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Stability of colloidal silica
Stability of Colloidal Silica III. Effect of Hydrolyzable Cations ~' 2 L A W R E N C E H. A L L E N ~ A~D E G O N M A T I J E V I C Institute of Colloid and Surface Science and Department of Chemistry, Clarkson College of Technology, Potsdam, New York 13676
Received April 10, 1970; accepted July 20, 1970 The stabilities of the silica hydrosols Ludox HS and Ludox AM have been studied as a function of pH in the presence of various concentrations of several hydrolyzable electrolytes. "Log electrolyte concentration-pH" domains are given for the HS sol in AI(NOa)a and A12(SO~)a solutions and for the AM sol in Al(NOa)a and La(NOa)a solutions. The ion-exchange mechanism of destabilization, which was previously established for the coagulation of silica with nonhydrolyzable electrolytes and for La(NO3)a at low pH, is shown to hold for Al(NOa)a and A12(SO0s along the coagulation boundaries at pit < 4.5 and [A1a+] < 3 X 10-2 M. Unhydrolyzed, hydrated A1a+ is the coagulant under these conditions for both of the aluminum salts. The difference in the coagulation behavior of aluminum sulfate and aluminum nitrate could be explained quantitatively by considering the formation of ALSO4+. Electrophoretic mobilities, determined by the moving boundary technique, are presented for conditions close to the coagulation boundary. Very little correlation was observed between the c.e.c, and the mobility. At low concentrations the shapes of the coagulation boundaries are determined by depletion of the bulk cation concentration due to exchange. Under these conditions the critical amounts of cation exchange of A1s+ and Ca~+ necessary for the destabilization of Ludox HS were calculated from the c.c.c, at two different sol concentrations. INTRODUCTION The stability of colloidal silica in the presence of simple electrolytes, particularly those containing cations which undergo no hydrolysis over the p H range studied, has been discussed in detail in the first paper of this series (1). I t was shown that ion exchange of such counterions for silanolic protons plays the dominant role in the destabilization of aqueous silica so]s (1, 2). Any effect of p H in these studies had to be interpreted in terms of the surface characteristics of silica particles.
When hydrolyzable counterions are added to a silica sol a change in the p H affects simultaneously the composition of the electrolyte medium and the properties of the particle surface. Thus both parts of the system should be well understood if one is to interpret the interactions of such ions with colloidal silica. The only other detailed studies of the effects of hydrolyzed metal ions upon the stability of silica seem to have been carried out by S t u m m and collaborators (3-6). These involve ferric and aluminum salts. Unfortunately, owing to very strong aging effects, which take place rapidly even at room temperature (7), ferric ions do not ]end themselves to a quantitative interpretation of stability phenomena. With aluminum salts, H a h n and S t u m m ' s investigations dealt mainly with the rate of coagulation.
1 Parts I and II, see references 1 and 2. 2 Supported in part by the Federal Water Pollution Control Administration, Grant WP00815. s Part of a Ph.D. thesis by L. I=[. Allen. The support of a Texaco Fellowship is gratefully acknowledged.
Journal of Colloidand InterfaceScience,Vol.35, No. 1, January 1971
66
STABILITY OF COLLOIDAL SILICA Here we report a systematic study of the stability of silica sols (Ludox HS and AM) in the presence of various concentrations of Al(NO3)3, A12(SO4)3, and L a ( N Q ) 3 as a function of pH. Certain observations with calcium salts will also be discussed. Aluminum and lanthanum ions have been chosen because the complex chemistry of their hydrolysis products is now fairly well understood (8-11). In addition the effects of these ions upon the stability of lyophobic colloids has been extensively studied (8, 9, 12-15). Thus, an analysis of the differences in the behavior of a hydrophilic colloid, such as silica, in the presence of the same ions is possible. The "silica-aluminum salt" systems are of
interest in many applications, among which water purification, flotation, cracking catalysts, and gel preparations are but few. EXPEI~IMENTAL The procedures for the determination of the critical coagulation concentrations (c.e.c.), the electrophoretic mobilities, and amounts of cation exchange have been described in great detail earlier (1, 2). Ludox HS and AM (E. I. du Pont de Nemours and Co.) have been used in this work. The lot numbers and the properties of these silica sols have also been given previously (1, 2). Precautions to avoid aging were taken when dilute solutions of hydrolyzing salts were required. In the preparation of aluminum and lanthanum salt solutions care was exercised in keeping the temperature below 25°C during the dissolution process. In the case of aluminum salts, solutions at concentrations < 2 X 10-2 M were prepared freshly for each experiment by diluting more highly concentrated stock solutions. The Ah(S04)3 stock solution, which showed a Tyndall cone when illuminated, was filtered through a 1Viillipore filter (0.22 pore diameter) until optically clear. All the salt solutions were analyzed for the metal ion content by means of appropriate gravimetric procedures. RESULTS
(a) Coagulatior~. The
coagulation process was followed by measuring the Rayleigh ratios of the silica sols in the presence of
O.3
67
I LUDOX AM 0.2 gm/lO0 ( [ hr 25°C --
0.2
--I.~
n
33t
U-
<~
~, o.~ gt:
2.8
5.0
5.2
3:4
5.6
3~
pH
FIG. 1. Rayleigh ratios as a function of pH for a Ludox AM sol in the presence of five different concentrations of aluminum nitrate. Al(NO3)3: (&) 1.12 X 10-2M; (O) 6.30X 10-a M; (~) 2.00 X 10-2M; (D) 5.01 X 10-2M; (V) 5.01 X 10-4M.
various concentrations of a given electrolyte at a constant pH, or at a constant concentration of the electrolyte with p H as the independent variable. Examples of the latter are shown in Fig. 1. Each coagulation curve is for a different, but constant, aluminum concentration and each point is for a different p H value as indicated on the abscissa. The p H of the systems was adjusted by the addition of very small amounts of concentrated nitric acid or sodium hydroxide with an automatic titrator. High Rayleigh ratios denote sols where coagulation has occurred. The critical coagulation concentration (c.c.c.) at a given p H or the critical p H at a given salt concentration corresponding to the c.c.c, was established by extrapolating the steepest portion of such coagulation curves to the Rayleigh ratio of the uncoagulated sol. All the data in Fig. 1 were taken 1 hour after -mixing the reacting components; this was found to be the "critical time" for colloidal silica 4 (1). Figure 2 gives the Rayleigh ratios of the Ludox AM silica sols as a function of the aluminum nitrate concentrations for two different p H values. Some of the coagulated systems settled before one hour, the time when the measurements were made. These systems were redispersed by shaking prior 4 For a detailed analysis of the data giving this critical time see the thesis by L. H. Allen: "Stability of Colloidal Silica," Clarkson College of Technology, Potsdam, New York, 1970.
Journal of Colloid and Interface Science,
Vol. 35, N o . 1, J a n u a r y
1971
68
ALLEN AND MATIJEVIC
0.4-
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LUDOX AM 0.2 gm/lO0 cc
~
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+1
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LUDOX COLLOIDAL i SILICA
250c
o
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/
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, I I I
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-2,0 LOG.
-2.5 MOLAR
-3.0 CONC. OF AI(NO~) 3
-3.5
FIG. 2. Rayleigh ratios as a function of the concentration of aluminum nitrate for a Ludox AM sol at p H 3.2 (circles) and pH 4.0-4.2 (triangles). Low Rayleigh ratios correspond to uncoagulated systems. Shaded symbols denote settled systems redispersed by shaking before the measurement. At a pH of 3.2 the sol exhibits both coagulation and reversal of charge; at p H 4.0-4.2 no stabilization was Observed before the onset of aluminum hydroxide precipitation. Dashed line gives the mobilities of the sol at pH 3.2 in the presence of various concentrations of AI(N03)~.
to making the measurements. A smooth line can be drawn through the tlayleigh ratio points so determined. At pH 3.2 two stability regions are observed, one at low and the other at higher molarities of Al(N03)3. The dashed line, which gives the corresponding mobilities of silica particles, shows that the restabilization at higher salt concentrations is accompanied by charge reversal. If a sufficient number of experiments, such as are shown as examples in Figs. 1 and 2, are carried out, the entire stability domain of the colloidal system can be obtained as a function of the salt concentration and pH (12, 14). The "log [AI(NOs)3]-pH" stability domains for Ludox AM and for Ludox HS (at two sol concentrations) are given in Fig. 3. Points obtained by either of the two procedures demonstrated in Figs. 1 and 2 lie on the same boundary. The dashed line indicates the conditions under which, with no silica sol present, the first traces of aluminum hydroxide precipitation were detected in a Tyndall beam as reported earlier (12) with the exception of the squares, which were determined in order to extend the dashed line to the silica coagulation
%
-
-
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~oLudox
HS , 0 . 2 g m / l O O c c _
• L u d 0 x H S , 2 clm/lO0 cc z~ & L u d c x A M , 0.2 gm/lO0 cc - - - A l u m i n u m Hydroxide
6
4
8
pH
FIG. 3. Log [AI(NO3)8]-pH domain of Ludox HS at two concentrations (0.2 gm/100 co, open circles, and 2 gm/100 cc, shaded circles) and of Ludox AM (0.2 gm/100 cc, triangles). In the latter case points obtained by different techniques are differentiated by flags; ( ~ ) and (A) designate [AI(NO3)~] and p H as the independent variable, respectively. The dashed line gives the conditions under which aluminum hydroxide precipitate was first detected in a Tyndall beam, in the absence of silica sol (12). Squares denote additional points for aluminum hydroxide precipitation added in this work.
