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MODERN COLLOIDCHEMICAL COXCEPTS OF T H E PHBNOMENON OF COAGULATION‘ ERNST A. HAUSER Massachusetts Institute of Technology, Cambridge, Massachusetts, and Worcester Polytechnic Institute, Worcester, Massachusetts Received May 16, 1960
In his book on Mineralogy and Geology, Johann Wolfgang von Goethe (17491832) made the following statement: “The History of Science is Science itself.” If one looks at this citation with the eyes of a modern scientist, it must be admitted that one could hardly find better words to express the need of a solid foundation for the development of modern scientific concepts. Only if one is fully familiar with their historical evolution will one be equipped to appraise them objectively and to make the best use of them. A real understanding of the latest colloidchemical concepts of the phenomenon of coagulation therefore needs a brief historical review as a basis. Colloidal sols are characterized by a tremendously subdivided dispersed phase; this results in a system exhibiting a very large interfacial area between the dispersion medium and the dispersed phase. According to the basic law of least free energy, the dispersed phase of a colloidal sol always will have the tendency to reduce its specific surface. This is most readily accomplished by forming aggregates of increasing size, a reaction termed “coagulation.” The dispersed phase can therefore retain its degree of dispersion only by applying specific factors for its stabilization. The most important stabilizing factors in colloidal sols are ( 1 ) the electric charge carried by the dispersed particle and (2) the solvation of the colloidal micelle. Since both these factors are based on the properties of the ions located in the surface of the dispersed phase and the ions present in the dispersion medium, it is only logical that any satisfactory attempt to explain colloidal phenomena based on the interfacial reactivity of the system calls for adequate knowledge of the morphology of the dispersed particles and their surface composition. The older theories make a clear-cut division between so-called electrocratic and lyocratic colloidal sols. According to the former, the colloidal sol owes its stability exclusively to the electric charge distribution between the surfaces of the dispersed particles and the ions in the dispersion medium. I n contrast thereto, lyocratic sols owe their stability to the affinity of the surface ions of the disperse phase for the dispersion medium as such. The oldest interpretation of the coagulation of electrocratic colloidal sols is based on the theory of the electric double layer originally formulated by Helmholtz (7). He assumed that the double layer consists of one layer firmly attached to the surface of the solid phase and a second layer located at a monomolecular
’
Presented a t the Symposium on Coagulation Fundamentals. which was held under the auspices of the Division of Water, Sewage and Sanitation Chemistry a t t h e 117th Meeting of the American Chemical Society, Detroit. Michigan, April 17-19,1950.
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distance in the dispersion medium (figure la). The stability of lyophilic colloidaI sols was attributed exclusively to the affinity of the surface of the colloidal particles for the dispersion medium. The Helmholtz theory becomes of questionable value, however, if applied to colloidal dispersions. According to this theory, a particle carrying a net positive charge on its surface would migrate in a perfectly dry condition through the surrounding liquid to the negative pole in an electric field imposed on the sol. This, however, is contrary to any known fact. It therefore was a considerable step forward when the French colloid chemist, J. Gouy (3), and the German colloid chemist, H. Freundlich (2), formulated an amended theory known today as the diguse double layer theory. They assumed that the potential gradient is not so abrupt as postulated by Helmholtz, but diffuses over a short distance into the dispersion medium. The charge on the surface of the solid body is
LIQUID
FIG.1 . Electric double layers: (a) Helmholtz; (b) Gouy-Freundlich. A, charges firmly attached t o particle; Ba, charges in liquid layer; Bb, ions in liquid layer firmly attached to particle; C, thickness of attached layer; D, diffuse ions in movable part of liquid. (The particle plus Ab and Bb is the colloidal micelle.) (4, 5)
distributed in the plane of the surface or it can even be embedded therein. The countercharges, however, are located in the surrounding dispersion medium in such a manner that the double layer extends considerably beyond monomolecular distances. The part of the double layer located in the liquid consists of two constituent layers: The first one lies in the liquid but is firmly attached to the surface of the dispersed particle, and thereby becomes an integral part of it. The second is located in the freely movable liquid and extends into it to the point where the mean electric charge reaches a zero value (figure lb). This theory explains why the particles of a colloidal sol cannot be considered as statically charged. When dealing with a colloidal sol whose dispersed phase has a pronounced affinity for the dispersion medium, it was assumed that coagulation was primarily due to the removal of the molecules of the dispersion medium from the dispersed phase, thereby depriving the dispersed particles of their protective layer of solvated ions located in the surface. Although it must be admitted that electric charge and solvation are the predominant factors governing the stability of electrocratic and lyocratic colloidal
