2366
E. B. GUTOFF,P. H. ROTH,AND A. E. STEIGMANN
Vol. 67
COAGULATION OF SILVER HALIDE SUSPENSIONS I N THE PRESENCE O F GELATIN BY EDGAR B. GUTOFF,PETER H. ROTH,AND ALBERTE. STEIGMAKN Emulsion Development Laboratory, Polaroid Corporation, Waltham, Massachusetts Received M a y 14, 1963 Silver halide suspensions formed in the presence of gelatin are the main product of the photographic industry. This paper reports studies of some of the conditions affecting the agglomeration and coagulation of these suspensions a t the time of their formation. We have corroborated and extended the work of earlier investigators showing that coagulation is reduced by diluting the reagents, having ammonia present, raising the operating temperature, using more gelatin (up to a limit), and using less iodide in bromoiodide systems. I n addition, it was found that the effect of ammonia is mainly due t o its p H effect, that the iodide content of the agglomerate in a bromoiodide system is less than the average value, that the protective power of a gelatin decreases with increasing reactivity toward silver ions and is relatively independent of the gelatin isoelectric point and viscosity, and that coagulation is reduced a t higher levels of agitation. A mechanism of coagulation is suggested for this type of system which takes into account the role of gelatin and explains the observed phenomena in terms of the electrical charge on the particles and on the adsorbed gelatin.
Introduction The formation of silver halide crystals by mixing a silver nitrate solution with an alkali metal halide solution in the presence of gelatin is the first step in the production of photographic emulsions. Under certain conditions the silver halide crystals can agglomerate to form large clusters which make the “emulsion” unfit for photographic use. This paper is a study of the conditions affecting the agglomeration process. This agglomeration process is familiar to workers in the field. I n a 1930 handbook on photography, Jahr‘ mentions that sedimentation occurs if the silver and halide solutions are too concentrated or if not enough gelatin is present. It was generally known2 that ammonia, which is used to form ammoniacal silver nitrate for the preparation of some types of emulsions, peptizes the silver halide suspension, and that relatively high gelatin and iodide concentrations in the halide solution lead to agglomerates which were called “pepper.” Glafkides, in his recent text13 notes that agglomeration is reduced by working at higher temperatures, by diluting the silver nitrate solution, by increasing the gelatin content (in the range of 1-3%), and in mixed bromoiodide systems, by reducing the iodide content. Photographic emulsions normally contain relatively large crystals, on the order of 1 p in diameter. The microcrystals formed during the precipitation stage are allowed to grow or ripen to the desired size by holding the suspension a t elevated temperatures, with or without the addition of more reagents. However, agglomeration, if it occurs, takes place during or immediately after the precipitation ~ t a g e . ~If. ~the agglomeration is limited to very small clumps, it may be an important mechanism in the crystal growth process.6 Zharkov and Dobroserdova7 noted a tendency toward agglomeration when approximately stoichiomet(1) R. Jahr in A. Hay, Ed., “Handbuch der wissen u. angew. Photographic," Vol. IV, Julius Springer, Vienna, 1930, pp. 217-226. (2) A. Steigrnann, Phot. K o T ~ .67, , 97 (1931).
(3) P. Glafkides, “Photographic Chemistry,” Vol. I, Fountain Press, London, 1958, pp. 298-303. (4) A. Hirata, Nzppon Shashan Gakkaa Kazshi, 23, 71 (1960): Chem. Abstr., 65, 14135d (1960). (5) V. N. Zharkov and E . P. Dobroserdova, Zh. Nauchn. Prakl. Fotogr. a Kznematoer., 1, 250 (1956). ( 6 ) A. deCugnac and H. Chateau, Sez. Ind. Phot., 33, 121 (1962). (7) V. N. Zharkov and E. P. Dobroserdova, Zh. Nauchn. % Prtkl. Fotogr. i Kznematogr., 1, 170 (1956). (8) V. N. Zharkov and E. P. Dobroserdova, zbad., 1, 337 (1956).
ric quantities of silver and halide mere used, and also that agglomeration is most extensive in pure silver iodide emulsion^.^ The silver halide suspensions normally are prepared by the single jet technique, that is, by adding the silver nitrate solution to the halide solution containing gelatin and excess halide.3 However, less agglomeration occurs when one uses the double jet techniqueI3.l0in which the halide and silver solutions are added simultaneously to a gelatin solution and in which it is possible to keep the excess halide concentration constant a t all times. According to the Derjaguin-Landau-Verwey-Overbeek theory,11r12the stability of suspended particles is due to the charge repulsion of the electrical double layers, while the coagulating tendency is caused by London-van der Waals attractive forces. When the effective charge on the particles is sufficiently reduced, coagulation occurs. This charge reduction may take place by compression of the double layer caused by added electrolyte, with higher valency counterions having a greater effect. Tezak, et aZ.,13 approached the question differently. They explained the effects of higher valency coagulating ions by assuming ion pair formation between the coagulating counterion and the stabilizing ion in the double layer, with a consequent reduction in the surface charge. Mirnikl4 developed this same theory further. PackterI5 reconciled and combined these two approaches and estimated that in extreme cases the surface potential may be reduced by ion pair formation to 407, of its original value. I n general, silver halides have been studied only in dilute solutions and in the absence of gelatin. The school of Tezak, and then Matijevic, have made the most complete study. 13*16--27 They have accumulated these findings : (9) V. N. Zharkov, E . P. Dobroserdova, and L. X. Panfilova. ibid., 2, 103 (1957). (10) H. Amrnann-Brass in 1% Sauvenier, Ed., Scientific Photography, Proo. of the International Colloquium a t Liege, 1959, Pergarnon, New York, N. Y., 1962, pp. 276-290. (11) B. Derjaguin a n d L. Landau, Acta Physicochim. U R S S . 14, 633 (1941). (12) E. J. W.Verwey and J. T. Overbeek, “Theory of the Stability of Lyophobic Colloids,” Elsevier Publishing Co., New York, S. Y., 1948. (13) B. Tezak, el al., J. Phvs. Chem., 57, 301 (1953). (14) M. Mirnik, Nature, 190, 689 (1961). (15) A. Paekter, 2. phycrik. Chem. (Leipsig), 214, 63 (1960). (16) B. Tezak, E . Matijevic, and K. Schulz, J . Phys. Chem., 66, 1557 (1951). (17) B. Tezak, E. Rlatijevic, and K. Schulz, ibid., 56, 1567 (1951).
