Effects of Added Silica Nanoparticles on the Nematic Liquid Crystal

Article pubs.acs.org/JPCB

Effects of Added Silica Nanoparticles on the Nematic Liquid Crystal Phase Formation in Beidellite Suspensions Jasper Landman,†,* Erwan Paineau,‡,* Patrick Davidson,‡ Isabelle Bihannic,§ Laurent J. Michot,§ Adrian-Marie Philippe,∥ Andrei V. Petukhov,† and Henk N. W. Lekkerkerker† †

Van’t Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands ‡ Laboratoire de Physique des Solides, UMR 8502 CNRS, Université Paris-Sud, Bâtiment 510, 91405 Orsay Cedex, France § Laboratoire Interdisciplinaire des Environnements Continentaux, UMR 7360 CNRS, Université de Lorraine, 15 avenue du Charmois, 54500 Vandoeuvre, France ∥ Laboratoire d’Energétique et de Mécanique Théorique et Appliquée, UMR 7563 CNRS, Université de Lorraine, 2 avenue de la Forêt de Haye, TSA 60604, 54518 Vandoeuvre, France ABSTRACT: In this article, we present a study of the liquid crystal phase behavior of mixed suspensions of the natural smectite clay mineral beidellite and nonadsorbing colloidal silica particles. While virtually all smectite clays dispersed in water form gels at very low concentrations, beidellite displays a first order isotropic−nematic phase transition before gel formation (J. Phys. Chem. B, 2009, 113, 15858−15869). The addition of silica nanospheres shifts the concentrations of the coexisting isotropic and nematic phases to slightly higher values while at the same time markedly accelerating the phase separation process. Furthermore, beidellite suspensions at volume fractions above the isotropic−nematic phase separation, trapped in a kinetically arrested gel state, liquefy on the addition of silica nanospheres and proceed to isotropic−nematic phase separation. Using small-angle X-ray scattering (SAXS), we probe the structural changes caused by the addition of the silica nanospheres, and we relate the modification of the phase transition kinetics to the change of the rheological properties.

1. INTRODUCTION

as 0.05 vol %, the interactions between the larger particles are altered significantly. Doshi et al.9,10 and Kleshchanok et al.11 investigated the change of the liquid-crystal phase behavior in mixed suspensions of charged plate-like Gibbsite colloids and alumina-coated silica spheres. They observed that the addition of the spheres significantly decreases the gibbsite concentration at which the isotropic−nematic transition9,10 and isotropiccolumnar transition11 take place. In addition to changes in the colloidal interactions and phase behavior, the addition of small silica colloids also leads to changes in the rheological properties of colloidal suspensions. This has been investigated extensively for suspensions of clay particles. Baird and Walz investigated the effect of added silica spheres on the structure and rheology of an aqueous suspension of disc-shaped kaolinite particles.12,13 Addition of silica colloids and salt (NaCl or KCl) to a kaolinite suspension caused the entire suspension to turn from a fluid to a stiff gel. Cousin et al. investigated aqueous mixtures of Laponite nanodiscs and silica-coated maghemite nanopar-

The ability of nonadsorbing polymers to promote aggregation and phase separation in colloidal suspensions has been known for a long time. In the 1920s, Fahraeus related the pathological enhanced aggregation of red blood cells to the increased concentrations of blood serum proteins fibrinogen, globulin and albumin.1,2 In the same period, Traube showed that adding plant and seaweed polysaccharides to a suspension of natural rubber latex particles led to a phase separation between a dilute and concentrated phase.3 The first satisfactory explanation of the underlying cause of these phenomena was given by Asakura and Oosawa.4 They showed that adding nonadsorbing polymer chains induces an effective attraction between particles with a hard core interaction. In the last 40 years, this so-called depletion interaction and its effect on aggregation and phase separation has been studied extensively (for an overview, see ref 5). The depletion effect is also operational in mixtures of large and small colloids. Walz and co-workers studied the effect of small charged silica colloids on the interaction between two larger charged particles both theoretically and experimentally.6−8 They observed that even at silica concentrations as low © 2014 American Chemical Society

