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Dispersions of Silica Particles in Surfactant Phases V. L. Alexeev,†,‡ P. Ilekti,† J. Persello,§ J. Lambard,| T. Gulik,⊥ and B. Cabane*,† Equipe mixte CEA-RP, Service de Chimie Mole´ culaire, CE-Saclay, 91191 Gif sur Yvette, France, St. Petersburg Nuclear Physics Institute, 188350 Gatchina, Russia, Electrochimie des Solides, Universite´ de Franche Comte´ , 25030 Besanc¸ on, France, Service de Chimie Mole´ culaire, CE-Saclay, 91191 Gif sur Yvette, France, and Centre de Ge´ ne´ tique Mole´ culaire, CNRS, 91198 Gif sur Yvette, France Received August 22, 1995. In Final Form: January 16, 1996X Addition of surfactants to aqueous dispersions of silica may either enhance or depress the colloidal stability of the dispersions. With a nonionic surfactant that forms micelles (Triton X-100), dilute silica dispersions have enhanced stability if the silica particles are fully covered with adsorbed surfactant micelles. Concentrated dispersions are also stable provided that the particles remain separated by at least one layer of micelles. Beyond full coverage, excess micelles may cause depletion flocculation of the silica particles. With a nonionic surfactant that forms vesicles and bilayers (polyoxyethylene alkyl ether C12E4), the effects of surface coverage are the same; however, the critical separation between particles is the thickness of two bilayers. Beyond full coverage, the surfactant forms a lamellar phase which excludes the particles.
Introduction Colloidal dispersions are stabilized by repulsions that keep the dispersed particles apart from each other. In water, the repulsions often originate from the overlap of counterion clouds that surround each particle. These repulsions are established during the synthesis of the particles. However, the aqueous media in which the particles are synthesized are often not the medium required for applications. For instance, it may be desired to disperse the particles in a different aqueous phase (different ions or pH), in a nonaqueous solvent, or else in a polymer matrix. Then the difficulty is to maintain the stability of the dispersion during the solvent exchange and in the final dispersing phase. In this paper we describe the transfer of mineral particles to surfactant phases. Depending on the nature of the particles, two types of transfer may be considered: lipophilic particles into the hydrocarbon domains or hydrophilic particles into the water domains of the surfactant phase. The former type of transfer is easily achieved by introducing small particles with grafted hydrocarbon chains into the lamellar phase of ionic surfactants; it has been studied extensively.1 The latter is more difficult to achieve, for reasons that will be explained below; however, it occurs in practice in cosmetic creams, pharmaceuticals, and some detergent mixtures. The aim of this paper is to demonstrate, on a model system, what are the possibilities and the limitations of transferring hydrophilic particles to surfactant systems. With ionic surfactants of opposite charge to the solid, the surfactant ions bind to the particle surfaces, which become hydrophobic.2 Such surfactant-covered particles cannot be kept in an aqueous environment, unless a double layer of surfactant molecules is adsorbed, which is difficult †
Equipe mixte CEA-RP, Service de Chimie Mole´ulaire. St. Petersburg Nuclear Physics Institute. § Universite ´ de Franche Comte´. | Service de Chimie Mole ´ culaire. ⊥ Centre de Ge ´ ne´tique Mole´culaire. X Abstract published in Advance ACS Abstracts, April 1, 1996. ‡
(1) Ponsinet, V.; Fabre, P.; Veyssie´, M.; Auvray, L. J. Phys. II 1993, 3, 1021. (2) Somasundaran, P.; Healy, T. W.; Fuerstenau, D. W. J. Phys. Chem. 1964, 68, 3562. Somasundaran, P.; Snell, E. D.; Fu, E.; Xu, Q. Colloids Surf. 1992, 63, 49.
to achieve with very small colloidal particles.3 With ionic surfactants of the same charge as the solid, the particles are expelled from the surfactant phase under the effect of depletion forces; this is well documented in the case of emulsions, where it leads to creaming.4 In this work, we used negatively charged silica particles and nonionic surfactants. The interaction of nonionic surfactants with mineral surfaces is well understood; in dilute dispersions it is known that surfactant micelles adsorb on the mineral surfaces.5-7 Our aim was to use this adsorption to produce silica particles that would be covered with micelles. The expectation was that the adsorbed micelles would act as “bumpers” between particles, preventing any direct contact between them. In this way the particles could not segregate from the surfactant phase and would remain dispersed in it. Materials and Methods Silica Sols. Synthesis. Silica particles were grown from aqueous silicate solutions neutralized by an acid in a manner described by Iler.8 The reaction produced silica particles dispersed in a salt solution at pH 9.9 The particles had a dense core of amorphous silica and a surface covered with silanol groups. They were examined through electron microscopy and quasielastic light scattering.10 Their diameters were narrowly distributed around an average diameter of 200 Å, with a standard deviation of 20 Å. Their shapes were roughly spherical. Their surface area, determined through BET, was 150 m2‚g-1. Washing and Concentration Processes. In order to remove the salt, each silica sol was washed with water. For this operation the sol was circulated in a tangential ultrafiltration module equipped with IRIS-PSS membranes (cutoff at 10 000 Da). The replacement water was also kept at pH 9. At the end of this operation the sol had a silica concentration of 10 g/L and a salt (NaNO3) concentration below 3 × 10-3 M. (3) Wong, K.; Cabane, B.; Duplessix, R.; Somasundaran, P. Langmuir 1989, 5, 1346. (4) Bibette, J.; Roux, D.; Pouligny, B. J. Phys. II 1992, 2, 401. (5) Levitz, P.; Van Damme, H.; Keravis, D. J. Phys. Chem. 1984, 88, 2228. Levitz, P.; Van Damme, H. J. Phys. Chem. 1986, 90, 130. (6) Lindheimer, M.; Keh, E.; Laini, S.; Partyka, S. J. Colloid Interface Sci. 1990, 138, 83. (7) Giordano-Palmino, F.; Denoyel, R.; Rouquerol, J. J. Colloid Interface Sci. 1994, 165, 82. (8) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (9) Persello, J.; Magnin, A.; Chang, J.; Piau, J. M.; Cabane, B. J. Rheol. 1994, 38, 1845. (10) Chang, J.; Lesieur, P.; Delsanti, M.; Belloni, L.; Bonnet-Gonnet, C.; Cabane, B. J. Phys. Chem. 1995, 99, 15993.
