E. MATIJEVI~, N. KOLAK,AND D. L,CATONE
3556
Interactions of Silver Halides with Metal Chelates and Chelating Agentslp2
by Egon Matijevik, Nicholas Kolak,s and David L. Catone4 Institute of Colloid and SuTface Science and Department of Chemistry, Clarkson College of Technology, Postdam, New York 13676 (Received February 18, 1969)
It is shown that the chelating agents ethylenediamine (en), 2,2'-dipyridyl (dipy), and 1,lO-phenanthroline (phen) can coagulate and reverse the charge of a silver bromide sol, the order of efficiency being phen > dipy > en. With increasing pH the critical coagulation concentration (ccc) and the critical stabilizationconcentration (csc) increase for en and decrease for dipy and phen. Apparently two different mechanisms apply to interactions of en and of dipy and phen with silver halide sols. These are discussed in light of the composition of the solute species along the ccc and csc boundaries. The addition of asmall amount of dipy (in concentrations considerablyless than the ccc of dipy) strikingly sensitizesthe sol toward Ni2+,the ccc of the latter being lowered by more than four orders of magnitude. If the ratio of the added [Niz+]to [dipy] is kept constant (1: 3), the charge of the sol can be reversed. The ir spectra show that the Ni(dipy)2+complex is formed at the interface. On the other hand, when the concentration of dipy is constant and [Ni2+]is varied, the charge reversal does not occur, yet the sol is stable again at higher nickel Concentration. In this case the sol mobility goes through a minimum, which coincides with the coagulation maximum.
Introduction
Experimental Section
In a recent publicationJs it was shown that certain metal chelates exhibit striking effects upon the stability of lyophobic colloids. These complexes coagulate at extremely low concentrations, and they also act as powerful reversal of charge agents. For example, for a silver bromide sol the critical coagulation concentration (ccc) of the trisphenanthroline nickel ion, Ni(phen)B2+,is 7 X low8M and the critical stabilization concentration due to charge reversal (csc) is 4 X lo-' M . In contrast, noncomplexed Ni2+ ion coagulates M and cannot reverse the charge at all. a t -1 X In addition, it was found that the enhanced coagulation and reversal of charge ability of metal chelates (as compared to noncomplexed metal ions) takes place over a broad pH range. This observation is surprising as the stability of some of these complexes in solution depends strongly on the pH. The purpose of this investigation is to examine the reasons for such a behavior of metal chelates. To this end the interactions of several chelating agents (ethylenediamine, 2,2'-dipyridyl, and 1,lO-phenanthroline) with a silver bromide sol in the absence and in the presence of metal ions have been studied. Also, the adsorption of the complex chelates and the chelating agents on silver halides has been followed by infrared spectroscopy. These results should contribute toward a better understanding of coagulation processes which are caused by charge neutralization owing to the chemisorption of counterions. They should also indicate the ways for modifications of lyophobic interfaces, particularly when charge reversal is desired. Finally, the investigated systems can serve as models for processes of interfacial conversion, such as adhesion, polymersolid interactions, etc.
A. Materials. Ethylenediamine, 2,2'-dipyridyl, and 1,lO-phenanthroline were commercially available chemicals, which were purified before use. The trisdipyridyl nickel(I1) chloride, [Ni(dipy)$]ClZ, was prepared following the procedure by Morgan and BurstalL6 All other chemicals were of the highest purity grade available and were used without further purification. All solutions were prepared at 25' using doubly distilled water from an all-glass still. I n order to prevent aging of the solutions upon standing, fresh solutions were prepared every few days. A Millipore filter with a pore size of 0.22 CI was used to remove any foreign particles from the stock solutions. B. Methods. The critical coagulation concentration (ccc) and the critical stabilization concentration (csc) due to charge reversal were determined by following the changes in light scattering of a silver bromide sol in statu nascendi according to a procedure described earlier.7~8 Scattering intensities were measured with a Brice-Phoenix Model D-M light-scattering photometer and were expressed in terms of Rayleigh ratios (RR).5 Mobilities of silver bromide particles were deter-
The Journal of Physical Chemistry
(1) Supported by the Federal Water Pollution Control Administration, Grant WP-00815. (2) Presented a t the 156th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 8-13, 1968. (3) Part of a n M.S. thesis by N. Kolak. (4) Part of a Ph.D. thesis by D. L. Catone. (5) E. Matijevii! and N. Kolak, J . Colloid Interfac. Sci., 24, 411 (1967). (6) J. T. rMorgan and F. H. Burstall, J . Chem. SOC.,2213 (1931). (7) E.Matijevi; and M. Kerker, J. Phys. Chem., 6 2 , 1271 (1958). (8) B. Teiak, E. Matijevi;, and K. Sohulz, ibid., 5 5 , 1557 (1951).
