pubs.acs.org/Langmuir © 2010 American Chemical Society
Rheology and Structure Formation in Diluted Mixed Particle-Surfactant Systems Stephanie Limage,† Jurgen Kr€agel,‡ Murielle Schmitt,† Christian Dominici,§ Reinhard Miller,‡ and Mickael Antoni*,† † Universit e d’Aix Marseille UMR-CNRS 6263 ISM2, France, ‡Max Planck Institut f€ ur Kolloid und Grenzfl€ achenforschung, Potsdam-Golm, Germany, and §Centre Pluridisciplinaire de Microscopie Electronique et de Microanalyse, Marseille, France
Received June 17, 2010. Revised Manuscript Received September 6, 2010 In the present work, we focus on the bulk rheology of mixtures consisting of surfactant modified silica nanoparticles in water. Depending on the ratio of surfactant and nanoparticle concentrations, significant modifications in the measured rheology are evidenced. A domain where the dispersions behave like viscoelastic media is observed. Outside this domain, the dispersions exhibit viscous properties. The changes in the bulk rheology characteristics are discussed in terms of interaction effects between the surfactant and the particles. The results obtained are seen in the context of diluted emulsions’ properties and characteristics.
Introduction The presence of particles in liquids can result in a variety of consequences, for example, the stabilization of emulsions and foams or the appearance of self-organized colloidal structures. The interfacial properties of particles can be modified by the addition of surfactants that usually allows for a controlled change of their wettability. Indeed, the surfactant molecules can adsorb onto the surface of the particles and confer to the latter a partially hydrophobic character, which increases with surfactant concentration. Such effects have been studied by Lan et al.1 Partially hydrophobized particles are shown to migrate to air/water interfaces and modify interfacial properties.2,3 Self-organized structures due to direct particle-particle interactions in the bulk phase of aqueous solutions have also been evidenced.4 The adsorption of surfactant on particle’s surface can also lead to particle aggregation. The role of particle flocculation on emulsion stability has been studied for instance by Binks et al.5,6 It has been shown that, for mixtures of negatively charged silica nanoparticles and cationic surfactants, oil-in-water emulsions were the most stable to creaming and coalescence when the flocculation of particles in the aqueous dispersion alone is maximal. This idea was extended in Ref 6 using positively charged silica particles in mixtures with an anionic surfactant. Finally, Hassander et al.7 observed that silica particle stabilized emulsions are more stable if they are prepared when the particles are first flocculated by addition of a surfactant. The wettability of particles at oil-water interfaces has been shown to be crucial in optimizing the stability of particle-stabilized
emulsions.8 Numerous surface rheology studies have also been achieved with the aim to understand the interfacial elastic and viscous properties in the presence of particles.9-11 In addition, several bulk rheology studies involving concentrated particle/ surfactant systems have been performed.12 Functionalized particles can indeed generate microstructures in the bulk phase that are expected to modify the bulk rheology properties.4 Consequently, for concentrated dispersions, foams, emulsions, and associative polymer systems, important theoretical and experimental investigations do exist, although some phenomena are still not yet well understood.13 In contrast, much less work has been performed for dilute dispersions. In a recent paper,14 we investigated Ramsden-Pickering emulsions consisting of water droplets in paraffin oil. The water droplets were composed of a mixture of silica nanoparticles and CTAB surfactant. Changing the amount of CTAB allows the tuning of the hydrophobicity of the particles. This modifies their action on the stability of the emulsions due to their self-assembling at the interface, which decreases the water-oil interfacial tension. Optical tomography studies were performed on these emulsions14,15 with different CTAB/silica particle mixtures and evidenced an irreversible deformation of the emulsion droplets at a critical CTAB/ silica particle ratio Rc. Below this critical ratio, emulsions consist of droplets with irregular shapes, while above this critical ratio droplets display their usual equilibrium spherical geometry. A solidlike behavior can appear at the water-oil interface, which influences the morphology of the droplets that appear in some cases as stiff polymorphous objects. In order to characterize this transition,
*To whom correspondence should be addressed. E-mail: m.antoni@ univ-cezanne.fr. (1) Lan, Q.; Yang, F.; Zhang, S.; Liu, S.; Xu, J.; Sun, D. Colloids Surf., A 2007, 302, 126–135. (2) Ravera, F.; Ferrari, M.; Liggieri, L.; Loglio, G.; Santini, E.; Zanobini, A. Colloids Surf., A 2008, 323, 99–108. (3) Ravera, F.; Santini, E.; Loglio, G.; Ferrari, M.; Liggieri, L. J. Phys. Chem. B 2006, 110, 19543. (4) Zaman, A. A.; Singh, P.; Moudgil, B. M. J. Colloid Interface Sci. 2002, 251, 381–387. (5) Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Langmuir 2007, 23, 3626–3636. (6) Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Langmuir 2007, 23, 7436–7439. (7) Hassander, H.; Johansson, B.; T€ornell, B. Colloids Surf., A 1989, 40, 93–105. (8) Binks, B. P.; Horozov, T. S. Colloidal particles at liquid interfaces: An introduction; Cambridge University Press: 2006; pp 1-74.