Nov., 1963
COAGULATION OF SILVER HALIDE SUSPENSIONS
(a) The coagulation concentration for a precipitating counterion is relatively independent of the silver ion Concentration (pAg), almost down to the isoionic point. 17.19 (b) The coa,gulation value of higher valency counterions, up to a charge of six, follows the expression of Tezak18 in that the log of the coagulating concentration decreases linearly with increasing valency.20$21Thus the coagulation conclentratioii drops from about 2 X 10-1 N for monovalent cations to about 9 X N for hexavalent cations in a given system. (c) Larger ions of the same valency have a slightly greater coagulating effect. l8 (d) The concentration of the silver sol has very little effect on the coagulation value of an added electr01yte.l~ (e) Using mixed electrolytes for coagulation, one can get additive, superadditive, and antagonistic effects.22 Some of the deviations from additivity can be explained by ionic association or by hydrolysis of the hydrated ions. (f) Small amounts--say 0.5-1OJo-of gelatin strongly retard the coagulatia'ri of silver halides. However, the gelatin has very little effect on the isoelectric maxima, where neither silver nor halide is in appreciable excess and the sol is ~ncharged.13.2~hluch of the above work has been summarized by Matijevic in a German paper.24 (g) At elevated temperatures the general picture is the same, but the stable sol region is narrower.25 Furthermore, the isoionic point of the sol, which normally is on the excess silver side of the equivalence point due to the greater adsorption of the halide ion, shifts towards the equivalence point because of the more nearly equal adsorption of silver ions.26 (h) Negatively charged silver iodide is least stable against multivalent positive coagulating ions, and negatively charged silver chloride is most stable. The reverse is true for monovalent coagulating ions. (i) The coagulating effect is greater, the more insoluble the salt formed by the coagulating ion and the stabilizing i ~ n That . ~ work ~ ~was~ done ~ using salts of different organic acids to coagulate positive silver halide sols. (j) Large simple organic cations, such as amines and materials having a quaternary nitrogen, have a much stronger coagulating action than n7ould be expected from their valence. Monovalent materials behave as simple tri- or tetravalent ions, and like them can reverse the charge and stabilize a negative ~ 0 l . ~ ~ ' Tamaki 2~ has shown that divalent organic cations have an even greater effect and that the sol concentration is important.30 The strong coagulating and charge reversal action is due to the adsorption of these mat'erials onto the silver halide.28,31Materials with more alkyl groups (18) B. Tezak, E. Matijevic, and K. Schulz, 1. Phvs. Chem., 69, 769 (1955). (19) M. Mirnik, F'. Flajsman, K. F. Schulz, and B. Teaak, ibid., 60, 1473 (1956). (20) E. lllatijevio and M. Kerker, ihid., 62, 1271 (1958). (21) E. Matijevic, D. Broadhurst. and &.IKerker. . ibid., 63, 1552 (1959). (22) B. Tezak, et al., Proc. Intern. Congr. Surface Activity, bnd, London, 3, 607 (1957). (23) B. Tezak and S. Kratohvil-Babic, Arhin Kern., 24, 67 (1952). (24) E. Matijevic, Chimia (Aarau). 9, 287 (1955). (25) G. Deaelic and B. Teaak, Croat. Chem. Acta, 30, 119 (1958). ( 2 6 ) J. Herak and B. Tezak, Arhiu Kern.. 27, 49 (1955). (27) J. Herak and B. Teaalr, ibid., 26, 1 (1954). (28) E. Matijevic and R. €1. Ottewill, J . Colloid Sci., 13, 242 (1958). (29) V. Pravdic and M. Mirnik, Croat. ChPrn. Acta, 82, 1 (1960). (30) K. Tamaki, Kolloid-Z., 177,45 (1961).
