Oct., 1963
COAGULATION EFFECTS OF EIGHLY CHARGED COUNTERIOKS
O X COAGULATION EFFECTS OF HIGHLY CHARGED COUSTERIOKS. BY
E.
AfATIJEVIk,
K. G.
M B T H A I , AND
1995
IF2
M. KERKER
Department of Chemistry, Clarkson College of Technology, Polsdam, n’ew York Received March 11, 1963 Critical coa,gulation concentrations and critical stabilization concentrations of 12-tungstosilicic acid, 9-tungstophosphoric acid, 12-molybdoceric(IV)-acid, and 12-tungstophosphoric acid were obtained as a function of pH. Both silver iodide sols in statu nascendi and “aged” AgI sols were used in these experiments and each type of sol gave the same results. Mobility measurements were carried out using a microelectrophoresis cell. For a given counterion, tkie C.S.C. and mobility were independent of pH, whereas the C.C.C. generally decreased with increasing
Introduction Aqueous solutions of heteropoly acids give highly charged counterions (3 to 8 electronic charges) which behave as powerful coagulatiiig agents for positively charged lyophobic colloidal sols. Indeed, critical coagulation concentrations (c.c.c.) as low as low8mole/l. are f o ~ n d . ~ -At~ concentrations slightly higher than the C.C.C.these solutions will stabilize the sols due to the adsorption of the heteropoly anions onto the colloidal particles with consequent reversal of charge. I n our earlier work there was no attempt to observe the influence of pH; here, we have extended these studies to include the effect of pH upon both the critical coagulation caoiicentratioii (c.c.c.) and the critical stabilization concentration (c.s.c.). The acids used were 12-tungstosilicic, 9-tungstophosphoric, 12-tungstophosphoric, and 12-molybdoceric(IV) acids. I n addition, elect rophoretic mobilities of the sol particles in the presence of various concentrations of heteropoly ions have been determined a t various pH’s. The experiments reported here have also been extended to include “aged” silver iodide sols as well as sols in statu nascendi
‘Experimental Materials.-l2-Tungstosilicic acid (12-TSA) H4SiW120A0, ~Z, 9-tungstophosphoric acid (9-TPA) H ~ P ~ W I ~ O12-tungsto(solid), H7PW1204z(as. phosphoric acid (12-TPA) HsPJJT~~040 s o h . ) , and the 12-molybdoceric(IV) acid (12-MCA) HsCeMiz0 4 2 , were prepared and purified as described earlier.3-6 The very low concentrations of solutions needed in the coagulation experiments were obtained by stepwise dilution of stock solutions of higher concentrations prepared by direct weighing of solid acids previously analyzed for the moisture content, and dissolution of these in doubly distilled water. 2 . Methods.--The procedure for determination of the critiral coagulation concentration (c.c.c.) has been described in detail e a ~ - l i e r .The ~ ~ ~reported i~catteringintensities correspond t o turbidities when particles are sufficiently small and the sol sufficiently dilute. Most of the experiments were carried out with sols in statu nascendi. Aged sols, prepared as described elsewhere ,7 were also used and a t leaist two determinations of the C.C.C. were made with such sols for each heteropoly acid a t two different pH’s. I n all experiments the concentration of the silver iodide in the sol was 2 >(: M and the excess concentration of the stabilizing silver ions was 6 X M . The pH was adjusted by adding appropriate quantities of nitric acid or sodium hydroxide. I n the Latter carie blankswere always prepared to ensure thnt no precipitation of silver hydroxide had taken place. Microelectrophoresis experiments were carried out in a Mattson 1.
(1) Supported by a Grant from the Army Research Office No. ARO-(D)31-124-6320. (2) Part I: E. Matijevib, D. Broadhurst, and M. Kerker, J . Phgs. Chem., 63, 1552 (1959). (3) E. MatijeviC and M. Kerker, ibzd.. 62, 1271 (1858). (4) E. MatijeviC and M. Kerker, J . Am. Chem. Sac., 81, 5560 (1959). ( 5 ) E. MatijeviC and M. Kerker, ibid., 81, 1307 (1VS). (6) B. Tesak, E. hlatijevii-, and K. Sohuls, J . f’hys. Chem., 55, 1557 (1951). (7) R , H, Ottewill and A , Watmabe, Kolloid-Z., 170, 38 (1960).
type cell,* as described earlier.9 Only aged sols were used for determination of electrophoretic mobilities. In view of obvious theoretical difficulties, no attempts were made to calculate the corresponding I-potentials and therefore measured mobility data are reported. All p H measurements were carried out using glass electrodes and a Beckman Model G pH meter calibrated regularly with appropriate buffer solutions.
