Stability multiphase systems

Citric acid is utilized in a large variety of food and industrial appHcations because of its unique combination of properties. It is used as an acid to adjust pH, a buffer to control or maintain pH, a chelator to form stable complexes with multivalent metal ions, and a dispersing agent to stabilize emulsions and other multiphase systems (see Dispersants). In addition, it has a pleasant, clean, tart taste making it useful in food and beverage products. 

A multiphasic system for a chemical reaction can be constituted by any combination of gaseous, liquid, and solid phases. If a catalyst is present, it can be homogeneous or heterogeneous, thereby adding further phases—and degrees of freedom—to the system. Extra phases add new variables to a reaction, and it is therefore necessary that this be done for an advantage, such as an easier separation of the products, improved rates and selectivity, improved catalyst stability. 

This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

Parameters Appearing in the Generalized Stability Criterion (Unbounded Analysis) for Various Multiphase Systems... 

In the previous section, stability criteria were obtained for gas-hquid bubble columns, gas-solid fluidized beds, liquid-sohd fluidized beds, and three-phase fluidized beds. Before we begin the review of previous work, let us summarize the parameters that are important for the fluid mechanical description of multiphase systems. The first and foremost is the dispersion coefficient. During the derivation of equations of continuity and motion for multiphase turbulent dispersions, correlation terms such as esv appeared [Eqs. (3) and (10)]. These terms were modeled according to the Boussinesq hypothesis [Eq. (4)], and thus the dispersion coefficients for the sohd phase and hquid phase appear in the final forms of equation of continuity and motion [Eqs. (5), (6), (14), and (15)]. However, for the creeping flow regime, the dispersion term is obviously not important. 

In Sections II and VI, the stability of multiphase systems was analyzed for unbounded and bounded beds, respectively. In the unbounded case, it was dispersion that was the most important parameter in deciding the stability of the system. An increase in the dispersion coefficient led to a... 

In multiphase systems, like in rubber modified plastics, stabilizers can partition between different phases of the system [37]. The actual stabilizer concentration in either elastomer or thermopl tic phases may differ very significantly from the average stabilizer concentration declared for the whole multiphase system. In polymers like ABS, the partitioning of stabilizers may exert a controlling influence on the final stabilization effect. A proper chemical modification of the stabilizer molecule can enhance its affinity to a phase of the multiphase system which is more sensitive to degradation. 

Diffusion requires cooperative motions of both the polymer and the diffusant [27] and is therefore only low below T , and severely restricted below Tg. The diffusion of various stabilizers was elucidated in amorphous and semicrystalline polymers and in multiphase systems. In semicrystalline polymers, diffusion takes place almost exclusively in the amorphous phase and the value of D is sensitive to the total crystallinity and the morphology. It is difficult to predict D within a homologuous series of stabilizers. For a given molecular weight, long and flexible molecules diffuse more rapidly than more rigid and compact structures. For a given stabilizer, the value of D is usually lower in PP and HDPE than in LDPE. It was demonstrated that typical AO molecules have a very restricted mobility in polymers. They are, however, insufficient experimental data to correlate the AO mobility and the AO efficiency. 

The attachment of the functionalized mercaptans proceeds with unsaturated polymers more efficiently than with saturated ones. The stabilizing moiety can therefore be targetted toward the most oxidation sensitive component in multiphase systems or blends, e.g. to the BR phase in ABS. 

This indicates that polymeric stabilizers having M > 800 have high resistance against extraction by components of foods, and, moreover, they do not enter into the metabolic cycle. Public concern over the contamination of the human environment can be thus satisfied. As a consequence, stabilizers like Poly AO-79 (97), Chimassorb 944 (161a) or Tinuvin 622 (146) may be legally used in stabilization of polymers in contact with food. Possibilities for application of other polymeric stabilizers for articles used in the home are open. Extractability problems may be thus overcome. Articles made from weather resistant butadiene based multiphase systems like ABS, MBS or MABS [84] stabilized in the most sensitive BR phase with polymer bound stabilizers may serve as an example. 

