Orientation-dependent depletion interaction in rodlike colloid-polymer mixtures

Depletion interaction in a suspension of rodlike colloids with added non-adsorbing polymer coils is theoretically studied. We calculate an overlap volume of depletion zone between two rodlike colloids, based on the second virial approximation. We examine nematic-isotropic phase transition (NIT) and two-phase coexistence between an isotropic and a nematic phase at low polymer concentrations. We find that the depletion interaction is dependent on the orientational order parameter of rodlike colloids and leads to a decrease in the NIT concentration on the addition of polymer. The coexistence curves have a leaning-chimney shape and are shifted to lower rod concentrations on increasing the polymer concentration. Received 23 May 2001 and Received in final form 18 July 2001     

Floating liquid phase in sedimenting colloid-polymer mixtures

Density functional theory and computer simulation are used to investigate sedimentation equilibria of colloid-polymer mixtures within the Asakura-Oosawa-Vrij model of hard sphere colloids and ideal polymers. When the ratio of buoyant masses of the two species is comparable to the ratio of differences in density of the coexisting bulk (colloid) gas and liquid phases, a stable "floating liquid" phase is found, i.e., a thin layer of liquid sandwiched between upper and lower gas phases. The full phase diagram of the mixture under gravity shows coexistence of this floating liquid phase with a single gas phase or a phase involving liquid-gas equilibrium; the phase coexistence lines meet at a triple point. This scenario remains valid for general asymmetric binary mixtures undergoing bulk phase separation.     

Ultrasoft colloid-polymer mixtures: structure and phase diagram

Binary mixtures of ultrasoft colloids and linear polymer chains were investigated by small-angle neutron scattering and liquid state theory. We show that experimental data can be described by employing recently developed effective interactions between the colloid and the polymer chains, in which both components are modeled as point particles in a coarse-grained approach, in which the monomers have been traced out. Quantitative, parameter-free agreement between experiment and theory for the pair correlations, the phase behavior and the concentration dependence of the interaction length is achieved.     

Fluctuation forces and wetting layers in colloid-polymer mixtures

We present confocal microscopy experiments on the wetting of phase-separated colloid-polymer mixtures. We observe that an unusually thick wetting layer of the colloid-rich phase forms at the walls of the glass container that holds the mixture. Because of the ultralow interfacial tension between the colloid-rich and the polymer-rich phases, the thermally activated roughness of the interfaces becomes very big and measurable. We observe that close to the critical point the roughness of the interface between the wetting layer and the polymer-rich phase decreases with decreasing layer thickness: large excursions of the interface are confined in the wetting layer. The measured relationship between the roughness and the thickness of the wetting layer is in qualitative agreement with the predictions of renormalization group theory for short-range forces and complete wetting.     

Critical behavior of colloid-polymer mixtures in random porous media

We show that the critical behavior of a colloid-polymer mixture inside a random porous matrix of quenched hard spheres belongs to the universality class of the random-field Ising model. We also demonstrate that random-field effects in colloid-polymer mixtures are surprisingly strong. This makes these systems attractive candidates to study random-field behavior experimentally.     

Influence of polymer-excluded volume on the phase-behavior of colloid-polymer mixtures

We determine the depletion-induced phase-behavior of hard-sphere colloids and interacting polymers by large-scale Monte Carlo simulations using very accurate coarse-graining techniques. A comparison with standard Asakura-Oosawa model theories and simulations shows that including excluded-volume interactions between polymers leads to qualitative differences in the phase diagrams. These effects become increasingly important for larger relative polymer size. Our simulation results agree quantitatively with recent experiments.     

Phase behaviour of rod-like colloid+flexible polymer mixtures

Summary The effect of non-adsorbing, flexible polymer on the isotropic-nematic transition in dispersions of rod-like colloids is investigated. A widening of the biphasic gap is observed, in combination with a marked polymer partitioning between the coexisting phases. Under certain conditions, areas of isotropic-isotropic-nematic or isotropic-nematic-nematic three-phase coexistence appear in the phase diagram of rod-polymer mixtures. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids Copanello, Italy, July 4–8 1994.     