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Langmuir, Vol. 12, No. 10, 1996 2393 particles in each aggregate. At higher Q values, it decays according to an interference function that describes the internal structure of the aggregates. For large, selfsimilar aggregates with a selfsimilarity exponent df, only the higher Q decay is observed, which goes according to a power law. Accordingly, dispersions containing very large aggregates can be recognized by a fit of the experimental scattering curve to
S(Q) ≈ Q-df
Figure 1. Schematic phase diagram for dispersions of colloidal silica in water. The boundary between the fluid and the soft solid state has been placed at the overlap of the ionic clouds that surround particles. At lower ionic strengths or higher volume fractions, these clouds overlap, and each particle interacts with all its neighbors (soft solid). At high ionic strengths (above 0.05 M) these repulsions are screened, and the particles aggregate spontaneously. At the highest volume fractions, above 0.55, the surface-to-surface separation is below 20 Å, and the dispersion turns into a hard solid that cannot be rehydrated. A further ultrafiltration stage without replacement of water was used to concentrate the sol to a volume fraction of 0.1. This sol could be stored at room temperature (20 °C) for many years without loss of colloidal stability. Further concentration of the silica dispersion was performed by the osmotic stress technique described below; with this technique, the silica dispersions could be concentrated reversibly up to φ ) 0.5 while keeping fixed the chemical potentials of all ionic species in the aqueous phase. Phase Diagram. The concentrated silica dispersions were examined according to their mechanical properties (solid or liquid), transparency (homogeneous or heterogeneous), and reversibility (ability to redisperse spontaneously in water).9 According to these criteria, silica dispersions made at different volume fractions and different ionic strengths can be grouped in four classes. At low concentrations and low ionic strengths the samples are transparent sols with a viscosity close to that of water. At high concentrations and low ionic strengths, where the ionic clouds of particles are forced to overlap, the dispersions become soft solids, still transparent.9 In this state they can be reswelled to give homogeneous, transparent sols. At still higher concentrations the dispersions become hard solids, similar to a piece of glass. In this state they are no longer reversible: upon addition of water these solids turn white instead of redispersing. Finally, at high ionic strengths, the dispersions aggregate spontaneously; the resulting flocs are also irreversible; i.e., they do not redisperse when the salt is washed out. Figure 1 shows the phase diagram of colloidal silica drawn according to these criteria. Structures. The organization of silica particles in the dispersions was determined through small angle X-ray scattering (SAXS). The general formula for the intensity scattered by a dispersion of identical spherical particles is11,12
I(Q) ) nI1(Q) S(Q)
(1)
where I is the scattered intensity, Q the magnitude of the scattering vector, n the number of particles in the dispersion, I1(Q) the intensity scattered by an isolated particle, and S(Q) an interference function that describes the interferences between rays scattered by different particles. In dispersions of non-interacting particles, the interferences between rays scattered by different particles cancel, so that S(Q) ) 1. Accordingly, the random dispersion of the particles can be recognized by fitting the experimental scattering curve to the scattering function of a spherical particle, which is
I1(Q) ) I1(0)[3(sin(QR) - QR cos(QR))/(QR)3]2
(2)
In dispersions of aggregated particles, the intensity scattered at low Q is increased, proportionally to the average number of
(3)
In dispersions of repelling particles, the silica particles take ordered positions that minimize the overlap of ionic clouds.10 Consequently, the intensity at low Q is depressed, and at higher Q there is a peak related to the average interparticle distance. Crystalline long range order is obtained when the volume fraction of the particles is so high that all the ionic clouds are forced to overlap. For dispersions of 200 Å silica particles in water at ionic strength I ) 10-3 M and pH ) 9, long range order is obtained at volume fractions above φ ) 0.23. Then the scattering is reduced to a set of diffraction peaks located at Q values determined by the spacings of reticular planes of particles. In the case of a face-centered cubic (fcc) structure with unit cell a, the first diffraction peak corresponds to the distance between the 111 planes; the spacing of these planes is d111 ) a/x3 and the peak position is at Q ) 2π/d111. For a bodycentered cubic (bcc) structure the first peak corresponds to the distance between the 110 planes spaced by d110 ) a/x3.. In both cases the separation h between the surfaces of neighboring particles of diameter D is obtained from
D + h ) (2π/Q)(x3/x2)
(4)
These distances vary with the volume fraction of the dispersion. For fcc and bcc structures the relations are respectively
φ ) (π/3x2)(D/D + h)3 (fcc)
(5)
φ ) (πx3/8)(D/D + h)3 (bcc)
(6)
When the particles are in contact (h ) 0), the volume fraction is 0.74 in the case of a fcc structure and 0.68 in a bcc structure. Surfactants. The main surfactant used was TRITON X100, obtained from FLUKA and hereafter abbreviated as TX 100. It is a mixture of alkylphenol polyoxyethylenes with x ) 9 or 10 oxyethylene monomers:
CH3C(CH3)2CH2C(CH3)2C6H4(OCH2CH2)xOH
(7)
The phase diagram of TX 100 in water has been determined by Beyer;13 our observations, which match this diagram, are shown in Figure 2. At low concentrations the surfactant forms oblate micelles, with a hydrocarbon core and a swollen PEO shell.5 At 20 °C this micellar phase extends to 38% TX 100 in the phase, where the micelles occupy all the available volume. Beyond this concentration, there is a mesophase made of rodlike or ribbonlike aggregates packed in a hexagonal array. At 57% TX 100, the isotropic phase is found again. According to electron microscopy (below), the surfactant aggregates in this phase are flat micelles stacked with lamellar short range order. Finally, beyond 80% TX 100, there is less than one water molecule per oxyethylene group, and the samples are homogeneous solutions of water in a liquid surfactant. This succession of phases is altered at higher temperatures; above 30 °C the micellar phase covers the whole range of concentrations, and at temperatures above 65 °C the surfactant separates from water. The dimensions of the surfactant aggregates and their distances in each phase were measured through small angle neutron scattering (SANS); they are given in Table 1. At low concentrations the diameter D of the micelles was calculated from the curvature of the SANS curve. This yields D ) 60 Å. At higher concentrations the intermicellar distance was calculated (11) Glatter, O.; Kratky, O. Small angle X rays scattering; Academic Press: New York, 1982. (12) Cabane, B. In Surfactant Solutions: New Methods of Investigation; Zana, R., Ed.; Marcel Dekker: New York, 1987; p 57. (13) Beyer, K. J Colloid Interface Sci. 1982, 86, 73.
