DISPERSITY O F SILVER HALIDES I N RELATION TO T H E I R PHOTOGRAPHIC BEHAVIOR’ RY FRANK E. E. GERMANN AND MALCOLM C. HYLAN
Introduction There has been of late considerable discussion among photographic chemists over the relation between size of the silver halide grain and the speed of the photographic plate. Wightman, Trivelli and Sheppard2 seem to find that the relative speed of emulsions increases rapidly with the average size and range of size of the particles contained therein. That this is not always true is shown by the following3: “An experimental emulsion was prepared, the grains of which measured up to 8 p in diameter and which had an H and D speed of only 38. In comparison with this emulsion a Royal Standard Lightning Plate from Kodak Ltd. was tested, the grains of which averaged up to 2 . 8 , ~in diameter and of which the H and D speed was 728. Thus it appears that emulsions containing grains of approximately I /3 the linear dimension are more than 19 times as sensitive. This is also true of individual grains in the same emulsion”. Koch and du Preldhave concluded that it is not possible, with the information at present available, to formulate a definite relation between grain-size and sensitivity, but that it is certain that the largest grains in an emulsion are by no means the most sensitive. Before going on to a further discussion of the experimental results obtained by various authors it will be well to consider, from a purely theoretical standpoint, what should be expected. If we accept t.he sub-halide theory of the photochemical action of light on the silver halide, the amount of halide affected in unit time becomes the determining factor in the speed of the plate. If we accept the nuclear theory it is the number of grains affected which determines the number of nuclei formed and therefore the speed of the plate. Supposing light to have a continuous wave-front, that is, not a quantum-like or discrete structure of radiation, then the amount of halide affected would depend upon the “effective area” of the particles, and the number of particles affected would depend upon their “relative number”. By “effective area” is meant the sum of the projectional areas of the particles. By “relative number” of particles is meant the number of particles present per unit weight of silver halide. Now both (‘effective area” and “relative number” are proportional to the dispersity. This will be readily seen from the following illustration. We may consider the grains to approxiExtract from a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy. J. Phys. Chem. 27, 1-51 (1923). Eastman Kodak Co. Monographs on the Theory of Phot,ography No. I, p. 104. Physik. Z. 17, 536 (1916).
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mate spheres. Suppose, then, we have a sphere of radius R. Its mass will be 4/37rR3p, where p is the density. Its projectional area will be nR2. Let us, then, take an equal mass of spheres of radius r =R/2. Then the mass of each sphere will be 4/37rr3p or 1/8 of the larger sphere. We will then have 8 small spheres. The projectional area of each will be ar?, and their total projectional areas will be gar2 or twice the projectional area of the large sphere. It is evident, then, that by halving the linear dimensions we double the “effective area” and mutiply the number of particles by 8, or to be more general, the “effective area” is inversely proportional to the linear dimensions of the.particles, and the number of particles, or “relative number”, is inversely proportional to the cube of the linear dimensions. Suppose, now, we consider light to consist of quanta, or discrete particles of radiation. Then the probability of each quantum coming in contact with silver halide will be proportional to the “effective area”, and while greater dispersity will reduce the probability of any single particle being hit, it will increase the probability of the quantum hitting some silver halide. Thus the amount of halide affected is again proportional to the dispersity. And, finally, the number of particles hit will also be greater the greater the dispersity. Let us suppose that instead of the number of particles affected it is the number of molecules affected which is the determining factor. Owing to photochemical extinction, the amount of light reaching any molecule will depend upon its distance from the surface of the particle. Here again the greater the dispersity the more molecules are at or near the surface. For example take the spheres used as an illustration above. Light of sufficient intensity to just penetrate to the center of the small spheres would penetrate but half way to the center of the larger one. Thus, viewed from a number of different angles, theoretically it would seem that the smaller-grained emulsions should be the faster. The apparent usual greater speed of coarse-grained emulsions must be due to factors other than purely dimensional ones. The work of the authors on the speed of silver iodide emulsions’, in which they found the apparent great difference between the speed of iodide and bromide to be due principally to the adsorption of potassium iodide by the former, led them to wonder if the apparent greater speed of coarse-grained emulsions, in direct opposition to the theoretically expected result, might not be due to a similar phenomenon. Assumption of such a cause for the discrepancy between theory and practice perfectly explains the experimental results obtained by many investigators. We will take up some of these in detail. We should expect, since in gelatin emulsions the silver halide is formed in the presence of excess soluble halide2, that the finer grained emulsions would contain much more adsorbed halide, the effect of which might neutralize, or 1 2
Germann and Hylan: J. Am. Chem. SOC.,45, 2486 (1923). Eastman Monograph, No. I , p. 27.