Received: January 2, 2014 Revised: March 31, 2014 Published: April 23, 2014 4913

dx.doi.org/10.1021/jp500036v | J. Phys. Chem. B 2014, 118, 4913−4919

The Journal of Physical Chemistry B

Article

Figure 1. TEM micrographs of (a) beidellite platelets and (b) silica Ludox AS-40 spheres used in this study.

ticles.14 The addition of the silica-coated maghemite spheres shifted the rheological fluid−solid transition to much lower Laponite concentrations. Ten Brinke et al.15 and Kleschanok et al.16 observed that the addition of small quantities of silica colloids to hectorite suspensions leads to strengthening of gels at high hectorite concentrations but may lead to gel collapse at lower hectorite concentrations. While virtually all smectite clays dispersed in water form gels at very low concentrations, aqueous suspensions of beidellite17 as well as nontronite18 exhibit a unique behavior with a first order isotropic−nematic phase transition before gel formation. In this study, we use beidellite suspensions to investigate the change of the rheological properties and the modification of the phase behavior upon addition of silica colloids. Oscillatory and steady shear rheological experiments performed on these mixed beidellite−silica dispersions demonstrate that, with increasing silica concentration, originally gel-like samples liquefy, in agreement with the observed liquid crystal phase behavior. Small-angle X-ray scattering (SAXS) was performed in order to assess the structural modification induced by silica. We find that silica sphere addition in the mixtures makes the distance between the clay platelets smaller in the coexisting isotropic and nematic phases indicating that the added silica spheres alter the interactions between the clay particles.

tion and redispersed in fresh Milli-Q water. The silica spheres used here have an average diameter of 22 nm as determined by TEM observations (Figure 1b). Samples were prepared by mixing appropriate volumes of beidellite suspension S3, Ludox AS-40 suspension and Milli-Q water. A 10−2 M NaCl solution was added to raise the ionic strength in each sample to a constant 10−4 M. For visual observations of the birefringence, the mixtures were transferred in 1.6 mL glass vials. 2.2. Rheological Measurements. Oscillatory and steady shear experiments on the samples mixtures were performed on a Physica Anton Paar (MCR-300) rheometer set up in cone−plate geometry. A microroughened plate was used together with a microroughened cone of angle 1.006° and diameter 24.978 mm. The cone was truncated at a distance of 49 μm. To prevent unwanted evaporation of the solvent, a solvent trap with complete liquid seal was used. After sample loading, the samples underwent a 300 s preshear treatment at a shear strain of 1000% and a frequency of 1 Hz. The preshear treatment was followed by a 1000 s waiting time. Frequency sweeps were recorded at a shear strain of 1%, after which a down ramp, shear-rate controlled linear shear measurement was performed on each sample. 2.3. Small Angle X-ray Scattering. The samples were held in flat 500 μm thick glass capillaries. All samples were stored vertically at 20 °C and they were checked by polarized-light microscopy before the measurements. SAXS experiments were carried out at the DUBBLE beamline (BM-26) of the European Synchrotron Radiation Facility (ESRF, Grenoble, France).20 The beam was focused by bending the second crystal of the monochromator (focusing in the horizontal direction) and by bending the mirror after the monochromator (vertical focusing). The beam diameter in the sample was approximately 500 μm. Measurements were carried out at a fixed wavelength of λ = 1.001 Å and a sample to detector distance of 7120 mm. The 2D scattering patterns were collected on a Pilatus 1 M photon counting detector with 172 μm pixel size. The typical accessible range of scattering vector modulus q was 0.01−0.6 nm−1 (q = 4π(sin θ)/λ, where 2θ is the scattering angle). The curves of scattering intensity I(q) were deduced from the azimuthal [0, 2π] angular integration of the SAXS patterns previously corrected for water and glass scattering.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Natural beidellite (SBId-1) was purchased from the Source Clays Minerals Repository of the Clay Mineral Society (Purdue University, West Lafayette, IN). Beidellite is a dioctahedral swelling clay mineral, whose average structural formula is (Si7.27Al0.73)(Al3.77Fe3+0.11Mg0.21)O20(OH)4(Na,Ca)0.67.19 Before use, raw samples were purified following a procedure adapted from Paineau et al.17 The clay powder was hydrated for 3 days, left to sediment to get rid of the coarse particles, and then three times exchanged with NaCl 1 M. The resulting suspension was dialyzed against Milli-Q water for 14 days until the conductivity dropped below 5 μS·cm−1. To reduce polydispersity, the stock suspension was then fractionated by centrifugation during 90 min at 7000g and then at 13000g; the sediment was collected and rediluted in Milli-Q water in both cases. Finally, the remaining suspension was concentrated by centrifugation at 35000g for 2 h. In the present study, we only focus on this size fraction labeled “size 3” (S3) in the following. A micrograph obtained by transmission electron microscopy (TEM) is presented in Figure 1a and shows the irregular (subhedral) shape of the clay platelets. As shown previously,17 particles of this latter fraction are not only rather small (average particle diameter D ∼ 200 nm) but are also perfectly exfoliated in suspension (thickness t = 0.7 nm). Commercial Ludox AS-40 colloidal silica spheres were sedimented by centrifuga-