In sections 7.3.1-7.3.4 we have considered only relatively simple dilute emulsions. Many pharmaceutical preparations, lotions or creams are, in fact, complex semisolid or stmc-tured systems which contain excess emulsifier over that required to form a stabilising mono-layer at the oil/water interface. The excess surfactant can interact with other components either at the droplet interface or in the bulk (continuous) phase to produce complex semisolid multiphase systems. Theories derived to explain the stability of dilute colloidal systems cannot be applied directly. In many cases the formation of stable interfacial films at the oil/water interface cannot be considered to play the dominant role in maintaining... 

Several classes of formulations of disperse systems are encountered in the chemical industry, including suspensions, emulsions, suspoemulsions (mixtures of suspensions and emulsions), nanoemulsions, multiple emulsions, microemulsions, latexes, pigment formulations, and ceramics. For the rational preparation of these multiphase systems it is necessary to understand the interaction forces that occur between the particles or droplets. Control of the long-term physical stability of these formulations requires the application of various surfactants and dispersants. It is also necessary to assess and predict the stability of these systems, and this requires the application of various physical techniques. 

Table 11 compares the effectiveness of a synergistic UV stabilizer (BHBM-B + EBHPT-B) with some commercial stabilizing systems for ABS added conventionally. The exceptional activity of the polymer-bound system is believed to be due to the fact that it is confined to the rubber phase of the polyblend (18), which is known to be more sensitive than the thermoplastic phase to the effects of both heat and light (36). This finding, if confirmed in other multiphase systems, could be of considerable importance for the stabilization of heterogeneous polymer blends. 

In the earliest stage of foaming, the rising foam is a frothlike multiphase system of expanding gas cells surrounded by liquid phase which contains out-of-phase carbamic acid and arylamine carbamates which tend to stabilize the foam. At about the time disubstituted urea can be measured by infrared in the foam, the system achieves its maximum rate-of-rise. These highly polar species begin to "thicken" the system retarding the rate-of-rise. 

Polymer-bound amines are of specific importance in multiphase systems where partitioning of migratable stabilizers may diminish the stability of more sensitive phases. System 193 representing a matrix and rubber-phase bound amine in acry-lonitrile/EPDM/styrene terpolymer is an example [297]. The superior performance was obtained when the migratable monomer 192 was melt-blended into the HAS-functionalized multiphase system 193. 

In Table 2 is presented the modulus evolution between 2 to 6 ageing weeks, after processing, for different multiphase systems. We can show that the introduction of fibres into the TPS matrix provokes a drastic decrease of this ageing. This is due to the fibre-matrix interactions which can create a physical 3D network (hydrogen network). These H bonds may stabilize the materials. Besides, we have tested the T PS-fi bres-polyester system to obtain a better water... 

The morphology of latex particles is controlled by the thermodynamic and kinetic factors. The thermodynamic factors determine the ultimate stability of the multiphase system, inherent in the production of a composite latex particle, while the kinetic factors determine the ease with which such a thermodynamically favored state can be achieved. The parameters affecting the thermodynamics of the system include the particle surface polarity, the relative phase volumes, and the core particle size. The parameters affecting the kinetics of the morphological development include the mode of monomer addition (monomer starved or batch) and the use of crosslinking agents. Of course, crosslinked core/shell latexes constitute IPNs, see Section 6.4.1. 

The question of polymer blend miscibility derives out of the nineteenth century development of thermodynamics and studies in the same periodic of binary mixtures of low molecular weight liquids. From a thermodynamic viewpoint, Gibbs (23) formulated the stability of multiphase systems in terms of the quantity G defined by (in modem notation (24))... 

The application of this strategy to commercially important polyolefin multiphase systems to produce tuned and/or stabilized morphologies is certainly attractive. However, the required interfacial agents for this purpose have... 

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