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Alexeev et al. tions, clear homogeneous mixed dispersions were obtained, indicating that most of the surfactant was transferred to the silica surfaces. The mixtures of silica sol and surfactants were placed in dialysis bags and immersed in aqueous solutions of poly(ethylene oxide) (PEO 35000, supplied by FLUKA). An equilibrium of the osmotic pressures was reached after 2 weeks. Afterward, the silica volume fraction was determined by drying part of the sample at 120 °C. The rest of the sample was used to measure distances between silica particles through X-ray or neutron scattering. In experiments with silica and TX 100 in the bag, no TX 100 was added to the PEO solutions outside the bag. It was verified that the amount of TX 100 passed through the membrane was less than 1%. Therefore the pressure outside the bag was that of the PEO solutions, and that inside the bag was that of the silica and surfactant dispersion. The osmotic pressure Π, in dyn/cm2, of the PEO solutions was calculated according to the weight percent w of PEO:15
Figure 2. Phase diagram of TX 100 in water, drawn according to our observations and to the results reported by Beyer.13 Table 1. Structures of Micellar Solutions of the Surfactant TX 100a TX mass %
dist (Å)
hc (Å)
EO + W (Å)
W/EO
25 35 70 80
116 89 50 46
8.8 9.4 10.6 11.2
107 80 39 35
10.5 6.5 1.5 0.9
a TX mass %, concentration of surfactant (TX 100) in the sample; dist, center-to-center distance between micelles, calculated from the peak position in SANS experiments, assuming lamellar ordering; hc, hydrocarbon thickness of surfactant micelles, calculated from the distance between micelles; EO + W, thickness of the layer made of water and oxyethylene chains separating the hydrocarbon cores of micelles; W/EO, number of water molecules per oxyethylene unit of the surfactant.
from the position of the intermicellar peak. Since this peak position varies linearly with concentration, a lamellar ordering was assumed. The other surfactant used in this study belongs to the group of alkyl polyoxyethylene surfactants CnH2n+1(OCH2CH2)mOH. We used tetra(ethylene glycol) monodecyl ether, hereafter abbreviated C12E4, obtained from FLUKA. The phase behavior of this group of the surfactants was comprehensively studied by Mitchell et al.14 For C12E4, in the temperature range 20-50 °C, the main phase is a lamellar phase which extends from 25% to 80% surfactant. At lower concentrations the surfactant forms a dispersion of lamellar domains in water. At higher concentrations it forms a homogeneous solution of water in the surfactant liquid. The lamellar phase is made of surfactant bilayers (average thickness 11 Å) separated by water layers (average thickness according to composition). The repetition of the bilayers is quite regular; it is demonstrated in scattering experiments and in the freeze fracture images shown below. Mixing and Deswelling. In this work, the main difficulty was to prevent or control aggregation of the silica particles. For this reason, the surfactant was always added to a dilute silica dispersion (volume fraction 0.1), and water was removed at a later stage through osmotic stress.15 This technique is particularly good at producing a uniform approach of the particles in a dispersion. Pure surfactants were mixed with a silica sol (volume fraction 0.1) by simple agitation at 50 °C. At this temperature, the system TX 100/water forms a micellar phase over most of the concentration range. This is easily diluted with the silica sol to give a homogeneous mixture. With C12E4 at 50 °C, the surfactant forms a dispersion of the lamellar phase in water; this phase may not be diluted with the silica sol. Still, at low surfactant concentra(14) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (15) Parsegian, V. A.; Rand, R. P.; Fuller, N. L.; Rau, D. C. In Methods in enzymology; Packer, L., Ed.; Academic Press: New York, 1986; Vol. 127, p 400.
log Π ) 1.61 + 2.72w0.21
(8)
Cycles of deswelling and reswelling were used to observe whether the concentrated dispersions of silica were redispersible. For this purpose, SAXS spectra taken before deswelling and after reswelling were compared. Finally, samples at a low volume fraction of silica but high concentration of surfactant were prepared by direct mixing of the silica sol with the pure surfactant, since their osmotic pressures were too high for osmotic stress.