boundaries (solid lines). It is evident that the domains of the two sols are fairly similar in position but differ to a small degree in shape. The precision of the data in the relatively vertical portions of the coagulation boundaries in such that the difference in the shapes of the two domains is significant. At the higher sol concentration (2 gin/100 co) the vertical portion of the coagulation boundary is shifted v~eryslightly to lower pH values as compared to the corresponding boundary at one-tenth this sol concentration. A cheek run using a Beckman DU and cells of several optical path lengths down to 0.05 em gave the same c.c.c, as the Briee Phoenix instrument. This would tend to indicate that the difference in coagulation boundaries is not due to multiple scattering in the more concentrated system. The stability boundaries show that silica
Journal of Colloid and Interface Science, Vol. 35, No. 1, January 1971
STABILITY OF COLLOIDAL SILICA
~
_
~
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L
LUDOX HS 02 gm/tO0cc I hr , 25~C
-I
I
69
LUDOX AM -I
312"1 d z
o
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g~ ~E ._i_~
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UNCOAGULATED k SOLS - - ~ ' "
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COAGULAI ////// \\\\\\
Am~(so~)~
A AI(NO3)3 ------
Hydroxide Precipitote Boundary
4
5
-5
6
pH
FIG. 4. The log [A13+]-pH domain for a Ludox HS sol (circles, solid line) in the presence of aluminum sulfate. Superimposed on these data is the corresponding domain for aluminum nitrate (triangles, solid line). The dashed line gives the conditions in aluminum sulfate solutions under which the first traces of hydroxide sol can be detected in a Tyndall beam, in the absence of silica sol (14). sols remain stable at low p H values even at aluminum nitrate concentrations as high as 0.1 M. This behavior is distinctly different from that observed using lyophobic colloids (12, 14). The stability domain of Ludox HS in the presence of aluminum sulfate is given in Fig. 4 (circles). Plotted on the ordinate of this figure is the molar concentration of A13+ on the basis t h a t aluminum sulfate dissociates completely to give only A13+ and SO 2-. For comparison, the corresponding domain for aluminum nitrate is superimposed (triangles). Below p H 4 larger concentrations of A12(SO4)3 t h a n of AI(NO~)3 are required to produce coagulation. The horizontal portions of the two domains are in excellent agreement. The dashed line indicates the boundary for the region in which a hydroxide precipitate is formed in aluminum sulfate solutions in the absence of colloidal silica (14). Two of the points for the aluminum sulfate coagulation boundary lie inside the hydroxide precipitate region. Since the initial
2
4
6 pH
8
I0
FIo. 5. Comparison of the log [AI(NO3)3]-pH domain of a Ludox AM sol with its log [La(NO3)3]pH domain. The hatched areas designate the coagulation regions. To the right of the dashed line bounding each coagulation zone lies the region where the corresponding hydroxide precipitate is formed in the absence of silica sol. For lanthanum nitrate the dashed line is calculated from the known solubility product (16), and the triangles denote data obtained in this work. concentration of aluminum ion, which is plotted on the ordinate, is depleted b y ion exchange, the equilibrium concentration of aluminum ion for these two points may be low enough to be outside the hydroxide precipitation range. A comparison of the coagulation domains of Ludox AM in the presence of aluminum nitrate and lanthanum nitrate (2) is given in Fig. 5. The shaded area with hatching of positive slope denotes the coagulation region of aluminum nitrate; that of negative slope is for the conditions for coagulation with lanthanum nitrate. The coagulation regions are bounded on the right b y their respective hydroxide precipitate boundaries. For lant h a n u m nitrate the position of this line was calculated from the solubility product as determined b y Feitknecht and Schindler (16). As a check several runs were made to determine the boundary conditions under which colloidal lanthanum hydroxide is formed (triangles). These were detected by
Journa$ of CoUoid and Intsr/acs Science, VoL 35, No. 1, J a n u a r y 1971
70
ALLEN AND MATIJEVIC I
E
~
0
[
L
LUDOX COLLOIDALS,L,CA
(N~z,
AI(NO~)~ A Lo(NO3)3~ o hi(N03)3 LudoxHS 001 M I Glycine-HNO~ Buffer •
-5
-I
-2 -3 LOG. MOLAR CONC. OF SALT
T
-4
-5
FIG. 6. Electrophoretic mobilities of Ludox AM and HS in the presence of various concentrations of Al(NOs)s and La(NOs)s. The mobilities of Ludox AM, at pH = 3.15, are given as a function of the logarithm of the molar concentrations of AlCNO3)s and La(NO3)3 by circles and triangles, respectively. The concentration ranges over which these salts produce coagulation at this pH are designated by the hatched intervals. Squares give the mobilities of Ludox HS in the presence of aluminum nitrate at pH 3.15. Blackened triangles and dashed line are for Ludox AM in the presence of La(NO~)s at pit = 2.0. observing the first appearance of the Tyndall beam in systems prepared in the same way as the coagulation runs, but without silica. The experimental points thus obtained are in excellent agreement with the solubility product line. (b) Electrophoresis. Eleetrophoretie mobil, ities of Ludox silicas as obtained from moving boundary measurements (1) in the presence of various eoncentrations~ of Al(N03)3 and La(NO3)s are given in Fig. 6. All curves but one are for pH 3.15. Thus, the mobilities were taken along the vertical portions of the aluminum nitrate coagulation boundaries, just outside the coagulation region of Ludox HS (Fig. 3). The coagulation boundary of Ludox AM extends to pH values lower than 3.15 over a small range of concentrations which is indicated by the upper hatched interval along the line of zero mobility. At the low concentration ends both of the mobility curves approach the mobility of their respective sols at p H 3.15 in the absence of aluminum nitrate. At concentrations of aluminum nitrate above 2.5 )4 10.3 M for Ludox tIS and 1.0 X 10-2 M for
Ludox AM the electrophoretic mobilities at a p H of 3.15 are positive. In contrast to their behavior in aluminum nitrate, neither of the colloidal silicas underwent a reversal of charge in the presence of high concentrations of lanthanum nitrate at pH 3.15. Represented by the lower hatched region on the zero mobility line is the range of concentrations of lanthanum nitrate over which the sol is coagulated (Fig. 5). The curve for the mobility of the sol in the presence of lanthanum nitrate (triangles) does not pass through zero, but, instead, remains in the region of negative mobilities. Further work at this pI-I with higher concentrations of lanthanum nitrate revealed a small, but still negative, mobility. Because the electrophoretic studies at p H 3.15 in the presence of various concentrations of lanthanum nitrate lacked data over the rather broad coagulation range, runs were also performed at a p H of 2.0, where there was no coagulation. The resulting mobility curve is given b y dashed lines and blackened triangles. The lack of any maxima or minima in the curve at pH 2.0 over the region of La(NO3)s concentrations from ~-~5 X I0 -4 to 4 X lO-2M suggests that such extrema are also absent in the mobility data at p H = 3.15, and that the curve through the circles is correct as drawn. It was not possible to determine whether or not lanthanum nitrate could reverse the charge of Ludox A~[ at concentrations greater than 0.1 M and at pH values higher than 3.15. A series of eleetrophoretic experiments in, say, 0.2 M lanthanum nitrate and for various p i t values between 3 and 6 would yield definitive information about the possible existence of positive mobilities. Unfortunately, however, at these electrolyte concentrations the conductivities of the suspending media are very high and the moving boundary technique breaks down. Because of this it was impossible to take electrophoretic data to resolve the problem. In the case of Ludox HS, a single run was performed in 0.1 M lanthanum nitrate at a ptI of 4.4. The mobility observed was zero. Just outside the low concentration ends o f the aluminum nitrate coagulation regions for the two sols (Fig. 3) a number of moving
Journal of Colloid and Interface Science, ¥ol. 35, No. 1, J a n u a r y 1971
STABILITY OF COLLOIDAL SILICA boundary electrophoretic measurements have been made. These were performed in 1.0 X 10-4 M aluminum nitrate and are represented b y triangles in Fig. 7. The open symbols denote mobilities of Ludox AM; shaded symbols are" for the HS sols. For comparison, the mobilities of the sols in the absence of aluminum nitrate (circles) are included in the figures. Both colloidal silicas in 1.0 X 10-4 M aluminum nitrate undergo an initial increase in their negative mobilities which is very similar to the increase in the absence of aluminum nitrate. At ptI = 2.75 the mobilities reach a maximum, and then, as the pH is increased, pass through a minimum at p H 3.5. The maxima and minima appear to lie at the same pH for both soD. Above p H ~-~ 3.5 the mobilities of the two sols increase rapidly and approach asymptotically their respective curves at zero aluminum nitrate concentration. A similar maximum and minimum were obselared in the presence of 1.0 X 10-~ M lanthanum nitrate (dashed line). The maximum and minimum in the mobility curve in the presence of lanthanum nitrate are situated at the same p i t values as in aluminum nitrate solution, and the chief difference in the two curves is that the mobility in -4
E
-.3 ~
~>
I
i
LUDOX COLLOIDAL SILICA 0.2 ~rn/~00cc
"~
!
I i
0
/
/ ' ~ ' ~ D ~
,
,
o --
>_"
0-I
o
~
, ,k Ludox HS o • Buffer onl zx, [AI(NO3)3]=IO -'~ M [:3 [Lo(NO~)3]=I0-4 M i
2
6
i
8
pH
FIG. 7. Electrophoretic mobilities ot Ludox AM (open symbols) and I-IS (shaded symbols) as a function of pH. The circles denote the mobility in the presence of buffers only; the triangles give the mobilities of the sols, buffered in the same way, in the presence of 1.0 X 10-4 M aluminum nitrate. Dashed line and squares give the mobilities in the presence of 1.0 X 10-4 M lanthanum nitrate.