Aggregates in mixtures of ionic liquids

Surfactant-like ionic liquids (C₁₆MIMCl, C₁₆MIMBF₄) aggregate in mixtures with another ionic liquid (EAN). Critical aggregate concentrations and estimations of object sizes are given and compared to aqueous systems and other relevant literature data. The investigated mixtures are stable up to more than 200 °C and can probably be used to extend the limited temperature range of water-based colloids.     

Wetting, drying, and layering of colloid-polymer mixtures at porous interfaces

The influence of interface porosity on the wetting properties of colloid-polymer mixtures is studied within density functional theory for the Asakura-Oosawa-Vrij model at the surface of a quenched hard-sphere matrix. While the porosity hardly changes the location of the transition from partial to complete wetting at colloidal bulk gas-liquid coexistence, the onset of wetting, as signaled by the first discontinuous layering transition, can be efficiently controlled by tailoring the porosity. We furthermore find that the penetrability of the porous interface induces complete drying into the matrix upon approaching capillary coexistence.     

Fluid-fluid phase separation in colloid-polymer mixtures studied with small angle light scattering and light microscopy

Mixtures of colloidal silica spheres and polydimethylsiloxane in cyclohexane with a colloid-polymer size ratio of about one were found to phase separate into two fluid phases, one which is colloid-rich and one which is colloid-poor. In this work the phase separation kinetics of this fluid-fluid phase separation is studied for different compositions of the colloid-polymer mixtures, and at several degrees of supersaturation, with small angle light scattering and with light microscopy. The small angle light scattering curve exhibits a peak that grows in intensity and that shifts to smaller wave vector with time. The characteristic length scale that is obtained from the scattering peak is of the order of a few μm, in agreement with observations by light microscopy. The domain size increases with time as , which might be an indication of coarsening by diffusion and coalescence, like in the case of binary liquid mixtures and polymer blends. For sufficiently low degrees of supersaturation the angular scattering intensity curves satisfy dynamical scaling behavior.     

Life at ultralow interfacial tension: wetting, waves and droplets in demixed colloid-polymer mixtures

Mixtures of colloids and polymers display a rich phase behavior, involving colloidal gas (rich in polymer, poor in colloid), colloidal liquid (poor in polymer, rich in colloid) and colloidal crystal phases (poor in polymer, highly ordered colloids). Recently, the colloidal gas-colloidal liquid interface received considerable attention as well. Due to the colloidal length scale the interfacial tension is much lower than in the atomic or molecular analog (nN/m instead of mN/m). This ultra-low interfacial tension has pronounced effects on the kinetics of phase separation, the colloidal gas-liquid profile near a single wall and the thermally induced fluctuations of the interface. The amplitudes of these thermally excited capillary waves are restrained by the interfacial tension and are for that reason of the order of the particle diameter. Therefore, in molecular systems, the capillary waves can only be seen indirectly in scattering experiments. In colloidal systems, however, the wave amplitudes are on a (sub) micrometer scale. This fact enables the direct observation of capillary waves in both real space and real time using confocal scanning laser microscopy. Moreover, the real space technique enables us to demonstrate the strong influence of interface fluctuations on droplet coalescence and droplet break up.     

Phase behaviour of a dispersion of charge-stabilised colloidal spheres with added non-adsorbing interacting polymer chains

We present a theory for the phase behaviour of mixtures of charge-stabilised colloidal spheres plus interacting polymer chains in good and θ -solvents within the framework of free-volume theory. We use simple but accurate combination rules for the depletion thickness around a colloidal particle and for the osmotic pressure up to the semi-dilute concentration regime. Hence, we obtain expressions for the free energy for mixtures of charged colloidal particles and non-adsorbing interacting polymers. From that, we calculate the phase behaviour, and discuss its topology in dependence on the competition between the charge-induced repulsion and the polymer-induced attraction. The homogeneous mixture of colloids and polymers becomes more stabilised against demixing when increasing the electrostatic repulsion. This charge-induced stabilisation is strongest for small polymer-to-colloid size ratios and is more pronounced for charged colloids mixed with polymers in a good solvent than for polymers in a θ -solvent. For the weakly charged regime we find that the phase diagram becomes salt-concentration-independent in the protein limit for charged colloids plus polymers in a θ -solvent. The liquid window, i.e., the concentration regimes where a colloidal liquid exists, is narrowed down upon increasing the charge-induced repulsion. Also this effect is more pronounced when charged colloids are mixed with polymer chains in a good solvent. In summary, we demonstrate that the solvent quality significantly influences the phase behaviour of mixtures of charged colloids plus non-adsorbing polymers if the range of the screened electrostatic repulsion becomes of the order of the range of the depletion-induced attraction.     