Adsorption of Surfactants on Silica Since the pioneering work of Levitz and Van Damme,5 it is known that nonionic surfactants such as TX 100 form adsorbed micelles on the silica surfaces. The adsorption mechanism is as follows.5-7 The first surfactant molecules adsorb on the silica surfaces through hydrogen bonding of their oxyethylene units to the surface silanols. Then there is an association process around these first adsorbed molecules, leading to the formation of surface micelles, bound to the silica surface by the same hydrogen bonds. The whole process occurs before the critical micelle concentration (cmc), since adsorbed micelles have a lower free energy than free micelles. We have determined the adsorption isotherm for TX 100 on dilute dispersions (φ ) 0.04) of the silica described above. Known amounts of silica and TX 100 were mixed, equilibrated, and then separated through centrifugation. The concentration of TX 100 in the supernatant was determined from its UV absorption at 2760 Å. The amount adsorbed was calculated from the difference between the initial and final concentrations of surfactant in water. The adsorption isotherm, shown in Figure 3, has the typical sigmoidal shape. The steep part of the isotherm, corresponding to the formation of adsorbed micelles, is situated below the cmc. The ratio of the concentrations for the formation of adsorbed micelles and free micelles yields the adsorption free energy per surfactant molecule, which is of the order of kT. The plateau is reached at the cmc of the surfactant in water; it corresponds to full coverage of the silica surface by surfactant micelles; and the adsorbed amount is 35 g of TX 100 per 100 g of silica. For C12E4, the saturation value of the adsorption was estimated from the composition where a pure lamellar phase is seen on top of the sediment after centrifugation. This value is 30 g of C12E4 per 100 g of silica. Osmotic Compression of the Dispersions Samples prepared at a fixed surfactant/silica ratio and concentrated through osmotic stress were used to measure the equation of state of the dispersions, which was taken as the relation of osmotic pressure to volume fraction of silica. For this purpose, the samples were equilibrated at
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Figure 3. Adsorption isotherm of TX 100 on silica. Horizontal scale: free surfactant concentration, in M units. Vertical scale: amount of bound surfactant (mg/m2 of surface). The formation of surface micelles starts at a free surfactant concentration of 10-4 M, and the adsorption saturates at the critical micelle concentration of free surfactant micelles, which is 0.33 M. The two sets of data correspond to different runs on batches of the same dispersion.
Figure 4. Osmotic pressures of silica dispersions and of TX 100 micellar solutions. Horizontal scale: volume fraction of silica or TX 100 in water. Vertical scale: osmotic pressures, in pascals. The osmotic pressures of surfactant solutions become extremely high upon approaching the boundary to the hexagonal phase. The osmotic pressures of silica dispersions rise progressively due to overlap of the ionic clouds that surrround the silica particles.
an osmotic pressure set by the aqueous polymer solution, then the volume fraction was measured, and finally the rest of the sample was used in a scattering experiment to measure the distances between silica particles. Osmotic Pressures of Pure Silica Dispersions. The equation of state (osmotic pressure as a function of volume) for a dispersion of silica particles in water is shown in Figure 4. Each data point was obtained by setting the osmotic pressure of the stressing solution as explained in Materials and Methods section and measuring the resulting volume fraction φ of silica. For comparison, we have also indicated the osmotic pressures of TX100 micellar
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Figure 5. Osmotic pressures of silica dispersions plotted vs the average separation between surfaces of silica particles. The osmotic pressures of the silica dispersions decay exponentially with increasing separation, due to the decreased overlap of the ionic clouds that surrround the silica particles.
solutions, calculated from the known pressures of poly(ethylene oxide) in water.15 Note that the surfactant solutions, at comparable volume fractions, have much higher osmotic pressures. The osmotic pressure curve of silica dispersions may be described as follows. At low volume fractions (φ ) 0-0.2), the ionic clouds surrounding each particle do not overlap, and the dispersions can be modeled as a gas of repelling particles, with an effective hard sphere diameter placed at a distance from the particle surface equal to the Debye length. The calculated pressures match the rise of the measured pressures until the volume fractions exceed 0.2. Beyond this point (φ ) 0.2-0.5) the measured pressures are below the pressures calculated for a gas of inpenetrable ionic clouds, indicating that the ionic clouds do overlap. In this range of volume fractions there is an exponential rise in pressure, which reflects the progressive overlap of ionic clouds as the volume fraction is increased. Finally, beyond φ ) 0.5, the osmotic pressures rise extremely fast with volume fraction, indicating that the repulsions between silica surfaces become quite strong. The same data are presented in Figure 5 as a function of the surface-to-surface distance between silica particles. These distances are calculated from the volume fractions according to eq 5; they match the distances measured through neutron or X-ray diffraction. In this representation the gaslike regime is located at distances beyond 100 Å, the regime where ionic clouds are forced to overlap is between 100 and 20 Å, and the rigid state is below 20 Å. Osmotic Pressures of Mixed Silica + TX 100 Dispersions. Figure 6 presents the results for the mixed silica + surfactant dispersions. These dispersions were prepared with a fixed weight ratio of TX 100/silica (respectively 0.1, 0.2, 0.3, and 0.5). This ratio may also be expressed as a fraction of saturation coverage of the silica surfaces (respectively 0.28, 0.57, 0.85, and 1.43). For comparison, we have calculated the osmotic pressures of the pure surfactant in the same amount of water; these pressures are shown in Figure 4. In the dilute range (φ ) 0-0.2) and for coverages below saturation, the pressures of the mixed dispersions are the same as for pure silica. This result confirms that all nonadsorbed surfactant is below the cmc. Indeed, if the added surfactant did form free micelles, they would give
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Figure 6. Osmotic pressures of mixed silica + TX 100 dispersions. Dots: pure silica dispersions. Squares: mixed dispersions at a mass ratio TX 100/silica ) 0.1. Circles: TX 100/silica ) 0.3. Triangles: TX 100/silica ) 0.5. Continuous line: pressures from pure dispersions, assuming that each particle has been expanded at a volume ratio (TX 100 + silica)/ silica ) 2; this ratio matches the expansion of particles if they adsorb TX 100 at a mass ratio TX 100/silica ) 0.5. Table 2. Distances between Silica Particles in Mixed Silica + Surfactant Dispersionsa Π (Pa)
L(φ) (Å)
L(SANS) (Å)
10 000 50 000 210 000
108 72 72
102 67 89
a Π, osmotic pressure of the mixed dispersion containing silica, surfactant (TX 100), and water; L(φ), calculated separations (faceto-face) between the silica particles, from the silica volume fraction according to eq5; L(SANS), measured separations, from the peak position in SANS spectra.