71
lanthanum nitrate rises less rapidly in the pH range from 4 to 6. DISCUSSION
(a) Coagulation. Of the two counterions used, aluminum hydrolyzes much more strongly than lanthanum. Aluminum apparently forms predominantly only one soluble polymeric cationie species, for which the formulations Als(OH)~ + (8) or A17(OH)~+ (10) have been suggested. A hydrolysis constant is available for the latter (10). The hydrolysis products of lanthanum consist of LaOH 2+, La20H 5+, and Las(OH)~ +, for which equilibrium constants have also been determined. The composition of the eleetrolyte solutions along the coagulation boundaries of the aluminum and lanthanum salt domains (Fig. 5) have been calculated, and it was found that except at relatively high aluminum salt concentrations ( > 3 X 10-2 M) the concentration of all hydrolysis products was negligible up to pH 4.5. Thus the data for the critical coagulation concentrations as a function of pH can be discussed in terms o f nonhydrolyzed counterions. It was found with nonhydrolyzed electrolytes (2) that. ion exchange plays a predominant role in the destabilization of colloidal silica. These studies included lanthanum at low pH. There is considerable evidence that ion exchange of unhydrolyzed aluminum ions on silica is responsible for the destabilization of Ludox HS and AM by aluminum salts at low pH. Unfortunately, the pH titration technique is not suitable for measuring amounts of ion exchange of aluminum because of hydrolysis. Figure 5 shows that the coagulation boundary for aluminum ion bears a striking resemblance to that of the same sol destabilized by lanthanum ion, where the ionexchange mechanism of coagulation has been shown to apply. The horizontal portions of the coagulation boundaries for La 3+ and A1a+ are found at almost the same salt concentration (2 × 10.4 M). It was demonstrated for La ~+ that the critical coagulation concentrations under these conditions are influenced by depletion of the metal ion
Journal of Cdloid and Interface Science, Vol. 35, N o . 1, J a n u a r y 1971
72
ALLEN AND MATIJEVIC
concentration b y exchange. The fact that the c.c.c, in this region is the same for both A1s+ and La 3+ is further evidence t h a t ion exchange of A13+ is an important factor in the coagulation process. As would be expected for a system destabilized b y ion exchange, there is a large sol concentration effect at low aluminum nitrate concentrations (Fig. 3) due to depletion of metal ions from the suspending medium. It is possible to treat this effect quantitatively; from the differences in c.c.e. ~t two different sol concentrations the critical degree of cation exchange for destabilization can be calculated. An example of this, based on the coagulation domains of Ca ~+, is given here. Figure 7 in reference 1 shows that at p H 9 the critical coagulation concentrations of the 0.20 and 2.0 gm/100 cc sols are 2.5 X 10-3 and 6.3 X 10-3 M, respectively. Letting X be the amount of Ca 2+ exchanged (expressed in moles/liter) at the c.c.c, of the 0.20 gm/100 ce sol, the equilibrium concentrations of Ca 2+ at the c.c.c, are (2.5 X 10-3 - X) M for the dilute sol, and (6.3 X 10-2 -- 10X) M for the concentrated sol. Since both of these systems are at the same p H these two equilibrium concentrations of Ca 2+ must be equal, to give the same degree of ion exchange at the c.c.c. Thus the expressions can be equated. Solving for X we find that 4.2 X 10-~ moles of Ca ~+ are exchanged per liter of sol in the 0.20 gin/100 cc system. This quantity is easily converted to moles of Ca 2+ per gram of Ludox, and thence to meq O H - / g m silica, assuming that two hydrogen ions are released per calcium ion exchanged. Values of the critical amount of ion exchange determined in this way for Ca 2+ are plotted (circles) against the pH in Fig. 8. For comparison, the critical exchange curve for Ludox HS, as determined from titration experiments with monovalent cations, is represented by the solid line (2); the squares denote data obtained using this technique with Ca 2+. In view of the relatively large experimental error involved in the method employing two sol concentrations, the data are in excellent agreement with the titration-derived data for Ca + (squares). The dashed, line as drawn through the points is strikingly similar to the assumed exchange line which was used in the attempt to
.c
O
[.2--LUDOX HS-25 ° C
200
o
I
-
_
I
7
I
8
9
_
l
I0
II
pH
FIG. 8. Critical amounts of exchange of calcium ion on Ludox ItS (circles) as calculated from the coagulation domain boundaries at two different sol concentrations. The solid line is the critical exchange curve of Ludox HS for monovalent cations, determined from t~tration and coagulation experiments (2). Squares denote data for calcium ion obtained using the latter method. quantitatively interpret the critical exchange curve for Ludox HS (reference 2, Fig. 11). For aluminum the critical amount of ion exchange, which was calculated in the same way as shown above for calcium, is 0.033 mmole Al3+/gm silica. This value holds over the pH range 3.8-4.5 and represents a reasonable extension of the critical exchange concept as represented for Ludox HS in Fig. 8 (solid line). This tends to substantiate that exchange of unhydrolyzed aluminum ions brings about destabilization. In view of the similarity in position of the log[Al(NOa)3]-pH domains of Ludox HS and AM (Fig. 3), it appears very probable that the coagulation of Ludox AM by aluminum nitrate (triangles) is also governed by the cation exchange of unhydrolyzed aluminum ions, at least for [Al(NO3)3] < 7 X 10-3 M. At the low concentration ends of the coagulation boundaries a somewhat lower c.c.c. is evident for the AM sol. This may possibly be due to dissolution of some of the alumina from the particles The coagulation domains for the destabilization of Ludox HS b y aluminum sulfate and by aluminum nitrate differ significantly (Fig. 4). At any given pH less than 4, it requires considerably more aluminum sulfate than aluminum nitrate to coagulate the sol. The discrepancy is particularly large at low pH, where the c.c.c.
Journal of Colloid and Interface Science, Vol. 35, N o . 1, J a n u a r y 1971
STABILITY OF COLLOIDAL SILICA of aluminum sulfate is about ten times as high as that of aluminum nitrate. Since the coagulation boundary for aluminum sulfate is also obtained over the concentration and pH range which precludes the existence of hydroxylated species, the observed differences should be due to aluminum sulfate complexing. The only cationic complex reported to exist in acidic solutions containing aluminum and sulfate ions is AISO4+. Since this complex carries a + 1 charge, it would require much higher concentrations and p}i values to be effective as a coagulant. Hence it is reasonable to assume that the unhydrolyzed and uncomplexed A13+ coagulates Ludox }IS from aluminum sulfate solutions, and that the increase in the c.c.c, is due to the removal of some of the A13+in the form of the complex ALSO4+. With these assumptions it is possible to calculate the concentrations of ALSO4+ and A13+ along the coagulation boundary using the stability constant for AISOd+: K -
[AISOd+] [A1i+] [SO~-] "
[1]
The equilibrium concentratior_s of free aluminum and sulfate ions can be expressed in terms of [ALSO4+] and [A13+],oo (the concentration of A13+ at the c.c.c.) as follows: [At3+] = [A13+]o,¢- [A1SOd+];
[2]
[SO~-] = 1.5 [AI~+]ooo- [AISOa+]
[3]
+ [g2SO~]; where [}I2SO4] is the concentration of sulfate introduced by adjusting the p}i with sulfuric acid. This term was negligible for [A12(S04)3] > 5 X 10-3 M and also for pH >5 for points along the coagulation boundary. At very low pH the species }ISOc is generated; however, calculations of the concentrations of this ion for conditions along the coagulation boundary showed it to be present in concentrations not exceeding 2 % of the equilibrium sulfate ion concentration, ~ which is considerably less than the experimental error. 5 In these computations the dissociation c o n s t a n t K I = [H+][SO~-]/[HSOc] = 1.03 X [0-~ m o l e / 1 w a s e m p l o y e d (17).
73
Combination of Eqs. [1]-[3] yields the quadratic equation [AISQ+]2 -- B[A1SO~+] ~- C -- 0,
[4]
where B = 2.5 [A13+]oco+ [H2SO4]+
1/K
[51
and C-- [A13+]co,(1.5 [A13+]ooc+ [H2SO41). [6] Four values of K are available from the literature: 1.1 X 102 1/mole (18), 3.7 X 102 1/mole (13), 1.6 X 103 i/mole (19), and 5.4 X 103 I/mole (20). The results of the calculations using K = 3.7 X 102 1/mole are presented in Table I. The first two columns give the data for nine points along the coagulation boundary of the log [AP÷]coc-pH domMn in Fig. 4. In the third and fourth columns are listed the calculated equilibrium concentrations of ALSO4+ and A1a+ at these points. The fifth column gives the c.c.c, of aluminum nitrate corresponding to the p}i values listed in the first column. Examination of the right-hand two columns of Table I reveals that the concentration of Al3+ at the c.c.c. of aluminum sulfate is, within the experimental error, the same as the c.c.c, of aluminum nitrate for every pH value. In fact, the agreement of corresponding values in the two columns is excellent in view of the steepness of the slopes of the coagulation boundaries at [Ala+],oc > 2.3 X 10-4 in Fig. 4. Calculations using the other values for the complex constant of ALSO4+ did not give consistent results. This correlation is quantitative evidence in support of the concept that unhydrolyzed A13+ is the coagulant in the destabilization of Ludox }IS for conditions along the coagulation boundary. The results also support the validity of the stability constant for ALSO4+ (K = 3.7 X 102 1/mole), which was originally obtained from stability studies with lyophobic sols (13). Thus far evidence has been presented which indicates that ion exchange plays a key role in the destabilization of both Ludox sols by aluminum nitrate at low p}i. At higher p}i values, i.e., within the coagulation region (shaded area in Fig. 5) by-
Journal of Colloid and Interface Science, VoL 35, No. 1, January 1971
74
ALLEN AND MATIJEVIC TABLE
I
COMPAriSON OF [AL ~+] AT C.C.C. OF AL2(SO4)8 AND AL(NO3)3 pH 3.43 3.39 3.42 3.46 3.51 3. 62 4. 00 5. 00 5.50
c.c.c, of A12(SO4)3a 1.0 5.0 2.0 1.0 5.0 1.0 2.3 2.3 2.3
X X X X )< X )< )< X
10-1 10.5 10-2 10-5 10.8 10-3 10-4 10 -4 10-4
[AISO4+1 at c.c.c,b 9.5 4.6 1.7 7.4 3.2 3.6 4.3 2.2 2.4
× × X X X X X X )
10-2 10-2 10-2 10-8 10-3 10-9 10-5 10-~ 10-8
[A18+] at c.c.c, of AI~(SO4)8 4.7 4.2 3.4 2.6 1.8 6.4 1.9 2.1 2.1
)< X X X )< ;< X X X
10-a 10.3 10-~ 10-8 10-8 10-8 10-4 10-4 10-4
c.c.c, of AI(NO~)~ 4.0 5.7 3.8 2.4 1.4 4.5 2.3 2.3 2.3
X }( X X )< X X X X
10-8 10-~ 10-8 10 -8 10-8 10-4 10-4 10-4 10-4
" I n i t i a l c o n c e n t r a t i o n of A13+ ( m o l e s / l i t e r ) o n t h e b a s i s t h a t A12(SO4)3 d i s s o c i a t e s c o m p l e t e l y t o A1~+ a n d SO~-. b K = 3.7 Y, 102 1 / m o l e (13).
