The Colloid Chemistry of Silica - American Chemical Society

7 Stability of Aqueous Silica Sols Thomas W. Healy

Downloaded by UNIV OF TEXAS EL PASO on August 15, 2014 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch007

School of Chemistry, The University of Melbourne, Parkville, VIC., 3052, Australia

The control of silica sol coagulation by pH and by addition of simple electrolytes is said to be "anomalous" in that it is not simply predicted by conventional Derjaguin-Landau-Verwey-Overbeck (DLVO) theory. This chapter describes a model based on the control of coagulation by surface steric barriers of polysilicate and bound cations. The model suggests new experimental directions.s.

THE COAGULATION-DISPERSION BEHAVIOR of aqueous

silica sols is central to almost all processes requiring their unique adsorption, dispersion, gelation, and sol-gel properties. Aqueous silica sols are of particular interest in colloid science because their coagulation-dispersion behavior is said to be "anomalous", that is, their stability in terms of electrolyte-pH control does not follow the pattern followed by almost all other oxide and latex colloidal materials. This chapter examines aqueous silica sol coagulation effects in light of studies of macroscopic silica-water interfaces and i n particular the electrical double layer at such interfaces. It is difficult to define precisely the term "aqueous silica sols" and thereby contrast them with other forms of silica (colloidal silica, colloidal quartz, pyrogenic silica, and so forth). Bulk chemical distinctions are not very useful. The definition chosen here follows Iler's terminology (J). Aqueous silica sols are characteristically composed of spherical particles nucleated and grown by alkaline hydrolysis of sodium silicate solutions. They are often monodisperse systems and have particle diameters in the range 1-100 nm (density, —2.2 g/cm ) that lead to sols that vary from optically transparent to opalescent. The colloidal behavior referred to i n the preceding text as "anomalous" helps to further define the term "aqueous silica sols". Her (2) noted, 3

0065-2393/94/0234-0147$08.00/0 © 1994 American Chemical Society

In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

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T H E C O L L O I D CHEMISTRY O F SILICA

in a second major monograph, that such silica sols were colloidally stable at their isoelectric point p H (pHi ). Her also noted their remarkable stability in high salt concentration at near-neutral p H . The first definitive study of the "anomalous" coagulation behavior of silica sols was by Allen and Matijevic (3) in 1969. They focused on the measurement of the "critical coagulation concentration" of Ludox A M & H S sols 15 nm in diameter [210 m /g Brunauer-Emmett-Teller (BET) surface area], as determined by standard Rayleigh ratio light-scattering measurements. Figure 1 is a summary of the key results of Allen and Matijevic (3). In particular, when Na+ and L i were the coagulating cations, the critical coagulation concentration decreased with increasing p H . Further, the silica sols were colloidally stable below the p H limit of Figure 1 and stable at the observed p H i of about p H 2 - 3 . For K+ and Cs+ the silica sols were stable from p H 2 - 3 to p H 6, and these ions showed an irregular series effect in the p H 6-11 range, as shown schematically in Figure 1. Dépasse and Watillon (4) in a similar study in 1970 observed the same "anomalous" coagulation trends, again using standard light-scattering techniques with 50-nm-diameter silica sol particles prepared in a way similar to the technique of Allen and Matijevic (3). Figure 2 is a summary of the N a and Li+ coagulation data of Dépasse and Watillon (4) for an addition of electrolyte of 1.5 M . The simplest way of focusing on the "anomalous" character of these results is to present the variation of the electrokinetic or zeta potential of the silica sols as a function of p H and added salt. The general form of the electrokinetic results obtained for a vitreous silica (5) and for all silica sols is shown schematically in Figure 3. The silica sols, and indeed all oxide sols, show an increasing negative zeta potential with increasing p H as the p H is raised above the pHiep. The magnitude of the zeta potential decreases uniformly at each p H as the salt concentration is increased. There are subtle effects in the electrokinetics as the counterion is varied from Li+ to Cs+, but these effects are minor compared with the general reduction in zeta potential as the p H is moved toward the isoelectric point or as 1:1 electrolyte is added at any p H . The usual result of such variation in the zeta potential with p H and 1:1 electrolyte concentration is that the critical coagulation concentration normally varies with p H as shown schematically by the theory line in Figure 4. The critical coagulation concentration increases as the p H is increased above the isoelectric point and peaks at high values as shown. Many examples of these trends are confirmed for oxide and latex colloids for which H and O H are potential-determining ions (6, 7). The inset of Figure 4 illustrates this normal or theoretical variation with p H expected of the critical coagulation concentration, together with the form of the observed results for silica sols. In this inset the high-PH critical coagulation ep

2


+

ep

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-


7.