Results Two typical coagulation curves using an aged silver iodide sol and lZmolybdoceric(1V) acid (12-MCA) as the coagulating agent are given as an example in Fig. 1. In these curves, the excess light intensity scattered by the sol, relative to the uncoagulated sol of the same concentration, is plotted as a function of Concentration of added heteropoly acid a t 10 and 60 min. after the addition of the heteropoly acid. Whereas the coagulation limit A (c.c.c.-obtained by extrapolation of the limit A to zero scattering intensity) is time dependent during the first hour, the stabilization limit B (c.s.c.obtained by extrapolation of limit B to zero scattering intensity) changes very little with t h e . For times longer than one hour, the C.C.C.does not change appreciably. The mobility carves (Fig. 1) show a strong reversal of charge taking place within the coagulation range. While there is some change in mobility with time within this range, indicating aging effects, there is virtually no time effect a t concentrations below the C.C.C.and in the stability range of recharged sols. Figure 2 demonstrates the effect of pH. I n this example aged silver halide sols were coagulated by 9-TPA over two different pH ranges. Here only the 60-min. turbidity curves are given. The lower pH range M in corresponds to systems which mere nitric acid, while the higher pH range corresponds to systems with no addition of either acid or base. The C.C.C. is higher a t lower pH (shift in limit A ) which is to be expected if the ionization of the heteropoly acid were repressed by the addition of acid. Corresponding electrophoretic mobilities are also plotted with the striking result that these appear to be independent of pH. The same effects were observed a t 10 min. and also with the other heteropoly acids as coagulating agents. Coagulation curves as shown in Fig. 1 and 2 were obtained over a wide range of pH’s and for all four heteropoly acids mentioned earlier. These results can be summarized by plots of the critical coagulation concentration and the critical stabilization concentration against pH for each heteropoly acid. Figure 3 gives such plots for 12-TSA where the results of the 10- and 60-min. curves for both aged sols and (8) S.Mattson, J . Phvs. Chem., 38, 1532 (1928); 37, 223 (1933). (9) E. Matijevib, K. G2 Mathai, R. HbOttewill, and M, h r k e r , ib.id.,. 66,
828 (106l)h
Vol. 67
E. MATIJEVI~, K. G. MATHAI,AND M. KERKER
1996
LOG MOLAR CONC OF I2-MCA.
Fig. 1.-Coagulation (circles, full lines), mobility (squares, dashed lines) in p/sec., and p H (triangles, dotted lines) curves of an “aged” silver iodide sol 10 min. (open signs) and 60 min. (blackened signs) after mixing of reacting components in the presence of various amounts of 12-molybdoceric(IV) acid ( 12MCA). Concentrations: AgI: 2 X $1,excess ,4gN03: 6 x loF4M . A , coagulation limit, B stabilization limit.
2
I 3
p4H.
I 5
I
6
Fig. 3.-Plot of the critical coagulation concentrations (c.c.c.) and critical stabilization concentrations ~c.s.c.) against pH for 12-tungstosilicic acid (12-TSA) for a silver iodide sol. c.c.c . 10 min. (O), 60 min. ( 0 ) ; c.s.c.: 10 min. (V), 60 min. (A) after mixing the reacting components. Open signs: h g I sols in statu nascendi; blackend signs : “aged” AgI sols. I O’
-60
-65
-7.0
I -75
I
21
-8.0
LOG. MOLAR CONC. OF 9-TPA.