SYMPOSIUM ON INTERFACIAL PHENOMENA mined in a microelectrophoresis cell of the Mattson type. g*10 A Beckman Model G pH meter with glass electrodes was used for pH measurements. The instrument was regularly standardized using appropriate buffer solutions. The infrared spectra of the adsorbed species on silver bromide particles were obtained with a Beckman IR-12 spectrophotometer. This instrument has a wave number reproducibility of 0.1 cm-l, a wave number accuracy of h0.2 cm-l a t 740 cm-l, and a resolution of 0.25 cm-l at 923 cm-I. The solutions containing the species to be adsorbed were stirred for about 5 days with a silver bromide sol. The mixture was then filtered, washed three times with 5-ml aliquots of doubly distilled water, and the solids were dried under vacuum at 25" for at least 4 days. The samples were then pressed into pellets. No additional support material needed to be added because silver bromide is a suitable medium itself.
35.57
ZOI
1.0
1
I
,
1
2.0 3.0 4.0 5.0 6.0 ZO
I
8.0 9.0
PH
Results The interactions of a silver bromide sol with three chelating agents have been studied in detail. These are ethylenediamine (en), 2,2'-dipyridyl (dipy), and 1,lO-phenanthroline (phen). I n Figure 1 are given several coagulation curves in which the Rayleigh ratio (RR) is plotted against the concentration of ethylenediamine. Each curve is for a different but narrow pH range. High and low RR represent coagulated and stable sols, respectively. The critical coagulation concentration (ccc) is obtained by
-LOG MOLAR-CONC.
OFETH;LENEDIAMINE
Figure 1. Coagulation curves (solid lines, open symbols) for a silver bromide sol in statu nascendi in the presence of ethylenediamine a t various pH values (dashed lines, blackened symbols). High and low Rayleigh ratios (RR) designate coagulated and stable sols, respectively. Concentrations: AgBr, 1.0 X 10-4 M ; excess Br-, 1.9 X lo-* M . RR measured 10 min after mixing the reacting components.
Figure 2. Plot of the critical coagulation concentrations (ccc, solid lines) and of the critical stabilization concentrations (csc, dashed lines) of a silver bromide sol in statu nascendi as a function of pH for ethylenediamine (en, A), 2,2'-dipyridyl (dipy, 0 e), and 1,lO-phenanthroline (phen, 0.). AgBr, 1.0 x lom4M ; excess Br-, 1.9 X IOmaM .