HEALY

149

Stability of Aqueous Silica Sols diam 15nm

1.50M

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LOOM

COAGULATE D


DISPERSED 0.50M

0.15M

12

pH

log

[MCI] V

/

—s

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\

pH

\N + a\

12

Figure 1. Schematic view of the coagulation domains of Ludox silica sols observed by Allen and Matijevic. (Adapted from reference 3.)

concentration values, for both theory and experiment, are shown coincid ing for reasons to be highlighted subsequently. The challenge for research ers is to seek an understanding in terms of classic DerjaguinLandau-Verwey-Overbeek (DLVO) theory, and more recent formulations of stability theory of φ β increased stability with decreasing p H and the observed stability at the p H i . ep


150


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Figure 2. Schematic view of the coagulation domains ofSi02 sols as determined by Dépasse and Watillon. (Adapted from reference 4.)

Origins of Colloid Stability The intention in this section is to define key concepts and terms; detailed descriptions of the forces between colloidal particles are in standards texts (β, 9). Electrostatic Repulsion. The electrostatic energy of repulsion (VR) between like charge surfaces arises from the overlap of the electrical double layers on the two particles. The range of interaction is of the order


7.

151

Stability of Aqueous Silica Soh

HEALY

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Figure 3. Plot of the zeta potential of a fused silica capillary versus pH in aqueous solutions of KCl as determined by the flat-plate streaming potential technique. (Adapted from reference 5.) of the D e b y e - H u e k e l reciprocal length (κτ )· F o r 1:1 electrolytes the value of κ- is approximately 10, 3, and 1.0 nm for concentrations of 10~ , 10~ , and ΙΟ- M , respectively. The magnitude of Vk at any given value of separation between two particle surfaces at given values of particle size, electrolyte concentration, and temperature is proportional to the surface potential (^ ) on each particle. A measure of φο and hence the repulsive energy of interaction can be obtained from the experimentally accessible, measured electrokinetic or zeta potential (C) by equating ζ to φά the diffuse-layer potential. The link between the surface charge and the potentials φ φα, and ζ is not trivial in the case of the silica-water interface (10). Unlike other oxides, the S1O2 pH-charge relationship shows a characteristically small charge at small values of ΔρΗ for p H values exceeding p H (II, 12). According to Healy and White (13), this relationship means that φα (=ζ), within 2 - 3 units of the isoelectric point, w i l l be expected to be smaller than that of oxides such as T1O2, AI2O3, and Fe203. Such a relatively slow increase i n potential is seen i n some studies of the ζ - ρ Η relationship of 1

1

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152


pH 2

4

6

8

10

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COAGULATED

Figure 4. Schematic variation of the DLVO theoretical stability domain for a pHiep colloid ofpH 2 (critical coagulation concentration, c.c.c; isoelectric point, i.e.p.). The insert shows this theoretical prediction compared to that observed for silica soh.

silica sols (3) but is less evident in studies of the zeta potential of macroscopic vitreous silica or quartz surfaces (5, 14). Again, silica, unlike other oxides, will not regulate charge during the approach of two surfaces (15) and may demonstrate a subtly different VR-separation curve. Recent computations (16) suggest such charge-regu-


7.

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153

Stability of Aqueous Silica Sols


lation effects will be small, but will be seen most especially around pHiep, at which point the S1O2 surface charge is low. Consideration of the stability of silica sols, that is, the so-called "anomalous" stability, will, in later sections of this chapter, focus on 1:1 electrolyte concentrations of 0.1 M and greater. At this salt concentration, the range of the repulsive electrostatic forces between particles is small, and any subtle differences in the electrostatics between silica and other oxide sols i n itself cannot provide the necessary repulsion to stabilize silica sols at such high electrolyte levels. v a n d e r W a a l s A t t r a c t i o n . The attractive van der Waals energy of interaction ( V A ) for spheres in the 10- to 100-nm size range for silica sols discussed here varies as the inverse of the separation distance, and at any separation V A is directly proportional to particle size. The Hamaker constant (A), which controls the magnitude of the variation of van der Waals attraction with particle radius (a) and separation (H ) between surfaces, is for silica-water-silica not a large number. Further, the known hydration-polysilicic acid formation at silica-water interfaces will further reduce the overall Hamaker constant in the silica sol-water-silica sol system. The simple attractive van der Waals term ( V A ) and the simple electrostatic (double-layer) repulsive interaction energy ( V R ) are now summed. Silica sol particles in the 10-100 nm radius size range, for salt concentrations of 0.1 M and greater and for p H values of 5 or more above the pHiep of silica are now considered. The isoelectric point is taken as p H 2 - 3 . Thus for 25-nm-radius particles the variation of total energy V T (= V A + V R ) with particle separation is attractive at all separations, and the sols are therefore expected to be unstable over this entire p H range above 5 as shown by the theory line in Figure 4 and in the inset in Figure 4. It is important to dispel any hope that adjustment of the electrostatic or van der Waals term will bring theory into line with experiment for silica sols. At about 0.1 M 1:1 electrolyte, reductions in the Hamaker constant to values near that of water itself or potentials of near - 1 0 0 m V at, for example, p H 9 - 1 0 would be required. The experimental evidence (17) clearly points to a value more like - 2 0 mV. Furthermore, several successful fits of double-layer theory to the observed charge and potential behavior of silicas are illustrated by the results (points) and theory (solid lines) in Figure 5, taken from the work of James (17) for a well-studied pyrogenic silica. James (17) invoked site dissociation, that is, 0

-SiOH

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H

+

and counterion binding, for example,


154

T H E C O L L O I D CHEMISTRY O F

-20

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Figure 5. Comparison of theory and experiment in the variation of surface charge and electrokinetic potential for a pyrogenic silica. (Adapted from reference 17.)