a large contribution to the osmotic pressure. In the concentrated range, the pressures of the mixed dispersions rise above from those of pure silica dispersions; then the mixed dispersions become nearly incompressible at a volume fraction of silica which depends on the TX 100/ silica ratio. This rise indicates that the adsorbed micelles make the removal of water more difficult. Indeed, it is quite difficult to dehydrate TX 100 micelles, since the transition from micellar to hexagonal phase is caused by the difficulty of dehydrating the micelles. Accordingly, for a dispersion made at a TX 100/silica weight ratio of 0.5, the volume fraction of the hydrated micelles equals that of silica. At the location of the steep rise in pressure, the volume fraction of silica is 0.3 and the combined volume fraction of silica and hydrated micelles is 0.6. Therefore it is not surprising that the dispersion becomes incompressible. The distances between silica particles in the dispersions can be calculated from the measured volume fractions, assuming a fcc ordering of the particles as in eq 5. Alternatively, they can be measured directly through SANS. A comparison of calculated and measured distances is presented in Table 2 for dispersions made at a weight ratio TX 100/silica ) 0.35. The calculated and measured distances are in good agreement. From the interparticle distances, the surface-to-surface separations between particles may be calculated. They can be compared with the sizes of TX 100 micelles, in
Figure 7. Osmotic pressures of mixed silica + TX 100 dispersions plotted vs average separation of silica surfaces. Dots: pure silica dispersions. Squares: mixed dispersions at a mass ratio TX 100/silica ) 0.1. Circles: TX 100/silica ) 0.3. Triangles: TX 100/silica ) 0.5. Dispersions brought to pressures exceeding 5 × 106 Pa or to surface-to-surface separations below 60 Å were found to be aggregated.
order to find out how many micelles separate neighboring particles. In Figure 7, the measured pressures are plotted according to the calculated separations between silica particles. For fully covered silica particles, it was expected that the resistance to osmotic compression would rise steeply when the separation would decrease below the thickness of two adsorbed layers, i.e. two diameters or 120 Å. This is not observed, indicating that adsorbed micelles can be displaced easily as long as the surfaces remain fully covered. The pressures do rise steeply when the separation is decreased below one micelle diameter or 60 Å. Thus the resistance to creating bare surfaces is high. To eject these micelles, a pressure of order of 5 × 106 Pa must be imposed. Osmotic Pressures of Mixed Silica + C12E4 Dispersions. Figure 8 shows the pressure vs separation curves obtained with C12E4 adsorbed on silica instead of TX 100. The results are quite similar. Full coverage of the silica surfaces is obtained when the weight ratio of C12E4/silica is 0.25. Dispersions with this composition show a steep rise of the pressure when the separation reaches 40 Å. This length matches the thickness of one bilayer in the lamellar phase of C12E4 and water.16 Thus, there is very little resistance to displacing one of the two bilayers which initially separate the silica particles, but there is a high resistance to removing the other one, because this produces bare silica surfaces. Aggregation of Surfactant-Covered Silica Aggregation was defined as a condition where the silica particles were stuck to each other, either directly (surfaceto-surface) or indirectly (through a surfactant aggregate), and could not be reseparated by dilution. The occurrence of aggregation was examined through visual observation of dilute dispersions (turbidity, sedimentation, or gelation) and through SAXS or freeze fracture examination of concentrated dispersions. (16) Cummins, P. G.; Staples, E.; Penfold, J. J. Phys. Chem. 1990, 94, 3740; 1991, 95, 5902.
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Figure 8. Osmotic pressures of mixed silica + C12E4 dispersions, plotted vs average separation of silica surfaces. Dots: pure silica dispersions. Squares: mixed dispersions at a mass ratio C12E4/silica ) 0.125. Circles: C12E4/silica ) 0.25. Triangles: C12E4/silica ) 0.5. Dispersions brought to surface-tosurface separations below 60 Å were found to be aggregated.