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(9) Fuller, G. G.; Stancik, E. J.; Melle, S. Colloidal particles at liquid interfaces; C.U. Press: 2006, pp 169-185. (10) Montreux, C.; Jung, E.; Fuller, G. G. Langmuir 2007, 23, 3975–3980. (11) Spigone, E.; Cho, G. Y.; Fuller, G. G.; Cicuta, P. Langmuir 2009, 25, 7457– 7464. (12) Larson, R. G. The structure and rheology of complex fluids; Oxford University Press: New York, 1999. (13) Wyss, H. M.; Miyazaki, K.; Mattson, J.; Hu, Z.; Reichmann, D. R.; Weitz, D. A. Phys. Rev. Lett. 2007, 98, 238303–238301. (14) Schmitt-Rozieres, M.; Kr€agel, J.; Grigoriev, D. O.; Liggieri, L.; Miller, R.; Vincent-Bonnieu, S.; Antoni, M. Langmuir 2009, 25, 4266–4270. (15) Antoni, M.; Kr€agel, J.; Liggieri, L.; Miller, R.; Sanfeld, A.; Sylvain, J. D. Colloids Surf., A 2007, 309, 280–285.
Published on Web 10/14/2010
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investigations of these emulsions with different mixing ratios based on cryo-scanning electron microscopy (cryo-SEM)16 have been performed. Cryo-SEM measurements indicated the existence of self-organized nanoparticle structures that are shown to be the origin of the transition in the droplet morphology.16 This transition from spherical to polymorphous droplets shows up when selforganized silica nanoparticle microstructures fill the entire droplet. We believe such structural changes should be detectable by bulk rheological techniques. Due to the prior mixing of the aqueous CTAB/silica solution, particle flocculation is observed depending on the mixing ratio, reaching from slight solution turbidity up to precipitation of particle aggregates. Therefore, it is obvious to associate the observations with a change in particle self-organization in the aqueous bulk phase. The rheological behavior of the aqueous particle/surfactant dispersions used in the emulsions of Refs 14 and 16 is indeed strongly affected by the details of the microstructure and by the interparticle forces, as will be shown below. Our aim here is hence not to perform emulsion rheology measurements but instead to focus only on the bulk rheological properties of the aqueous dispersion before emulsification. Bulk rheology measurements are commonly used to investigate the dynamical and structural properties of complex fluids. A ubiquitous behavior of many metastable complex fluids is that they show distinct nonlinear viscoelasticity. With increasing oscillatory-strain amplitude above a critical strain, the elastic modulus decreases continuously as the strain amplitude increases, whereas the viscous modulus has a distinct peak before it decreases at larger strain. Yziquel et al.17 explained these universally observed phenomena for dispersions of fumed silica particles by a breakdown of aggregates into smaller clusters which are more dissipative. Rheological measurements of complex microstructured fluids are often complicated, since the deformation in the samples is not uniform. In such microstructured liquids, shear-banding is often observed.18-20 This phenomenon consists of localized regions where flows in the samples are separated by regions where they remains unsheared. Further experimental complications arise from the highly sensitive nature of self-organized microstructures and from the fact that such systems are often in a nonequilibrium state. A consequence of the latter is that the viscoelastic moduli are usually time dependent. Such behavior can be associated with slow structure changes, phase transitions or gelation.21 Creaming and sedimentation processes can also affect the overall time dependent changes in the samples. The aim of the present experimental study is to propose a bulk rheological description of aqueous dispersions containing surfactant modified silica nanoparticles at given CTAB/silica composition. Depending on the ratio of surfactant and nanoparticle concentrations, significant modifications in the measured bulk rheology are evidenced. These changes are discussed and coupled with cryo-SEM observations. The latter allow the visualization of the nanoparticle self-organized microstructures that are generated by the adsorption effects of the surfactant on the particles’ surface. Aggregated microstructures are evidenced and shown to depend on the CTAB/silica concentrations. A domain where the solutions have a viscoelastic behavior is also evidenced. Outside this domain, they behave as viscous Newtonian fluids. These features (16) Limage, S.; Schmitt, M.; Vincent-Bonnieu, S.; Dominici, C.; Antoni, M. Colloids Surf., A 2010, 365, 154-161. (17) Yziquel, F.; Carreau, P. J.; Tanguy, P. A. Rheol. Acta 1999, 38, 14–25. (18) Hu, Y. T.; Palla, C.; Lips, A. J. Rheol. 2008, 52, 379–400. (19) Manneville, S. Rheol. Acta 2008, 47, 301–318. (20) Ovarlez, G.; Rodts, S.; Chateau, X.; Coussot, P Rheol. Acta 2009, 48, 831– 844. (21) Stokes, J. R.; Frith, W. J. Soft Matter 2008, 4, 1133–1140.