extrapolation of the steepest part of a coagulation curve to zero RR. Similar experiments were carried out with dipy and phen. With these substances the coagulation curves showed a maximum in RR. (One example of such a curve can be seen in Figure 6.) Figure 2 gives a summary of the results as obtained with the three chelating agents. Above the solid lines the sols are coagulated and above the dashed lines they are restabilized. Thus, with dipy and phen the coagulation occurs only between the corresponding solid and dashed boundaries. The results of electrophoretic measurements on a silver bromide sol in the presence of the three chelating agents are given in Figure 3. Each system was studied a t two different pH values. The mobilities show that all three substances can reverse the charge of the soI, with the order of efficiency being phen > dipy > en. This would explain the restabilization phenomenon of phen and dipy as observed by light scattering (Figure 2). The experiments described give evidence that the chelating agents interact with silver halide sols even when they are not complexed with a metal ion. I n order to see whether the presence of the metal ion has any effect upon the behavior of the chelating agents, coagulation and mobility studies were carried out with (9) 8.Mattaon, J. Phys. Chem., 37,223 (1933); 32,1632 (1928). (10) E. Matijevi;, K. G. Mathai, R. H. Ottewill, and M. Kerker, ibid., 65,826 (1961). Volume 79, Number 11
November 1969
E. MATIJEIVI~, N. KOLAK,AND D.L.CATONB
3558
50
IOmin., 25'C
-LOG MOLAR CONC. OF EN,DIPY,OR PHEN
Figure 3. Electrophoretic mobilities of a silver bromide sol in statu nascendi (AgBr, 1 , O X lo-' M ; excess Br-, 1.9 x 10-8 M) as a function of varying concentrations of ethylenediamine (en), 2,2'-dipyridyl (dipy), and 1,lO-phenanthroline (phen) at pH values as indicated in the diagram. ~
[Ni+*]:[DIPY]-
1.3
'AgBr;lr l6M ' pBr:2.70
~ I O m i o . 25OC ,
I
Coagulation curve (solid line) of a silver bromide sol i n statu nascendi. (AgBr, 1.0 x 10-4 M ; excess Br-, 1.9 X M ) as a function of Ni(N03)2at pH 3.5 in the presence of 2,2'-dipyridyl (dipy). The concentration of dipy in each system was equal to 3X the concentration of Nit+ ions. Figure 4.
Dashed line gives the corresponding electrophoretic mobilities.
silver bromide sols to which the chelating agent and the metal ion were simultaneously added. In Figure 4 a coagulation curve is given as a function of the nickel nitrate concentration. Each system contained dipy in amounts sufficient to maintain a constant ratio [Ni2+]:[dipy] = 1:3. The ccc and the csc of the nickel ion are approximately the same as when the complex Ni(dipy)a2f is added to the sol. The mobility measurements (dashed line) indicate strong charge reversal. The J O U Tof ~Phyaioal ChmistTy
Figure 5. Coagulation curves of a silver bromide sol (AgBr, 1.0 X 10-4 M; excess Br-, 1.9 X M ) as a function of Ni(NO& in the presence of a constant amount of 2,2'-dipyridyl (3.0 X 10+ M ) at two different pH (0, V, 3.3; 0,5.7). Dashed lines give the corresponding mobilities. Circles and diamonds represent a sol in statu nascendi, whereas triangles, are for a sol aged for 30 min before addition of Ni(NO&.
An unexpected result was obtained when the 2,2dipyridyl was added to the silver bromide sol in constant concentration, whereas the concentration of Ni(NO&? was systematically varied. This is shown in Figure 5 in which the two systems are for different pH values. Again coagulation maxima appear but the stability a t higher concentrations of the nickel salt is not due to charge reversal. The corresponding curves for the electrophoretic mobilities show that the sols remain negatively charged over the entire salt concentration range. A minimum in the mobility coincides with a maximum in turbidity. It should be noted that the amount of dipyridyl added is considerably below the ccc of this compound for the same sol. Most surprisingly, the amount of nickel salt needed to coagulate the silver bromide is at least three orders of magnitude lower in the presence of the very small amount of dipyridyl than in the absence of it. Discussion 1. Stability Efects by Chelating Agents. Since a11 of the three chelating agents used in this work can coagulate the negatively charged silver bromide sols and reverse their charge, one would expect them to be present in aqueous solutions in the form of cationic species. It is known that en, dipy, and phen behave as organic bases which may undergo protonation in aqueous solutions. The corresponding stability constants are given in Table I.
