Homogeneous fluids structural properties

In this section, we will review some of the results obtained for homogeneous fluids. The focus of the section strongly reflects the author s particular interest rather than a complete review of all work done in this area. To a large extent, we will concentrate on aspects that have not been reviewed previously, or on areas that developed since those reviews. The first section deals with the influence of electrostatic interactions on the structure factor, and we stress the decoupling of dipole-dipole interactions from the structure factor, although there is a strong effect on particular g y r) s. In Section V.B we consider the dielectric constant obtained from the CSL equation with particular reference to the influence of shape forces in the dielectric properties. Section V.C considers the application of interaction site theories to calculate thermodynamic properties and fluid phase equilibria. 

The supercritical fluids have received considerable attention as solvents for the synthesis of a number of ceramic, metal, and other materials. These new applications have been developed in order to improve the characteristics of the obtained powders, such as chemical homogeneity or unique structural properties. Unlike previously described processes (RESS, SAS, and PGSS) which are principally physical transformation, the chemical transformation of materials in supercritical fluid is the principle of different reactive processes. 

Brinkman, H. C. and J. J. Hermans. 1949. The effect of non-homogeneity of molecular weight on the scattering of hght by high polymer solutions. Journal of Chemical Physics. 17,574. Broccio, M., D. Costa, Y. Liu, and S. H. Chen. 2006. The structural properties of a two-Yukawa fluid Simulation and analytical results. Journal of Chemical Physics. 124,084501. Brooks, C. L., M. Kaiplus, and B. M. Pettitt. 1988. Proteins A Theoretical Perspective of Dynamics, Structure and Thermodynamics. Vol. 71, Advances in Chemical Physics Hoboken, NJ Wiley. 

Traditionally, the thermodynamics of fluids used in engineering is essentially macroscopic. Fluids are treated as homogeneous molecular structure and fluctuations are ignored. Size and surface effects disappear in the thermodynamic limit in which the volume V and the number of particles N tend to infinity while the molecular density of the substance, p = NjV, remains finite. Macroscopic thermodynamics often eliminates the size of the system by reducing the extensive thermodynamic properties by the number of particles, mass, or volume. The actual scale is restored only in the stage of engineering design. 

Thus far, the focus of this review has been homogeneous fluids. For many interesting phenomena observed in biological and soft material systems, the micro- or mescoscale structure determines the properties of the system. DFT provides a valuable tool to predict mesoscale structure and interfacial properties assuming a suitable free energy functional can be developed. Excellent reviews of DFT for associating molecules have been written [92-94], so only a brief introduction will be provided here. 

The Emerman model described in the previous section is hardly applicable to the carbon black-filled CCM as the black particles have sizes of hundreds angstrom and such a composite, compared with the molding channel size, may be considered as a homogeneous viscous fluid. Therefore, the polymer structure, crystallinity and orientation play an important role for such small particles. The above-given example of manufacture of the CCM demonstrates the importance of these factors being considered during processing of a composite material to and article with the desired electrical properties. 

For applications where only mechanical properties are relevant, it is often sufficient to use resins for the filling and we end up with carbon-reinforced polymer structures. Such materials [23] can be soft, like the family of poly-butadiene materials leading to rubber or tires. The transport properties of the carbon fibers lead to some limited improvement of the transport properties of the polymer. If carbon nanotubes with their extensive propensity of percolation are used [24], then a compromise between mechanical reinforcement and improvement of electrical and thermal stability is possible provided one solves the severe challenge of homogeneous mixing of binder and filler phases. For the macroscopic carbon fibers this is less of a problem, in particular when advanced techniques of vacuum infiltration of the fluid resin precursor and suitable chemical functionalization of the carbon fiber are applied. 

Ultimately physical theories should be expressed in quantitative terms for testing and use, but because of the complexity of liquid systems this can only be accomplished by making severe approximations. For example, it is often necessary to treat the solvent as a continuous homogeneous medium characterized by bulk properties such as dielectric constant and density, whereas we know that the solvent is a molecular assemblage with short-range structure. For an example of this tool with supercritical fluids see Ting et al., 1993. 

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