2367
on the with larger head groupslZ8and with greater chain lengths (up t o a limit)33have a greater coagulating and charge reversal action. Above the critical micelle concentration these materials completely protect the nIathai and O t t e ~ i 1 1showed ~~ that nonionic surfactants also are strongly adsorbed, and tend to protect the sol, therefore requiring larger quantities of electrolyte for coagulation. Ottewill, et uZ.,36-38have developed a theory for the coagulation by large organic ions, and along with Pravdic and Mirnik,29have shown the strong effect of these ions on the { potential of the system. Barr and D i c k i n ~ o nfound ~ ~ that the {-potential of a silver bromide sol is a function of particle size. The isoelectric pAg of a 1 ,u sol is 5.15, of a 1000 p sol 4.17, and the largest particles resisted positive charging by either excess silver ions or cationic compounds such as quaternary salts. Gelatin above its isoelectric pH is anionic and increases the isoionic pAg of the 40 while below the isoelectric pH it is cationic and decreases the isoionic pAg of the sol. Herz and Helling40 also showed that raising the pH of a pure aqueous bromide or bromoiodide sol increases the isoionic pAg, probably because of the adsorption of hydroxyl ions, as demonstrated by Nicolae and L a b a ~ . ~ l The studies reviewed above are helpful in understanding the more complex photographic emulsion containing concentrated silver halide suspensions in the presence of gelatin. Our experimental program to increase this understanding now will be described. Experimental Measurement of Agglomeration.-To measure the extent of agglomeration, two methods were used. One consists of examining a drop of the silver halide suspension under a 400 power microscope. This method can be used to rank different degrees of agglomeration approximately, but does not give a quantitative value. In the other method the volume of settled agglomerates is measured in a graduate. The suspension is first filtered through a silk screen having an average pore diameter of 150 ,u, and the coarse agglomerates retained on the screen are then gently washed three times with 500-ml. portions of distilled water a t 35'. The agglomerates not peptized by the washing are transferred to a 25ml. (or larger if needed) graduated cylinder and allowed to settle for 5 min. The volume is iread directly, and these volume measurements are reproducible to within about 1 ml. There are disadvantages of this technique. (a) The measured agglomeration depends greatly on the settled density of the material. Thus, in the complete absence of gelatin there will be complete and total agglomeration, and yet the volume of the agglomerates is low due to the high settled density. However, the packed density should not vary too g e a t l y with gelatin present. (b) Small agglomerates will pass through the screen. (e! Some fraction of the agglomerates will be peptized during washing. In spite of these disadvantages, this measure of the volume of agglomerates can be a useful tool in studying these systems. Turbidity measurements did not prove to be useful in these concentrated systems containing up to 10% of gelatin. (31) R . W.Horne, E. Matijevic, R. H. Ottewill, and J. W. Weyrnouth, ibid., 161, 50 (1958). ( 3 2 ) K. Tamaki, ihid., 170, 113 (1960). (33) B. Tamamushi and K. Tamaki, ibid., 163, 122 (1959). (34) K. G. Mathai and R. H. Ottewill, ibid., 185, 55 (1962). (38) R. H. Ottewill, M. C. Rastogi, and A. Watanabe, Trans. Faraday Soc., 66, 854 (1980). (36) R. H. Ottewill and hZ. C. Ramtogi, ibid., 56, 866 (1960). (37) R. H. Ottewill and M. C. Rastogi, ibid., 5 6 , 880 (1960). (38) R. H. Ottewill and A. Watanabe, Kolloid-Z., 170, 132 (1960). (39) J. Barr and H. 0. Dickinson, J . Phot. Sci., 9, 222 (1961). (40) A. H. Herz and J. 0. Helling, J . Colloid Sci.. 16, 199 (1961). (41) M. Nicolae and V. Labau, J . Phot. Sci., 10, 170 (1962).
E. B. GUTOFF,P. H. ROTH,AND A. E. STEIGMANN
2368
-E,
20/~.,
d
added. After 2 min., solution B is dumped in while agitating the mixture. After 5 min., the silver halide suspension is cooled t o 35" and the volume of agglomerate is determined. If the measurement of agglomeration is done visually with the microscope, the cooling may be omitted.
With NH40H
&P- \
HzSO,
\
y
I
\
PH.
Fig. 1.-Effect
25
of pH on the volume of agglomerates a t 60".
t
1
a
\o
I O C
O 40
I 45
50
I
I
I
55
65
70
I 60 Temperature, "C.
Basic Formulation.-A published bromoiodide formulation42 was slightly modified and, in order to obtain measureable quantities of agglomerates, the water content was greatly reduced. I n many cases some of the remaining water was replaced by ammonium hydroxide. Because agglomeration takes place during or immediately after the nucleation ~ t a g e ,as ~ ,mentioned ~ in the Introduction, only one rapid addition of silver nitrate was used. The halide content, a t 470 mole % of the total silver, and the iodide content, a t 10 mole % of the total silver, is therefore much higher than in any complete emulsion formulation. Measurements were made 5 min. after the silver nitrate addition. The basic formulation is Solution A Water plus ammonium hydroxide Ammonium bromide Potassium iodide Gelatin Solutioii H Water Silver nit,rate Solution B is preheated to 40'
225 ml. 40 g. 1 . 5 g. 8 g. 30 ml. 15 g.
Solution A in an 1800-ml. beaker is brought to temperature, llormally and then t,he hydroxide, if used, is .
..
(42) A . St~eigir~nrin, C(~i,,rr.Ch17th. Jnrl. Phot., 7 , 120 (lU3lj).
isBB , , ~ ~ s n c l a6. ,5
Results pH and Ammonia Concentration.-Figure 1illustrates the reduction of agglomeration on increasing the pH from 5.0 to 11.5 using both amnionium and sodium hydroxides. With ammonium hydroxide the total volume was kept constant, but with sodium hydroxide considerable dilution took place. For a pH of 11.5, 210 ml. of 2 N caustic was used. A succinyl-derivatized limed bone gelatin43with an isoelectric point below pH 5.0 mas used. In one test the ammonium bromide was replaced by an equivalent amount of potassium bromide, and a t a pH of 11.5, volume of agglomerates increased from 0 to 6.5 ml. Temperature.-When using the basic formulation with 30 ml. of 2 N ammonium hydroxide to give a pH of about 8.2, increasing the temperature from 40 to 70" greatly reduced the volume of agglomerates, as shown ill Fig. 2 , Agitation and Addition Rates.-Reducing the degree of agitation increased the volume of agglomerates, as shown in Table I. Microscope studies showed that it made no noticeable difference whether the silver nitrate is dumped rapidly or added over a period of 15 sec. TABLE I EFFECTOF AGITATIOS ON AGGLOMERATION Standard formulation with 30 ml. of 2 N ammonium hydroxide R.p.m. of paddle
No. of baffles
Amount of agglomerate, ml.