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in the bulk rheology characteristics are discussed and shown to be related to self-organized microstructures of the nanoparticles. The transition in the droplet shape observed in Ref 16 when emulsifying the solution in paraffin oil is also considered and shown to be independent of the rheology of the droplets’ composition. As a conclusion, this emulsion drop shape transition is not due to modifications in the bulk rheology properties of the aqueous dispersions.
Materials and Methods The solutions under investigation are aqueous mixtures containing the cationic surfactant CTAB (Fluka 52365) and amorphous silica nanoparticles (provided by Starck, Levasil 200) with excellent stability and resistance to gelling. The latter are delivered as a 30% mass fraction alkalic aqueous dispersion with pH 9.2, an effective surface of 200 m2/g and a typical size diameter of 15 nm. Gelling transition in the silica solution occurs for pH < 5.5. This solution was used after 20 min of centrifugation, in order to eliminate preexisting aggregates due to possible skin formation at the liquid/air interface generated by water evaporation. The Levasil 200 dispersion is diluted in HPLC grade water in order to obtain given stock solutions of 0.3% (pH 9.4) and 3% (pH 9.6) mass fraction. The cationic surfactant CTAB (CMC of 9.10-4 mol/L or 0.33 g/L) was used without any further purification and diluted in HPLC grade water. These silica particle dispersions and CTAB stock solutions were further mixed to finally obtain 30 different dispersions with controlled compositions. The compositions of the samples are organized in a matrix-like structure, that is, elaborated in a way to have several samples with given CTAB (respectively SiO2) concentrations but with varying SiO2 (respectively CTAB) concentrations. This matrixlike structure of the compositions is motivated by the previous studies focusing on morphological transitions in the shape of droplets in diluted emulsion.16 It has the important advantage to allow cross comparisons between the samples while maintaining one of the concentration fixed. Besides its composition, each sample will be characterized by a parameter R = [CTAB]/[SiO2] introduced in Ref 14. The precise compositions of the samples, the value of R and the corresponding final pH values are given in Table 1. The bulk rheological measurements are performed with the MCR 301 rheometer from Anton Paar Physica, Germany. The measuring geometry is a double gap concentric cylinder. The temperature of the samples is controlled by Peltier elements and fixed at 20 C. For each measurement, 7.8 mL of solution is required. The experiments have been performed under stress-controlled conditions (amplitude and frequency sweep experiments) and the precise positioning of the inner cylinder in the outer one is controlled by the hardware through a TruGap function. The procedure for the rheological measurements for each sample is twofold. First, after the filling of the rheometer, a 10 min delay is used, for temperature equilibration. Then, amplitude sweep measurements are performed, at different time intervals, with amplitudes ranging between 0.1 and 10% and a constant angular frequency of 10 rad/s. The total duration of each experiment is about 100 min. Frequency sweep measurements with constant amplitude of 0.5% and a frequency range between 0.1 and 100 rad/s were also performed. The results of these measurements are not presented here, since they do not provide additional informations. Flow curve experiments have also been performed in order to observe the shear thinning behavior with increasing shear rate. These results are also not reported here either, since the microstructures we aim to investigate will be destroyed by the continuous shear. Cryo-scanning electron microscopy observations are performed with a Philips XL30 SFEG STEM scanning electron microscope. For each experiment, four specimens of about 2 mm3 of the considered dispersion were taken with a syringe and deposited on a brass sample holder. The latter is then plunged into slush nitrogen at -196 C and transferred into a preparation chamber at low DOI: 10.1021/la102473s
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Table 1. Sample Names, Compositions, pH (after its stabilization), and Corresponding Value of the Ratios R = [CTAB]/[SiO2] (both concentrations are expressed in g/L) sample
[CTAB] (g/L)
[SiO2] (g/L)
R
pH
S03 S05 S06 S07 S08 S09 Sll S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S27 S28 S29 S30 S30bis S31 S32 S33 S34 S35 S36 S37 S38
0.05 0.05 0.05 0.12 0.12 0.12 0.26 0.25 0.25 0.25 0.25 0.61 0.65 0.65 0.64 0.65 0.65 1.34 1.28 1.29 1.27 1.27 1.29 1.28 0.65 0.65 0.25 0.12 0.12 0.05
