . : Kapillarchemie, 4th edition, T'ol. 11, p. 616. Akademische (3) FREUSDLICH, Verlagsgesellschaft, Leipzig (1932). (4) FREUNDLICH, H . : Thixotropy. ActualitBs Scientifiques e t Industrielles. Paris (1935). (5) FREUNDLICH, H., ASD GORDON, P. S.: Trans. Faraday Soc. 32, 1415 (1936). (6) FREUNDLICH, H . , A N D JOSES, A. D.: J . Phys. Chem. 40,1217 (1936). ( 7 ) FREUXDLICH, H . , ASD JULIUSBURGER: Trans. Faraday Soc. 30, 333 (1934). (8) FREUSDLICH, H., ASD JTLICSBCRGER: Trans. Faraday Soc. 31, 920 (1935). H . , ASD KROCH:2. physik. Chem. 124, 155 (1926). (9) FREUSDLICH, (10) FREUSDLICH, H . , ASD K R ~ G E R MISS , D.: Trans. Faraday SOC.31, 906 (1935). H . , A N D LOEBMANN: Z. physik. Chem. 139, 368 (1928); Kolloid(11) FREUSDLICH, chem. Beihefte 28, 391 (1929). (12) FREUSDLICH, H., AKD POSSJAK:Kolloidchem. Beihefte 3, 517 (1912). H., ASD v. RECKLJNGHAUSES: Z. physik. Chem. A167, 325 (1931). (13) FREUSDLICH, (14) FREUKDLICH, H., ROGOWSKI, A N D SOLLXER: Z. physik. Chem. A160,469 (1932); Kolloidchem. Beihefte 37,223 (1933). (15) FREUSDLICH, H., A N D SOLLXER: Kolloid-Z. 46, 348 (1928). (16) FREONDLICH, H . , ASD SOLLKER:Trans."Faraday Soc. 32, 966 (1936). (16a) GORTSER,R. A , : Trans. Faraday Soc. 26,678 (1930). (17) HAMAKER: Rec. trav. chim. 66, 1015 (1936);66,3 (1937). (18) HARrLEY, G. S.: Trans. Faraday Soc. 27, 10, 1 (1931). (19) HATSCHEK: Trans. Faraday Soc. 32, 787 (1936). (20) HAUSER, E. A . : Kolloid-Z. 48,57 (1929). (21) HELLER:Kolloid-Z. 60, 125 (1930). (22) HEYMANS, E.: Trans. Faraday Soc. 31,846 (1935). (23) H E Y Y A N N , E.: Trans. Faraday Soc. 32, 462 (1936), concerning this whole paragraph. (24) HEYMANN, E. : Unpublished experiments. ASD PLEASS:Biochem. J. 21,1356 (1927). (25) JORDAS-LLOYD JORDAX-LLOYD A N D MARRIOTT: Trans. Faraday S o r . 32, 932 (1936). AND PIRQUET: Trans. Faraday Soc. 32, 445 (1936). (26) JELIUSBURGER (27) KALLMASS,H . , ASD X I L L S T A T T E R , MISS 11.: Saturirissenschaften 20, 952 (1932). (28) KISTLER:J . Am. Chem. SOC.68,901 (1936). (29) K ~ N T Z E Biochem. L: Z. 209, 326 (1929). (30) LAISGASD LICBAIS: J. Chem. soc. 117,1506 (1920). (31) LOEB,J. : Proteins and the Theory of Colloidal Behavior. McGraw-Hill Book Co., Inc., New York (1922). (32) MARIXESCO: Compt. rend. 194, 1824 (1932).
910 (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46)
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NORTHROP A N D KUNITZ:J. Gen. Physiol. 8,317 (1926). OSBORNE REYNOLDS: Phil. Mag. [5] 20,469 (1885); Nature 33,429 (1886). OSTWALD, Wo. : Kleines Praktikum der Kolloidchemie, 5th edition, p. 80. PROCTER: J. Chem. Soc. 106, 313 (1914). PROCTER ANDWILSOS, J. A , : J. Chem. Soc. 109,317 (1916). v. RECKLINGHAUSEN: Kolloid-Z. 60,34 (1932). RIDEAL:Trans. Faraday Soc. 32,3 (1936). SOLLNERAND BONDY:Trans. Faraday Soc. 32,616 (1936). SPRISG: Bull. soc. Belg. geol. 17, No. 13 (1903). SZEGVARI .4ND SCHALEK, L I I S S : Kolloid-Z. 32, 318; 33,326 (1923). TEORELL: Proc. Katl. Acad. Sei. U. S. 21,152 (1935). THOVERT: Ann. chim. phys. [7] 26,419 (1902); Ann. phys. [9] 2, 405 (1914). WILLIAMSOX, R. V.: J. Phys. Chem. 36,354 (1931). WILLSTATTER, R., KRAUT,AXD LOBINGER: Ber. 61,2280 (1928); 62,2027 (1929). WILSON,J. A.: In Bogue’s The Theory and Application of Colloidal Behavior. Vol. I, p. 1. McGralv-Hill Book Co., Inc , New York (1924).