180 180 90 90
4
18.0 18.5 25.0 26.5
1
4 1
TABLE I1 EFFECT O F GELATIN CONCENTRATION ON AGGLOMERATION Xeutral, more dilute formulation
Fig. 2.-Effect of temperature on the volume of agglomerates, using a formulation containing ammonia.
.
Vol. 67
Gelatin
Succinyl-derivatized limed bone Limed bone Acid pig
7Wt. % of halide 6olution-Concn. range Optimum concn.
0.09-7.0 0.09-7 . O 0.09-10.0
2.0-4.0 4
7
Concentration of Reagents.--A full factorial design of 81 experiments was run using the same basic formulation containing 30 ml. of 2 N ammonium hydroxide and keeping the total volume constant. Each of the following factors was varied over the listed three levels, and the middle level is that of the basic formulat1on. Final tot,al silver concentration, moles/l. Weight ratio of gelat,in t,o silver metal T'olume ratio of silver nitratc phase to bromide phase Mole ratio of bromide t o silver
0.147
-0 293
-0 586
0.42
-0.84
-I
0.076
-0.218 - 4 62
-0.433 -9.24
2.37
68
The results of this series were analyzed statistically t,o determine significance. The data show these results. (a,) 'Hie i ~ ) l u i u eof aggloinerates is directly related
(lQ35); N r i . I n d . (,I:$)
Ir. C. Yutdy atid (i. V. I'raiue,
U.S.I k t c n t 2,fil
Nov., 1963
COAGULATION OF SILVERHALIDESUSPENSIOSS
to the total quantity of silver nitrate added, as shown in Fig. 3a. (b) Diluting the silver phase with water from the bromide phase greatly reduces agglomeration, as shown in Fig. 3b. This has been confirmed by other experiments which show that, to eliminate agglomeration, it is more effective l o dilute the silver nitrate phase than to add the same volume of water to the bromide phase. (c) The ratio of the halide ion concentration in its phase to the silver ion concentration in its phase appears to play an important role, as shown in Fig. 3c. Despite the sca,tter of the data, one can see agglomeration is at a maximum when the bromide ion concentration is just slightly less than the silver concentration, and falls off more rapidly on the dilute silver side than on the dilute bromide side. (d) The mole ratilo of bromide to silver and the total bromide concentration were also statistically highly significant, but this is probably due to the strong effects of factors already described in (a) and (c). (e) The effects of gel to silver ratio and its interactions, such as total gel concentration and gel to bromide ion concentration, were not statistically significant. In other tests, however, usiiig visual examination under the microscope to determine agglonieratioii it wa,s found that agglomeration mas reduced with increasing gelatin concent rations. Above an optimum concentration the agglomeration slowly increased. The data are presented in Table I1 for an ammonia-free formulation that was made more dilute in order to have only slight amounts of agglomeration. (f) The agglomerates tended to be coarser at higher silver nitrate concentrations. Increasing the iodide content up to double its standard value gave increased agglomeration as observed under the microscope. Decreasing the iodide content to zero lowered the volume of agglomerates from about 23 to 9.5 ml. I n these cases a neutral formulation was used. Additional Silver Nitrate Solution.-In this test an ammonia-free, all bromide formulation was used. Silver nitrate of the same coiicentration as the basic formulation was used, but the amounts were 0.25, 0.50, 1.0, and 2.0 times that of the basic formulation. The volumes of agglomerates were 3.5, 6.9, 9.5, and 16.5 ml. The volumes of agglomerates did not increase as rapidly as did the volumes of silver nitrate used, showing that the silver nitrate in the later stages of the dump addition gives rise to a lesser volume of agglomerates than the first stages. Composition of the Agglomerate.-In one test where the iodide content of the system was 10.2 mole % based on silver, the washed agglomerate had only 6.5 mole % iodide based on silver, and the nonagglomerated suspension passing through the silk screen analyzed 9.3 mole % iodide based on silver. A silver analysis showed that the agglomerate was 98% silver halide, and a semimicro Kjeldahl analysis showed that the adsorbed gelatin content could not be over 1% based on silver. Effect of Different Gelatins.--The effect of different gelatins on agglomeration was t ested in several mays. ['sing the basic forinulatioii with 30 nil. of 2 N ani-
0.2
0.I
0.3
2369
0.6
0.4
0.8
1.0
F i n a l Total Silver Concentration, moleslliter.
Fig. 3a.-Effect of total quantity of silver on agglomeration using a formulation containing ammonia. Each point is the average of 27 results.
monium hydroxide at 50" and measuring t'he volume of agglomerate gave the results presented in Table 111. TABLE I11 ACTIONOF DIFFERENT GELATINS ON AQGLOMERATION Basic formulation a t 50" with 30 ml. of 2 N. ammonium hydroxide Isoelectric PH
Viscosity of 10% s o h . at 40°, cp.
Volume of agglomerate, ml.
