Multicomponent systems, water contents

COMPARISON OF PREDICTED AND EXPERIMENTAL WATER CONTENTS OF MULTICOMPONENT SYSTEMS (From Reference 2)... 

The lacking special description of the Gibbs phase rule in MEIS that should be met automatically in case of its validity is very important for solution of many problems on the analysis of multiphase, multicomponent systems. Indeed, without information (at least complete enough) on the process mechanism (for coal combustion, for example, it may consist of thousands of stages), it is impossible to specify the number of independent reactions and the number of phases. Prior to calculations it is difficult to evaluate, concentrations of what substances will turn out to be negligibly low, i.e., the dimensionality of the studied system. Besides, note that the MEIS application leads to departure from the Gibbs classical definition of the notion of a system component and its interpretation not as an individual substance, but only as part of this substance that is contained in any one phase. For example, if water in the reactive mixture is in gas and liquid phases, its corresponding phase contents represent different parameters of the considered system. Such an expansion of the space of variables in the problem solved facilitates its reduction to the CP problems. 

The food technology literature contains a substantial number of references to enzyme activity at low water content (Acker, 1962 Drapron, 1985 Potthast etal., 1975). Drapron (1985) gives tables of the hydradon level for the onset of activity of various enzymes. Much of this work consisted of monitoring the activity of a particular enzyme, as a function of relative humidity, for a sample (e.g., a food product) in which the enzyme was not the principal component. Such measurements have the difficulties of interpretation associated with multicomponent systems—... 

Porous silicas are usually mesoporous materials and they can be made with a variety of pore dimensions. In particular, silica glasses can be made with well-defined pore diameters, typically in the range 30-250 A, using sol-gel methods. Such a system provides a good model for testing the models of relaxation behaviour of fluids in porous solids. It is normally found that the two-site fast-exchange model for relaxation described above for macroporous systems is still valid. For instance, H and relaxation times have been measured during both adsorption and desorption of water in a porous silica. Despite hysteresis in the observed adsorption isotherms, it was found that the relaxation times depended solely on water content.For deuterated water in some porous silicas, multicomponent relaxation behaviour for T2 and Tip has been observed, and this has been attributed to the fractal nature of the pore structure. 

For a specified solvent system, water or aqueous solutions for example, there are two variables that must be considered in the solubilization process (1) the molecular nature, purity, and homogeneity of the surfactant and (2) the chemical nature of the additive. From a technological viewpoint, it is important to understand exactly what surfactant structural features serve to maximize the desired solubilizing effect, and the best way to achieve that understanding is through a fundamental knowledge of the molecular and thermodynamic processes involved. In addition, since most technological applications of solubilization involve complex multicomponent systems, such factors as temperature, electrolyte content, and the presence of polymeric species and other solutes must be examined. 

Li et al. [27] used NIR spectroscopy to troubleshoot and optimize a wet granulation process. During development phases, authors realized that out-of-trend content uniformity values were obtained for some sieve cuts due to the interaction of the excipients and the binding solution during the coalescence process. A qualitative method was put in place to determine within different sieve fractions the change in content uniformity. Other process optimization methods involving NIR were described by Miwa et al. [28-30]. They used moisture levels predicted by NIR for independent components to estimate a lower and an upper limit for the amount of water needed to granulate a multicomponent system. 

The water activity of some selected foods is given in Table 7.10. The water content of many foods and thus their activities vary according to humidity of the ambient air (and thus temperature), as there is a constant sorption or desorption ofwater. The term water activity should be used only for systems that are in thermodynamic equilibrium. Foods are often multicomponent multiphase systems and only if there is a thermodynamic equihbrium between all phases is the water activity in the whole system equal, which does not happen often. It frequently requires hours, weeks or takes even longer to achieve the equihbrium state. Many food systems may also be in a non-equihbrium metastabile state (e.g. sugar melts). 

A microemulsion is a multicomponent (3-4 components) system, e.g., water in hydrocarbon (water/oil) or hydrocarbon in water (oil/water), surfactant, and cosurfactant, and generally it exists only in small concentration ranges. Nevertheless, the capacity for reactants and variability of solubilization properties are high and of practical interest [76]. On the basis of microemulsions Menger and co-workers developed a method for an economical environmental cleanup of chemical warfare contamination [77]. As an example of organometallic catalysis in a microemulsion, Beletskaya [78] performed palladium-catalyzed C—C coupling reactions in aqueous medium with a very high content of surfactant. 

As an example for multivariate modeling, we consider the simultaneous determination of several components in low-selective analytical systems (multicomponent analysis). These components can be elements, compounds, or chemical/physical properties. By means of multicomponent analysis, constituents of pharmaceutical formulations can be determined in the UV range, the water and protein content of cereal can be estimated from NIR spectra or chemical elements, and technological parameters of coal are predictable on the basis of infrared (IR) spectra. The limited selectivity of chemical sensors can also be overcome by applying the principles of multicomponent analysis. 

Substitution at polynuclear aqua-ions is often much slower than at the respective parent mononuclear aqua-ions, as has been demonstrated for, e.g., aluminum(III), iron(III), zirconium(IV), and hafnium(IV). A further example has been provided by aluminum, where the Al,3 polynuclear oxo-aqua-species takes several months to equilibrate with lactate. Very different rates of complex formation are used to sort out speciation in multicomponent polynuclear/mononuclear aqua-cation systems. The problems involved in this approach are well illustrated by a Al NMR study of the numerous reaction pathways in the Al " /ferron [ferron = 7-iodo-8-hydroxyquinoline-5-sulfonic acid, (18)] reaction. The slowness of reaction of polynuclear species can cause difficulties in the determination of total metal content, e.g., of aluminum by ferron or Alizarin S (19), in natural waters. In certain situations these... 

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