drolyzed aluminum or lanthanum species become more abundant and ion exchange is not necessarily the only mechanism for sol destabilization. Within the coagulation region a number of different processes, isolated or simultaneous, can take place as enumerated below: 1. The ion exchange of the nonhydrolyzed eounterion. 2. The cation exchange of hydrolyzed species such as:
with colloidal alumina, and vice versa, at pH 4. 5. The enmeshment of silica particles in aluminum hydroxide floes. This has been suggested to occur in the "sweep zone", i.e., the aluminum hydroxide precipitation region (30). The last two eases will be discussed in greater detail in a subsequent communication (31). The experimental procedures employed in this work do not permit a distinction between ~ S i O H + L a ( O H ) (H20)~ + ,~the mechanisms listed above, especially in ~ S i O L a ( O H ) (H~O) ~+ + H~O + the absence of aluminum hydroxide precipitates. Indeed, it would be very difficult to This type of exchange has been suggested distinguish between mechanisms (2) and (3) for aluminum (21-24) and for a number of by any other experimental technique. other hydrolyzable metal ions (22, 25). (b) Electrophoresis. For nonhydrolyzing How easily polymeric hydrolysis products coagulants it was previously remarked that can exchange with protons and whether there is very little correlation between the such exchange, if it takes place, is equivalent stability of silica and its eleetrophoretie are still open questions. mobility. This appears to be also true for 3. The formation of surface complexes by hydrolyzable coagulants. With increasing the condensation reaction of hydrolyzed ions aluminum nitrate concentrations the mowith surface silanol groups, such as bilities of both Ludox AM and HS at pH 3.15 decrease to zero and then become positive -=--SiOH + L a ( O H ) (H2C)s2+ (Fig. 6). The charge reversal has no effect on ~SiOLa(H20)~ + + H20 the shape of the coagulation domain of This reaction has been suggested for hy- Ludox HS (Fig. 3); log e.e.e, is a linear drolysis products of Fe 3+ (27) and Cu 2+ function of pH irrespective of particle (28). charge. Ludox AM does have a bend in its 4. The mutual coagulation of positively coagulation domain near its point of charge charged alumina particles and the negative reversal at pH 3.15. However, at high eoaguparticles of silica; Iler (29), for example, has lant concentrations there is no large stabilizashown that silica particles can be coated tion region in the domain (Fig. 3) comparable Journal of Colloid and Interface Science, Vol. 35, No. 1, January 1971
STABILITY OF COLLOIDAL SILICA to those observed with hydrophobic sols (12, 14). The minima in the plots of the mobilities of Ludox HS and AM as a function of pH in 1.0 X 10-~ M aluminum nitrate (Fig. 7) also exist without any corresponding effect in the shapes of the coagulation domains in Fig. 3. The reason for these minima is not understood. Kohlschiitter et al. (21) observed a very strong uptake of aluminum by silica gel over the pH range 3.8-4.2 at higher concentrations of aluminum chlm~de ( > 1 X 10-~ M), which they attributed to the exchange of hydrolyzed species. On the other hand, a similar minimum in the electrophoretic mobility of Ludox AM was observed over this p i t range in 1.0 X 10-4 M lant h a n u m nitrate (Fig. 7). Under these conditions there is no appreciable hydrolysis of La 3+ in solution according to calculations based on the available stability constants (11). One should, however, consider the possibility that surface hydrolysis can take place even when the conditions in the bulk do not produce hydrolyzed species. The cause for charge reversal of silica b y aluminum ions is not clear at this point. I t is now a well-established fact that the charge of lyophobic colloids is reversed by the adsorption of hydrolyzed metal ions (32). This has been demonstrated with aluminum salts which can reverse the charge of silver halides or latices when added in low concentrations if the p H is sufficiently high ( > 4 ) (8, 12, 14). I t was also shown that very strongly hydrated sols, such as microerystalline cellulose (33), cannot be recharged by hydrolyzed metal ions regardless of concentration and pH. Charge reversal of silica takes place only at rather high concentrations of aluminum salts. Under these conditions even at p H 3-4 the concentration of hydrolyzed species may possibly become sufficient to cause charge reversal, most likely due to oxygen bridging with the surface silanol groups. O'Melia and Stumm (4) have observed charge reversal of silica b y ferric ions in much lower concentrations. However, this is consistent with the fact that ferric salts hydrolyze considerably easier than the aluminum salts.
75
REFERENCES 1. ALL~N, L. H., AND MATIJ~Vld, E., J. Colloid Interface Sci. 31,287 (1969). 2. ALLEN, L. tI., AND MATIJEW~, E., J. Colloid Interface Sci. 33, 420 (1970). 3. STUMM,W., AND O'MELIA, C. R., J. Amer. Water Works Ass. 60,514 (1968). 4. O'MELIA, C. R., AND STUMM,W., J. Colloid Interface Sei. 23,437 (1967). 5. HAHN, H. H., AND STU~M, W., J. Colloid Interface Sci. 28,134 (1968). 6. HAHN, It. H., AND STUMM,W., Advan. Chem. Set. 79, 91 (1968). 7. MATIJEVId,E., ANDJANAVER,G. E., J. Colloid Interface Sci. 21,197 (1966). 8. MATIJ~Vld, E., MATgAI, K. G., OTTE~VILL, R. H., AND KERKER, M., J. Phys. Chem. 65,826 (1961). 9. MATIJEWd, E., AND ST~YKER,L. J., J. Colloid Interface Sci. 22, 68 (1966). 10. BIEDERMANN, G., Svens~ Kent. Tidskr. 76, 362 (1964). 1l. BIEDERMANN, G., AND CIAVATTA, L., Acta Chem. Scan& 15, 1347 (1961). 12. MATIJEVld, E., JANAUER, G. E., AND KERKER, M., J. Colloid Sei. 19,333 (1964). 13. STRYKER,L. J., AND ~/[ATIJEVId,E., J. Phys. Chem. 73, 1484 (1969). 14. MATIJEVI~, E., AND FORCE, C. G., KolloidZ. Z. Polym. 225, 33 (1968). 15. MATIJEVld, E., AND ALLEN, L. H., Environ. Sci. Technol. 3, 264 (1969). 16. F]~ITKNECHT, W., AND SCHINDL]~R, P., Pure Appl. Chem. 6, 130 (1963). 17. DAWES, C. W., JONES, H. W., AND MONK, C. B., Trans. Faraday Soc. 48,921 (1952). 18. NANDA, R. ~5~., AND ADITYA, S., Z . Phys. Chem. (Frankfurt am Main) 35,139 (1962). 19. B~Ha, B., AND WnNDT, H., Z. Elektroehem.
66, No. 3, 223 (1962). 20. NISHIDE, T., AND TSUCHIYA,R., Bull. Chem. Soc. Jap. 38, 1398 (1964). 21. KOHLSCH{.~TTER, I7I. W., GETROST, 1-I., AND MIED~AN~, S., Z. Anorg. Allg. Chem. 308,
190 (1961). 22. VVl)RA, F., AND GALBA, J., Z. Anal. Chem. 235, 166 (1968); Collect. Czech. Chem. Commun. 32, 3530 (1967). 23. BOEnM, H. P., AND SCH~EDER, M., Z. Anorg. Allg. Chem. 316, 128 (1962). 24. STIGTER, D., BOSMAN, J., AND DITMARSCH, R., Ree. Tray. Chim. Pays-Bas 77,430 (1958). 25. CLARK, S. W., AND COOKE, S. R. B., Trans. A I M E 241,334 (1968). 26. BOEnM, H. P., AND SCI~NmDER,M., Z. Anorg. Allg. Chem. 301,326 (1959).
Journa~ of Colloid and Interface Science, Vol. 35, No. 1, January 1971
76
ALLEN AND 1V~ATIJEVI~
27. MACKENZIE, J. M. W., Trans. A I M E 235, 28. 29. 30. 31.
82 (1966). ]SKI{A, J., AND LASKOWSKI, J., Inst. Mining Met., Trans. Sect. C 78, 113 (1969). IL~I~, IZ. K., J. Amer. Ccram. Soc. 47, 194 (1964). PACKHAM,R. F., Proc. Soc. Water Treat. Exam. 12, 15 (1963). MAWlJEVlS, E., MANGRAVITE,F. J., JI~., AND
CASSELL, E. A., J. Colloid Interface Sci., Submitted. 32. MATIJEVIS, E., in "Principles and Applications of Water Chemistry," pp 328-369. Wiley, New York, 1967. 33. KRATOHVII~,S., JANAUER, G. E., AND MATIJEVIS, E., J. Colloid Interface Sci. 29, 187 (1969).