9.3 4.7 6.0
15.8 21.0 5.9
22.0 17.0 5.0
4.2
22.5 4.5
18.0 15.0
Limed ossein As is Phthaloyl derivatized Acid degraded
5.0 4.0
15.6 18.0 3.5
19.5 17.5 13.5
Limed calf hide
5.1
15.6
17.5
Gelatin
Acid pig As is Succinyl derivatized Ammonia degraded Succinyl derivatized limed ossein As is Acid degraded
Similar experiments were performed in more dilute systems where much less agglomeration results, and the gelatins were ranked by microscopic examination. In both neutral and ammonia systems, it was found that: (a) an acid pig gelatin with an isoelectric pH of 9.0 and a limed ossein gelatin with an isoelectric pH of 5.0 both have abolut. the same agglomerating tendencies; (b) derivatized gelatins-namely gelatin derivatives made with phthalic anhydride or benzene sulfoiiyl chloride-have greater agglomeration teiidencies and give more agglomeration a t higher degrees of derivatizat'ion; (c) an alkali-degraded gelatin which inhibited silver halide crystal growth gave very large quantities of agglomerates. The size of the individual agglomerates was small when using this gelatin in a formulation with ammonia. Gelatin in the Silver Nitrate.--An ammonia-free, all broniide forniulation was used, wit I i a deioiiizcxl
E. B. GUTO~W, P. H. ROTH,ASD A.
2370
E.STEIGMANN
Vol. 67
TABLE IV EFFECTOF SURFACTANTS ON AGGLOMERATION Surfactant
Control Igepon T-77 [sodium N-methyl-N-oleoyl taurate] Ethoquad 0/12 [oleylmethyldi (hydroxyethy1)quaternary ammonium chloride] Igepal CO-530 [nonyIphenoxypoIyoxyetherethanol]
Ionic charge
Concn., M
Minus
0.003
Plus Zero
2c
IE
-E v)
+ 0
&
IC
E rn rn
a
u0 0
-s
5
0
>
0
U
0.06 0.08 0.1
L
0.2
.
U 0.3
C
0.4
Volume Ratio O f Silver Nitrate Phose t o Bromide Phase,
Fig. 3b.-Effect of the distribution of water between the silver nitrate phase and the bromide phase. Each point is the average of 27 results.
.002 ,003
i'I
2-Propanol
19.0 16.0
Sudsy
13.0 53.0
Rapid settling Sticky
TABLE V Amount of agglomeration by microscope
Zero (control)
Slight amount About same as control More than control More than control More than control About same as control More than control
10-6 10-6
0
NaCl
Type of agglomerates
Over-all concentration of €IrSiW120io,
10-7
20/
Volume of agglomerates
halide phase of the standard formulation, and the ammonia was used. The results are presented in Table IV. I n a neutral formulation, by microscopic examination neither 0.0'2 ill sorbital (a nonionic material) nor the same weight of Kekal BA-75 (a sodium alkyl naphthalenesulfonate) had a noticeable effect on agglomeration. B. Electrolytes.-Several anionic heteropoly salts were added to the silver to try to reduce agglomeration by rapidly reversing the charge of the positive silver halide salts formed in the silver nitrate stream. A neutral formulation was used, with 50 g. of potassium bromide in place of the ammonium bromide and with 6 g. of a limed ossein gelatin. The silver nitrate was diluted to give just a slight amount of agglomeration. amAmmonium 9-phosphotungstate, ("4)d32W~&~, monium nickel-3-molybdeno-3-tungstate, (NH4)&iMo31V~02~,and 12-silicotungstic acid, H4SiW12040, were used. A precipitate formed when the nickel molybdenotungstate was added to the silver nitrate. At over-all concentrations of M none of the salts helped. The effects of silicotungstic acid are shown in Table V.
10-8
0
Impurities
10-4 10-3
E
a IO 8
I 0.I
I
I I I I I I I
I
I
3 Normality of NHsBrlNorrnality of AgNO,
0.2
0.3
0.5
0.7
1.0
2
I
.
I
4 5
Fig. 3c.-Effect of the ratio of bromide concentration to silver nitrate concentration. Each point is the average of 9 results.
limed ossein gelatin in order to eliminate the formation of a precipitate between impurities (such as chloride or sulfate) in the gelatin and the silver. The volume of each phase was kept constant. The results are given here in tabular form. Grams of gelatin in bromide phase Grams of gelatin in silver phase Amount of agglomeration by microscope
8 0 Little
4 0
4 4
Much Very little
Effect of Additives. A. Surfactants.-Anionic, nonionic, and cationic sinfactants were added to thc
A test was run which confirmed published results18 that this heteropoly salt will discharge and coagulate aiid lop6 N a positive silver bromide sol between and mill reverse the charge and stabilize the sol at concentrations above lo-&A'. The silicotungstic acid did not reduce the coagulation of a negative silver bromide or bromoiodide sol in gelatin-free systems and systems diluted 100-fold. It did not matter whether the acid was added to the silver phase or to the bromide phase. Gelatin Tests.-To test the suggestion of l l a t i j e ~ i c ~ ~ that the agglomerating effect of diflerent gelatins might be relatcd to their ability to react with silver ions, several gelatins were titrated with silver nitrate. The results are tabulated in Table VI. The data show that the derivatized gelatins give rise to higher pAg values than the nonderivatized gelatins. This means they tie up more silver ions than do the nonderivatized gelatins. Discussion Coagulation of a sol takes place when the charge 011 the particles, and therefore the electrostatic forces of (44) E. Matqevic, personal coinmuorcation, Oct. 10, 1962.
COAGULATION OF SILVER HALIDESUSPENSIONS
Nov., 1963
TABLE VI TITRATIONS OB GELATINS WITH SILVER NITRATE Samples of 100 ml. of gelatin solution were used. Meter readings of pAg are given for the water blank. For gelatins, the differences from the water value a t the same titer are given. Vol. 0.01 Ai AgNOa using 1.5% gelatins, ml.