Description of the fluctuating colloid-polymer interface

To describe the full spectrum of surface fluctuations of the interface between phase-separated colloid-polymer mixtures from low scattering vector q (classical capillary wave theory) to high q (bulklike fluctuations), one must take account of the interface's bending rigidity. We find that the bending rigidity is negative and that on approach to the critical point it vanishes proportionally to the interfacial tension. Both features are in agreement with Monte Carlo simulations.     

Phase diagrams for hard disc mixtures

Phase diagrams for binary hard disc mixtures are predicted for diameter ratios between 0.15 and 1, using a new equation of state for the liquid based on recent calculations of the fifth virial coefficient, and a cell model for the solid. At diameter ratios close to 1, a substitutional solid solution is formed, and both azeotropic and eutectic phase behaviour are predicted. At smaller diameter ratios, a square sodium chloride lattice and a trigonal AB₂ lattice are observed.     

Depolarized light scattering study of equilibrium sedimentation profiles of colloidal dispersions

Summary Equilibrium sedimentation profiles of concentrated suspensions of charged optically anisotropic colloids are accurately determined by performing depolarized light scattering measurements. From the data we derive the osmotic pressure πvs. the particle volume fraction Φ. The π(Φ) values obtained with strongly screened colloids are in excellent agreement with the predicted hard-sphere equation of state. The data suggest that, besides gravitation, there is an additional force acting on the particles, probably due to the build-up of an electrostatic sedimentation potential. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids Copanello, Italy, July 4–8, 1994.     

Viscosity behaviour of binary mixtures of non-polar solvents

The viscous nature of binary liquid solutions of non-polar solvents,viz. n-heptane, benzene, carbon tetrachloride and paraffin oil has been studied in terms of the deviation from the ideal law. The ideal law is supposed to take the form of (weighted) geometric mean. The viscous behaviour has been observed over a range of temperatures (298–322°K). The relative excess volume and relative excess enthalpy of activation have been evaluated to study the increase in viscosity associated with the shrinkage in mixing of its components.     

Phase diagram of two-dimensional binary Yukawa mixtures

We have used Ramakrishnan–Yussouff (RY) density functional theory (DFT) to explore the topology of the phase diagram of two-component charge stabilised colloidal suspensions confined to a two-dimensional plane. The particles of the system interact via purely repulsive soft core Yukawa potential. Pair correlation functions (PCFs) used as input informations in DFT were calculated by solving both the hypernetted chain (HNC) and Percus–Yevick (PY) integral equation theories. To test the relative performance of the HNC and PY theories in the context of phase transitions, we have also studied the corresponding one-component systems. We found that RY DFT with HNC PCFs does not stabilise solid in both the one- and two-component cases, whereas the PY theory does. By considering the freezing into the substitutionally disordered triangular solid, we found that the temperature-composition phase diagrams of the binary mixture are narrow spindles whose thickness depends on the symmetry of the mixture components and the value of the screening constant of the Yukawa potential. Although the phase diagram obtained by RY DFT with structural inputs calculated by the PY theory is found to be shifted to higher temperature region in the temperature-composition plane, however, it captures qualitatively all the essential features of the phase diagram. Our results are in principle verifiable through computer simulations and experiments.     

SANS and light scattering of nonylammonium chloride micelles in aqueous solutions of sodium chloride

Summary Aqueous solutions of nonylammonium chloride (NAC) containing sodium chloride were studied by using light scattering and small-angle neutron scattering (SANS) techniques. It was found that the aggregation number of NAC is independent of the surfactant concentration and increases smoothly with increasing the ionic strength. Micelles of NAC in D₂O with [NaCl]=0.3422 have prolate ellipsoidal shape and their average aggregation number is greater than that found in H₂O at the same ionic strength. A possible explanation of this difference may arise from a solvent effect. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.