2.6 9.1 25.1 4.5 15.0 25.0 2.6 4.5 9.0 25.4 75.0 2.5 9.0 15.0 60.0 150.4 240.1 2.7 4.5 9.1 25.4 15.0 89.8 50.0 4.5 25.0 50.0 9.0 75.0 75.0
0.02 0.005 0.002 0.03 0.008 0.005 0.10 0.06 0.03 0.01 0.003 0.24 0.07 0.04 0.01 0.004 0.003 0.49 0.28 0.14 0.05 0.08 0.01 0.03 0.14 0.03 0.005 0.01 0.002 0.001
7.8 9.1 9.3 7.8 8.5 9.6 7.1 7.5 9.1 9.5 9.6 6.8 7.5 8.8 9.5 9.4 9.3 6.4 6.4 6.8 8.4 7.3 9.4 9.2 6.6 9.2 9.6 8.9 9.6 9.5
temperature. The four samples are fractured using the tip of a cold scalpel blade and left for 1 h at a temperature of -100 C and 10-5 Pa pressure. This last procedure is performed to remove a controlled amount of water from the considered specimens and therefore to evidence their inner structure. After this sublimation step, the samples are introduced in the analysis chamber. All the images were obtained using the backscattered electron (BSE) mode that provides good contrast between different chemical species and X-ray analysis for their composition. This analysis is important to complete usual visual inspection of the images by a reliable spectral analysis and hence to eliminate possible artifacts due to either water recoiling mechanisms or misleading bright regions obtained in BSE mode. The interpretations of the pictures obtained from cryo-SEM were performed as carefully as possible to detect artifacts due to sample preparation.22 Frozen aqueous solutions with particles might, for example, show striations that are due to excess particles sandwiched between frozen water regions.11,23 Still, the existence of self-organized networks is considered to be reminiscent of structures that are actually present at room temperature. Indeed, in spite of the possibility of artifacts, the consistent changes in frozen morphology with water chemistry are believed to be related to actual particle microstructures, even if they are not direct representations.22 Unwanted microstructures can also appear during the sublimation procedure. Crystallization phenomena of water were indeed observed when the experimental conditions were not fully controlled. For consistency, the SEM images were systematically compared with X-ray and BSE analysis as well as by experiments with nonsublimated samples.
Results and Discussion The viscoelastic properties were measured for all 30 samples with composition shown in Table 1. Figure 1 displays 5 successive (22) Mikula, R. J.; Munoz, V. A. Colloids Surf., A 2000, 174, 23–36. (23) Binks, B. P.; Kirkland, M. Phys. Chem. Chem. Phys. 2002, 4, 3727–3733.
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Figure 1. Elastic (a) and viscous (b) moduli for sample S07 (see Table 1). Curves with markers are the measurements at successive times (expressed in minutes). Full black lines corresponds to the time averaged moduli ÆG0 æ and ÆG00 æ.
amplitude sweep experiments for sample S07. This figure exhibits a linear regime for both elastic (G0 ) and viscous (G00 ) moduli when strain amplitude A is smaller than 1%. This indicates that the deformations generated by the rheometer do not change the colloidal microstructures in the sample. When A is higher than 1%, a decrease shows up in both G0 and G00 , indicating that these microstructures are influenced by the generated shear field. Time repetition of the amplitude sweep experiments shows almost the same shapes of G0 and G00 dependencies, suggesting a recovery of the colloidal structures of the sample after each amplitude sweep. A detailed investigation of Figure 1 reveals a change of the moduli with time for a given amplitude A and a shift of the linear viscoelastic regime to smaller values of A. Due to the broad range of compositions of the samples studied herein, a large variety of time-dependent behaviors is observed. When running over all the results obtained, rheology measurements indicate that samples can have either a viscoelastic or a viscous rheological behavior. The increasing behavior of G0 and G00 with time observed here is typical for all the samples that will be considered in the following as viscoelastic ones. Indeed, from one viscoelastic sample to the other, differenet values of G0 and G00 can show up, but for a given amplitude A, the increasing trend always occurs. Qualitatively different shapes for G00 can be observed, with reduced linear viscoelastic regimes and/or increasing maxima in the amplitude dependence. Langmuir 2010, 26(22), 16754–16761
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Figure 3. Ratio q = ÆG0 æ/ÆG00 æ of the time averaged elastic and viscous moduli as a function of amplitude for samples S03, S11, S13, S14, S27, and S31. The full horizontal black line corresponds to q = 1.
Figure 2. Elastic (a) and viscous (b) moduli for sample S05 (see Table 1). Curves with markers are the measurements at successive times (expressed in minutes). Full black lines corresponds to the time averaged moduli ÆG0 æ and ÆG00 æ.