Water blank
,--Regular Acid pig
0 5 10 20
6.95 3.83 3.58 3.33
$1.50 + O . 41 .22 .ll
+ +
+ O . 14 .87 .43 .26
+ + +
$0.76 $1.07 $0.64 .34
+
+0.37 +1.22 +0.75 .42
+0.37 +1.11 +0.72 .41
+
$0.62 $1.59 $1.04 +0.58
$1.70 $2.00 +1.48 t0.93
$1.28 +2.00 +1.45 $0.91
+0.20 $ .79 .44 .25
$0.05 +l.62 +1.08 $0.59
$1.16 +1.75 +1.28 +0.76
$0.79 $1.98 +1.48 +0.91
gelatins-Limed bone
YDerivatised gelatinsLimed Acid bone pig
+
Using 2.5% gelatins
0
5 10 20 Using 3.0% gelatins
0 5 10 20
+ +
repulsion, are reduced almost to zero. In gelatinsilver halide photographic emulsions the silver halide particles are formed where the silver nitrate jet enters the halide solution, in a region of high silver concentration, and therefore are initially positively charged. The halide concent r:ztion varies from zero in the silver nitrate jet to full strength in the bulk solution, while the silver concentration is full strength in the jet and almost zero in the bulk solution. The postively charged silver halide particles formed in the vicinity of the jet must migrate to the bulk solution with its high excess halide concentration, and so the original positive charge on the particles must decrease to zero and then be reversed before the suspension is stabilized. We feel that coagulation takes place only during this period before the particles reach the bulk solution and are stabilized. The gelatin in the system plays an important role in the prevention of agglomeration. It stabilizes the silver halide suspension, for without gelatin complete coagulation 1akes place. For example, the ammonium ion concentraltion in the basic formulation used here is much higher than the 0.2 N monovalent cation concentration that is sufficient to coagulate a silver halide sol. However, gelatin also can exert a coagulating effect. Gelatin a t concentrations under 0.3% flocculates silver It is thought that the polar groupings halide of the gelatin are adsorbed to the polar surface of the silver halide, leaving the predominantly hydrocarbon groups facing outwards. Aqueous suspensions of these particles with organophilic surfaces would coagulate. 45 On further additionti of gelatin, the hydrocarbon groups of the gelatin would be attracted to the hydrocarbon surface, leaving the charged polar groups facing outward and thereby stabilizing the particles. This same reasoning is used to explain the stability of negative silver halide sols in higher concentrations of cationic surfactants.28 On the other hand, Pouradier and Roman46believe that the added gelatin, not having (45) S. E. Sheppard, R. H. Lambert, a n d D. Swinehart, J. Chem. Phys., 13, 372 (1944). (483 J. Pouradlei and J. E oman, Scz. Ind. Phot., 23, 4 (1952).
2371
sufficient space on the surface, is just partly adsorbed with part of the molecule, including some of the polar groups extending outward. This also explains the increased adsorption a t higher molecular weights. We think that a t least part of the agglomeration is due to the coagulating action of the low concentrations of gelatin in the vicinity of the silver nitrate jet. As evidence, we have noted in the results that a demineralized gelatin added to the silver nitrate appears to reduce agglomeration. The over-all process of stabilization might well be a result both of the charge reversal by the excess halide and of the adsorption of additional gelatin in the bulk solution. The decrease in agglomeration with increasing gelatin content shown in Table I1 agrees with previous photographic emulsion e~perience.~The higher gelatin concentrations reduce the time interval during which the silver halide particles in the neighborhood of the silver nitrate jet are exposed to the low coagulating quantities of gelatin migrating toward the jet. The slight decrease in the protective action of the gelatin a t still higher concentrations noted in the same table has also been reported previously.2 This may well be due to the increased viscosities of the more concentrated gelatin solutions reducing the effectiveness of agitation and also reducing the diffusivities of the various components. Because of these opposing effects the degree of agglomeration goes through a maximum with increasing gelatin content. However, the effect of gelatin viscosity is not great compared to the other gelatin properties and hardly affects the over-all protective power of the gelatin, as seen in Table 111. Gelatin, like all proteins, is amphoteric with both anionic carboxylic groups and cationic amino groups. Under alkaline conditions above the isoelectric pH more of the carboxylic groups are ionized and therefore the gelatin carries a net negative charge. Below the isoelectric pH the protonated amino groups give the gelatin a net positive charge. Gelatin, a t low concentrations, has its greatest coagulating action on dilute silver bromide sols a t or near the isoelectric point46 which is the pH where the net charge on the gelatin molecule is zero. At high and low values of pH there is less a g g l ~ m e r a t i o n . ~On ~ negative quartz suspensions, Kragh and L a n g ~ t o nfound ~ ~ that the coagulating action of small amounts of gelatin is greatest when the pH is just slightly below the isoelectric pH, about 4.8 for gelatin of isoelectric pH of 5.0 and about 6.0 for gelatin of isoelectric pH of 8.4. The small net positive charge with both gelatins is about the same.47 At the pH of the greatest agglomerate formation the adsorption of the gelatin is a t a maximum. That the electrophoretic mobility of the quartz suspension in aqueous gelatin is not a t a minimum a t the optimum pH for flocculation demonstrates that the net charge on the particles plus adsorbed gelatin is not the major factor in agglomeration. The authors suggest that the main effect of pH is on the configuration of the adsorbed molecules and believe Coagulation is caused by gelatin molecules bridging the gap between particle^.^' However, the explanation presented earlier that coagulation takes place a t low gelatin coverages when the surface is organophilic, and is retarded when the surface is highly polar, seems more reasonable. At or near the iso(47) A. M. Kragh and W. I?. Lungston, J. Collozd Sri., 17, 101 (1962).
