Microstructure colloidal dispersions

Characteristic microstructural properties of TiOj membranes produced in this way are given in Table 2.5. Mean pore diameters of 4-5 nm were obtained after heat treatment at T < 500°C. The pore size distribution was narrow in this case and the particle size in the membrane layer was about 5 nm. Anderson et al. (1988) discuss sol/gel chemistry and the formation of nonsupported titania membranes using the colloidal suspension synthesis of the type mentioned above. The particle size in the colloidal dispersion increased with the H/Ti ratio from 80 nm (H /Ti = 0.4, minimum gelling volume) to 140 nm (H /Ti " — 1.0). The membranes, thus prepared, had microstructural characteristics similar to those reported in Table 2.5 and are composed mainly of 20 nm anatase particles. Considerable problems were encountered in membrane synthesis with the polymeric gel route. Anderson et al. (1988) report that clear polymeric sols without precipitates could be produced using initial water concentrations up to 16 mole per mole Ti. Transparent gels could be obtained only when the molar ratio of H2O to Ti is < 4. Gels with up to 12 wt.% T1O2 could be produced provided a low pH is used (H /Ti + < 0.025). 

Our objective in this chapter is to establish the quantitative connections between interparticle forces and colloid stability. Before we consider this it is instructive to look at the role of interaction forces in a larger context, that is, the relation between interparticle forces and the microstructure of dispersions and the factors that determine such a relation. These aid us in appreciating the underlying theme of this chapter, namely, the manipulation of interparticle forces to control the properties of dispersions. 

Maranzano, B. J. and Wagner, N. J. 2002. Flow-small angle neutron scattering measurements of colloidal dispersion microstructure evolution through the shear-thickening transition. J. Chem. Phys. 117 10291-10302. 

In describing the mechanical response of microstructured fluids, e.g., polymers, emulsions, colloidal dispersions, etc., one needs to determine the pair distribution function - the probability density P(r) for finding a particle at a position r given a particle at the origin in suspensions, or the probability density of the end-to-end vector in polymers, or a measure of the deformation of drops in an emulsion. This probability density satisfies an advection-diffusion or Smoluchowski equation of the following (when suitable approximations have been made) form ... 

Sato, K Ueno, S. Crystallization, transformation and microstructures of polymorphic fats in colloidal dispersion states. Curr Opin Colloid Interface Sci, 2011 16 384-90. 

Emphasizing equilibrium phenomena, flow, transport, and stability, Intcrfacial Phenomena Equilibrium and Dynamic Effects, Second Edition presents a concise and current summary of the fundamental principles governing interfacial interactions. This new edition features updated and expanded topics in every chapter. It highlights key experimental techniques that have expanded the scope of our understanding, such as in mass transfer, microstructure determination in colloidal dispersions, and surfactant-polymer interactions. 

Bohlin has developed a statistical theory, applicable to colloidal dispersions, in which macroscopic flow is the consequence of co-operative rearrangements of microscopic elements. The model is based on a lattice microstructure whose elements can exist in stressed or relaxed states. Stress relaxation at constant strain is co-operative owing to the presence of an energy of interaction between neighbouring elements in different states. In common with all lattice models, the predictions are sensitive to the (somewhat arbitrary) choice of co-ordination number. 

The flow properties of a colloidal system are very much dependent on its microstructure, as determined by the molecular arrangement and interaction of its components. ME systems show flow typical of a Newtonian liquids, for which the shear stress is directly proportional to the shear rate. Since viscosity measurements are dynamic experiments, they will give information on dynamic properties of the ME. These will depend on the miCTOStructure, type of aggregates, or interactions within the ME, which in turn are determined by the concentration of the various components and the temperature. The dispersion of one component in another, e.g., water in oil, will generally increase the bulk viscosity in comparison to the individual components (oil and water) [58]. For at true colloidal dispersion, viscosity will increase with increasing volume fraction of dispersed phase according to the formula generated by Einstein ... 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

In fact, even in pure block copolymer (say, diblock copolymer) solutions the self-association behavior of blocks of each type leads to very useful microstructures (see Fig. 1.7), analogous to association colloids formed by short-chain surfactants. The optical, electrical, and mechanical properties of such composites can be significantly different from those of conventional polymer blends (usually simple spherical dispersions). Conventional blends are formed by quenching processes and result in coarse composites in contrast, the above materials result from equilibrium structures and reversible phase transitions and therefore could lead to smart materials capable of responding to suitable external stimuli. 

Of course, the processing engineer or the colloid chemist seldom worries about the fundamental aspects of the microstructure of the product and its relation to rheology, but many of the topics we discuss in this chapter are useful for gaining the conceptual basis necessary to deal with the practical aspects of dispersion rheology. 

Alany et al. [11,35] reported on the phase behavior of two pharmaceutical ME systems showing interesting viscosity changes. The viscosity of both systems increased with increasing volume fraction of the dispersed phase to 0.15 and flow was Newtonian. However, formation of LC in one of the two systems, namely the cosurfac-tant-free system, resulted in a dramatic increase in viscosity that was dependent on the volume fraction of the internal phase and a change to pseudoplastic flow. In contrast, the viscosity of the bicontinuous ME was independent of water volume fraction. The authors used two different mathematical models to explain the viscosity results and related those to the different colloidal microstructures described. 

Very often, the microstructure and the macroscopic states of dispersions are determined by kinetic and thermodynamic considerations. While thermodynamics dictates what the equilibrium state will be, kinetics determine how fast that equilibrium state will be determined. While in thermodynamics the initial and final states must be determined, in kinetics the path and any energy barriers are important. The electrostatic and the electrical double-layer (the two charged portions of an inter cial region) play important roles in food emulsion stability. The Derjaguin-Landau-Verwey-Oveibeek (DLVO) theory of colloidal stability has been used to examine the factors affecting colloidal stability. 

Mesocopic flows are important to understand because they hold the key to the interaction between the macroscopic flow and the microstructural inhomogeneities. This is especially true in colloidal flows, which involve colloidal mixtures, thermal fluctuations and particle-particle interactions. Dynamic processes occurring in the granulation of colloidal agglomerate in solvents are severely influenced by coupling between the dispersed microstructures and the global flow. On the mesoscale, this... 

The importance of the amorphous glassy microstructure of soft particle dispersions is reflected by the great influence that the particle elastic modulus has on yielding and flow. The yield stresses of colloidal pastes and of emulsions scales like the shear modulus [13, 133, 134]. In Sect. 5, the flow curves of soft particle glasses will be shown to exhibit a remarkable universal behavior in terms ofa unique microscopic time scale tliat involves the shear modulus [13]. In Sect.4, the slip velocity... 

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