3. RESULTS 3.1. Phase Behavior. A series of pure beidellite suspensions, of volume fractions ϕclay, ranging from 0.27% to 0.50%, held in glass vials, was observed between crossed polarizers with the naked eye. All freshly prepared samples showed strong flow-birefringence that persisted longer for samples of higher concentration. After a week, the beidellite suspensions of volume fraction ranging from 0.32% to 0.37% already showed a clear phase separation between an isotropic and a nematic phase. After a month, a phase separation was also 4914



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Figure 2. Aqueous beidellite suspensions observed between crossed polarizers one month after preparation. Volume fractions of clay (ϕclay) are as follows: (a) 0.27%; (b) 0.32%; (c) 0.35%; (d) 0.37%; (e) 0.40%; (f) 0.41%. Note that the tiny bright layer just at the glass bottom of the vial in part a is not due to a nematic phase but is a light reflection artifact.

Figure 3. Mixed beidellite/silica suspensions observed between crossed polarizers one month after preparation: (a−c) ϕclay = 0.32% and ϕsilica = 0, 0.069 and 0.138%; (d−f) ϕclay = 0.37% and ϕsilica = 0, 0.034 and 0.138%; (g−i) ϕclay = 0.41% and ϕsilica = 0, 0.034 and 0.138%. In panel d, a slight motion before taking the picture has induced flow birefringence in the isotropic phase. Note that the tiny bright layer just at the glass bottom of the vial in parts b and c is not due to a nematic phase but is a light reflection artifact.

observed in samples of volume fractions larger than 0.37% but smaller than 0.40%. The sample at 0.40% is also biphasic but may not have fully equilibrated, even after a year. Samples of higher beidellite volume fraction show strong birefringence colors with no sign of phase separation, even after a year. We consider these dynamically arrested states to be gels. Figure 2 presents a series of samples of volume fractions increasing from 0.27% to 0.41% that display an isotropic phase (Figure 2a), coexisting isotropic and nematic phases (Figure 2b−e), and a gel state (Figure 2f). We emphasize that in Figure 2e full equilibrium may not yet have been reached. The addition of a small amount of silica particles significantly affects this scenario, as can be seen in Figure 3. We can distinguish three main effects. First of all, at beidellite

concentrations where a small amount of nematic phase was observed (ϕclay ∼ 0.32), the addition of silica causes the entire system to become isotropic (Figure 3a). Second, for beidellite concentrations where a significant amount of nematic phase is observed (ϕclay ∼ 0.37), addition of silica leads to a decrease of the overall proportion of nematic phase (Figure 3b). Finally, for beidellite concentrations where the system is in the gel state (ϕclay ∼ 0.41), addition of silica leads to a fluid nematic phase and eventually to a phase separated I/N system (Figure 3c). All these observations are gathered in the phase diagram shown in Figure 4. Besides these changes in the phase diagram, the addition of silica spheres accelerates the I/N phase separation process. For the system presented in Figure 3b, 4915



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Here ηr is the relative viscosity of the suspension, η∞ is its viscosity under infinite shear, ηf is the viscosity of the suspending fluid, σc is a critical shear stress that is reached when hydrodynamic effects are comparable to Brownian motion and interaction energy. ϕsph is an effective equivalent hard sphere volume fraction by considering the whole volume trapped by the freely rotating anisotropic platelets: φsph =