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even reverse, the purely dimensional effect of the grains. To quote Trivelli and Sheppard’, “The smaller the crystal, the larger its relative surface, and the more it is liable to contamination with dissolved and adsorbed foreign molecules.” In an emulsion, then, containing considerable excess of soluble halide we should expect the speed to be inversely as the adsorbed halide, or, in other words, directly pToporLiona1 to the grain size. Probably t h r difference Wightman, Trivelli and Sheppartl2 found between their own experimentally prepared emulsions and the Royal Standard Lightning Plate from Kodak Ltd. lies here, in the difference in excess of soluble halide. The effect of variation of size can also be explained by adsorption. Adsorption, in general, is proportional to the “specific surface” of the adsorbent. “Specific surface”, which may be defined as the surface per unit mass of material, is proportional to the dispersity. Again taking spheres as examples, a sphere of radius R would have a surface of 47rR2. As we have shown above, if we take an equal mass of spheres of radius r = R/z we would have 8 small spheres. The surface of each would be 4 m 2 , and the total surface of the 8 would be twice that of the single large sphere. Thus the specific surface is inversely proportional to the linear dimensions of the particles. We have taken spheres as examples because they have less surface per given mass than any other solid figure, and hence the ratios we have developed for them are a minimum. If the crystals were very irregular in shape the increase of surface per decrease of linear dimensions might be considerably greater. In emulsions, then, having considerable range in the size of the paiyticles, the smaller grains, present together with the larger ones, adsorb relatively more halide and leave the larger ones relatively purer than if there were only large ones present. The “ripening” effect of allowing emulsions to stand a t room temperature or higher for some time before use has been thought by some to be due to increase in size of grain, similar to “digesting” a barium sulfate precipitate before filtering. But to again quote Trivelli and 8heppard3, “modern highspeed emulsions, relatively coarse grained, are not produced by the ripening of emulsions which would otherwise be slow and fine grained. The two types are produced under relatively different initial conditions, and, as pointed out by Luppo-Cramer and Mees. are practically discontinuous.” And again, “ripening by way of recrystallization depends mainly on the elimination of adsorbed impurities.” As adsorption phenomena are more accentuated in silver iodide than in other silver halides, n study of the relations between size of grain and sensitiveness in plates of this material seemed worth while.
Experimental Various methods were attempted for preparing simultaneously two emulsions, of which one would contain relatively mall, and the other relatively 1
Eastman Monograph, No. I , p. 29. J. Phys. Chem. 27, 1-51 (1923). Eastman Monograph, No. I , p. 27.