Journal of Colloidand Interface Science, Vol. 35, No. 1, January 1971
Received April 10, 1970; accepted July 20, 1970 The stabilities of the silica hydrosols Ludox HS and Ludox AM have been studied as a function of pH in the presence of various concentrations of several hydrolyzable electrolytes. "Log electrolyte concentration-pH" domains are given for the HS sol in AI(NOa)a and A12(SO~)a solutions and for the AM sol in Al(NOa)a and La(NOa)a solutions. The ion-exchange mechanism of destabilization, which was previously established for the coagulation of silica with nonhydrolyzable electrolytes and for La(NO3)a at low pH, is shown to hold for Al(NOa)a and A12(SO0s along the coagulation boundaries at pit < 4.5 and [A1a+] < 3 X 10-2 M. Unhydrolyzed, hydrated A1a+ is the coagulant under these conditions for both of the aluminum salts. The difference in the coagulation behavior of aluminum sulfate and aluminum nitrate could be explained quantitatively by considering the formation of ALSO4+. Electrophoretic mobilities, determined by the moving boundary technique, are presented for conditions close to the coagulation boundary. Very little correlation was observed between the c.e.c, and the mobility. At low concentrations the shapes of the coagulation boundaries are determined by depletion of the bulk cation concentration due to exchange. Under these conditions the critical amounts of cation exchange of A1s+ and Ca~+ necessary for the destabilization of Ludox HS were calculated from the c.c.c, at two different sol concentrations. INTRODUCTION The stability of colloidal silica in the presence of simple electrolytes, particularly those containing cations which undergo no hydrolysis over the p H range studied, has been discussed in detail in the first paper of this series (1). I t was shown that ion exchange of such counterions for silanolic protons plays the dominant role in the destabilization of aqueous silica so]s (1, 2). Any effect of p H in these studies had to be interpreted in terms of the surface characteristics of silica particles.
When hydrolyzable counterions are added to a silica sol a change in the p H affects simultaneously the composition of the electrolyte medium and the properties of the particle surface. Thus both parts of the system should be well understood if one is to interpret the interactions of such ions with colloidal silica. The only other detailed studies of the effects of hydrolyzed metal ions upon the stability of silica seem to have been carried out by S t u m m and collaborators (3-6). These involve ferric and aluminum salts. Unfortunately, owing to very strong aging effects, which take place rapidly even at room temperature (7), ferric ions do not ]end themselves to a quantitative interpretation of stability phenomena. With aluminum salts, H a h n and S t u m m ' s investigations dealt mainly with the rate of coagulation.
1 Parts I and II, see references 1 and 2. 2 Supported in part by the Federal Water Pollution Control Administration, Grant WP00815. s Part of a Ph.D. thesis by L. I=[. Allen. The support of a Texaco Fellowship is gratefully acknowledged.
Journal of Colloidand InterfaceScience,Vol.35, No. 1, January 1971
66
STABILITY OF COLLOIDAL SILICA Here we report a systematic study of the stability of silica sols (Ludox HS and AM) in the presence of various concentrations of Al(NO3)3, A12(SO4)3, and L a ( N Q ) 3 as a function of pH. Certain observations with calcium salts will also be discussed. Aluminum and lanthanum ions have been chosen because the complex chemistry of their hydrolysis products is now fairly well understood (8-11). In addition the effects of these ions upon the stability of lyophobic colloids has been extensively studied (8, 9, 12-15). Thus, an analysis of the differences in the behavior of a hydrophilic colloid, such as silica, in the presence of the same ions is possible. The "silica-aluminum salt" systems are of
interest in many applications, among which water purification, flotation, cracking catalysts, and gel preparations are but few. EXPEI~IMENTAL The procedures for the determination of the critical coagulation concentrations (c.e.c.), the electrophoretic mobilities, and amounts of cation exchange have been described in great detail earlier (1, 2). Ludox HS and AM (E. I. du Pont de Nemours and Co.) have been used in this work. The lot numbers and the properties of these silica sols have also been given previously (1, 2). Precautions to avoid aging were taken when dilute solutions of hydrolyzing salts were required. In the preparation of aluminum and lanthanum salt solutions care was exercised in keeping the temperature below 25°C during the dissolution process. In the case of aluminum salts, solutions at concentrations < 2 X 10-2 M were prepared freshly for each experiment by diluting more highly concentrated stock solutions. The Ah(S04)3 stock solution, which showed a Tyndall cone when illuminated, was filtered through a 1Viillipore filter (0.22 pore diameter) until optically clear. All the salt solutions were analyzed for the metal ion content by means of appropriate gravimetric procedures. RESULTS
(a) Coagulatior~. The
coagulation process was followed by measuring the Rayleigh ratios of the silica sols in the presence of
O.3
67
I LUDOX AM 0.2 gm/lO0 ( [ hr 25°C --
0.2
--I.~
n
33t
U-
<~
~, o.~ gt:
2.8
5.0
5.2
3:4
5.6
3~
pH
FIG. 1. Rayleigh ratios as a function of pH for a Ludox AM sol in the presence of five different concentrations of aluminum nitrate. Al(NO3)3: (&) 1.12 X 10-2M; (O) 6.30X 10-a M; (~) 2.00 X 10-2M; (D) 5.01 X 10-2M; (V) 5.01 X 10-4M.
various concentrations of a given electrolyte at a constant pH, or at a constant concentration of the electrolyte with p H as the independent variable. Examples of the latter are shown in Fig. 1. Each coagulation curve is for a different, but constant, aluminum concentration and each point is for a different p H value as indicated on the abscissa. The p H of the systems was adjusted by the addition of very small amounts of concentrated nitric acid or sodium hydroxide with an automatic titrator. High Rayleigh ratios denote sols where coagulation has occurred. The critical coagulation concentration (c.c.c.) at a given p H or the critical p H at a given salt concentration corresponding to the c.c.c, was established by extrapolating the steepest portion of such coagulation curves to the Rayleigh ratio of the uncoagulated sol. All the data in Fig. 1 were taken 1 hour after -mixing the reacting components; this was found to be the "critical time" for colloidal silica 4 (1). Figure 2 gives the Rayleigh ratios of the Ludox AM silica sols as a function of the aluminum nitrate concentrations for two different p H values. Some of the coagulated systems settled before one hour, the time when the measurements were made. These systems were redispersed by shaking prior 4 For a detailed analysis of the data giving this critical time see the thesis by L. H. Allen: "Stability of Colloidal Silica," Clarkson College of Technology, Potsdam, New York, 1970.
Journal of Colloid and Interface Science,
Vol. 35, N o . 1, J a n u a r y
1971
68
ALLEN AND MATIJEVIC
0.4-
'
o
\0.6
I
LUDOX AM 0.2 gm/lO0 cc
~
~
+1
"%_1
--
__ ~ _ _ [ h r
,
LUDOX COLLOIDAL i SILICA
250c
o
o~
""r MOB.
/
1 R.
$Z
>:
_s
z I
I R.R.
I
, I I I
=o
i
o •
-2,0 LOG.
-2.5 MOLAR
-3.0 CONC. OF AI(NO~) 3
-3.5
FIG. 2. Rayleigh ratios as a function of the concentration of aluminum nitrate for a Ludox AM sol at p H 3.2 (circles) and pH 4.0-4.2 (triangles). Low Rayleigh ratios correspond to uncoagulated systems. Shaded symbols denote settled systems redispersed by shaking before the measurement. At a pH of 3.2 the sol exhibits both coagulation and reversal of charge; at p H 4.0-4.2 no stabilization was Observed before the onset of aluminum hydroxide precipitation. Dashed line gives the mobilities of the sol at pH 3.2 in the presence of various concentrations of AI(N03)~.
to making the measurements. A smooth line can be drawn through the tlayleigh ratio points so determined. At pH 3.2 two stability regions are observed, one at low and the other at higher molarities of Al(N03)3. The dashed line, which gives the corresponding mobilities of silica particles, shows that the restabilization at higher salt concentrations is accompanied by charge reversal. If a sufficient number of experiments, such as are shown as examples in Figs. 1 and 2, are carried out, the entire stability domain of the colloidal system can be obtained as a function of the salt concentration and pH (12, 14). The "log [AI(NOs)3]-pH" stability domains for Ludox AM and for Ludox HS (at two sol concentrations) are given in Fig. 3. Points obtained by either of the two procedures demonstrated in Figs. 1 and 2 lie on the same boundary. The dashed line indicates the conditions under which, with no silica sol present, the first traces of aluminum hydroxide precipitation were detected in a Tyndall beam as reported earlier (12) with the exception of the squares, which were determined in order to extend the dashed line to the silica coagulation
%
-
-
o o, -4
~oLudox
HS , 0 . 2 g m / l O O c c _
• L u d 0 x H S , 2 clm/lO0 cc z~ & L u d c x A M , 0.2 gm/lO0 cc - - - A l u m i n u m Hydroxide
6
4
8
pH
FIG. 3. Log [AI(NO3)8]-pH domain of Ludox HS at two concentrations (0.2 gm/100 co, open circles, and 2 gm/100 cc, shaded circles) and of Ludox AM (0.2 gm/100 cc, triangles). In the latter case points obtained by different techniques are differentiated by flags; ( ~ ) and (A) designate [AI(NO3)~] and p H as the independent variable, respectively. The dashed line gives the conditions under which aluminum hydroxide precipitate was first detected in a Tyndall beam, in the absence of silica sol (12). Squares denote additional points for aluminum hydroxide precipitation added in this work.