χ = (η∞/η0)1/2

(3)

On the other hand, if the suspension exhibits a yield stress at rest, then χ becomes negative and, in that case, the yield stress can be derived as σy = −χσc (4)

the phase separation proceeds in a matter of days instead of weeks. 3.2. Rheological Properties. The viscoelastic behavior of suspensions of beidellite is dramatically affected upon addition of small silica spheres, as can be seen in oscillatory frequency sweep measurements (Figure 5a). For example, at ϕclay ∼ 0.50, the storage G′ and loss G″ moduli exhibit a limited frequency dependence, a feature typical of a gel.21 However, upon addition of silica spheres, both moduli show a frequency dependence and drop from about 2 to 0.1 Pa, which is indeed the commonly accepted transition range between a gel and a fluid.22,23 Furthermore, the viscosity is strongly reduced with increasing silica concentration (Figure 5b). We have shown previously that viscosity measurements under shear stress on anisometric particles can be described by the extended hard sphere model,21 based on Quemada’s approach.24

Using such a model, the flow curves (viscosity vs. shear stress) can be fitted by three parameters, η∞, χ, and σc. Fits of the experimental flow curves to eq 1 are shown in Figure 5b while Figure 6 represents the variation of the obtained yield stress as a function of the silica volume fraction. As expected, σy decreases dramatically with increasing ϕsilica, confirming the macroscopic observations represented in Figure 3. Interestingly, although the addition of silica in beidellite suspensions reduces markedly the viscosity at rest, all flow curves of the mixtures approach the viscosity of the initial beidellite suspension at high shear stress. 3.3. Structure of the Suspensions. In order to gain insight in the structural rearrangements brought upon by addition of silica, we performed SAXS measurements on dispersions of beidellite and silica. We present in Figure 7a the variation of the scattered intensity I as a function of q. The scattering is due to the two contributions of the clay platelets and the silica spheres. Because the scattering of a particle is

αφsph φ0* αφsph φ∞*

(2)

The parameter α adjusts the value of ϕsph by taking into account the possible orientation of the particles. It is equal to 1 for a fully isotropic system and reaches a value of (3/2)(t/D) for fully aligned platelets. Finally, ϕ0* and ϕ∞* are defined as the critical packing fractions at rest and under infinite shear, respectively. If only shear-thinning behavior occurs, then χ is positive and has the value:

Figure 4. Experimental phase diagram of aqueous beidellite/silica suspensions. Open and lower half black circles correspond to isotropic (I) and biphasic (I + N) samples, respectively while star symbol marks the samples in the gel state (G). The boundary between the isotropic and the biphasic samples was obtained from naked-eye observations of the test-tubes while the sol−gel transition line was determined by rheological measurements.

2 1− η ⎛ 1 + σ /σc ⎞ ηr = ∞ ⎜ ⎟ with χ = ηf ⎝ χ + σ /σc ⎠ 1−

2D φ 3 t clay

(1)

Figure 5. (a) Storage moduli (G′, filled symbols) and loss moduli (G″, open symbols) as a function of frequency and (b) the corresponding flow curves (viscosity vs steady shear stress) for aqueous beidellite suspensions (ϕclay = 0.5%) with increasing silica volume fractions. In panel b, the solid lines are the fits of the experimental viscosity data to the extended hard-sphere model in eq 1. 4916



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of the N phase. Remarkably, we note that the distances in the coexisting isotropic phase also decrease, i.e., the concentration of clay platelets in the isotropic phase also becomes larger. This actually agrees with the observation that the amount of nematic phase decreases and the amount of isotropic phase increases upon addition of silica spheres. The decrease in volume of the clay-rich nematic phase results in a surplus of clay material that needs to be accommodated in each of the phases, thus increasing the concentration in both phases. The dependence of ⟨d⟩ with clay volume fraction is the so-called “swelling law” of the system (Figure 8b).17,25 In the nematic phase, ⟨d⟩ ∼ ϕclay−1 whereas in the isotropic phase, ⟨d⟩ ∼ ϕclay−1/3. The crossover between these two regimes provides a good qualitative estimate of the position of the I/N phase transition. This position shifts to larger clay volume fractions upon silica addition. For the highest silica concentration, the ϕclay−1 scaling law disappears, indicating that the system does not become fully nematic. All these features are in good agreement with the phase diagram presented in Figure 4. Although we did not assess directly and quantitatively the partition coefficient of the silica between the two I and N phases, their SAXS patterns show that both clay plates and silica spheres are present in each phase (Figure 9). Indeed, the patterns display diffraction peaks that arise from the clay particles and also a weak intensity modulation due to the silica particles.