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systems, it was demonstrated some time ago that such an absolutely clear distinction cannot be drawn between the two types. The first proof was offered when it was discovered that a gelatin sol becomes turbid upon the addition of alcohol, and that the presence of ultramicrons could be ascertained by ultramicroscopic studies. When such a sol is subjected to electrophoresis, the particles migrate to the anode, where they coagulate (9). Working with an agar-agar sol it could be demonstrated that the particles originally carrying a negative charge can be discharged by the addition of small quantities of electrolyte, without losing their stability, however. Only after adding a dehydrating agent such as alcohol did coagulation occur. If such a sol is first dehydrated very carefully, one nevertheless obtains a sol of noticeable stability which exhibits typical electrocratic properties (8). I t can then be flocculated by the addition of minute quantities of electrolyte. On the other hand, HYDROPHOBIC CONDITION
HYDROPH ILIC CONDITION DEHYDRATING AGENT
-
t
-
/ .
WATER
t +
+
ADDITION O f SMALL AMOUNTS OF ELECTROLYTES (DISCHARGE)
ADDITION OF SMALL AMOUNTS OF ELECTROLYTES (DISCHARGE)
n ADDITION OF DEHYDRATING AGENT (DEHYDRATION)
.
0
FLoccuTioN
FIG.2. Electric neutralization and dehydration of colloidal particles
it is known that colloidal sols which so far have been classified as typically electrocratic, as, for example, sols of clays of the bentonite type, exhibit very pronounced lyocratic properties ( 5 ) . The stability of a sol therefore depends on the electric charge as well as on the degree of solvation of the ions located in the surface of the colloidal particle. This is schematically expressed in figure 2. Another phenomenon which also falls into this group is generally known as the salting-out reaction. I t is known that lyophilic sols can be precipitated by the addition of electrolytes, although the quantities needed therefor are large in comparison with those needed to coagulate typically lyophobic systems. The mechanism of this coagulation is associated with the hydration of the ions employed. Ions of a great degree of hydration will have a pronounced coagulating effect on lyophilic sols, because they take up the water which was originally used to form the hydrated surface of the suspended lyophilic particle. On the basis of what has been discussed so far, it is evident that coagulation is the result of the removal of two stabilizing factors: namely, the electric charge and the solvated layer attached to the dispersed phase of the system. In the
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salting-out reaction, the first small quantity of electrolyte added causes a discharge of the colloidal micelle and the continued addition of electrolyte then causes dehydration of the suspended particle. In the case of truly electrocratic sols, the addition of electrolyte will first cause a reduction of the lyosphere, followed by electric neutralization of the charge carried by the particle. Although many phenomena of coagulation can find a more or less satisfactory explanation on the basis of these concepts, it must be admitted that one factor has SO far been completely overlooked. When dealing with colloidally dispersed matter, one constantly must bear in mind that such systems are characterized by a tremendous preponderance of surface over volume. Colloidally dispersed systems will therefore have many ions located at the interfaces and these ions are chemically unsaturated in comparison with those located in the interior of the solid. The stability of the sol and most of its properties will depend on the reactions these ions can undergo with those present in the dispersion medium and on the extent to which the latter can solvate the surface ions of the dispersed phase. An important fact, which however has only recently been given consideration, is that many surface phenomena exhibited by colloidal matter can be explained on the basis of the principles which govern the atomic structure of crystals. The possibility of polarization effects which result from asymmetrical force fields must also be considered for a complete understanding of surface phenomena (1). Crystal structures are most usually described by applying the coordination number, the symmetry, and the internuclear distance of the average atoms present, but this method does not apply to atoms located in the surface. Most of the extensive work so far carried out to prove that the surface structure of crystals is not identical with that of the interior has been devoted to siliceous matter. Therefore its importance in a general discussion of the surface chemistry of matter will be limited in this paper to silicates. It should be understood, however, that the same considerations apply to any other colloidal dispersion. It is well known that silicon in combination with oxygen, if present in the condensed state, has a valency of four. Systematic x-ray studies coupled with chemical analysis