-SiO"

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Na

+

—*

-SiO"Na

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and fitted simultaneously (as shown in Figure 5) the observed charge and potential data. Similar fits to observed charge and potential data for many forms of silica, including silica sols, have been made, and only minor adjustment of the (sodium) ion binding constants are required. Steric Stabilization and "Anomalous" Stability. A n explanation of the "anomalous" stability of Iler's silica sols i n terms of steric stabilization effects requires that oligomeric or polymeric silicate species are present at the silica-water interface and that steric repulsion results during overlap of such layers. This mechanism is appealing in that soluble silicates, usually sodium silicates, are universal dispersants of many


7.

HEALY

155

Stability of Aqueous Silica Soh

electrostatic colloids. Again, well-hydrated silicas (2) and other colloids exposed to aqueous silicate (18) acquire high adsorption densities of aqueous silica. To construct a model for "anomalous" stability, observations of silica sols at p H 11 are used. Allen and Matijevic (19) observed a critical coagulation concentration of approximately 0.15 M for N a at p H 11.0. If they take this p H 11 sol to, for example, p H 8.0, it remains stable in salt concentrations much greater than 0.15 M , and the critical coagulation concentration increases even further as the p H is reduced from p H 8 to p H 6. At p H 11.0, if the silica sol is treated as a " n o r m a l " 30-nm-diameter silica (without adsorbed silicate layers), D L V O calculation generates a coagulation condition (Vrmax = 0) at just under 2-nm surface separation. The electrostatic repulsion is inside this separation and cannot overcome the van der Waals attraction. A key element of this postulate is that the silica sol is " n o r m a l " at p H 11.0; that is, it has no protective (steric) layer of silicate. Her (2) and more recently Furlong et al. (18) noted the desorption (or lack of adsorption) of oligomeric and polymeric aqueous silica at p H values above 10.5. Conversely, these workers observed adsorption of oligomeric and polymeric aqueous silica below p H 10.5. Thus, as the p H is changed from H t o 8 i n 0 . 1 5 M salt, silicate steric layers are generated. For these layers to stabilize the sol under these conditions, they must extend farther than the Vrmax = 0 condition of 2 nm. The hypothesis is therefore that a 2-nm steric silicate oligomer-polymer barrier protects a silica sol at p H 8.0 in 0.15 M salt. To explore this very simple picture further, the ion-exchange behavior of silica sols must be considered. In their 1970 paper Allen and Matijevic (19) linked the observed coagulation behavior to the observed ion exchange. In this elegant study they introduced the concept of a "critical exchange curve", namely, the increase in exchange capacity with increasing p H at the critical coagulation concentration condition at each p H . These ion-exchange results can be used to consider what happens as salt is added to the p H 8, 0.15 M salt case that we have proposed is stabilized by a steric silicate polymer layer. The stabilizing adsorbed polysilicate layer is the actual exchange volume or layer, and addition of salt above 0.15 M must lead to further binding of N a to the stabilizing layer. It is therefore important to view the barrier at the surface as a M - p o l y s i l i c i c acid coating, which will increase in thickness as the p H is decreased because of decreased - S i O H ionization; conversely, it will decrease in thickness as the amount of bound cation increases (i.e., it will "exchange") in the language of Allen and Matijevic. Indeed, the presence of increasing amounts of bound N a must switch off the electrosteric contribution of the adsorbed polysilicate layer, and,


+

+

+

+



156

T H E COLLOID

CHEMISTRY

O F SILICA

for the p H 8.0 sol at its critical coagulation concentration, which is greater than 0.15 M , a secondary minimum well of >2kT must open up as a result of the compression of the thickness and screening length of the adsorbed layer (k, Boltzmann constant; T, temperature). Similar effects will operate at p H 6.0, initially at 0.15 M salt, where again the sol is electrosterically stabilized. As salt is added at p H 6, again the thickness and range of the electrosteric coating will eventually decrease to yield a >2kT secondary minimum at the critical coagulation concentration, which is very much greater than 0.15 M . These effects are summarized schematically i n energy-distance curves i n Figure 6. The sequence is as follows: 1. p H 11; 0.15 M NaCl—unstable " n o r m a l " sol behavior 2. p H 8.0; 0.15 M NaCl—stable because of electrosteric coating 3. p H 8.0; >0.15 M NaCl—secondary minimum coagulation 4. p H 6.0; 0.15 M NaCl—stable; electrosteric barrier 5. p H 6.0; » 0 . 1 5 M NaCl—secondary minimum coagulation The proposed mechanism has several interlocking postulates that will need experimental testing. They are as follows: • At p H 11, oligomeric-polymeric silicate is absent from the sol particle surface. • A t p H

The Colloid Chemistry of Silica - American Chemical Society

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