SYMPOSIUM ON INTERFACIAL PHENOMENA
3559
Table I: Stability Constants for Metal Chelates and Chelating Agents”
+ +
en zH+ ~2 enHz+ dipy zH + $ dipyH” + phen zH+ phenH”+ Ni2+ zen e Ni(en),a+ Ni2+ zdipy e Ni(dipy),2+ Ni*+ zphen s Ni(phen),Bf
+ + + +
Log Kt
Log Ka
Log Ka
10.17 4.44 4.86 7.51 6.80 8.60
7.44 0.52 0.70 6.35 6.46 8.10
4.42 5.20 7.55
” The values have been selected from “Stability Constants of Metal-Ion Complexes,” compiled by L. G. Sillen and A. E. Martell, The Chemical Society, London, 1964, and “Dissociation Constants of Organic Bases in Aqueous Solution,” compiled by D. D. Perrin, Butterworth and Co. Ltd., London, 1965.
The coagulation effects of ethylenediamine can be explained in terms of protonation. In Table I1 the composition of the en solutions for various ccc is given as a function of the corresponding pH. I n all cases the concentration of the doubly protonated species enHzz+ is reasonably constant. The average value M is what one would expect for of 6.4 f 0.6 X counterions of 2+ charge for the silver bromide sol used. The ccc is somewhat lower than for simple divalent counterions for the same sol,ll but this is to be expected since enHzZ+ is a larger species. The Concentration of the monoprotonated ion becomes larger with increasing pH, but even at pH 8.0 this is much too low for a monovalent counterion to affect the rate of coagulation; hence the ccc will not be influenced by enH+. It appears, then, that the amount of en needed to coagulate the sol is such to produce at a pH the sufficient concentration of the coagulating species, which is in this case enHZ2+. Electrophoretic measurements (Figure 3) indicate that the adsorption of en is negligible over the ccc range, which explains why the enH2+ counterion obeys the Schulze-Hardy rule.12 The stability effects of dipy and phen are considerably more involved. The following experimental observations should be considered. Both substances coagulate and also reverse the charge of the silver bromide sol. The ccc and the csc decrease with increasing pH until above a given pH these values remain constant. The composition of the solutions along the coagulation boundary and the limit marking the restabilization due to charge reversal (Figure 2) are given in Table 11. The concentration of protonated species decreases rapidly as the pH becomes higher. Thus, the protonated species cannot be responsible for charge reversal and may adsorb less strongly than the unprotonated chelating agents. This is corroborated by the mobility measurements which indicate strong charge reversal even at pH 7-8 (Figure 3) a t which condition there are no protonated species present in the solution.
Infrared adsorption studies confirm that the uncharged species adsorb on silver bromide sol particles. Table I11 gives the characteristic frequencies of dipy and the protonated dipy (dipynHC104) in the absence of silver bromide. The same frequencies of the most intense bands for unprotonated dipy are observed for the species adsorbed on silver bromide for samples equilibrated over the pH range 3-7. The peaks become more intense with increasing pH. All of this indicates that the uncharged dipy adsorbs and causes coagulation and charge reversal, which at first would seem to be a rather unusual result. The following mechanism is suggested by way of explanation of these effects. The neutral dipy molecules adsorb by an ion-exchange mechanism in which they substitute the potential determining bromide ions. In doing so they may form a chelate with the silver ion a t the silver bromide-solution interface. The removal of bromide ions at first destabilizes the particles due to charge neutralization and then restabilizes the sol when more bromide ions than necessary to establish the point of zero charge are exchanged. The silver-dipyridyl chelate is rather stablela and will not easily desorb. The formation of the Ag-dipy ions a t the interface cannot be distinguished by ir because the characteristic peaks due to this chelate are identical with those of 2,2’-dipyridyl alone. The suggested mechanism certainly would explain the inapplicability of the SchulaeHardy rule to this case. It is noteworthy that at the csc the concentration of the unprotonated dipy ( L ) remains constant over the entire pH range (Table 11). The same is true for the ccc except at pH