of siliceous matter have revealed that the Si4+ion is always surrounded by four oxygen ions. This results in the SiO, tetrahedron, which can be considered as the universal building unit of all silicates. I t is also known that the Si04 tetrahedra can interlink in a variety of patterns and form chains, double chains, ring structures, or even three-dimensional networks. Two SiOI groups have never been found to have more than one oxygen atom in common, however. This is due to the stability of the electron configuration of the Si4+ (complete octet shell) and its strong positive force field (11, 12, 13). The high charge of the small silicon ion is responsible for the strong Coulomb forces which its ions exert upon the oxygen ions and which lead to the stability of the Si04 tetrahedron. Two SiO, groups sharing a face or even an edge would make the energy content of this unit too high to remain stable, because strong repulsion forces exist between two Si'+ ions in close proximity. Sharing of an edge would decrease their distance to 58 per cent of that necessary for sharing a
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corner. In order to share a face instead of a corner of their tetrahedra, the two central Si4+ ions would have to approach one-third of the original distance. The relatively high polarizability of the large oxygen ion as compared with the small silicon ion makes it imperative that the surface be formed by oxygen ions alone. The mobility of the surface atoms makes it possible for the 02-ions t o be pushed into the surface layer and for the Si4+ions to be pushed into the next layer. This atomic arrangement leads to oriented dipoles, which form the electrical double layer. The surface layer will rearrange so that the surface free energy of the system is lowered even at the expense of the lattice energy of the following atomic layers. One sees that from this point of view the surface exerts a depth action and must be treated as a defective crystal. I t is npt sufficient to consider only one or two atomic layers, if one wants to derive surface properties (13). If the exterior and interior building units of a silica gel are studied very carefully, they can be described by chemical formulas of stoichiometric composition: namely, Si02 and Si02.0.5H20. This is not possible for the new surface unit ho\+ever, because its formula would then have to be written as SiO2.0.50, which means that the surface contains excess oxygen ions. The excess oxygen makes it a defective unit, because this oxygen ion is exposed to only oh, Si4+ion and therefore has no balancing positive field on its other side. This resdts in the formation of a dipole moment,' the dipoles being oriented so that their centers of positive charge point towards the interior and their centers of negative charge towards the exterior of the particle. This dipole orientation produces what one might term a solid electrical double layer. By adsorbing positive ions or polar molecules which can balance the force field, some of the original symmetry can be restored. However, as has been clearly demonstrated recently, these surface units can also undergo an electron transfer which leads to divalent silicon and atomic oxygen (6).
Colloidal siliceous matter falls within this concept. Therefore its surface has bo readjust itself in order not to violate the structural principles. This results
in chemical reactivity with the environment. Figure 3 (13) is a schematic picture of a crystallized and a vitreous form of silica. Following its fracture one can distinguish two types of Si4+ ions. One type retains its complete fourfold coordination, but the other does not. The first type has an excess of oxygen ions over the stoichiometric ratio because it shares only three of its four oxygen ions with neighboring Si4+ ions. This Si4+ ion therefore has an excess negative charge. At the other side of the fracture one finds an Si4+ion which has an oxygen deficiency and an excess positive charge. In a moist atmosphere or in water, these positively charged surface centers attract water molecules and combine with their hydroxyl groups, thus releasing protons. Since these protons are not balanced by anions in the water the latter carries a net positive charge, whereas the gel is charged negatively. Applying these recent findings to the phenomenon of coagulation of colloidal
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sols quite generally, one must admit that an entirely new way of approach is opened up for many problems which have so far found no fully satisfactory explanation. I t is well known, for example, that acids have a stronger coagulating effect on negatively charged particles of a pure silica sol than other electrolytes, even if their cations are of higher valency than that of hydrogen. It has been assumed that this is entirely due t o the fact that hydrogen is the most potent cation in the lyotropic series (IO). Systematic research, details of which will soon be reported, has revealed, however, that the main reason is the fact that even when absolutely pure water is used as the dispersion medium, the colloidal particles have an electrokinetic potential which calls for the presence of counter ions. This new concept also makes it much easier for us to understand why the introduction of hydrogen ions into a colloidal sol of sodium clay will
-
.