The origin of these behaviors is not well understood so far in the case of diluted dispersions. However, such a phenomenon has been observed for highly concentrated dispersions in the neighborhood of a glasslike state24 and considered as a deviation from the linear viscoelastic range. Conversely, for viscous samples, values of G0 are almost zero while G00 generally shows a constant behavior over all amplitudes. The typical amplitude and time evolution of G0 and G00 for such viscous samples are displayed in Figure 2 for S05. Since the aim here consists of assessing the differences observed in the emulsification of the samples under consideration in Refs 14 and 16, we limit the description of the rheological properties to the time averaged values of both G0 and G00 that are plotted in Figures 1 and 2 as a full black line and noted ÆG0 æ and ÆG00 æ. Figure 2a shows a comparatively small average value of ÆG0 æ. This is due to the fact that many measurement points are very close to zero. For A > 2, most of the G0 values are smaller than 10-5 Pa. Figure 3 shows the amplitude dependence of ratio q = ÆG0 æ/ÆG00 æ for six samples from Table 1. For all the samples, q is shown to decrease with increasing shearing amplitude A. This indicates that the decreasing of the averaged elastic modulus ÆG0 æ with increasing A is stronger than the decrease of ÆG00 æ. The ratio q refers here to the usual loss factor tan(δ) = ÆG00 æ/ÆG0 æ, since samples can be (24) Siebenb€urger, M.; Fuchs, M.; Winter, H.; Ballauff, M. J. Rheol. 2009, 53, 707–726.
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viscous and hence lead to divergences due to vanishing ÆG0 æ values. This graph suggests that the detailed investigation of ÆG0 æ is sufficient to obtain relevant informations about the precise rheological properties of the solutions. We define ÆG0 æA (respectively ÆG00 æA) as the amplitude averaged value of ÆG0 æ (respectively ÆG00 æ). The considered amplitude averaged sequence ranges from A = 0.1% to A = 10% with 23 different values, whereas time is averaged over 10 measurements running over almost 100 min. These new parameters aim to give typical elastic and viscous moduli, not only for successive time measurement sequences but also for all the used amplitudes. Both quantities are plotted in Figure 4 as function of the CTAB and SiO2 concentrations. The surfaces resulting from a nonlinear fitting of the data are also represented. As mentioned in the introduction, during emulsification of the solutions of Table 1 in paraffin oil, a transition occurs in the shape of the droplets when the ratio R = [CTAB]/[SiO2] reaches the critical value Rc ≈ 0.03. The surface resulting from the nonlinear fitting shows an edge along the line defined by Rc for which both ÆG0 æA and ÆG00 æA increase for increasing [CTAB] and [SiO2]. This edge defines a symmetric plane (in a log-log plot) apart from which both ÆG0 æA and ÆG00 æA decrease. The relative simplicity of the surfaces displayed in Figure 4a allows the use of a simple criterion to distinguish the different rheological properties of the solutions of Table 1. Indeed, in the following, all the samples having values of ÆG0 æA higher than 1 Pa will be considered viscoelastic, whereas all the others will be viscous. This choice is also motivated by the fact that, for viscous samples, the measured elastic modulus tends to vanish. Figure 5 displays a 2D representation of the rheological properties of the samples according to the previous criterion. A domain where the solutions are viscoelastic clearly shows up around the transition line [CTAB]/[SiO2] = Rc, surrounded by regions where the dispersions are viscous. The viscoelastic domain broadens with increasing concentrations. Its lower limit is reached when both [CTAB] < 0.1 g/L and [SiO2] 1 Pa). The thick black line corresponds to the transition line [CTAB]/[SiO2] = Rc. The gray ones emphasize the limit between viscous and viscoelastic samples. The names of the samples observed in SEM (see below) are also represented.
Figure 4. 3D plot representation of the time and amplitude averaged elastic modulus ÆG0 æA (a) and viscous modulus ÆG00 æA (b) as a function of [CTAB] and [SiO2]. Surfaces correspond to nonlinear fittings of the data, with correlation coefficients close to 0.95. The transition shows up as an edge along the R = Rc direction.
