Colloidal Phase Separation Research Articles

Wetting of Emulsions Droplets: From Macroscopic to Colloidal Scale

By using large oil-in-water droplets covered with ionic surfactant we measure contact angles to deduce the adhesive energy between macroscopic interfaces as a function of the temperature $T$ and the salt concentration $C$. A wetting transition takes place at a well defined temperature ${T}^{*}(C)$. At the colloidal scale, we have observed that submicron droplets covered by the same monolayers undergo a phases separation. We show that the phases diagram can be quantitatively predicted from the macroscopic contact angles. However, to describe the colloidal phase separation we have to account for both the entropy and the deformation induced by the wetting of the droplets. Finally, our results show how the macroscopic wetting transition can be shifted at the colloidal scale where droplets entropy plays an important role.

Polymerization-Induced Phase Separation: A Maximum in the Intensity of Scattered Light Associated with a Nucleation-Growth Mechanism

The appearance of a maximum in the intensity of scattered light at a nonzero wave vector for systems undergoing a polymerization-induced phase separation (PIPS) has been considered in the past as conclusive evidence of the presence of a spinodal demixing mechanism. However, recent results from light scattering studies of colloidal aggregation and phase separation in aqueous biopolymers systems and polymer blends prove that the maximum may also be generated by a nucleation-growth process (NG). The origin of this scattering behavior is the presence of a layer surrounding dispersed-phase particles that contains less solute concentration than the bulk (depletion layer). We apply this concept to a system undergoing PIPS through an NG mechanism. The analysis is constrained to the generation of a diluted dispersion of spherical particles where concentration profiles around particles may be analytically derived. Both Rayleigh−Gans and Mie scattering theories are used to describe the patterns of scattered light. I...

Novel Ceramic Processing Methods Using Responsive Polymers

Optimal ceramic powder processing methods have to fulfil many requirements. Some of these requirements are obvious - like high reliability and good performance of the sintered material - but there is also a strong need of simple processing concepts which can lower the manufacturing cost, e.g. near net-shape, high flexibility, few process steps and water-based methods [1]. The high cost of manufactured parts is today probably the limiting factor of a large scale commercial implementat on of high performance ceramics. Many different approaches are being taken in this continuing effort of developing more robust and cost-effective ceramic processing methods. An important feature of optimal processing methods is versatility and high flexibility; it should be possible to exert a precise control of important suspension parameters like colloidal stability (degree of flocculation), rheology, phase separation and suspension microstructure and permeability. Depending on the forming method, the ideal conditions might vary drastically during the different processing steps [2]. For example, mixing of multi-component powder systems and removal of large, hard aggregates are best performed in well dispersed, low viscosity suspensions. Tape casting also requires stable suspensions while it may be beneficial to use weakly flocculated suspensions in slip casting and pressure filtration. Several alternative forming methods, e.g. freeze forming (Quickset) 131, get casting 141, direct coagulation casting (DCC) 151, and temperature induced flocculation (TIF) 161, require a well dispersed, low viscosity suspension of high concentration which should be transformed to a stiff green body in the mould. The details of these complex systems are starting to be unravelled with an improved knowledge of the fundamental forces and physical parameters acting in these systems, e.g. interparticle forces, dispersant interactions, solvent effects. This knowledge is being used in the design and development of novel processing methods, e.g. DCC 151 and vibraforming of salt induced flocculated suspensions [7]. Research is also being devoted to the development of ceramic processing systems with better suspension control using different additives. In this extended abstract, the characteristics of an important class of additives -responsive polymers- will be described and the current and future potential regarding their use in ceramic processing will be discussed. Responsive polymers - which are characterised by a reversible response to an external stimuli, such as temperature, pH and light -are the focus of considerable research efforts in many areas, e.g. controlled drug delivery, recycling, paints and coatings, and the potential in ceramic applications is high. Responsive polymers undergo conformational and phase changes in response to a change in e.g. temperature. Typically, the response is characterised by a transition from a polymer solution to some type of aggregated state or get structure. A polymer get is a fluid-containing, self-supporting system which implies some type of connectivity. This is also true for an aggregated or precipitated polymer solution but the concentration may be too low for a cell-spanning get to be formed. Hence, the responsive function can be identified with the breaking and forming ofjunctions between the polymer molecules. Now, there are many types of processes that can create junctions and thus gels, e.g.