Figure 6. Yield stress σy of various aqueous beidellite suspensions with increasing silica volume fractions.

proportional to the square of its volume, the contribution of the clay platelets (Vclay ∼ 2 × 104 nm3) is much larger than that of the silica spheres (Vsilica ∼ 3 × 103 nm3). Moreover, since the clay volume fraction is larger than that of the silica in our samples, we therefore consider in the following only the scattering of the clay platelets. Furthermore, in another approximation, we neglect the interplay of positional and orientational correlations of the platelets. Then, their scattered intensity is simply the product of their form factor, Pclay(q), by their structure factor, Sclay(q). In this scattering vector range, Pclay(q) follows the classical q−2 dependence (Figure 7a). Therefore, we plot q2I(q) vs q to represent Sclay(q), which dominates the signal (Figure 7b). The most striking features in Sclay(q) are the strong periodic oscillations that are often observed (Figure 7b). These oscillations were already reported before for pure beidellite suspensions17,25 and are due to short-range positional correlations of the platelets. For each of the patterns, a typical average distance, ⟨d⟩ = 2π/q between platelets can be estimated from the position of the maxima. These distances are plotted as a function of the system composition in Figure 8. We see that silica sphere addition in the mixtures makes the distance between the clay platelets smaller (Figure 8a). This means that the concentration of clay platelets in the nematic phase becomes larger. At this time, we do not have a quantitative model of the effect of silica on the organization

4. DISCUSSION The experimental results presented above clearly demonstrate that the addition of nonadsorbing silica spheres at concentrations as low as 0.1 vol % changes the liquid crystal phase behavior and rheological properties of beidellite suspensions substantially. The addition of silica spheres shifts the coexisting isotropic−nematic beidellite concentrations to higher values. In contrast, in the case of charged gibbsite plate-silica sphere mixtures,9−11 the addition of silica spheres shifts the liquid crystal phase transitions to lower gibbsite concentrations. In this latter case, the silica concentrations required to bring about significant changes in the liquid crystal phase behavior are typically 1 vol %, an order of magnitude higher than in the present case. A possible explanation for this difference may be the high surface charge density of the beidellite platelets (−0.11 C·m−2)25 compared to gibbsite surfaces (0.04 C·m−2)26 leading to a repulsion dominated system.27−29

Figure 7. (a) Intensity I(q) and (b) the structure factor q2I(q) for aqueous beidellite suspensions (ϕclay = 0.5%) with increasing silica volume fractions. The black line in panel a represents the q−2 dependence of the scattered intensity, arising from the form factor Pclay(q) of the clay platelets. 4917



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Figure 8. (a) Interparticle distances ⟨d⟩ as a function of the silica volume percent ϕsilica in mixed beidellite/silica suspensions with different clay volume fraction ϕclay. (b) Swelling laws at various silica volume fractions for mixed beidellite/silica suspensions. The interparticle distances are reported in nanometers.

crystal concentrations in suspensions of charged colloidal platelets has indeed been observed.31 Moreover, the addition of silica spheres leads to a drop of the storage G′ and loss G″ moduli and the viscosity is strongly reduced. This is accompanied by a marked effect on the phase separation process. The phase separation proceeds much faster. At volume fractions where the beidellite suspension without added silica spheres is trapped in a kinetically arrested gel state, the addition of silica colloids may lead to an isotropic−nematic phase separation. While a microstructural explanation for this effect is lacking, our rheological data would suggest that some lowering of the repulsive interaction between the beidellite clay platelets could lie at the basis of this effect.32

5. CONCLUSIONS This work shows that small amounts of nonadsorbing silica nanoparticles can be used to modify the phase behavior and rheological behavior of beidellite clay platelet suspensions. Presence of silica colloids in beidellite suspensions causes the phase separation to occur faster and for certain concentrations, induces a phase separation in an otherwise gelled suspension. While the precise mechanisms involved in these phenomena at the microscopic scale are not clear at present, the observations suggest that in these mixtures a complex interplay of electrostatic and depletion interactions is at work. This demonstrates that a wide range of different behaviors for mixed colloidal systems may still be discovered.