viscous properties. It is also interesting to note here that samples can behave in a viscoelastic way although concentrations are small, and that the CMC of the CTAB does not influence the rheology of the samples. These observations raise several questions not only about the interactions between the surfactant and the particles but also about the interparticle interactions themselves. The rheological observations support the idea that there must be an interaction between the modified particles due to the rising elastic moduli of the samples. Viscoelastic properties are indeed reminiscent of organized silicon dioxide structures in the samples, otherwise no energy storage could be possible and the elastic modulus would be close to zero as for viscous solutions. Still, the measured values support that the microstructures formed must be weak since the elastic moduli are comparatively small.9 This is the reason why these fragile systems were studied under soft shearing conditions (i.e., small amplitudes and low frequencies). The weakness of the self-organized silica microstructures is confirmed by a simple and direct visual inspection of the samples. No gel-like behavior was observed, even for samples with the largest viscoelasticity. This suggests that the structures of the colloidal microstructures are too weak to generate significant macroscopic changes in the bulk properties. The differences observed in the rheology of the samples of Table 1 naturally raise the problem of the nanoparticles’ 16758 DOI: 10.1021/la102473s
self-organization in the solutions. Figure 6a displays vessels of solutions S06, S14, and S30, 24 h after their preparation. In these solutions, CTAB concentration varies and SiO2 concentration is fixed to 25 g/L. Figure 6b shows samples S11, S14 (again), and S15 for which SiO2 concentration varies with [CTAB] ≈ 0.25 g/L. Depending on the [CTAB]/[SiO2] ratio, up to three phases can appear. A precipitate (P1) constituted of aggregated silica nanoparticles can show up in a variable quantity. Above, there is a phase (P2) that can either be transparent (e.g., for S30 and S11) or cloudy (S06, S14, and S15). In certain samples, formation of foam (P3) has been observed above the solution, visible here in Figure 6a for S30. Figure 7 shows SEM images for samples S06, S14, and S30 with several magnifications. The idea here is to try to get insights of the silica microstructures at room temperature. All these pictures evidence a structuring of the silica nanoparticles which is confirmed by X-ray analysis. As already mentioned, it is important to note that the striations of silica nanoparticles visible in the SEM images are induced by the cryo-freezing of the samples. Freezing artifacts must hence be accounted for, but it is commonly believed that the microstructures observed in such images are representative of the actual organization of the nanoparticles in the considered samples at room temperature.23 Here, it was evidenced with original Levasil solutions that these striations constitute a signature of an almost isotropic distribution of the silica nanoparticles (or submicrometer-scaled aggregates of nanoparticles) at room temperature. Their thickness and density depend on the silica nanoparticle concentration. Figure 7a-c corresponds to sample S06, which is viscous with R < Rc. These panels show densely packed and smooth silica nanoparticle striations. Figure 7c provides the largest magnification and allows a direct visualization of the nanoparticles. Although cryo-freezing induces the silica layers in the samples, nanoparticles appear in these structures as spherical 15 nm diameter objects as in the original Levasil dispersion. They have hence kept their original shape, indicating that in S06 they were not significantly modified by the addition of CTAB. Their chemical properties are hence almost unchanged, and the corresponding dispersion is stabilized by electrostatic repulsion. Langmuir 2010, 26(22), 16754–16761
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Figure 6. Photograph of vessels containing solutions S06, S14, S30 (a) and S11, S15 (b). Pictures were taken 24 h after mixing the CTAB and the SiO2 nanoparticle solutions. In (a), [SiO2] is fixed, whereas in (b) it is [CTAB] that is fixed. P1, P2, and P3 indicate the different phases observed in the samples. S14 is represented twice to ease interpretation of images.
Figure 7. Cryo-SEM pictures of samples S06 (a-c), S14 (d-f ), and S30 (g-i). [CTAB] is increased from top to bottom, and [SiO2] takes a value close to 25 g/L (see Table 1). Scales are given on each image. Magnification increases from left to right.
In other words, the balance between CTAB and SiO2 concentrations does generate solutions with rheological properties similar to the ones of the original Levasil solution. Figure 7d-f corresponds to sample S14 which is viscoelastic with R < Rc. These panels show again silica particle striations that look more porous and interconnected than the ones of Figure 7a. The detailed analysis of Langmuir 2010, 26(22), 16754–16761
these latter is possible with Figure 7f that indicates the presence of spherical and densely packed objects, as already observed in Figure 7c for S06. However, in S14, these objects have a larger radius than the original nanoparticles. The size distribution of the particles hence broadens. Moreover, they seem fused together conversely to what can be observed in Figure 7c where nanoparticles DOI: 10.1021/la102473s
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Figure 8. Cryo-SEM pictures of samples S11 (a-c) and S15 (d-f ). [SiO2] is increased from top to bottom, and [CTAB] is maintained to a value close to 0.25 g/L (see Table 1). Magnification increases from left to right, but for (c) it is smaller than that in (f). The arrows in (a) point at aggregates, and the ones in (c) emphasize nanoparticle wires.