boundaries (solid lines). It is evident that the domains of the two sols are fairly similar in position but differ to a small degree in shape. The precision of the data in the relatively vertical portions of the coagulation boundaries in such that the difference in the shapes of the two domains is significant. At the higher sol concentration (2 gin/100 co) the vertical portion of the coagulation boundary is shifted v~eryslightly to lower pH values as compared to the corresponding boundary at one-tenth this sol concentration. A cheek run using a Beckman DU and cells of several optical path lengths down to 0.05 em gave the same c.c.c, as the Briee Phoenix instrument. This would tend to indicate that the difference in coagulation boundaries is not due to multiple scattering in the more concentrated system. The stability boundaries show that silica
Journal of Colloid and Interface Science, Vol. 35, No. 1, January 1971
STABILITY OF COLLOIDAL SILICA
~
_
~
J
L
LUDOX HS 02 gm/tO0cc I hr , 25~C
-I
I
69
LUDOX AM -I
312"1 d z
o
-:5
g~ ~E ._i_~
j
\
2~
UNCOAGULATED k SOLS - - ~ ' "
I
o
,' -- -- -- ~
8J -4
;~"
COAGULAI ////// \\\\\\
Am~(so~)~
A AI(NO3)3 ------
Hydroxide Precipitote Boundary
4
5
-5
6
pH
FIG. 4. The log [A13+]-pH domain for a Ludox HS sol (circles, solid line) in the presence of aluminum sulfate. Superimposed on these data is the corresponding domain for aluminum nitrate (triangles, solid line). The dashed line gives the conditions in aluminum sulfate solutions under which the first traces of hydroxide sol can be detected in a Tyndall beam, in the absence of silica sol (14). sols remain stable at low p H values even at aluminum nitrate concentrations as high as 0.1 M. This behavior is distinctly different from that observed using lyophobic colloids (12, 14). The stability domain of Ludox HS in the presence of aluminum sulfate is given in Fig. 4 (circles). Plotted on the ordinate of this figure is the molar concentration of A13+ on the basis t h a t aluminum sulfate dissociates completely to give only A13+ and SO 2-. For comparison, the corresponding domain for aluminum nitrate is superimposed (triangles). Below p H 4 larger concentrations of A12(SO4)3 t h a n of AI(NO~)3 are required to produce coagulation. The horizontal portions of the two domains are in excellent agreement. The dashed line indicates the boundary for the region in which a hydroxide precipitate is formed in aluminum sulfate solutions in the absence of colloidal silica (14). Two of the points for the aluminum sulfate coagulation boundary lie inside the hydroxide precipitate region. Since the initial
2
4
6 pH
8
I0
FIo. 5. Comparison of the log [AI(NO3)3]-pH domain of a Ludox AM sol with its log [La(NO3)3]pH domain. The hatched areas designate the coagulation regions. To the right of the dashed line bounding each coagulation zone lies the region where the corresponding hydroxide precipitate is formed in the absence of silica sol. For lanthanum nitrate the dashed line is calculated from the known solubility product (16), and the triangles denote data obtained in this work. concentration of aluminum ion, which is plotted on the ordinate, is depleted b y ion exchange, the equilibrium concentration of aluminum ion for these two points may be low enough to be outside the hydroxide precipitation range. A comparison of the coagulation domains of Ludox AM in the presence of aluminum nitrate and lanthanum nitrate (2) is given in Fig. 5. The shaded area with hatching of positive slope denotes the coagulation region of aluminum nitrate; that of negative slope is for the conditions for coagulation with lanthanum nitrate. The coagulation regions are bounded on the right b y their respective hydroxide precipitate boundaries. For lant h a n u m nitrate the position of this line was calculated from the solubility product as determined b y Feitknecht and Schindler (16). As a check several runs were made to determine the boundary conditions under which colloidal lanthanum hydroxide is formed (triangles). These were detected by
Journa$ of CoUoid and Intsr/acs Science, VoL 35, No. 1, J a n u a r y 1971
70
ALLEN AND MATIJEVIC I
E
~
0
[
L
LUDOX COLLOIDALS,L,CA
(N~z,
AI(NO~)~ A Lo(NO3)3~ o hi(N03)3 LudoxHS 001 M I Glycine-HNO~ Buffer •
-5
-I
-2 -3 LOG. MOLAR CONC. OF SALT
T
-4
-5
FIG. 6. Electrophoretic mobilities of Ludox AM and HS in the presence of various concentrations of Al(NOs)s and La(NOs)s. The mobilities of Ludox AM, at pH = 3.15, are given as a function of the logarithm of the molar concentrations of AlCNO3)s and La(NO3)3 by circles and triangles, respectively. The concentration ranges over which these salts produce coagulation at this pH are designated by the hatched intervals. Squares give the mobilities of Ludox HS in the presence of aluminum nitrate at pH 3.15. Blackened triangles and dashed line are for Ludox AM in the presence of La(NO~)s at pit = 2.0. observing the first appearance of the Tyndall beam in systems prepared in the same way as the coagulation runs, but without silica. The experimental points thus obtained are in excellent agreement with the solubility product line. (b) Electrophoresis. Eleetrophoretie mobil, ities of Ludox silicas as obtained from moving boundary measurements (1) in the presence of various eoncentrations~ of Al(N03)3 and La(NO3)s are given in Fig. 6. All curves but one are for pH 3.15. Thus, the mobilities were taken along the vertical portions of the aluminum nitrate coagulation boundaries, just outside the coagulation region of Ludox HS (Fig. 3). The coagulation boundary of Ludox AM extends to pH values lower than 3.15 over a small range of concentrations which is indicated by the upper hatched interval along the line of zero mobility. At the low concentration ends both of the mobility curves approach the mobility of their respective sols at p H 3.15 in the absence of aluminum nitrate. At concentrations of aluminum nitrate above 2.5 )4 10.3 M for Ludox tIS and 1.0 X 10-2 M for
Ludox AM the electrophoretic mobilities at a p H of 3.15 are positive. In contrast to their behavior in aluminum nitrate, neither of the colloidal silicas underwent a reversal of charge in the presence of high concentrations of lanthanum nitrate at pH 3.15. Represented by the lower hatched region on the zero mobility line is the range of concentrations of lanthanum nitrate over which the sol is coagulated (Fig. 5). The curve for the mobility of the sol in the presence of lanthanum nitrate (triangles) does not pass through zero, but, instead, remains in the region of negative mobilities. Further work at this pI-I with higher concentrations of lanthanum nitrate revealed a small, but still negative, mobility. Because the electrophoretic studies at p H 3.15 in the presence of various concentrations of lanthanum nitrate lacked data over the rather broad coagulation range, runs were also performed at a p H of 2.0, where there was no coagulation. The resulting mobility curve is given b y dashed lines and blackened triangles. The lack of any maxima or minima in the curve at pH 2.0 over the region of La(NO3)s concentrations from ~-~5 X I0 -4 to 4 X lO-2M suggests that such extrema are also absent in the mobility data at p H = 3.15, and that the curve through the circles is correct as drawn. It was not possible to determine whether or not lanthanum nitrate could reverse the charge of Ludox A~[ at concentrations greater than 0.1 M and at pH values higher than 3.15. A series of eleetrophoretic experiments in, say, 0.2 M lanthanum nitrate and for various p i t values between 3 and 6 would yield definitive information about the possible existence of positive mobilities. Unfortunately, however, at these electrolyte concentrations the conductivities of the suspending media are very high and the moving boundary technique breaks down. Because of this it was impossible to take electrophoretic data to resolve the problem. In the case of Ludox HS, a single run was performed in 0.1 M lanthanum nitrate at a ptI of 4.4. The mobility observed was zero. Just outside the low concentration ends o f the aluminum nitrate coagulation regions for the two sols (Fig. 3) a number of moving
Journal of Colloid and Interface Science, ¥ol. 35, No. 1, J a n u a r y 1971
STABILITY OF COLLOIDAL SILICA boundary electrophoretic measurements have been made. These were performed in 1.0 X 10-4 M aluminum nitrate and are represented b y triangles in Fig. 7. The open symbols denote mobilities of Ludox AM; shaded symbols are" for the HS sols. For comparison, the mobilities of the sols in the absence of aluminum nitrate (circles) are included in the figures. Both colloidal silicas in 1.0 X 10-4 M aluminum nitrate undergo an initial increase in their negative mobilities which is very similar to the increase in the absence of aluminum nitrate. At ptI = 2.75 the mobilities reach a maximum, and then, as the pH is increased, pass through a minimum at p H 3.5. The maxima and minima appear to lie at the same pH for both soD. Above p H ~-~ 3.5 the mobilities of the two sols increase rapidly and approach asymptotically their respective curves at zero aluminum nitrate concentration. A similar maximum and minimum were obselared in the presence of 1.0 X 10-~ M lanthanum nitrate (dashed line). The maximum and minimum in the mobility curve in the presence of lanthanum nitrate are situated at the same p i t values as in aluminum nitrate solution, and the chief difference in the two curves is that the mobility in -4
E
-.3 ~
~>
I
i
LUDOX COLLOIDAL SILICA 0.2 ~rn/~00cc
"~
!
I i
0
/
/ ' ~ ' ~ D ~
,
,
o --
>_"
0-I
o
~
, ,k Ludox HS o • Buffer onl zx, [AI(NO3)3]=IO -'~ M [:3 [Lo(NO~)3]=I0-4 M i
2
6
i
8
pH
FIG. 7. Electrophoretic mobilities ot Ludox AM (open symbols) and I-IS (shaded symbols) as a function of pH. The circles denote the mobility in the presence of buffers only; the triangles give the mobilities of the sols, buffered in the same way, in the presence of 1.0 X 10-4 M aluminum nitrate. Dashed line and squares give the mobilities in the presence of 1.0 X 10-4 M lanthanum nitrate.