Dilute Surfactant Solutions. The colloidal stability of silica sols mixed with dilute solutions of TX 100 was found to vary with the coverage of the silica surfaces, as already observed by Giordano-Palmino et al.7 When the silica surfaces were fully covered by TX micelles, the sols were stable at all temperatures up to 90 °C; they could also be frozen down to -10 °C and reheated into the liquid stated without aggregation of the particles. This stability implies that the adsorbed micelles protect the silica surfaces, since the same silica particles without adsorbed surfactant aggregate when the temperature is raised above 40 °C and also when it is cooled below freezing. When the silica surfaces were partly covered (e.g. between 1/3 and 2/3 of full coverage), the dispersions made at volume fraction 0.1 evolved into translucent gels. As explained by Giordano-Palmino et al.,7 this evolution results from bridging by micelles. The condition for optimum bridging was found to be near half coverage; this is indeed the composition where the silica surfaces maximize their adsorption free energy by sharing adsorbed micelles. Note that the length of the bridges (a micelle diameter is 60 Å) is much below the average interparticle distance (800 Å at φ ) 0.01). Therefore the formation of a gel requires a growth mechanism with the following steps: (i) particles collide and remain stuck in their configuration of first contact; (ii) such clusters of particles also bind to each other, forming fractal aggregates; (iii) these aggregates occupy more and more volume as they grow larger (a typical growth law is M ≈ R2); and a gel is obtained when they entrap the whole sample volume. If, instead, reordering after a collision was possible, then dense clusters would be formed, and sedimentation would be observed instead of gelation. Similar observations were made with silica particles covered by C12E4 bilayers; gel formation was also observed in silica dispersions (volume fraction 0.1) covered with unsaturated bilayers (C12E4/silica weight ratio up to 0.25). Stoichiometric Mixtures. Concentrated dispersions obtained through osmotic stress were also examined for
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aggregation. In this case the procedure was to let the samples reswell through dialysis in water, measure the extent of reswelling, and in some cases determine the structures of the reswelled dispersions through SAXS. Dispersions with a stoichiometric amount of surfactant (enough to saturate the silica surfaces, i.e. mass ratio TX 100/silica ) 0.3) could be deswelled and reswelled reversibly as long as the uppermost pressure reached in the cycle was below the steep rise of the equation of state. This boundary corresponds to a volume fraction of 0.3, a separation of 60 Å between surfaces, and an osmotic pressure of 5 × 106 Pa (Figure 7). Thus, when particles are separated by a single layer of micelles, an additional rise in pressure causes ejection of these micelles and aggregation of the particles rather than compression and dehydration of the micelles. Dispersions with excess surfactant gave a steep rise in pressure at a lower volume fraction (Figure 6). As indicated above, this steep rise results from the resistance to dehydration of the TX 100 micelles; it occurs earlier if the dispersion contains more micelles. Again, aggregation was found to occur during this steep rise, indicating that it is easier to aggregate the particles than to dehydrate the micelles. Dispersions with a large amount of excess surfactant could not be prepared through osmotic equilibrium with aqueous polymer solutions, because their osmotic pressures were too high. Instead they were prepared through direct mixing of silica sol and pure surfactant liquid, as explained below. Concentrated Surfactant Phases. Silica sols were also mixed with concentrated surfactant phases. The state of the dispersion was found to vary with the concentration of surfactant. Since the amount of surfactant was vastly in excess of the amount adsorbed on the surfaces, these variations resulted from the properties of the phases formed by excess surfactant and water. Generally, it was found that only micellar phases were miscible with the silica dispersion; hexagonal phases, lamellar phases, and pure liquid surfactant took up the water from the dispersion and rejected the particles. Some typical cases are listed below. Concentrated Micellar Phase (TX 100, 70%; water, 30%). Dispersions containing silica at a volume fraction of 0.03 in this phase were examined visually, through SAXS and through freeze fracture electron microscopy. Visually, the samples were transparent fluids. SAXS spectra gave the scattering from the particles with respect to the background made of surfactant and water. A spectrum is shown in Figure 9: it matches the theoretical scattering curves of independent spherical particles, calculated from eq 2. Aggregation of the particles would have been recognized as a rise of the intensity at low Q relative to this theoretical curve. Therefore this spectrum demonstrates that the particles were dispersed in the micellar phase. An image of the fracture surface from a frozen dispersion is shown in Figure 10. The ridges or waves in the surface of the replica reflect the edges of stacks of flat micelles. The small dark dots scattered throughout the image are “pits” or “bumps” corresponding to the dispersed silica particles. In addition, some particles remained on the replica, and they are seen as dark dots in the (white) shadowed regions. A few aggregates of particles are also seen; however, these aggregates are quite small and not numerous. Comparison with the SANS curves suggests that these aggregates are the cause of the very slight rise in the intensity at low Q above the theoretical curve of independent particles.
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Figure 9. SAXS curves from dispersions of silica in a concentrated micellar solution (TX 100, 70%; water, 30%). Circles: initial dispersion, volume fraction of silica ) 0.03. Full line: calculated scattering curve for independent spheres, radius 100 Å. Triangles: same dispersion after drying and rehydration to the initial water content.
Figure 10. Freeze-fracture image from the initial dispersion of silica (volume fraction 0.03) in a concentrated micellar solution (TX 100, 70%; water, 30%). The irregular ridges in the surface of the replica reflect the edges of stacks of flat micelles. The silica particles (diameter 200 Å) are seen as small pits on a dark background or as dots on a white background.
Homogeneous Liquid Surfactant (TX 100, 80%; water, 20%). Dispersions containing silica at a volume fraction of 0.02 in this phase were examined visually, through SAXS and through freeze fracture electron microscopy. Visually, the samples were transparent fluids. However, SAXS spectra show a very strong rise of the scattered
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Figure 11. SAXS curves from silica in a homogeneous surfactant phase. Circles: dispersion in a phase at TX 100, 80%; water, 20%; the Q-2 slope corresponds to bushy aggregates of silica particles. Triangles: same, after drying; the Q-3 slope corresponds to dense aggregates.
