Finely Dispersed Systems

Institute for Technology of Nuclear and Other Mineral Raw Materials, Belgrade, Serbia and Montenegro 

Chemical Engineering and Scales — Macro, Micro, Nano, and Atto. 3 

Processes in Nano Continua, Discontinua, and Spaces of Interactions. 7 

Classical chemical engineering has been intensively developed during the last century. Theoretical backgrounds of momentum, mass, energy balances, and equilibrium states are commonly used as well as chemical thermodynamics and kinetics. Physical and mathematical formalisms are related to heat, mass, and momentum transfer phenomena as well as to homogeneous and heterogeneous catalyses. Entire object models, continuum models, and constrained continuum models are frequently used for the description of the events, and for equipment designing. Usual, principal. 

Molecular engineering still suffers substantial development. Besides heat, mass, and momentum transfer phenomena, commonly used in classical chemical engineering, it is also necessary to introduce the electron transfer phenomenon. Description of the events is based on molecular mechanics, molecular orbits, and electrodynamics. Principal tools and equipment are micro-reactors, membrane systems, micro-analytical sensors, and micro-electronic devices. Output is, generally, demonstrated as molecules, chemicals (solutions), and biochemicals. 

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For reasons that are not fiiUy understood, PPSF exhibits generally improved compatibiUty characteristics over either PSF or PES in a number of systems. An example of this is blends of PPSF with polyaryletherketones (39,40). These blends form extremely finely dispersed systems with synergistic strength, impact, and environmental stress cracking resistance properties. Blends of PPSF with either PSF or PES are synergistic in the sense that they exhibit the super-toughness characteristic of PPSF at PSF or PES contents of up to 35 wt % (33,34). The miscibility of PPSF with a special class of polyimides has been discovered and documented (41). The miscibility profile of PPSF with high temperature (T > 230° C) polysulfones has been reported (42). 

Phase inversion along the dilution path (by addition of water to the oil/surfactant mixture) followed for nanoemulsion preparation was confirmed by conductivity measurements, and was found to be essential for obtaining finely dispersed systems, as transparent dispersions were not obtained if the order of addition of the components was changed following an experimental path with no phase inversion (Figure 6.2). 

Finely dispersed systems of antifoams prove to be economically more convenient in industry [26-28]. The only possibility to use low molecular antifoams (fatty alcohols, acids, etc.) of short lasting defoaming ability, is by spraying their solutions. 

As shown above, the minimal of microcells is in the range of 10 jum (R < 1000 A) and S in the range of 10 cm. Thus, recent data on the structure of oligomeric foams, at least those described above, allow assignment of these foams to high or fine dispersion systems. 

The assignment of ol meric foams to fine dispersion systems is of considerable importance since it enables to approach the study and modification of structure by using the concepts of dispersion media. The advantages of such an approach are evident if one takes into account the variety of ideas and methods developed in the study of high dispersion systems of non-polymeric nature and the first results of the application of this approach to oligomeric foams 1. 

The fact that foamed plastic can be classified among fine-dispersed systems is highly important since their structure may be studied not only from the conventional polymer point of view, but also by applying a different novel and promising approach — the physics and chemistry of disperse materials. 

Up to now we have considered interfacial phenomena in systems where the interfacial boundaries separating coexisting phases were essentially flat (i.e., with large radius of curvature). The interfacial curvature changes the thermodynamic properties of systems and is responsible for a number of important phenomena, such as capillary effects. The large interfacial curvature is typical of finely dispersed systems, and hence one has to take into account its effects on the thermodynamic properties of such systems. 

The processes of generation and growth of nuclei form the basis of the condensational formation of disperse systems. Finely dispersed systems can... 

The condensation methods play an especially important role in the preparation of finely disperse systems, which are impossible to make by simple dispersion process. These methods also allow one to control the degree of dispersion (and the degree of polydispersity) in the systems formed. 

In order to describe the stability of fine disperse systems stabilized by diffuse ionic layers, one has to use the total free energy of interaction between particles, instead of the energy per unit film area, and compare the barrier height,, to the thermal energy, kT. For us to be able to use the solution derived for the case of plane-parallel surfaces, let us introduce some effective area of particle contact, Se[. Then the potential barrier height for the particles can be expressed as = A5 max St(. When diffuse part of electrical double... 

The range of disperse systems of interest in colloid science is very broad. These include coarse disperse systems consisting of particles with sizes of 1 pm or larger (surface area S < 1 m2/g), and fine disperse systems, including ultramicroheterogeneous colloidal systems with fine particles, down to 1 nm in diameter, and with surface areas reaching 1000 m2/g ( nanosystems ). The fine disperse systems may be both structured (i.e. systems in which particles form a continuous three-dimensional network, referred to as the disperse structure), and free disperse, or unstructured (systems in which particles are separated from each other by the dispersion medium and take part in Brownian motion and diffusion). 

The principal peculiarity of fine disperse systems is the presence of highly developed interfaces. These interfaces and the interfacial phenomena occurring at them affect the properties of disperse systems, primarily due to the existence of excessive surface (interfacialf energy associated with interfaces. The excess of interfacial energy reveals its action along the interface in the form of interfacial tension, which tends to decrease interfacial... 

The enrichment of the interfacial layers is attended by a decrease in free energy which is the greater the larger the area involved. The adsorption, therefore, stabilizes the finely dispersed system of droplets which constitutes an emulsion. 

Similar to the liquid-liquid system, the volume-surface diameter of dispersed phase particles in a liquid-gas flow is determined by the initial size of the bubbles at the gas input points [81-83]. An increase of the liquid-gas flow rate leads to an increase of the shear deformation influence of the dispersed phase particles and therefore, to a decrease in the diameter of the gas bubbles in the input area of the device. Finally, it leads to the formation of a finely dispersed system in a device with an increase of the liquid-gas flow rate (Figure 2.23). Fast chemical reactions in two-phase gas-liquid systems usually occur in a gas-phase excess, so it is reasonable to analyse the influence of the gas content in a flow on the change of phase contact surface. 

The upper limit for an increase of the two-phase reaction mixture flow rate and the degree of reactor wall profiling, for the formation of finely dispersed systems, is determined by the fast growth of the input-output pressure drop, leading to an increase... 

A new idea, using deterministic approach, has been applied for the elucidation of the electron and momentum transfer phenomena at both the rigid and deformable interfaces in finely (micro, nano, atto) dispersed systems. Since the events at the interfaces of finely dispersed systems have to be considered at the molecular, atomic, and entities level, it is inevitable to introduce the electron transfer besides the classical heat, mass, and momenrnm transfers commonly used in chemical engineering [8]. Therefore, an entity can be defined as the smallest indivisible element of matter that is related to the particular transfer phenomenon. Hence, the entity can be either a differential element of mass or demon, or an ion, or a phonon as quantum of acoustic energy, or an infon as quantum of information, or a photon, or an electron [9,10]. 

A possible approach is proposed to the general formulation of the links between the basic characteristics, levels of approximation, and levels of abstraction related to the existence of finely dispersed systems (DS) [11]. At first, for simplicity and easy physical and mathematical modeling, it is convenient to introduce the terms homo-aggregate (phases in the same state of aggregation [HQA]) and hetero-aggregate (phases in a more than one state of aggregation [HEA]). Now the... 

Spasic, A.M., Babic, M.D., Marinko, M.M., Djokovic, N.N., Mitrovic, M., and Krstic, D.N., A new classification of finely dispersed systems. Abstract of Papers, Part 5, in Proceedings of the Fourth European Congress of Chemical Engineering, Granada, Spain, September 21-25, 2003, p.5.2.39. 

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