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U
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(b)
(a)
FIG.3. Surface structure of fracture of (a) crystalline and (b) amorphous silica (13)
cause coagulation much more readily than the introduction of a cation of much higher valence. On the basis of the previous discussion it becomes evident that the donation of protons by the clay particle in the presence of water has destabilized it even prior t o any addition of electrolyte. Finally, attention should be drawn to the well-known fact that certain lyophilic colloids coagulate upon being heated. This phenomenon is known as denaturation. I t is due t o a chemical change in the micelle, resulting in a release of protons which act as counter ions and cause a decrease in their stability. On the basis of these facts it must be admitted that when dealing with problems of coagulation, particularly when siliceous matter is involved, more attention must be paid to the principles of their atomic structure and how it affects their colloidal properties. SUMMARY
-4brief historical review of the theories pertaining to coagulation is offered. In the light of our present knowledge, the shortcomings of the Helmholtz double
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layer theory and of the Gouy-Freundlich diffuse double layer theory are discussed. It is pointed out that a definite line of demarcation can no longer be drawn between electrocratic and lyocratic colloids in regard to their stability, and experimental evidence for this concept is offered. Attention is drawn to the importance of the morphology and ion distribution in the surface of colloidal particles and why these ions must act differently than when located under the surface. New concepts of solid double layers caused by dipole formation in the surface layer of colloidal matter are offered and their importance is explained on the basis of recent experimental results. REFERENCES (1) FAJARS, K., ASD KREIDL,N. J.: J. Am. Ceram. SOC.31, 105 (1948). (2) FREUNDLICH, H . : Kapilhrchemie, Vol. I , 4th edition, pp. 356 ff. Akademische Verlapsgesellschaft, Leipzig (1930). (3) GOUY,G.: J. phys. radium 9,457 (1910). (4) HAUEER,E . A , : Colloidal Phenomena, pp. 91 ff. McGraw-Hill Book Company, Inc., New York (1939). (5) HAUSER, E. A , : Chem. Revs. 37,287 (1945). (6) HAUSER,E. A., LE BEAU,D. S., A R D PEVEAR,P. P.: J. Phys. C Colloid Chem. 56, 68 (1951). 17) HELMHOLTZ, H . v.: Wied. Ann. 7, 337 (1879). (8) KRUYT,H. R . , A N D JONG, H . G. B. ~ ~ : K o l l o i d c h e m Beihefte28,l . (1928). 0.: Kolloid-2. 15, 8 (1914). (9) SCARPA, (10) VOER,A , : Chem. Weekblad 36, 113 (1938). (11) WEYL,W. A , : J. Am. Ceram. SOC.32, 36i (1949). (12) WEYL.W. A , : J. SOC.Glass Technol. 32, 247 (1948). (13) WEYL,W. A , : Private communication.
COMMUNICATIONS TO THE EDITOR T H E GIBBS-A
RATIONAL UNIT FOR ADSORPTIOS
Adsorption data have been reported in a large number of irrational units. Some of these, such as z/m, have been made necessary in cases where the surface area was unknown. The B.E.T. isotherm is usually stated in terms of the volume adsorbed per gram of adsorbent. Recent papers (McMillan and Teller: J. Phys. & Colloid Chem. 66,17 (1951); Zettlemoyer, Healey, and Fetsko: Paper presented before the Division of Colloid Chemistry a t the 117th Meeting of the American Chemical Society, Houston, Texas, March, 1950) have shown that this and similar isotherms measure the number of sites available for adsorption; hence moles adsorbed would seem to be more logical than volumes adsorbed. On the other hand, surface concentrations of insoluble monolayers are usually reported in terms of the inverse function “area per molecule” comparable to the outmoded reporting of volumes per mole originally used for electrolyte solu-