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large particles of silver halide. The method finally found to be most successful for obtaining fine-grained emulsions consists, in general, of mixing concentrated solutions of silver nitrate and potassium halide. The most successful method for preparing coarse-grained emulsions consists of adding the required amount of crystalline silver nitrate to a dilute solution of potassium halide, and stirring the mixture only after the nitrate and halide have had time to react. I n order to get a fair comparison of the effect of size of halide grain on the speed of the emulsion, the emulsions compared must contain equal weights of silver halide. The maximum amount of silver which could be deposited from each would be the same. The excess of potassium halide in the two emulsions must also be equal in order to give equal Mass Action effects. Taking these facts into consideration, emulsions were prepared as described below. For the fine-grain erriulsion ggm of silver nitrate and ggm of potassium iodide were accurately weighed on the balance. The potassium iodide was dissolved in gcc of distilled water, together with something less than a gram of gelatin. The silver nitrate was dissolved in 3cc of distilled water and quickly added to the gelatin-potassium iodide solution. The resulting silver iodide emulsion was then added to 240cc of distilled water in which zogm of gelatin had previously been dissolved, and the whole stirred vigorously to insure thorough mixing. For the coarse-grain emulsion o.ggm of silver nitrate and o.ggm of potassium iodide were accurately weighed. The potassium iodide was dissolved in 25cc of distilled water, together with zgm of gelatin. When solution was complete the crystalline silver nitrate was added, and, after allowing time for the reaction of the iodide and the nitrat,e, the emulsion was thoroughly stirred. A slide for microscopic examination was prepared from each emulsion by pouring some of the warm fluid emulsion over the slide, rotating it to get it to spread evenly over the surface, and pouring off the excess. The emulsions were then painted on cards and set away to dry. When dry, a card from each was exposed for one minute and developed for five minutes in alkaline metol solution. In each case, the card having the large grains gave a fair picture, while the card having the small grains gave a faint picture or no picture a t all. A card from each emulsion was bathed for five minutes in metol sulfite sensitiser, dried, exposed for one second, and developed in alkaline sulfite solution for 30 seconds. I n each case the card having the smaller grains gave much the better picture. For details as to the metol sulfite sensitiser, the alkaline sulfite developer, and the alkaline metol developer the reader is referred to a previous article by the authors’ on “The Photographic Sensitiveness of Silver Iodide”. 1
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Photomicrographs of several of the experimental emulsions were also taken to serve as a record. The equipment used consisted of a Bausch and Lomb photomicrographic bench, mounted on a concrete pier to reduce vibrations to a minimum. The illumination was furnished by a 5 ampere handfeed arc light, fitted with a Bausch and Lomb aspheric condensor, and a Wratten filter. The microscope was a Spencer, fitt,ed with a 4mm oil-immersion 1.25NA objective, a 1oXHuygens occular, and a Spencer 1.3NA substage condensor. The pictures were taken on Wratten M plates.
Discussion It is to be noted that in every case where a distinct difference in the average grain size was observed, the emulsion having the coarser grains was the faster if not sensitised, but that the finer grained emulsion was the faster after sensitisation. This is in perfect harmony with the theoretically expected results as discussed in the introduction. The authors' in their article on "The Photographic Sensitiveness of Silver Iodide" have shown that the principal effect of sensitising these emulsions is the removal of adsorbed potassium iodide. In the unsensitised emulsion, then, the excess of potassium iodide is adsorbed on the silver halide grains, and the smaller grains adsorbing relatively more than the larger ones are retarded much more, so that, naturally more sensitive, they are actually rendered less sensitive than the larger grains. In the case of the sensitised emulsions the adsorbed potassium iodide has been removed and the relative sensitiveness of the large and small grains appears in its true light. As should be expected, the smaller-grained emulsions show the greater speed. Svedberg3 has found that the percentage number of grains made developable 'after a certain exposure increases approximately exponentially as the cross section of t8hegrains increases. He claims, however, that this rule holds only for grains formed under very nearly the same conditions, such as the grains within one and the same emulsion. He assumes that the product of the light action in the halide grain consists of small centers distributed thru the grain, or thru the light-affected part of the grain, according to the laws of chance, and explains his experimental results in the following manner. "A certain grain will become developed if it contains one or more developable centers. Now according to the laws of chance the percentage probability for thP occurrcnce of n centers in a grain is e-"vn Pn=roo--n!
where v is the average number of centers per grain. Thus the percentage probability that the grain will cont,ain a t least one center is P=100 (1-e-I.) Germann and Hylan: J. Am. Chem. Sor. 1. e. Phot. Jour. 62, 186 (1922).
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