Fig. 2.--Coagulation (circles, full lines), mobility (squares, dashed lines) in p/sec., and p H (triangles, dotted lines) of an “aged” silver iodide sol in the presence of various amounts of 9tungstophosphoric acid (9-TPA) 60 min. after mixing of reacting components. Open signs represent a run a t higher and blackened signs a run at lower pH.
sols in statu nascendi are shown. The two lower curves correspond to the C.C.C. and the upper curve to the I n the latter case there was no time effect. I n C.S.C. both cases (c.c.c. and c.s.c.) the results were independent of the mode of the sol preparation (“aged” or statu nascendi sols). The area between these curves represents concentrations of heteropoly acid that coagulate the silver iodide sol. Above the uppermost curve the sols are stabilized due to the reversal of charge. Below the C.C.C. there are insufficient coagulating counterions to produce coagulation. For 12-STA the C.C.C. shows only a small drop with increasing pH, and remains practically constant above pH -4. The C.S.C. curve shows a quite different trend, characteristic only for 12-STA. Figure 4 gives a similar plot for 9-TPA. Here, the C.C.C. falls much more sharply and to lower values than those for 12-TSA. The lower values are to be expected in view of higher charge of the 9-TPA ion. However, the C.C.C. has not leveled off a t even the highest pH. The c.s.c., on the other hand, appears to be practically independent of pH.
The results obtained with the 12-MCA are presented in Fig. 5 and are similar to those for 9-TPA. The individual coagulation curves below pH -5 a t the C.C.C. all look very similar to those shown as an example in Fig. 1. However, a t higher pH’s, the coagulation curves become oscillatory in the coagulation range, showing two or more maxima. One example of this effect is given in Fig. 6, in which the coagulation curve for 12-MCA exhibits two distinct maxima. We had observed the same effect earlier with AgBr sols (see Fig. 4, ref. 4). The appearance of these maxima with both types of silver iodide sols, vix., statu nascendi and aged, indicate that these oscillations in the coagulation curve are independent of the composition of the sol or its method of its preparation. Mobility curyes exhibit similar oscillations under the same conditions. The effect is less marked a t earlier times than 60 min. after the addition of the coagulating electrolyte. Figure 7 gives the c . c . ~and C.S.C.curves for the 12tungstophosphoric ion. Riost of the experiments were carried out with 12-TP-4. A few measurements mere madke with the sodium salt and the results were very similar to those for 12-TPA a t the same pH. From Fig. 7 it is apparent that the c.c.c decreases sharply with pH in much the same manner as 9-TPA and 12MCA except that it levels off a t about pH > 5 and then remains constant. For pH > 5 at the c.c.c., the coagulation curves for 12-TPA also exhibit oscillations in turbidities within the coagulation range similar to those shown for the 12-MCA (Fig. 6). Such effects had also been observed
Oct., 1963
COAGULATION EFFECTS OF
I
HIGHLY CHARGED
COUNTERIONS
1997
I
-6.0 I
2
COAGULATION REGION
I-
I
cn - 6 5 LL
0. cj
LOG. MOLAR CONC. OF 12-MCA.
z
Fig. 6.--Coagulation (circles, full line) and p H (triangles, dotted line) curves of an “aged” silver iodide sol in the presence of various amounts of 12-molybdoceric(IV) acid (12-MCA).
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Fig. 7 . -The same plot as in Fig. 3 for 12-tungstophosphoric acid (12-TPA). Diamonds indicate experiments with the sodium salt of 12-TPA.
