A NEW ASPECT OF T H E COLLOIDAL GOLD REACTIONS‘ HANS M. CASSEL
Research Department, Colgate-Palmolive-Peet Company, Jersey City, New Jersey Received July 1, 1968 THE BASIC IDEA
There is a general agreement about the r6le of the [-potential as a stabilizing factor. The {-potential is a measure of the repelling forces between neighboring particles, hence a sufficient reduction in potential leads to coagulation. But this is not the only possible way of explaining the collapse of colloidal dispersions; there must be another principle.2 If the attractive forces are sufficiently increased the repulsion will be overcome, with coagulation as the result. Such an effect, indeed, should be expected if a “colloidal agent” condenses in the capillary interspaces between the particles during their collision. I n figure la two spherical particles are shown in a given volume containing under pressure P a gas which is adsorbed on the surface of the particles. Now suppose the two particles to be brought in contact. Immediately (figure lb) the pressure must decrease, owing to capillary condensation taking place between the particles. I n the case of very strong adsorption forces the pressure may become almost imperceptible. To establish the initial state by separating the particles, an energy of activation is required which might well exceed by far the work afforded by the rc we have t o deal with a tendency toward condensation in concavities, hence agglutination can be expected. On the other hand, when 21‘ < cr the tendency is t o condense preferentially upon convex interfaces of smaller radii. Hence, emulsification and protection should be possible. As shown in a previous publication this theory agrees with experiences in emulsification. Nothing seems t o be in the way of applying this view to the reactions of hydrophilic sols with hydrophobic suspensoids in general, disregarding for a first approximation departures from the spherical symmetry of crystalline particles. as to what will happen depends on the sign of the term
COMPARISON WITH EXPERIMENTAL FACTS
From the present standpoint the whole behavior of hydrophilic sols toward gold sols depends entirely on the individual shape of the adsorption isotherm. Unfortunately, not a single isotherm is known as yet, either on plane or on curved surfaces. Thus we have to make conjectures according to the known facts which consist chiefly in agglutination numbers (minimum amounts t o produce “sensitization”) and in protection numbers (minimum amounts to inhibit agglutination). I n not a single case have the actual concentrations been determined! Figure 4 shows two hypothetical adsorption isotherms drawn in a r-c diagram and representing the adsorption on a particle of radius r. The whole field is divided into two fields by the straight line 21- = rc. Those parts of the isotherms rising above this discriminant line define a zone of possible agglutination, while those parts lying beneath the discriminant, where 2r < T C , outline a zone of possible protection. We read from the diagram that agglutination can occur at much lower concentrations than protection, which is in agreement with experiment, Furthermore, it becomes clear that no protection at all can be provided in case the adsorption is too strong (figure 5 ) . The addition of electrolyte will either increase or decrease the adsorption. The first, as a pre-stage of salting-out, seems generally t o be the case in the determination of Zsigmondy’s gold numbers. These conditions are supposed t o be represented by the higher adsorption isotherm in figure 4. Here we see the protection zone for the particle size r consider-
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ably reduced. However, protection remains possible for larger sized particles, as illustrated by the discriminant r’ having a steeper slope than r. This is the way the theory accounts for the characteristic feature of the gold sol reactions with hydrophilic sols, in that the disturbance of stability through electrolytes does not induce complete precipitation but ends in reestablishing stable conditions for larger aggregates, in spite of the strong electrolytes present.
I
Au,,
?
Au,
?
C
FIG.4. The adsorption on a particle of radius T is shown in the presence and in the absence of electrolyte by curves A’ and A, respectively. The s t r a k h t lines T and T ’ represent the discriminants for particles of radii T and T‘ > T . Au, and A U , ~ indicate the corresponding gold number concentrations. The question marks indicate “upper gold numbers.”
C
FIG,5 . This figure exemplifies strong adsorption where the discriminants even of rather large particles lie beneath the isotherm.
Zsigmondy’s definition of the gold number implies the assumption that protection should always be enhanced by increasing the concentration of the colloids. However, while the chief concern has been to find out the best conditions for well-defined color changes, in actual practice this supposition has often been omitted. I n violation of the original definition many so-called gold numbers were determined by the minimum amount of substance required to produce color changes instead of by those required t o prevent them!
NEW ASPECT O F COLLOIDAL GOLD REACTIONS
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In any case caution is advisable in interpreting the presumably characteristic figures available in the literature, which may represent either the lower or the upper limit of the stability zone without apparent discrimination. Thus, for albumin, gum arabic, sodium oleate, and sodium laurate “upper gold numbers’’ are reported, as involuntarily demonstrated by the statement of Zsigmondy and Thiessen (4) that certain substances even a t rather high concentrations do not effect any color changes, either spontaneously or on the addition of sodium chloride. These substances are those just mentioned! From the present point of view such a behavior of hydrophilic sols can be foreseen, As already emphasized, the specificity of their reaction depends entirely on the shape of the adsorption isotherm. Let us imagine a type of curve which, up to rather high concentrations, always lies below the discriminant of the particle size chosen. Consequently, according to the theory, neither agglutination nor sensitization can be expected in the whole range of concentration limited by the “upper gold number.” As a matter of fact, the gold numbers of the last-mentioned substances are representative of the higher values in Zsigmondy’s classification. There is one more argument in favor of the present view. If the low gold numbers correspond to the lower stability limit and the higher values to the upper limit they should be differently affected by changes in the size of the particles. It follows from the diagram (figure 4) that for discriminants of larger particles the protection zone is widened towaids lower concentrations a t the lower limit, but the opposite holds a t the upper limit. Herewith it agrees that the gold numbers of soaps, dextrin, and albumin, which we recognized as “upper gold numbers,” increase with increasing particle size, whereas the lower class colloids behave diametrically differently. No one seems to have realized the possibility of gold numbers in the wider sense of the word, with the remarkable exception of such a critical observer as Wilhelm Biltz (1) who, studying the colloidal properties of dextrins, stated as early as in 1913 that two and more gold numbers may occur in different concentration ranges of the same substance. So far as I can see, no contradictions are known to the present attempt to gain a unitarian aspect of the reactions between hydrophobic colloids and hydrophilic substances. The results seem to encourage further experimental checking of the theory. If the present view is correct it should be promising to consider the behavior of gold sols as a prototype for certain antigen-antibody reactions. In closing I wish to express my gratitude to Mr. Robert B. Colgate, Chairman of the’Industria1 Research Council, for his part in making this paper possible.
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REFEREXCES (1) BILTZ,IY.:Z. physik. Chem. 83, 695 (1913). (2) CASSEL,H. M.: J. Phys. Chem 42,475 (1938). (3) GIBBS,J. WILLARD:Collected Works, Vol. I, p. 219 Longmans, Green and Co , N e x York (1928). (4) ZSIGYOSDY, R., AND THIESSEN, P. h.:Das kolloide Gold. Kolloidforschung in Einaeldarstellungen Akademische Verlagsgesellschaft, Leipzig (1925)