2372
E. B. GUTOFF,P. H. ROTH,AND A. E. STEIGMAKN
electric pH the ionization of the gelatin molecule would be a t a minimum and so the surface would be least hydrated and most organophilic. The gelatin coatings of two particles then would attract each other. As the pH diverges from the isoelectric point the gelatin coating would become more and more charged. Therefore, the coated particles would tend to repel each other, irrespecbive of the net charge on the complete gelatinparticle system. Also the polar gelatin would now be more highly hydrated and this too would aid disperion.^^ This discussion adequately explains the pH behavior observed here. The volume of agglomerates decreases as the pH rises above 5.3, as shown in Fig. 1. The isoelectric point of this gelatin is under pH 5.0. One point indicates that agglomeration may also decrease as the p H is lowered; but this could not be confirmed with this derivitized gelatin which, by itself, flocculates a t lower values of pH. The action of both ammonium and of sodium hydroxides on agglomeration, as shown in Fig. 1, seems to be mainly of pH. The slight displacement of one curve from the other is due to slight differences in experimental conditions, for these differences persist even when no alkali is added (at a pH of about 5.3). The complexing and solubilizing action of ammonia on silver does not seem to be important, although one test a t a pH of 11.5 indicates that ammonium bromide may give less agglomeration than potassium bromide. The isoelectric point of the gelatin does not seem to be of major importance in differentiating between the protective action of different gelatins, as shown in Table 111. The one gelatin which gives markedly less agglomeration had been degraded by ammonia. Microscopic examination of the dispersion formed using this gelatin in a neutral formulation showed only slightly less agglomeration than when using other gelatins. The agglomerates were smaller in size, however, and so in the test reported in Table I11 many clumps probably passed through the 150 p opening in the silk screen used in filtering out the agglomerates. As a class, the derivatized gelatins give rise to more agglomeration than the nonderivatized gelatins. From Table V one may note that the derivatized gelatins react more with silver than do the nonderivatized gelatins. Herak and Tezak,26*27 in studying the effect of monovalent organic anions on coagulation, have shown that those that form more insoluble silver salts more readily coagulate a positive silver halide sol. These reactive gelatins undoubtedly are adsorbed onto the surface of the silver halide particles to a greater extent, The agglomerating tendencies therefore would be stronger, just as higher molecular weight gelatins are more strongly adsorbed and more readily agglomerate silver halide sols.46 Increased agitation rates reduce agglomeration, as shown in Table I. This may be a result of several factors: reduction of the time interval between particle formation and stabilization; reduction of the time iiiterval that two particles are in the neighborhood of each other; and irreversible peptization of the agglomerates coagulated by the gelatin, as has been noted in the case of quartz suspension^.^^ ildding the silver nitrate at a slightly lower rate, over 15 SPC. instead of a rapid dump, did riot noticeably af'fect agglonieratioii.
1701.
61
The addition rate would not be expected to have much effect on the length of the period of instability. Diluting the system reduced agglomeration by decreasing the probability of particles approaching each other before they are stabilized in the bulk solution. The silver ion concentration in the neighborhood of the particles must drop from its initial value in the vicinity of the silver nitrate jet almost to zero before the particles are stabilized. Therefore decreasing the silver nitrate concentration is very effective in reducing the period of instability and also agglomeration. Decreasing the bromide ion concentration, however, has two counteracting effects. One is that of greater dilution, which would be expected to decrease agglomeration by increasing the particle separation. The other effect of bromide dilution is to increase the time interval before the particle reaches a high enough bromide concentration to give it a stable negative charge. These counteracting tendencies probably account for the maximum in Fig. 3c, where the volume of agglomerates is plotted against the ratio of bromide ion to silver ion concentrations. The greater effectiveness of diluting the silver is demonstrated in Fig. 3b. Figure 3a, showing that agglomeration increases with the amount of silver nitrate added, indicates that agglomeration occurs throughout the whole dump addition. It was also shown in the Results, however, that a greater volume of agglomerates is formed in the earlier stages of the dump addition, probably because of the greater dilution of the system towards the end of the addition. This does not contradict Fig. 3c, which indicates that under certain conditions dilution of the bromide can reduce agglomeration, while under other conditions it can increase agglomeration. In a bromoiodide system the heavier and larger iodide ions would diffuse more slowly to the region of the silver nitrate jet. This could explain the observation that the agglomerate contains less iodide than the over-all system, as shown in the Results. Only when the particles reach the bulk solution would the full iodide concentration be available to the growing particles. It is also possible that silver iodide may have a lower rate of nucleation than the silver bromide, but this has not been determined. On the other hand, it is possible that silver iodide does form first but that the size of the particles and agglomerates is small enough to pass through the 150 p screen. Our observation that higher iodide contents cause increased agglomeration confirms the findings reported in the l i t e r a t ~ r e and ~ . ~ is in agreement with a published study showing that pure silver iodide emulsions have more agglomeration than bromide or chloride emulsion^.^^^-^ Tezak, et al., have shown that negative silver iodide sols without gelatin are slightly less stable towards multivalent coagulating ions, and slightly more stable toward monovalent coagulating ions than negative sols of pure bromide or chloride. Bromide and iodide sols are equally stable toward cationic surfactants**and both are less stable than the chloride. The positive silver iodide sol is the most stable toward the sulfate These results do not help in explaining the higher agglomerating tendencies of iodide in bromoiodide systemscontaining gelatin. Perhapsmixed crystals are less stable than pure crystals because of distortions of the lattice. Perhaps the adsorption of