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Fig.5.--The same plot as in Fig. 3 for 12-molybdoceric(IV) acid (IZMCA),
earlier with silver bromide sols (Fig. 5, ref. 3) indicating again the generality of this effect. These experiments with all four heteropoly acids have demonstrated that neither C.C.C. nor C.S.C.depends on the particular method of the sol preparation. Furthermore, while C.S.C. is in general independent of pH (except in the case of the 12-TSA), C.C.C.decreases with pH. Also, the mobilities of the sol particles a t the same concentration of the heteropoly acid are practically independent of pH. Discussion The object of either a theoretical or experimental elucidation of the Schulze-Hardy rule is to establish the relation between the C.C.C. of the coagulating electrolyte and the magnitude of the charge of the counter-
1998
E. MATIJEVI~, K. G. MATHAI,
ion. I n order to do this, other factors such as ionic size, specific adsorption, dielectric constant of the medium, surface potential of the colloidal particle, etc., must be held constant, while counterion charge is varied. The heteropoly acids used as coagulating agents in this study provide a variety of highly charged ions, of comparable size and chemical properties. However, in order to obtain a definite C.C.C.related to a definite ionic species, the conditions under which these acids are conipletely ioiiized must be established and that is one of the reasons that earlier basicity studies? l o and these present studies were undertaken. In two cases, 12-TSA and 12-TPA, a limiting C.C.C.was obtained with increasing pH a t pH -4 and 3, respectively, suggesting that a t these limits the ionization is complete and that the prevalent species are charged -4 and -7. The corresponding values of tlie C.C.C. were 2.2 X and 8 X :1.1 (60 min.). This is quite consistent with our earlier basicity work. For the 9-TPA and 12-AICA, the c.c.c. does not level off and even at pH 6 where the values were 2 X 1.3 X d l . However, the basicity work’” has shown that these acids are completely ionized under these conditions so that it is iiot obvious at this time just why the c.c.c does not reach a limit in these tn-o cases. These results point up the extreme difficulty of obtaining precise data for qusntitatii e interpretation of the Schulze-Hardy rule for such highly charged systems. First, the values of the C.C.C.reported here are based on the total amount of added electrolyte rather than the actual concentration of free counterion in the electrolytic niedium of the coagulated sol. These two quantities may differ due to adsorption of counterions on the colloidal particles. For lower charged ions, the adsorbed quantities are sniall compared to tlie C . C . C . and ~ ~ . ~can, ~ therefore, be neglected. (For a similar sol, the C.C.C. of NOj- is 1.1 X lop2 131, and for SO4- it is 6.5 x M . I 3 ) However, for these highly charged ions, the C.C.C. itself is so small that the amounts adsorbed may comprise a significant portion of the total amount of added electrolyte. Accordingly, the actual values of the C.C.C.niay be considerably lower than those we report. The mobility measurements do show that adsorption takes place since there is reversal of charge at concentrations only slightly higher than the C.C.C. This effect can account for the fact that the C.C.C. for these highly charged counterions appears to drop off much more slom-ly with increasing charge than for counterions of lower charge. What we have measured as the C.C.C. niay be higher than the actual ambient concentrations in the medium. On the other hand, this “leveling” effect which we observe may he real. The adsorption of highly charged counterions affects the C.C.C.in still another way. With increasing adsorption of counterions the surface potential of colloidal particles decreases, which in turn causes a lowering of the c.c.c. Thus, the results of absorption, Le., lowering of counterion concentration in the medium and lowering of surface potential, affect the apparent (lo)
J. R. Keller, E. Matijevit, and h1. Kerker, J . Phys. Chem., 66,56 (1961). (11) E. MatijeviC, 11. B. Abiamson, R. €1. Ottenrlll, I 5 remains to be discussed. The ordinate of such coagulation curt.es is a measure of the rate of coagulation so that (19) J. F. Keggin, Proc. R o y SOC.(London), A144, 75 (1934); L. Pauling, J . Am. Chem. Soc., 61, 2868 (1929). (201 M. Kerker, J. Keller, J. Siau. and E. MatijeviC, Trans. Faraday SOL.,57, 780 (1961).
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oscillations of the type exhibited here must arise from several competitive processes which cause the rate of coagulation either to decrease or increase. Above the C.C.C. and below the C.S.C.the rate of coagulation will depend on the concentration of the free heteropoly ions in solution and 011the surface potential of the colloidal particles. Both quantities will depend, for a given initial concentration of the heteropoly acid, on the amount of the adsorbed acid. For a given particle size the relative ratio of the adsorbed to the free acid will change with the initial concentration of the heteropoly acid. I n addition, in sols in statu nascendi the size of primary particles may also be irifluenced by the concentration of the heteropoly acid. Thus, several of such competitive processes influencing the rate of coagulation may result in the oscillatory pattern. Similar effects have been observed earlier when silver bromide and silver iodide sols were coagulated by certain surface active agents.21 DISCUSSION A. W. NACMANS (Union Carbide Corp.).-Would you please cdmment on the nature of the driving force that gives rise to adsorption and charge reversal?
E. MATIJEV16.-wre believe that metal ions only when hydrolyzed reverse the charge and that this reversal of charge is caused by the chemisorption of hydrolyzed ions on the surface of lyophobic colloidal particles. (21) E. Matijevii. and R. €1. Ottewill, J . Colloid Sei., 13, 242 (1958).