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Figure 9. Scattered intensity q2I(q) (in Kratky representation) for (a) the isotropic and (b) the nematic phases of an aqueous beidellite− silica system with overall volume fractions ϕclay = 0.37%; ϕsilica = 0.069%. The solid arrows point to the diffraction peaks arising from the clay particles while the dotted arrow points to the first minimum of the intensity modulation due to the form factor of the silica spheres.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: (J.L.) [email protected]. *E-mail: (E.P) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.L. acknowledges financial support from Schlumberger Cambridge Research.

Walz et al. observed that adding nonadsorbing silica at concentrations as low as 0.05 vol % can alter the interactions between the large charged colloidal particles significantly.6,7 They noted that the addition of silica spheres may lower the Debye length as was earlier observed by Pashley and Ninham in the case of double−layer forces in ionic micellar solutions.30 That lowering the Debye length may lead to higher liquid

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REFERENCES

(1) Fahraeus, R. The Suspension-Stability of the Blood. Acta Med. Scand. 1921, 55, 3−7.

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(2) Fahraeus, R. The Suspension Stability of the Blood. Physiol. Rev. 1929, 9, 241−274. (3) Traube, J. Konzentrierung von Kautschukmilchsften. Gummi Z. 1925, 39, 434. (4) Asakura, S.; Oosawa, F. Surface Tension of High-Polymer Solutions. J. Chem. Phys. 1954, 22, 1255. (5) Lekkerkerker, H. N. W.; Tuinier, R. Colloids and the Depletion Interaction; Springer: Berlin, 2011. (6) Walz, J. Y.; Sharma, A. Effect of Long Range Interactions on the Depletion Force between Colloidal Particles. J. Colloid Interface Sci. 1994, 168, 485−496. (7) Sharma, A.; Walz, J. Y. Direct Measurement of the Depletion Interaction in a Charged Colloidal Dispersion. J. Chem. Soc. Faraday Trans. 1996, 92, 4997−5004. (8) Sharma, A.; Tan, S. N.; Walz, J. Y. Effect of Nonadsorbing Polyelectrolytes on Colloidal Interactions in Aqueous Mixtures. J. Colloid Interface Sci. 1997, 191, 236−246. (9) Doshi, N.; Cinacchi, G.; van Duijneveldt, J. S.; Cosgrove, T.; Prescott, S. W.; Grillo, I.; Phipps, J.; Gittins, D. I. Structure of Colloidal Sphere−Plate mixtures. J. Phys.: Condens. Matter 2011, 23, 194109. (10) Doshi, N. Colloidal Mixtures of Spheres and Plates. Ph.D. Thesis. University of Bristol: Bristol, U.K., 2011. (11) Kleshchanok, D.; Petukhov, A. V.; Holmqvist, P.; Byelov, D. V.; Lekkerkerker, H. N. W. Structures and Phase Behavior in Mixtures of Charged Colloidal Spheres and Platelets. Langmuir 2010, 26, 13614− 13621. (12) Baird, J. C.; Walz, J. Y. The Effects of Added Nanoparticles on Aqueous Kaolinite Suspensions: I. Structural Effects. J. Colloid Interface Sci. 2006, 297, 161−169. (13) Baird, J. C.; Walz, J. Y. The Effects of Added Nanoparticles on Aqueous Kaolinite Suspensions: II. Rheological Effects. J. Colloid Interface Sci. 2007, 306, 411−420. (14) Cousin, F.; Cabuil, V.; Grillo, I.; Levitz, P. Competition between Entropy and Electrostatic Interactions in a Binary Colloidal Mixture of Spheres and Platelets. Langmuir 2008, 24, 11422−11430. (15) ten Brinke, A. J. W.; Bailey, L.; Lekkerkerker, H. N. W.; Maitland, G. C. Rheology Modification in Mixed Shape Colloidal Dispersions. Part II: Mixtures. Soft Matter 2008, 4, 337−348. (16) Kleshchanok, D.; Meester, V.; Pompe, C. E.; Hilhorst, J.; Lekkerkerker, H. N. W. Effects of Added Silica Nanoparticles on Hectorite Gels. J. Phys. Chem. B 2012, 116, 9532−9539. (17) Paineau, E.; Antonova, K.; Baravian, C.; Bihannic, I.; Davidson, P.; Dozov, I.; Impéror-Clerc, M.; Levitz, P.; Madsen, A.; Meneau.; et al. Liquid-Crystalline Nematic Phase in Aqueous Suspensions of a Disk-Shaped Natural Beidellite Clay. J. Phys. Chem. B 2009, 113, 15858−15869. (18) Michot, L. J.; Bihannic, I.; Maddi, S.; Funari, S. S.; Baravian, C.; Levitz, P.; Davidson, P. Liquid-Crystalline Aqueous Clay Suspensions. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16101−16104. (19) Post, J. L.; Cupp, B. L.; Madsen, F. T. Beidellite and Associated Clays from the DeLamar Mine and Florida Mountain Area, Idaho. Clays Clay Miner. 1997, 45, 240−250. (20) Borsboom, M.; Bras, W.; Cerjak, I.; Detollenaere, D.; van Loon, D. G.; Goedtkindt, P.; Konijnenburg, M.; Lassing, P.; Levine, Y. K.; Munneke, B.; et al. The Dutch-Belgian Beamline at the ESRF. J. Synchrotron Radiat. 1998, 5, 518−520. (21) Paineau, E.; Michot, L. J.; Bihannic, I.; Baravian, C. Aqueous Suspensions of Natural Swelling Clay Minerals. 2. Rheological Characterization. Langmuir 2011, 27, 7806−7819. (22) Winter, H. H.; Morganelli, P.; Chambon, F. Stoichiometry Effects on Rheology of Model Polyurethanes at the Gel Point. Macromolecules 1988, 21, 532−535. (23) Almdal, K.; Dyre, J.; Hvidt, S.; Kramer, O. Towards a Phenomenological Definition of the Term ‘Gel’. Polym. Gel Networks 1993, 1, 5−17. (24) Quemada, D.; Berli, C. Energy of Interaction in Colloids and its Implications in Rheological Modeling. Adv. Colloid Interface Sci. 2002, 98, 51−85.

