Gel, glass and nematic states of plate-like particle suspensions: charge anisotropy and size effects (original) (raw)
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Structure of the repulsive gel/glass in suspensions of charged colloidal platelets
Journal of Physics: Condensed Matter, 2008
Rheological, optical and structural properties of colloidal suspensions of charge-stabilized gibbsite platelets across the sol-gel transition region are investigated. In this work we focus on samples with a low salt content (10 −4 M). While at a gibbsite concentration of 300 g l −1 , a nematic-columnar phase separation is observed, an arrested state with a nematic signature and highly elastic response has been observed for a concentration of 400 g l −1 . A temporal evolution of the structure of the arrested state, which leads to stronger interparticle correlations, has been observed on a timescale of 20 months. The results suggest that the arrested state develops into a glass with a columnar nematic structure.
Monte Carlo Simulations of a Clay Inspired Model Suspension: The Role of Charge Anisotropy
We present a theoretical investigation of a model clay dispersion in 1-1 salt solutions varying the particle volume fraction, ionic strength as well as the charge distribution on the clay platelets. The platelets are modeled as discs with charged sites distributed on a hexagonal lattice. The edge sites can be positively charged while the remaining sites are negative giving rise to a charge anisotropy. Simulations are carried out using a Monte Carlo method in the canonical ensemble. The interactions between the platelet sites are described with a screened Coulomb potential plus a short range repulsive potential. Simulations show a complex phase behavior. When the charge anisotropy is strong, a repulsive liquid phase is found at low volume fraction and ionic strength. When increasing the latter an attractive liquid phase forms. At these volume fractions the platelets aggregate in an "Overlapping Coins" configuration. With increasing volume fraction the dispersion becomes unstable and the pressure goes through a van der Waals loop. A liquid crystalline phase, Smectic B, forms in the thermodynamically unstable region. On the other side of the van der Waals loop a stable gel phase is found. A phase separation between a liquid and a gel is thus predicted. The threshold value of the volume fraction at which the phase separation occurs is found to increase with the salt concentration. The gel structure is a mixture of "Overlapping Coins" and "House of Cards" configurations. When the charge anisotropy is intermediate, no phase separation occurs. Instead, a gel forms from a sol of clusters of individual particles randomly oriented that progressively growth with the volume fraction. These results are discussed in light of experimental observations on clay suspensions.
Interplay of anisotropy in shape and interactions in charged platelet suspensions
The Journal of Chemical Physics, 2014
Motivated by the intriguing phase behavior of charged colloidal platelets, we investigate the structure and dynamics of charged repulsive disks by means of Monte Carlo simulations. The electrostatic interactions are taken into account through an effective two-body potential, obtained within the nonlinear Poisson-Boltzmann formalism, which has the form of anisotropic screened Coulomb potential.
Devitrification of the glassy state in suspensions of charged platelets
Journal of Physics: Condensed Matter, 2009
Colloidal suspensions of charged gibbsite platelets at salt concentrations of 10 −2 M and below and with a sufficiently high particle concentration form a kinetically arrested, glassy state. We study the evolution of the glassy state in suspensions of three different gibbsite systems. Despite differences in size and polydispersity, we observe small, iridescent grains of the hexagonal columnar phase, for all these systems after periods of months to years. The connections between this devitrification phenomenon and the structure of the glassy state are discussed.
The Journal of Physical Chemistry B, 2001
Monte Carlo simulations are performed in the Canonical ensemble to determine the equilibrium configurations of the counterions and co-ions in the vicinity of two charged hard disks at fixed separations. Derivation of the electrostatic energy and the ionic configurational entropy is used to determine the relative stability of two charged platelets as a function of their separation and relative orientation. An effective pair potential is derived from the free energy variation and may be used in the framework of the one component plasma to model suspensions of charged platelets. Finally, the variation of the free energy of two charged colloids in the presence of salt may be reproduced by Yukawa potential with a residual charge equal to 7% of the nominal electric charge of the colloids.
The Journal of Physical Chemistry B, 2008
We study gel formation in a mixture of equally-sized oppositely charged colloids both experimentally and by means of computer simulations. Both the experiments and the simulations show that the mechanism by which a gel is formed from a dilute, homogeneous suspension is an interrupted gas-liquid phase separation. Furthermore, we use Brownian dynamics simulations to study the relation between gel formation and the equilibrium phase diagram. We find that, regardless of the interaction range, an interrupted liquid-gas phase separation is observed as the system is quenched into a state point where the gas-liquid separation is metastable. The structure of the gel formed in our experiments compares well with that of a simulated gel, indicating that gravity has only a minor influence on the local structure of this type of gel. This is supported by the experimental evidence that gels squeezed or stretched by gravity have similar structures, as well as by the fact that gels do not collapse as readily as in the case of colloid-polymer mixtures. Finally, we check whether or not crystallites are formed in the gel branches; we find crystalline domains for the longer ranged interactions and for moderate quenches to the metastable gas-liquid spinodal regime.
Strong effect of weak charging in suspensions of anisotropic colloids
Soft Matter, 2014
Suspensions of hard colloidal particles frequently serve as model systems in studies on fundamental aspects of phase transitions. But often colloidal particles that are considered as "hard" are in fact weakly charged. If the colloids are spherical, weak charging has a only a weak effect on the structural properties of the suspension, which can be easily corrected for. However, this does not hold for anisotropic particles.
Computer simulations of charge and steric stabilised colloidal suspensions
Current Opinion in Colloid & Interface Science, 2001
Computer simulations of colloidal suspensions are prohibited by slow equilibration as very different length and time scales are involved for the various species. This is the reason that most simulations involve some degree of coarse-graining, whereby the degrees of freedom of the microscopic particles are traced out, and the mesoscopic particles interact with an effective potential, resulting in a coarse-grained, effective one-component description of the suspension. The focus of this paper is on recent simulation work of charge and steric stabilised colloidal suspensions. We discuss both direct simulations of the true colloidal mixture and coarse-grained approaches of the suspension involving effective interactions for the colloids.
The Journal of Chemical Physics, 2012
We use a fundamental-measure density functional for hard board-like polydisperse particles, in the restricted-orientation approximation, to explain the phase behaviour of platelet colloidal suspensions studied in recent experiments. In particular, we focus our attention on the behavior of the total packing fraction of the mixture, η, in the region of two-phase isotropic-nematic coexistence as a function of mean aspect ratio, polydispersity, and fraction of total volume γ occupied by the nematic phase. In our model, platelets are polydisperse in the square section, of side length σ , but have constant thickness L (and aspect ratio κ ≡ L/ σ < 1, with σ the mean side length). Good agreement between our theory and recent experiments is obtained by mapping the real system onto an effective one, with excluded volume interactions but with thicker particles (due to the presence of long-ranged repulsive interactions between platelets). The effect of polydispersity in both shape and particle size has been taken into account by using a size distribution function with an effective mean-square deviation that depends on both polydispersities. We also show that the bimodality of the size distribution function is required to correctly describe the huge two-phase coexistence gap and the nonlinearity of the function γ (η), two important features that these colloidal suspensions exhibit.