Nov., 1963
THERMODYNAMIC PROPERTIES
gelatin and ids coagulating action is different on silver bromide and silver iodide sols. The amount of gelatin irreversibly a,dsorbed onto silver bromide is less than 10/04646 and is less than our analytical techniques could measure. Increasing the temperature of the system has multiple effects. Fjrst, it reduces the viscosity of the system, and this, a5 explained earlier, should reduce the agglomeration by facilitating the movemeiit of the initially formed positive particles to the bulk solution where they become stabilized. Second, the adsorption of gelatin should be reduced a t higher temperatures, and this should reduce agglomeration. Third, the solubility of silver salts of gelatin are increased, which should again reduce the adsorption and therefore the agglomeration. Fourth, the solubility of the silver halides is increased and so the silver ion concentration does not have to be reduced to the same extent to stabilize the particles with the bromide ions. Fifth, relative to the bromide ions, the silver ions are more readily adsorbed than at lower temperatures, as evidenced by the shift of the isoionic pAg from the silver excess region towards the equivalence point. 26 Added polyvalent anions-the heteropoly acids and salts-did not reduce agglomeration. However, Mathai and O t t e ~ i 1 have 1 ~ ~ shown that in the presence of nonionic surfactants the effect of coagulating electrolytes is greatly reduced, and above a minimum concentration (the critical micelle concentration for surfactants) is nil. I t would appear that the adsorbed gelatin reduces the effect of added electrolyte. However, in the absence of gelatin, where coagulation was much greater, and in the presence and absence of gelatin using solutions diluted 100-fold, the heteropoly salts still did not reduce agglomeration noticeably. We suggest that in the presence of gelatin, the gelatin reduces the effect of added electrolyte, while in the absence of gelatin the over-all system is unstable be-
OF
3-AZABICYCLO NOSANE NO SANE
2373
cause the over-all electrolyte concentration makes the particles coagulate. Added surfactants, a8 shown in Table IV, did not reduce agglomeration materially, and in one case increased it. The cationic surfactant did reduce the volume of agglomerates, but the agglomerates settle rapidly and probably had a much higher packed density. One would expect a cationic surfactant to tend to flocculate particles protected by an anionic gelatin. The anionic surfactant had little effect, for it would not act much differently than the anionic gelatin. The nonionic material greatly increased the volume of agglomerate. We suspect that it could have been preferentially adsorbed and yet, because of its lack of charge, it may not have protected the particles sufficiently. Coiiclusions To summarize, we have confirmed earlier work that agglomeration of silver halide emulsions may be reduced by: (a) diluting the silver nitrate; (b) working at a pH above neutrality as by using ammonia; (c) working a t higher temperatures; (d) increasing the gelatin content (up to a certain point) ; and (e) reducing the iodide content in bromoiodide systems. We have also found that: (a) the isoelectric point and viscosity of the gelatin have little effect on agglonieration during the precipitation stage; (b) increased agitation reduces agglomeration; (c) the iodide content of the agglomerate in a broimoiodide system was less than the average value; and (id) the agglomeration tendency of gelatins increases with increasing reactivity toward silver nitrate. Acknowledgment.-The exploratory work of Jerome Reid is gratefully acknowledged. Much of the experimental program was cairried out by Richard Varney and George Whitehouse. Analyses were done by Peter Kliem. The heteropoly salts were kindly supplied by Prof. Matijevic of Clarkson College.
HEAT CAPA.CITIES AND THERMODYNAMIC PROPERTIES OF GLOBULAR MOLECULES. V. 3-AZABICYCLO [3,2,2]NO;"\'ANEFROM 5 TO 350°K. BY
CAROLYX
31. B.4RBER
ASD
EDGAR F. T~ESTRUM, J R . ~
Deparlmenl of Chemistry, Unzuersity of Michigan, Ann Arbor, Michigan Received M a y 14, 2963 The heat capacity of the globular molecule 3-azabicyclo[3,2,2]nonanewas determined by adiabatic calorimetry from 5 to 350°K. A transition to the plastically crystalline ("rotator") state was found a t 297.78'K. with an associated transitional entropy increment of 11.63 cal./mole-OK. At 298.15"K.the entropy ( S O ) , the enthalpy - H'o)/T], and the Gibbs free energy function [ ( G O - H " o ) / T ]are 56.14, 33.39, and -22.75 function [(HC cal./mole-'N:., respectively.
Introduction In conjunction with a series of studies on the thermodynamics of the transitions involved in the formation and fusion of the "p1,astically crystalline" or "rotator" phaseJ2+ the family of molecules of which bicyclo[2,2,2]octane is the prototype has proven interesting for study. Previoudy, triethylenediamine (1,4-diaza(1) To whom correspondence concerning this work should be addressed. S.8. Chang and E. F. 'Weatrum, Jr., J. Phgs. Chem., 64, 1547 (1960). (3) S. S. Chang and E. F. Westrum, Jr., i b i d . , 64, 1651 (1960). (4) S. S. Chang a.nd E. F. 'Westrum, Jr., ihid., 66, 834 (1962). ( 5 ) Jl. H. I'ltyne and E. F'. Weahrum, Jr., ( b i d . , 66, 748 (1962).
bicyclo [2,2,2]octane) has been studied over the low3 and intermediate6 temperature ranges, and low temperature data on iiorbornylane have received preliminary mention by Guthrie and McC~llough.~Another member of the family, 3-azabicyclo [3,2,2]iionane (CBHISN, hereafter AZBK) , offers further interesting possibilities for investigation of the nature of the plastically crystalline phase, for the t'ransitioii producing this phase occurs
(2)
( 6 ) . I . ( 2 . 'Tronhndee and E. F. \Vestrum, Jr., i b i d . , 67, 2381 (1963). ( 7 ) C;. 13. (:utllrie c t i i d ,I. 1'. h I ~ C u l l o u p 1 .I. ~ , P h p . Chsm. Solids, 16, 53 (I!Jtil),