The effect of an electrolyte on phase separation in colloids

Abstract We consider an influence of the solvent ionic strength on thermodynamic properties and conditions at the onset of instability of a monodisperse colloid, as well as on the coefficient of mutual Brownian diffusion of colloidal particles. Special attention is given to collective effects due to competition of the particles for ions to be adsorbed, which are proved to lead to a noticeable drop in the particle electric charge and potential as well as inthe ionic strength outside the double electric layers. An increase in the ionic strength above a region of relatively small electrolyte concentrations favours the phase separation of the colloid under otherwise identical conditions and causes the mutual diffusivity to markedly decrease, the more so the larger is the concentration of the particles.

Kinetics of phase separation in colloids II. Non-linear evolution of a metastable colloid

We study the evolution of a system of dropwise aggregates suspended in a macroscopically homogeneous metastable colloid with account of both the reduction in metastability (the decrease in the parent colloid supersaturation) and the continuing initiation of new nuclei in the metastable surroundings. The growing aggregates are distributed over size and the distribution density is governed by an equation of the Fokker-Planck type with allowance for variations in the growth rate of a single aggregate. A complementary equation relates the supersaturation to the total rate of absorption of colloidal particles by the aggregates. A method of solving those equations is worked out. It permits the supersaturation and diverse characteristics of the distribution density to be found as functions of time. The former quantity is monotonously declining to zero and the dispersion of the latter one is either decreasing or increasing with time depending on the level of random fluctuations of the growth rate. If the fluctuations are absent, the evolving system of the aggregates is tending to a monodisperse one, what calls in question the validity of underlying assumptions inherent to the classical theory of Ostwald ripening.

Kinetics of phase separation in colloids: I. Formation of initial aggregates

We consider an initial stage of the origination of aggregates in a colloid made metastable by either a sudden drop in its temperature or a shift of its concentration, when emerging nuclei are still not large enough to affect colloid properties appreciably. Although the theory is developed along the lines specific to those of nucleation in molecular systems, there are some essential distinctions. Expressions for the critical nucleus size, for the growth rate of a supercritical particulate aggregate and for the frequency of nucleation are presented. They are discussed in connection with the aggregation in ferrocolloids.

Phase separation of magnetic colloids and concentration domain patterns

The concept of magnetic condensation as a mechanism of formation of periodic concentration patterns of magnetic colloids is developed. Their characteristics in plane slots have been determined. It has been shown that a family of equilibrium concentration patterns can be found as eigenfunctions of a self-adjoint integro-differential operator. Types of pattern transformation caused by magnetic instabilities of microdrops of the condensed phase have been described. It should be noted that there is an analogy of the dynamics of two-dimensional magnetic pattern formation with diffusion-limited growth processes. The data of numerical experiments indicating a possible formation of fractal magnetic objects are given.

Living anionic synthesis and characterization of poly(epichlorohydrin-g-styrene) copolymers

Poly(epichlorohydrin-g-styrene) copolymers, P(ECH-g-S), of a wide range of compositions have been prepared in an argon atmosphere by coupling or deactivation of living polystyrene anions, PS−, at the chloromethyl groups of polyepichlorohydrin, PECH, in solution in benzene, C6H6, toluene, CH3C6H5, tetrahydrofuran, THF, and C6H6-THF solvent mixtures, usually at 20°C. When both polystyryl lithium, PS− Li+, and PECH were in C6H6 or toluene, grafting reactions were accompanied by cross-linking due to lithium-halogen interchange, colloidal phase separation and degradation of the backbone polymer. When the PS−Li+ in either C6H6 or THF was coupled with PECH in THF, cross-linking was avoided and degradation was minimized. Some non-degradative termination reactions also occurred on coupling in THF and THFC6H6 mixtures. A polar solvent was, however, found to be favourable for the coupling reaction. The graft copolymers were purified by fractional precipitation techniques and were characterized by infra-red spectroscopy, ultraviolet spectroscopy, microanalysis, gel permeation chromatography and solubility.