Isotropic−Nematic Phase Separation in Suspensions of Polydisperse Colloidal Platelets

Kooij, F.M. van der; Beek, D. van der; Lekkerkerker, H. N. W.

doi:10.1021/jp0031597

Cited by 97 publications

(105 citation statements)

References 38 publications

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“…The experimental phase boundaries are found to be somewhat lower (φ ) 0.18 and φ ) 0.30). 8 Equal phase densities are found at a volume fraction of approximately 0.24.…”

Section: Discussionmentioning

confidence: 96%

“…Replacing the excess free energy cB 2 in (6) by (23) gives us the Onsager-Parsons free energy for a binary mixture of hard platelets. Accordingly, for the isotropic phase, we must replace c by cf CS in the last term of (8). Recalculation of the osmotic pressure and chemical potentials for the isotropic phase is now straightforward, using the definitions (17) and (18).…”

Section: Theorymentioning

confidence: 99%

“…The dimensions given in Table 1 thus apply to the grafted gibbsite platelets. The thickness of the polymer layer has been estimated at 4 nm, 8 and the ratio of the core volume V core to the total volume V of the platelet can thus be calculated using the values from Table 1, giving V 1 core /V l ≈ 0.55 and V 2 core /V 2 ≈ 0.75. Figure 3 clearly reveals that a density inversion will indeed take place during the I-N phase separation.…”

Section: Isotropic-nematic Phase Coexistence: Density Inversionmentioning

confidence: 99%

“…8 The polydispersity in thickness, although difficult to determine accurately, was estimated at 50%. The high polydispersity is caused by the presence of a significant number of very thick platelets, as observed on transmission electron microscopy (TEM) micrographs of the gibbsite samples.…”

Section: Introductionmentioning

confidence: 99%

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Isotropic−Nematic Density Inversion in a Binary Mixture of Thin and Thick Hard Platelets

2001

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We study the phase behavior of a binary mixture of thin and thick hard platelets, using Onsager's second virial theory for binary mixtures in the Gaussian approximation. Higher virial terms are included by rescaling the excluded volume part of the Onsager free energy using a modified form of the Carnahan-Starling free energy for hard spheres (Parsons' approach). Our calculations provide a simple explanation for the isotropicnematic (I-N) density inversion, as experimentally observed in systems of polydisperse gibbsite platelets by Van der Kooij et al. (J. Phys. Chem. B 2001, 105, 1696. In these systems, a nematic upper phase was found to coexist with an isotropic bottom phase. We confirm the original conjecture of the authors, which states that the phenomenon originates from a pronounced fractionation in thickness between the phases, such that the thick platelets are largely expelled from the nematic phase and preferentially occupy the isotropic phase. Our calculations show that the inverted state is found in a major part of the I-N coexistence region. In addition, a nematic-nematic demixing transition is located at sufficiently high osmotic pressures for any thickness ratio L 2 /L 1 > 1. The N-N coexistence region is bounded by a lower critical point which shifts toward lower values as the thickness ratio is increased. At high thickness ratios (L 2 /L 1 > 3.3), a triphasic coexistence is found at which two nematic phases coexist with an isotropic phase. We show that the demixing transition is driven by a small O(L/D) contribution to the excluded volume entropy.

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“…The experimental phase boundaries are found to be somewhat lower (φ ) 0.18 and φ ) 0.30). 8 Equal phase densities are found at a volume fraction of approximately 0.24.…”

Section: Discussionmentioning

confidence: 96%

Section: Theorymentioning

confidence: 99%

Section: Isotropic-nematic Phase Coexistence: Density Inversionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Isotropic−Nematic Density Inversion in a Binary Mixture of Thin and Thick Hard Platelets

2001

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“…To explain that the IN interface is easily deformed under gravity, it was estimated from a scaling relation that the interfacial tension in these systems could be as low as 0:01 N=m [7]. In this Letter we present a measurement of the IN interfacial tension in suspensions of colloidal platelets and carry out density functional calculations [8] and Monte Carlo (MC) simulations of the free IN interface and wetting at a wall using a microscopic model of platelets with continuous degrees of freedom.…”

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Isotropic-Nematic Interface and Wetting in Suspensions of Colloidal Platelets

et al. 2006

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We study interfacial phenomena in a colloidal dispersion of sterically stabilized gibbsite platelets, exhibiting coexisting isotropic and nematic phases separated by a sharp horizontal interface. The nematic phase wets a vertical glass wall and polarized light micrographs reveal homeotropic surface anchoring both at the free isotropic-nematic interface and at the wall. On the basis of complete wetting of the wall by the nematic phase, as found in our density functional calculations and computer simulations, we analyze the balance between Frank elasticity and surface anchoring near the contact line. Because of weak surface anchoring, the director field in the capillary rise region is uniform. From the measured rise (6 m) of the meniscus at the wall we determine the isotropic-nematic surface tension to be 3 nN=m, in quantitative agreement with our theoretical and simulation results. (IN) transition of anisometric colloidal particles. The spontaneous orientational ordering of the particles in the nematic phase is attributed to the strongly anisotropic excluded volume pair interactions that favor parallel alignment of the particles and overcome the orientational entropy of the isotropic state. However, the precise particle shape matters, as ordering of thin rods can be quantitatively described at the level of second virial theory, yielding a density jump of 20% at the IN transition, whereas-to quote Onsager-''we can hardly hope for more than that our result will describe concentrated solutions of (platelike) particles qualitatively rather than quantitatively '' [2]. Indeed, computer simulations [3] have shown that for (infinitely thin) hard platelets the density jump is only 8%.In recent years the entropy-driven IN transition in suspensions of sterically stabilized [4] as well as charge stabilized colloidal platelets [5] has been studied both experimentally and theoretically [6]. To explain that the IN interface is easily deformed under gravity, it was estimated from a scaling relation that the interfacial tension in these systems could be as low as 0:01 N=m [7]. In this Letter we present a measurement of the IN interfacial tension in suspensions of colloidal platelets and carry out density functional calculations [8] and Monte Carlo (MC) simulations of the free IN interface and wetting at a wall using a microscopic model of platelets with continuous degrees of freedom. For detailed studies of interfacial properties of rectangular platelets with discrete orientations (Zwanzig model) see Refs. [9,10]. We find indeed a very low IN interfacial tension from the capillary rise measured in experiment, in quantitative agreement with our theoretical and simulation results. Our theory and simulations show complete wetting of the wall by the nematic phase, with its onset occurring only remarkably close to bulk IN coexistence.We use a model system of hard disks consisting of sterically stabilized colloidal gibbsite [Al OH 3 ] platelets dispersed in toluene [4], with average diameter D 240 nm and thickness L 18 nm, and a polyd...

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Mineral Liquid Crystals: Liquid‐Crystalline Suspensions of Mineral Nanoparticles

Davidson

2014

Handbook of Liquid Crystals

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Isotropic−Nematic Phase Separation in Suspensions of Polydisperse Colloidal Platelets

Cited by 97 publications

References 38 publications

Isotropic−Nematic Density Inversion in a Binary Mixture of Thin and Thick Hard Platelets

Isotropic−Nematic Density Inversion in a Binary Mixture of Thin and Thick Hard Platelets

Isotropic-Nematic Interface and Wetting in Suspensions of Colloidal Platelets

Mineral Liquid Crystals: Liquid‐Crystalline Suspensions of Mineral Nanoparticles

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