have kept their individuality. One possible explanation of this observation is the progressive screening of the electrostatic repulsion between the nanoparticles due to particle surface coverage by the increasing CTAB concentration. At room temperature, nanoparticles can hence come closer to each other and organize themselves in larger structures. In the following, these spherical structures will be denoted as silica globules (SiG). Owing to their spherical shape and relatively large radius, particle-particle aggregation is probably the basic mechanism of SiG formation. For S30, which behaves as viscoelastic with R > Rc, Figure 6 shows that the sample is almost entirely filled with precipitate. The corresponding SEM images are displayed in Figure 7g-i. When comparing them to the cryo-SEM pictures of S06 and S14, qualitatively different microstructures show up. They now appear as silica aggregates (SiA) that are present in a large quantity and almost fill the entire sample. Moreover, only few striations are present, indicating that a very limited amount of nanoparticles remains dispersed, as already suggested by the transparent phase P2 for S30 (see Figure 6a). The large magnification of this sample given in Figure 7i displays an assembly of objects having a size distribution slightly broader than the one of the nanoparticles of S06 but clearly narrower than the SiG of S14. Similarly to S14, the nanoparticles also seem to be organized in SiG but with smaller size. This indicates that, from the nanoparticle point of view (i.e., particles size), S30 is intermediate between S06 and S14. However, from the SiA point of view (i.e. the presence of aggregates), S30 has its own specificity due to the presence of many aggregates as suggested by its important P1 phase illustrated in Figure 6a. Cryo-SEM images for samples with varying SiO2 concentration and CTAB concentration fixed to 0.25 g/L are displayed in Figure 8 for S11 and S15. These pictures come in addition to Figure 7d-f for sample S14 that has the same CTAB concentration. The values of ÆG0 æA for S11 and S15 are, respectively, 0.02 and 0.27 Pa, indicating that both samples are viscous. Figure 8(a), (b) and (c) show again striations of silica nanoparticles, but with limited interconnections. They also appear as loose and porous when compared to the microstructures displayed in Figure 7a. This is due to the smaller silica nanoparticle concentration. Small aggregates also show up as illustrated by the arrows in Figure 8a. Figure 6b for S11 shows two separated phases P1 and P2, with P2 16760 DOI: 10.1021/la102473s
that is almost transparent due to a small silica particle concentration. Cryo-SEM loose striations of Figure 8a and b result from this last phase. Among nanoparticles, Figure 8a also shows that, similarly to Figure 7f and i, the association of silica nanoparticles generates SiG that can be identified in Figure 8c. Their size is typically 5 times larger than the single silica nanoparticles of the original Levasil solution. This figure also evidences the existence of nanoparticle wires (indicated by arrows in Figure 8c) that are probably generated by the freezing procedure but that indicate the presence of a fraction of unmodified nanoparticles at room temperature. Conversely to S06, where the CTAB concentration was too small for significant silica nanoparticle coverage, in S11 it is now the silica particle concentration that becomes too small with regard to CTAB. The balance between silica nanoparticles and CTAB is hence in favor of an important coverage of the nanoparticles that can yield SiG, similar to the ones of Figure 7f, coexisting with dispersions of nanoparticles. Sample S15 shows two separated phases P1 and P2 (see Figure 6b) with cloudy phase P2. Figure 8d-f presents cryo-SEM pictures of this solution. They evidence again striations similar to the ones already discussed in Figure 7a due to the dispersed nanoparticles at room temperature. One important difference with S06 is the existence of both striations and aggregates embedded in them. Such a configuration can be seen in the right-hand side of Figure 8e. A higher magnification of the striations is given in Figure 8d where 15 nm diameter nanoparticles can again be identified. As for S06, the nanoparticles seem here to have kept their individuality, indicating that at room temperature they were actually dispersed. However, owing to its opacity (see Figure 6b), this phase probably also contains SiG similar to the one identified in Figure 7f and i. The silica structure in the assemblies of Figures 7c,f,i and 8c,f together with the coexisting phenomena between SiA, SiG, and nanoparticles allow now to assess again the rheological measurements previously discussed. The time and amplitude averaged elastic modulus ÆG0 æA indicates that S06 is viscous (ÆG0 æA ≈ 0.11 Pa) whereas both S14 and S30 are viscoelastic (ÆG0 æA ≈ 1.72 Pa and ÆG0 æA ≈ 2.85 Pa, respectively). Sample S06, which contains essentially dispersed nanoparticles, exhibits viscous rheological properties, whereas S30 is viscoelastic and actually contains a large amount of SiA. As energy storage is less effective in S06 than Langmuir 2010, 26(22), 16754–16761
Limage et al.