71
lanthanum nitrate rises less rapidly in the pH range from 4 to 6. DISCUSSION
(a) Coagulation. Of the two counterions used, aluminum hydrolyzes much more strongly than lanthanum. Aluminum apparently forms predominantly only one soluble polymeric cationie species, for which the formulations Als(OH)~ + (8) or A17(OH)~+ (10) have been suggested. A hydrolysis constant is available for the latter (10). The hydrolysis products of lanthanum consist of LaOH 2+, La20H 5+, and Las(OH)~ +, for which equilibrium constants have also been determined. The composition of the eleetrolyte solutions along the coagulation boundaries of the aluminum and lanthanum salt domains (Fig. 5) have been calculated, and it was found that except at relatively high aluminum salt concentrations ( > 3 X 10-2 M) the concentration of all hydrolysis products was negligible up to pH 4.5. Thus the data for the critical coagulation concentrations as a function of pH can be discussed in terms o f nonhydrolyzed counterions. It was found with nonhydrolyzed electrolytes (2) that. ion exchange plays a predominant role in the destabilization of colloidal silica. These studies included lanthanum at low pH. There is considerable evidence that ion exchange of unhydrolyzed aluminum ions on silica is responsible for the destabilization of Ludox HS and AM by aluminum salts at low pH. Unfortunately, the pH titration technique is not suitable for measuring amounts of ion exchange of aluminum because of hydrolysis. Figure 5 shows that the coagulation boundary for aluminum ion bears a striking resemblance to that of the same sol destabilized by lanthanum ion, where the ionexchange mechanism of coagulation has been shown to apply. The horizontal portions of the coagulation boundaries for La 3+ and A1a+ are found at almost the same salt concentration (2 × 10.4 M). It was demonstrated for La ~+ that the critical coagulation concentrations under these conditions are influenced by depletion of the metal ion
Journal of Cdloid and Interface Science, Vol. 35, N o . 1, J a n u a r y 1971
72
ALLEN AND MATIJEVIC
concentration b y exchange. The fact that the c.c.c, in this region is the same for both A1s+ and La 3+ is further evidence t h a t ion exchange of A13+ is an important factor in the coagulation process. As would be expected for a system destabilized b y ion exchange, there is a large sol concentration effect at low aluminum nitrate concentrations (Fig. 3) due to depletion of metal ions from the suspending medium. It is possible to treat this effect quantitatively; from the differences in c.c.e. ~t two different sol concentrations the critical degree of cation exchange for destabilization can be calculated. An example of this, based on the coagulation domains of Ca ~+, is given here. Figure 7 in reference 1 shows that at p H 9 the critical coagulation concentrations of the 0.20 and 2.0 gm/100 cc sols are 2.5 X 10-3 and 6.3 X 10-3 M, respectively. Letting X be the amount of Ca 2+ exchanged (expressed in moles/liter) at the c.c.c, of the 0.20 gm/100 ce sol, the equilibrium concentrations of Ca 2+ at the c.c.c, are (2.5 X 10-3 - X) M for the dilute sol, and (6.3 X 10-2 -- 10X) M for the concentrated sol. Since both of these systems are at the same p H these two equilibrium concentrations of Ca 2+ must be equal, to give the same degree of ion exchange at the c.c.c. Thus the expressions can be equated. Solving for X we find that 4.2 X 10-~ moles of Ca ~+ are exchanged per liter of sol in the 0.20 gin/100 cc system. This quantity is easily converted to moles of Ca 2+ per gram of Ludox, and thence to meq O H - / g m silica, assuming that two hydrogen ions are released per calcium ion exchanged. Values of the critical amount of ion exchange determined in this way for Ca 2+ are plotted (circles) against the pH in Fig. 8. For comparison, the critical exchange curve for Ludox HS, as determined from titration experiments with monovalent cations, is represented by the solid line (2); the squares denote data obtained using this technique with Ca 2+. In view of the relatively large experimental error involved in the method employing two sol concentrations, the data are in excellent agreement with the titration-derived data for Ca + (squares). The dashed, line as drawn through the points is strikingly similar to the assumed exchange line which was used in the attempt to
.c
O
[.2--LUDOX HS-25 ° C
200
o
I
-
_
I
7
I
8
9
_
l
I0
II
pH
FIG. 8. Critical amounts of exchange of calcium ion on Ludox ItS (circles) as calculated from the coagulation domain boundaries at two different sol concentrations. The solid line is the critical exchange curve of Ludox HS for monovalent cations, determined from t~tration and coagulation experiments (2). Squares denote data for calcium ion obtained using the latter method. quantitatively interpret the critical exchange curve for Ludox HS (reference 2, Fig. 11). For aluminum the critical amount of ion exchange, which was calculated in the same way as shown above for calcium, is 0.033 mmole Al3+/gm silica. This value holds over the pH range 3.8-4.5 and represents a reasonable extension of the critical exchange concept as represented for Ludox HS in Fig. 8 (solid line). This tends to substantiate that exchange of unhydrolyzed aluminum ions brings about destabilization. In view of the similarity in position of the log[Al(NOa)3]-pH domains of Ludox HS and AM (Fig. 3), it appears very probable that the coagulation of Ludox AM by aluminum nitrate (triangles) is also governed by the cation exchange of unhydrolyzed aluminum ions, at least for [Al(NO3)3] < 7 X 10-3 M. At the low concentration ends of the coagulation boundaries a somewhat lower c.c.c. is evident for the AM sol. This may possibly be due to dissolution of some of the alumina from the particles The coagulation domains for the destabilization of Ludox HS b y aluminum sulfate and by aluminum nitrate differ significantly (Fig. 4). At any given pH less than 4, it requires considerably more aluminum sulfate than aluminum nitrate to coagulate the sol. The discrepancy is particularly large at low pH, where the c.c.c.
Journal of Colloid and Interface Science, Vol. 35, N o . 1, J a n u a r y 1971
STABILITY OF COLLOIDAL SILICA of aluminum sulfate is about ten times as high as that of aluminum nitrate. Since the coagulation boundary for aluminum sulfate is also obtained over the concentration and pH range which precludes the existence of hydroxylated species, the observed differences should be due to aluminum sulfate complexing. The only cationic complex reported to exist in acidic solutions containing aluminum and sulfate ions is AISO4+. Since this complex carries a + 1 charge, it would require much higher concentrations and p}i values to be effective as a coagulant. Hence it is reasonable to assume that the unhydrolyzed and uncomplexed A13+ coagulates Ludox }IS from aluminum sulfate solutions, and that the increase in the c.c.c, is due to the removal of some of the A13+in the form of the complex ALSO4+. With these assumptions it is possible to calculate the concentrations of ALSO4+ and A13+ along the coagulation boundary using the stability constant for AISOd+: K -
[AISOd+] [A1i+] [SO~-] "
[1]
The equilibrium concentratior_s of free aluminum and sulfate ions can be expressed in terms of [ALSO4+] and [A13+],oo (the concentration of A13+ at the c.c.c.) as follows: [At3+] = [A13+]o,¢- [A1SOd+];
[2]
[SO~-] = 1.5 [AI~+]ooo- [AISOa+]
[3]
+ [g2SO~]; where [}I2SO4] is the concentration of sulfate introduced by adjusting the p}i with sulfuric acid. This term was negligible for [A12(S04)3] > 5 X 10-3 M and also for pH >5 for points along the coagulation boundary. At very low pH the species }ISOc is generated; however, calculations of the concentrations of this ion for conditions along the coagulation boundary showed it to be present in concentrations not exceeding 2 % of the equilibrium sulfate ion concentration, ~ which is considerably less than the experimental error. 5 In these computations the dissociation c o n s t a n t K I = [H+][SO~-]/[HSOc] = 1.03 X [0-~ m o l e / 1 w a s e m p l o y e d (17).
73
Combination of Eqs. [1]-[3] yields the quadratic equation [AISQ+]2 -- B[A1SO~+] ~- C -- 0,
[4]
where B = 2.5 [A13+]oco+ [H2SO4]+
1/K
[51
and C-- [A13+]co,(1.5 [A13+]ooc+ [H2SO41). [6] Four values of K are available from the literature: 1.1 X 102 1/mole (18), 3.7 X 102 1/mole (13), 1.6 X 103 i/mole (19), and 5.4 X 103 I/mole (20). The results of the calculations using K = 3.7 X 102 1/mole are presented in Table I. The first two columns give the data for nine points along the coagulation boundary of the log [AP÷]coc-pH domMn in Fig. 4. In the third and fourth columns are listed the calculated equilibrium concentrations of ALSO4+ and A1a+ at these points. The fifth column gives the c.c.c, of aluminum nitrate corresponding to the p}i values listed in the first column. Examination of the right-hand two columns of Table I reveals that the concentration of Al3+ at the c.c.c. of aluminum sulfate is, within the experimental error, the same as the c.c.c, of aluminum nitrate for every pH value. In fact, the agreement of corresponding values in the two columns is excellent in view of the steepness of the slopes of the coagulation boundaries at [Ala+],oc > 2.3 X 10-4 in Fig. 4. Calculations using the other values for the complex constant of ALSO4+ did not give consistent results. This correlation is quantitative evidence in support of the concept that unhydrolyzed A13+ is the coagulant in the destabilization of Ludox }IS for conditions along the coagulation boundary. The results also support the validity of the stability constant for ALSO4+ (K = 3.7 X 102 1/mole), which was originally obtained from stability studies with lyophobic sols (13). Thus far evidence has been presented which indicates that ion exchange plays a key role in the destabilization of both Ludox sols by aluminum nitrate at low p}i. At higher p}i values, i.e., within the coagulation region (shaded area in Fig. 5) by-
Journal of Colloid and Interface Science, VoL 35, No. 1, January 1971
74
ALLEN AND MATIJEVIC TABLE
I
COMPAriSON OF [AL ~+] AT C.C.C. OF AL2(SO4)8 AND AL(NO3)3 pH 3.43 3.39 3.42 3.46 3.51 3. 62 4. 00 5. 00 5.50
c.c.c, of A12(SO4)3a 1.0 5.0 2.0 1.0 5.0 1.0 2.3 2.3 2.3
X X X X )< X )< )< X
10-1 10.5 10-2 10-5 10.8 10-3 10-4 10 -4 10-4
[AISO4+1 at c.c.c,b 9.5 4.6 1.7 7.4 3.2 3.6 4.3 2.2 2.4
× × X X X X X X )
10-2 10-2 10-2 10-8 10-3 10-9 10-5 10-~ 10-8
[A18+] at c.c.c, of AI~(SO4)8 4.7 4.2 3.4 2.6 1.8 6.4 1.9 2.1 2.1
)< X X X )< ;< X X X
10-a 10.3 10-~ 10-8 10-8 10-8 10-4 10-4 10-4
c.c.c, of AI(NO~)~ 4.0 5.7 3.8 2.4 1.4 4.5 2.3 2.3 2.3
X }( X X )< X X X X
10-8 10-~ 10-8 10 -8 10-8 10-4 10-4 10-4 10-4
" I n i t i a l c o n c e n t r a t i o n of A13+ ( m o l e s / l i t e r ) o n t h e b a s i s t h a t A12(SO4)3 d i s s o c i a t e s c o m p l e t e l y t o A1~+ a n d SO~-. b K = 3.7 Y, 102 1 / m o l e (13).