Figure 12. Freeze-fracture image from a dispersion of silica (volume fraction 0.03) in a homogeneous surfactant phase (TX 100, 80%; water, 20%). The surfactant phase appears as a smooth background, and the silica particles as aggregated dots.
intensity near Q ) 0, indicating that the particles were aggregated (Figure 11). An image of the fracture surface from a frozen dispersion is shown in Figure 12. The smooth background corresponds to the fracture running through the homogeneous surfactant phase. Silica aggregates are seen all over the image. Note that the image corresponds to a fracture through the aggregates; therefore, the apparent dimensions of the aggregates on the images may be smaller than their overall sizes. Still, it is clear that there are
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Figure 14. Stability diagram of mixed dispersions of colloidal silica and TX 100 micelles. Dispersions with compositions below the boundary contain independent silica particles dispersed in the surfactant phase. Dispersions with compositions above the boundary contain aggregated silica particles. Rehydration of these dispersions results in partial desegregation of the particles. Figure 13. Freeze-fracture image from a dispersion of silica (volume fraction 0.02) in a lamellar surfactant phase (C12E4, 70%; water, 30%). The silica dispersion gives the granular texture seen in places of the replica; it is completely segregated from the lamellar phase, which gives the regular set of steps in the replica of the fracture.
very many small aggregates and that all aggregates have a tenuous structure. Denser aggregates were observed in dispersions that had been dehydrated completely, and fully dense ones in dispersions that had been equilibrated at 80 °C and then dehydrated. After three weeks of equilibration, macroscopic phase separation was observed where the bottom phase contained silica aggregates and surfactant and the top phase surfactant only. Thus, there is a slow reordering process that operates in these homogeneous surfactant phases; this process completes the phase separation by turning tenuous silica aggregates into dense ones. Cycles of Dehydration and Rehydration. Dispersions of silica in the concentrated micellar phase were dehydrated into the homogeneous surfactant phase until all water was removed and then rehydrated back into the concentrated micellar phase. The state of the sample was followed through SAXS (Figure 9). Initially, the dispersion was nonaggregated, as shown by the SAXS curve, which matches the theoretical curve of independent spheres (circles). After dehydration the particles were massively aggregated, as shown by the strong rise in the scattered intensity (see the spectrum of the dried dispersion in Figure 11, dots). Attempts to disperse the particles through ultrasound were unsuccessful. However, upon rehydration, the intensity came back down close to that of non aggregated particles, but not quite (Figure 9, triangles). Therefore there is a spontaneous redispersion process that operates in these concentrated micellar phases. This process is capable of reversing some of the aggregation processes that occur in the dehydrated state. Hexagonal and Lamellar Phases. Dispersions of silica in the hexagonal phase of TX100 and water were always turbid, indicating that the particles could not be dispersed in the hexagonal phase.
Dispersions containing silica at a volume fraction of 0.02 in a lamellar phase (C12E4, 70%; water, 30%) could not be examined visually, since the lamellar phase is already turbid. Electron microscopy images show that there is complete phase separation. The fracture surface from a frozen dispersion is shown in Figure 13. The traces of the bilayers on the fracture surface are seen as curved “staircases”. The particles appear to be aggregated and completely excluded from the stacks of bilayers. This is confirmed by examination of other areas in the replica. These features indicate that all the water from the dispersion has been absorbed by the lamellar phase, leaving the silica particles in direct contact with each other. Discussion The original aim of this work was to determine the conditions according to which silica particles may be dispersed in a surfactant phase. Ordered mesophases (hexagonal and lamellar) exclude the silica particles from their aqueous layers. This is not surprising, since the aqueous layers of such phases are quite thin (30-60 Å) and filled with hydrated EO chains. Thus, incorporation of silica particles would create large defects in the structure of the mesophase, and it is certainly more favorable for the system to avoid all such defects in the bulk mesophase. It is quite possible that a swollen lamellar phase would accomodate the silica particles without suffering such structural defects; indeed, the swelling ratios resulting from steric undulation forces may be quite large.17 Micellar phases can incorporate some silica particles. However the amounts that can be dispersed in this way are not large, as judged from the results presented in the section on Aggregation of surfactant-covered silica. In the following, we examine this limitation. Aggregation Boundary. This boundary separates regions of composition where the silica particles are kept apart by repulsive forces from regions where they aggregate spontaneously. Spontaneous aggregation and (17) Helfrich, W. Z. Naturforsch., A 1978, 33, 305.
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redispersion were observed in fast dehydration/rehydration cycles of dispersions made at very high surfactant concentrations. From these experiments, a boundary separating a region of stable dispersions from a region of reversible aggregation was determined. Irreversible aggregation was observed in slow dehydration/rehydration cycles of stoichiometric dispersions. From these experiments, a boundary separating a region of stable dispersions from a region of irreversible aggregation was determined. These results have been compiled to give the boundary of the region of stable dispersions. Figure 14 presents the boundary determined at a temperature of 30 °C, where the surfactant/water system forms a micellar phase at all compositions. Remarkably, this boundary corresponds to a volume fraction of silica which decreases linearly with increasing concentrations of TX 100 in water. This suggests that the available water is simply partitioned between the silica and the surfactant: if there is more surfactant, it uses more water and leaves less of it for the silica. This idea can be formalized according to a simple phase separation model. Assume that the silica dispersion and the surfactant solution are phase separated but exchanging water through an osmotic equilibrium. Let x be the volume fraction of surfactant in the total volume, y that of silica, wx the volume fraction of the water associated with the surfactant, and wy the volume fraction of the water associated with the silica. Conservation of the total volume gives a linear relation between those volume fractions:
x + wx + y + wy ) 1
(9)
Assume that the amount of water associated with the surfactant is proportional to the amount of surfactant. Then the osmotic equilibrium with the surfactant imposes
x/(x + wx) ) R
(10)
Assume that the silica particles aggregate when their volume fraction in water exceeds a critical value, β. Then the aggregation boundary is determined by
y/(y + wy) ) β
(11)
From these three conditions, a linear relation may be derived between the volume fraction of surfactant, x, and that of silica, y, along the boundary. This relation is
(x/R) + (y/β) ) 1
(12)
Comparison with the experimental boundary yields R ) 0.8 and β ) 0.3. In other words, the surfactant has a minimum hydration of one water molecule per surfactant molecule (see Table 1). When pure surfactant is mixed with the silica dispersion, it extracts this amount of water from the dispersion; this causes aggregation of the silica if the remaining water is insufficient to separate the silica particles. This model could be improved through an experimental determination of the amounts of water that would be associated with silica and with surfactant in a true phase separation experiment. In this experiment, silica and water would be on one side of the dialysis membrane, with surfactant and water on the other. For each value of the osmotic pressure, wx may then be determined from the equation of state of the surfactant, and wy from the equation of state of the silica dispersion. The aggregation boundary would then be calculated from the aggregation condition, eq 11, and the conservation of total volume, eq 9.