(25) Paineau, E.; Bihannic, I.; Baravian, C.; Philippe, A. M.; Davidson, P.; Levitz, P.; Funari, S. S.; Rochas, C.; Michot, L. J. Aqueous Suspensions of Natural Swelling Clay Minerals. 1. Structure and Electrostatic Interactions. Langmuir 2011, 27, 5562−5573. (26) Hiemstra, T.; De Wit, J. C. M.; Van Riemsdijk, W. H. Multisite Proton Adsorption Modeling at the Solid/Solution Interface of (Hydr)oxides: A New Approach: II. Application to Various Important (Hydr)oxides. J. Colloid Interface Sci. 1989, 133, 105−117. (27) Norrish, K. The Swelling of Montmorillonite. Discuss. Faraday Soc. 1954, 18, 120−134. (28) Callaghan, I. C.; Ottewill, R. H. Interparticle Forces in Montmorillonite Gels. Faraday Discuss. Chem. Soc. 1974, 57, 110−118. (29) Lubetkin, S. D.; Middleton, S. R.; Ottewill, R. H. Some Properties of Clay-Water Dispersions. Philos. Trans. R. Soc. London, A 1984, 311, 353−368. (30) Pashley, R. M.; Ninham, B. W. Double-Layer Forces in Ionic Micellar Solutions. J. Phys. Chem. 1987, 91, 2902−2904. (31) Van der Beek, D.; Lekkerkerker, H. N. W. Liquid Crystal Phases of Charged Colloidal Platelets. Langmuir 2004, 20, 8582−8586. (32) Abend, S.; Lagaly, G. Sol−Gel Transitions of Sodium Montmorillonite Dispersions. Appl. Clay. Sci. 2000, 16, 201−207.

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Effects of Added Silica Nanoparticles on the Nematic Liquid Crystal

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