Figure 9. Schematic overview of the different rheological domains of Figure 5 with the corresponding silica structures as a function [CTAB] and [SiO2]. SiA stands for silica aggregates, and SiG for silica globules. The dashed arrows indicate the decrease in silica nanoparticles coverage by CTAB.
in S30, nanoparticles dispersed in S06 have a weaker interaction than the SiA of S30. A similar remark could be drawn when comparing S30 with S15 that contains both nanoparticles and SiA. As a result, smaller ÆG0 æA values can be explained by the interplay between two effects: weakly coupled coarse grained colloidal domains (such as small SiA) and/or repulsion between the nanoparticles. Both lead to more fragile colloidal microstructures that easily reorganize (or break) under small shear deformations. Consequently, small CTAB concentrations do not allow the establishment of mechanically resistant colloidal structures. For larger amounts of CTAB, however, surface modification of the nanoparticles is increased. They can hence come closer to each other since electrostatic repulsion is reduced. The nanoparticles are no longer repulsively stabilized as in the original Levasil dispersion. Steric interactions can come into play and allow the appearance of weakly interacting silica microstructures such as SiA or SiG. This occurs in S30 and S14. They are both viscoelastic with values of ÆG0 æA of the same order of magnitude, although their aspect (see Figure 6) and their cryo-SEM structures (see Figure 7f,i) are different. This suggests that viscoelasticity can have different origins. For S30, the fraction of SiA is large and its viscoelasticity results from the direct interaction between the aggregates. For S14, however, viscoelasticity results from the coupling between SiA, SiG, and dispersed nanoparticles. It is hence the interplay between all these different silica organizations that finally generates mechanically resistant structures in the samples at room temperature. When focusing only on viscous samples (S06, S11, and S15), it is important to note that they all display cryo-SEM structures consisting of silica particles with a narrow size distribution. Although they share this property, their aspects at room temperature and their cryo-SEM structures are quite different. Again, two different mechanisms come into play here to explain their low elasticity. For S06 and S15, R < Rc and nanoparticle modification by CTAB is limited due to the too small CTAB concentration regarding the silica nanoparticle one. The distribution of silica nanoparticles in these samples is hence comparable to the one of
Langmuir 2010, 26(22), 16754–16761
Article
the original Levasil solution. Particles are repulsively coupled when approaching one another, and this prevents mechanically resistant structures from appearing. This is verified with cryoSEM pictures revealing that nanoparticles have almost kept their original size and individuality. For S11, R > Rc and CTAB modification of the nanoparticles is important, but their concentration is now too small to generate large neighboring SiA. Therefore, no significant coupling can occur, these latter being too far apart from each other. As for S06 and S15, such a dispersion, even if mainly composed of SiA, cannot generate any mechanically resistant structures at room temperature. The previous discussion shows that it is finally the relative amount of aggregates and the coverage of silica particles that explains the different rheological properties of the samples of Table 1. Figure 9 presents a schematic view of the overall outputs of the present work. From S11 to S30, R > Rc and it is essentially the increasing size and amount of SiA that explains the increasing values of ÆG0 æA. The relative quantity of dispersed nanoparticles and SiG are indeed not significantly modified. From S30 to S14, the SiA size and quantity decreases while the amount of SiG and dispersed nanoparticles increases. When R = Rc, the coupling between coexisting SiA, SiG, and nanoparticles leads to structures that have the largest mechanical resistance as illustrated in the 3D plot of Figure 4. For R < Rc, the contribution of SiA decreases whereas the one of SiG and nanoparticles becomes dominant in the elasticity. In S15, SiA is no longer the dominant phase and SiG also tends to disappear in favor of single dispersed nanoparticles. ÆG0 æA then decreases, since particle modification by CTAB becomes too small to generate sufficiently coupled SiA and SiG. Finally from S15 to S06, both SiA and SiG disappear and only dispersed nanoparticles remain in the solution. ÆG0 æA then vanishes.
Conclusion The rheological properties of CTAB and SiO2 mixtures diluted in water are investigated. Amplitude sweep experiments indicate both qualitative and quantitative changes in the elastic and viscous moduli that evidence either viscous or viscoelastic regimes. The largest elastic modulus are observed along a region where the concentration ratio [CTAB]/[SiO2] is close to 0.03. A criterion to distinguish viscous and viscoelastic samples based on the value of the time and amplitude averaged elastic modulus is proposed. Cryo-SEM observations are performed to document the underlying colloidal organization in the samples. Colloidal structures of silica particles are discussed as well as their relative elasticity on the basis of CTAB and SiO2 concentrations. We believe that the rheological measurements performed here can provide important and general insights into the understanding of the nature of interactions between nanoparticles and surfactants. This work also indicates that the transition recently observed in the shape of emulsion droplets cannot be directly related to changes in the rheology of their dispersed phase. However, it indicates that the excess of surfactant might enter into play in the explanation of the origin of this transition. Acknowledgment. The work was financially supported by projects of the European Space Agency (FASES MAP AO-99052), Centre National Etudes Spatiales (CNES), and the DFG SPP 1273 (Mi418/16-2), GdR Mousses and GdR MFA.
DOI: 10.1021/la102473s
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