drolyzed aluminum or lanthanum species become more abundant and ion exchange is not necessarily the only mechanism for sol destabilization. Within the coagulation region a number of different processes, isolated or simultaneous, can take place as enumerated below: 1. The ion exchange of the nonhydrolyzed eounterion. 2. The cation exchange of hydrolyzed species such as:
with colloidal alumina, and vice versa, at pH 4. 5. The enmeshment of silica particles in aluminum hydroxide floes. This has been suggested to occur in the "sweep zone", i.e., the aluminum hydroxide precipitation region (30). The last two eases will be discussed in greater detail in a subsequent communication (31). The experimental procedures employed in this work do not permit a distinction between ~ S i O H + L a ( O H ) (H20)~ + ,~the mechanisms listed above, especially in ~ S i O L a ( O H ) (H~O) ~+ + H~O + the absence of aluminum hydroxide precipitates. Indeed, it would be very difficult to This type of exchange has been suggested distinguish between mechanisms (2) and (3) for aluminum (21-24) and for a number of by any other experimental technique. other hydrolyzable metal ions (22, 25). (b) Electrophoresis. For nonhydrolyzing How easily polymeric hydrolysis products coagulants it was previously remarked that can exchange with protons and whether there is very little correlation between the such exchange, if it takes place, is equivalent stability of silica and its eleetrophoretie are still open questions. mobility. This appears to be also true for 3. The formation of surface complexes by hydrolyzable coagulants. With increasing the condensation reaction of hydrolyzed ions aluminum nitrate concentrations the mowith surface silanol groups, such as bilities of both Ludox AM and HS at pH 3.15 decrease to zero and then become positive -=--SiOH + L a ( O H ) (H2C)s2+ (Fig. 6). The charge reversal has no effect on ~SiOLa(H20)~ + + H20 the shape of the coagulation domain of This reaction has been suggested for hy- Ludox HS (Fig. 3); log e.e.e, is a linear drolysis products of Fe 3+ (27) and Cu 2+ function of pH irrespective of particle (28). charge. Ludox AM does have a bend in its 4. The mutual coagulation of positively coagulation domain near its point of charge charged alumina particles and the negative reversal at pH 3.15. However, at high eoaguparticles of silica; Iler (29), for example, has lant concentrations there is no large stabilizashown that silica particles can be coated tion region in the domain (Fig. 3) comparable Journal of Colloid and Interface Science, Vol. 35, No. 1, January 1971
STABILITY OF COLLOIDAL SILICA to those observed with hydrophobic sols (12, 14). The minima in the plots of the mobilities of Ludox HS and AM as a function of pH in 1.0 X 10-~ M aluminum nitrate (Fig. 7) also exist without any corresponding effect in the shapes of the coagulation domains in Fig. 3. The reason for these minima is not understood. Kohlschiitter et al. (21) observed a very strong uptake of aluminum by silica gel over the pH range 3.8-4.2 at higher concentrations of aluminum chlm~de ( > 1 X 10-~ M), which they attributed to the exchange of hydrolyzed species. On the other hand, a similar minimum in the electrophoretic mobility of Ludox AM was observed over this p i t range in 1.0 X 10-4 M lant h a n u m nitrate (Fig. 7). Under these conditions there is no appreciable hydrolysis of La 3+ in solution according to calculations based on the available stability constants (11). One should, however, consider the possibility that surface hydrolysis can take place even when the conditions in the bulk do not produce hydrolyzed species. The cause for charge reversal of silica b y aluminum ions is not clear at this point. I t is now a well-established fact that the charge of lyophobic colloids is reversed by the adsorption of hydrolyzed metal ions (32). This has been demonstrated with aluminum salts which can reverse the charge of silver halides or latices when added in low concentrations if the p H is sufficiently high ( > 4 ) (8, 12, 14). I t was also shown that very strongly hydrated sols, such as microerystalline cellulose (33), cannot be recharged by hydrolyzed metal ions regardless of concentration and pH. Charge reversal of silica takes place only at rather high concentrations of aluminum salts. Under these conditions even at p H 3-4 the concentration of hydrolyzed species may possibly become sufficient to cause charge reversal, most likely due to oxygen bridging with the surface silanol groups. O'Melia and Stumm (4) have observed charge reversal of silica b y ferric ions in much lower concentrations. However, this is consistent with the fact that ferric salts hydrolyze considerably easier than the aluminum salts.
75
REFERENCES 1. ALL~N, L. H., AND MATIJ~Vld, E., J. Colloid Interface Sci. 31,287 (1969). 2. ALLEN, L. tI., AND MATIJEW~, E., J. Colloid Interface Sci. 33, 420 (1970). 3. STUMM,W., AND O'MELIA, C. R., J. Amer. Water Works Ass. 60,514 (1968). 4. O'MELIA, C. R., AND STUMM,W., J. Colloid Interface Sei. 23,437 (1967). 5. HAHN, H. H., AND STU~M, W., J. Colloid Interface Sci. 28,134 (1968). 6. HAHN, It. H., AND STUMM,W., Advan. Chem. Set. 79, 91 (1968). 7. MATIJEVId,E., ANDJANAVER,G. E., J. Colloid Interface Sci. 21,197 (1966). 8. MATIJ~Vld, E., MATgAI, K. G., OTTE~VILL, R. H., AND KERKER, M., J. Phys. Chem. 65,826 (1961). 9. MATIJEWd, E., AND ST~YKER,L. J., J. Colloid Interface Sci. 22, 68 (1966). 10. BIEDERMANN, G., Svens~ Kent. Tidskr. 76, 362 (1964). 1l. BIEDERMANN, G., AND CIAVATTA, L., Acta Chem. Scan& 15, 1347 (1961). 12. MATIJEVld, E., JANAUER, G. E., AND KERKER, M., J. Colloid Sei. 19,333 (1964). 13. STRYKER,L. J., AND ~/[ATIJEVId,E., J. Phys. Chem. 73, 1484 (1969). 14. MATIJEVI~, E., AND FORCE, C. G., KolloidZ. Z. Polym. 225, 33 (1968). 15. MATIJEVld, E., AND ALLEN, L. H., Environ. Sci. Technol. 3, 264 (1969). 16. F]~ITKNECHT, W., AND SCHINDL]~R, P., Pure Appl. Chem. 6, 130 (1963). 17. DAWES, C. W., JONES, H. W., AND MONK, C. B., Trans. Faraday Soc. 48,921 (1952). 18. NANDA, R. ~5~., AND ADITYA, S., Z . Phys. Chem. (Frankfurt am Main) 35,139 (1962). 19. B~Ha, B., AND WnNDT, H., Z. Elektroehem.
66, No. 3, 223 (1962). 20. NISHIDE, T., AND TSUCHIYA,R., Bull. Chem. Soc. Jap. 38, 1398 (1964). 21. KOHLSCH{.~TTER, I7I. W., GETROST, 1-I., AND MIED~AN~, S., Z. Anorg. Allg. Chem. 308,
190 (1961). 22. VVl)RA, F., AND GALBA, J., Z. Anal. Chem. 235, 166 (1968); Collect. Czech. Chem. Commun. 32, 3530 (1967). 23. BOEnM, H. P., AND SCH~EDER, M., Z. Anorg. Allg. Chem. 316, 128 (1962). 24. STIGTER, D., BOSMAN, J., AND DITMARSCH, R., Ree. Tray. Chim. Pays-Bas 77,430 (1958). 25. CLARK, S. W., AND COOKE, S. R. B., Trans. A I M E 241,334 (1968). 26. BOEnM, H. P., AND SCI~NmDER,M., Z. Anorg. Allg. Chem. 301,326 (1959).
Journa~ of Colloid and Interface Science, Vol. 35, No. 1, January 1971
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ALLEN AND 1V~ATIJEVI~
27. MACKENZIE, J. M. W., Trans. A I M E 235, 28. 29. 30. 31.
82 (1966). ]SKI{A, J., AND LASKOWSKI, J., Inst. Mining Met., Trans. Sect. C 78, 113 (1969). IL~I~, IZ. K., J. Amer. Ccram. Soc. 47, 194 (1964). PACKHAM,R. F., Proc. Soc. Water Treat. Exam. 12, 15 (1963). MAWlJEVlS, E., MANGRAVITE,F. J., JI~., AND
CASSELL, E. A., J. Colloid Interface Sci., Submitted. 32. MATIJEVIS, E., in "Principles and Applications of Water Chemistry," pp 328-369. Wiley, New York, 1967. 33. KRATOHVII~,S., JANAUER, G. E., AND MATIJEVIS, E., J. Colloid Interface Sci. 29, 187 (1969).
Journal of Colloidand Interface Science, Vol. 35, No. 1, January 1971