Forces between Silica Particles. The ease with which the aggregation boundary can be reproduced through such phase separation models is impressive, owing to their simplicity. Actually, there may be oversimplification of the processes that lead to aggregation and phase separation. Indeed, in these models, it is assumed that the silica particles phase separate into a pure silica dispersion and then aggregate if their separations in this aqueous phase are too short. However, this two-step process is never observed. Instead, the silica particles aggregate directly to form bushy aggregates dispersed in the surfactant phase, as shown in Figure 11. In time, these aggregates capture more particles, becoming larger and denser; true phase separation is observed only at very long times or after complete dehydration. Thus, direct aggregation is the basic process, and the measured boundary must reflect the conditions where the interaction between silica particles in the surfactant phase switches from repulsive to attractive. In this respect, it is useful to consider the processes by which silica particles are pushed into direct contact. Consider two silica particles, covered with adsorbed micelles and immersed a in micellar solution. If adsorption is weak, then it is possible for the particles to come to surface/surface separations shorter than one micellar diameter, through expulsion of adsorbed micelles. In this configuration, the particles are pushed toward each other by the pressure from the excess micelles; this pressure is opposed by the direct electrostatic repulsion between silica surfaces. Thus the onset of aggregation is determined by the equilibrium between the osmotic pressure from the micelles and the osmotic pressure of the silica dispersion. This osmotic equilibrium was the essential ingredient in the phase separation models presented above; thus, the surprising success of the phase separation models originates entirely from the fact that they relied on this osmotic equilibrium. The aggregation described above must be irreversible, because when the surfaces of particles are pushed in direct contact, they bind through condensation of surface silanols. This matches the observed aggregation behavior for silica dispersions in water, in stoichiometric mixtures, or in a moderate excess of TX 100. However, at the highest surfactant concentrations, reversible flocculation was also observed: dispersions in a micellar phase containing 70% TX 100 could be evaporated to 80% TX 100, where they aggregated, and then rehydrated to 70% TX 100, where they redispersed. This reversible flocculation may have the same origin as the irreversible aggregation observed at lower surfactant content. For instance, the pressure from excess micelles may cause silica particles to come closer together when the separation of their surfaces exceeds a micellar diameter but is not sufficient to allow the presence of two micellar layers. This effect would keep the particles bound at a separation of one micellar layer until water was added to decrease the pressure of excess micelles. Such forces resulting from the pressure of excess micelles in the aqueous phase are well-known. The effect that pushes particles into direct contact has been called a “depletion” attraction,4 and the effect that keeps them bound at a separation of one micellar diameter has been called a “structural force”.18 If these forces are responsible for the observed aggregation behavior, then the essential ingredient is not the range of the force but its magnitude. Indeed, the aggregation boundary of particles in dispersions with excess surfactant does not follow a line of fixed silica volume fraction. Therefore it does not correspond (18) Richetti, P.; Ke´kicheff, P. Phys. Rev. Lett. 1992, 68, 1951.
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to a critical distance between surfaces, such as a micellar diameter, but to a condition where the pressure from excess micelles exceeds the direct repulsion between silica surfaces. This is consistent with the known effect of depletion forces for emulsion droplets dispersed in micellar solutions of ionic surfactants.4 Conclusion In this work we examined the compatibility of aqueous mixtures containing mineral particles and nonionic surfactants. Our expectation was that the weak attractive interactions between both components would ensure their compatibility in the mixed dispersions. In fact, segregation was found in most circumstances. The lamellar phase, the hexagonal phase, and the pure surfactant liquid were all found to absorb the water from the dispersion and expell the silica particles. Even in the micellar phase, substantial compatibility was obtained only when the water content was sufficient to fully hydrate the oxyethylene chains of the surfactant, thereby avoiding competition between silica and surfactant for the available water.
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This competition for water is a key to the stability of multicomponent mixtures. The argument can be summarized in a simple way. Each component in the mixture has its own affinity for water, which is expressed by an equation of state, i.e. the curve of osmotic pressure vs volume fraction. In general, different components have different equations of state in water. Therefore, upon dehydration, the osmotic pressure of one will rise much faster than those of the other ones. When the osmotic pressure difference is large, the component with the higher pressure will extract water from the other ones, causing them to phase separate. Thus, in order avoid segregation of the components during dehydration, it is necessary to choose conditions such that the osmotic pressures of all components will remain comparable. Acknowledgment. Small-angle neutron scattering (SANS) experiments were performed on the instrument PAXE (LLB, CEN-Saclay), and for X-ray small angle scattering we used the Bonse-Hart Camera constructed in Service de Chimie Mole´culaire (CEN-Saclay). LA950707C
