Dielectric property mixture

Sigvartsen, T. Songstadt, J. Gestblom, B. Noreland, E., Dielectric properties of solutions of tetra-isopentylammonium nitrate in dioxane-water mixtures, J. Solut. Chem. 

In general, most reactions that can be carried out under thermal heating can be performed and accelerated by microwave irradiation. As discussed in Section 2.2, the efficiency of the microwave heating is highly dependent on the dielectric properties of the reaction mixture. Most results suggesting rate enhancements and improved yields can be explained in terms of simple thermal effects. However, for two main reasons, some reactions may not be suitable for performance in micro-wave reactors ... 

PCBs are attractive for industrial applications because of their stability and dielectric properties [351-354]. Figure 1 shows the structure of the biphenyl molecule along with examples of chlorination that can occur at any of the positions on the rings. The physical and chemical properties of both isomers and mixtures used in industrial applications depend upon the degree and position of the chlorine atoms [355 -358]. There are 209 possible chlorobiphenyl isomers and Table 4 lists the number of isomers for various degrees of substitution. However, many of these isomers do not occur in significant amounts in commercial products, and isomers with four or five chlorine atoms on one ring but none on the other are not detectable in PCB mixtures [359-362]. 

In microwave-assisted synthesis, a homogeneous mixture is preferred to obtain a uniform heating pattern. For this reason, silica gel is used for solvent-free (open-vessel) reactions or, in sealed containers, dipolar solvents of the DMSO type. Welton (1999), in a review, recommends ionic liquids as novel alternatives to the dipolar solvents. Ionic liquids are environmentally friendly and recyclable. They have excellent dielectric properties and absorb microwave irradiation in a very effective manner. They exhibit a very low vapor pressure that is not seriously enhanced during microwave heating. This makes the process not so dangerous as compared to conventional dipolar solvents. The polar participants of organic ion-radical reactions are perfectly soluble in polar ionic liquids. 

A good discussion of the dielectric properties of a mixture, independent of a specific model, has been given by Landau and Lifshitz (1960) we also recommend a paper by Niklasson et al. (1981) for its discussion of several aspects of the mixture problem. 

Finally, an area which is in need of much further research is that of the dielectric properties of two-phase systems such as frozen foods, emulsions, whips and foams. It is well known that the dielectric behavior of particles of one dielectric property imbedded in a substrate of another, behave very differently from a distributive mixture of both. Fricke (1955) developed a model for randomly oriented oblate spheroids suspended in a continuous medium. It is expected that this model may be used successfully to model two-phase food systems, but to date there is very little literature reporting such studies. 

Kraszewski, A. 1977. Prediction of dielectric properties of two-phase mixtures. Journal of Microwave Power 12(3) 215-222. 

Roebuck, B., Goldblith, S. and Westphal, W. 1972. Dielectric properties of carbohydrate-water mixtures at microwave frequencies. Journal of Food Science. 37 199-204. 

Haase and co-workers investigated electro-optic and dielectric properties of ferroelectric liquid crystals doped with chiral CNTs [495, 496]. The performance of the doped liquid crystal mixture was greatly affected even by a small concentration of CNTs. The experimental results were explained by two effects (1) the spontaneous polarization of the ferroelectric liquid crystal is screened by the 7t-electron system of the CNT and (2) the CNT 7i-electrons trap ionic impurities, resulting in a significant modification of the internal electric field within liquid crystal test cells. 

As was mentioned above, every efficient application of microwave energy to perform chemical syntheses requires reliable temperature measurement as well as continuous power feedback control, which enable heating of reaction mixtures to a desired temperature without thermal runaways. Moreover, power feedback control systems that are operated in the most microwave reactors enable a synthesis to be carried out without knowing the dielectric properties or/and conductive properties of all the components of the reaction mixture in detail. On the other hand, temperature control during microwave irradiation is a major problem that one faces during microwave-assisted chemical reactions. In general, temperature in microwave field can be measured by means of ... 

Skaf MS, Ladanyi BM. 1995. Molecular dynamics simulation of the wave vector-dependent static dielectric properties of methanol-water mixtures. J Chem Phys 102 6542-6551. 

M. S. Skaf, T. Fonseca and B. M. Ladanyi, Wave-vector-dependent dielectric relaxation in hydrogen-bonding liquids a molecular-dynamics study of methanol, J. Chem. Phys., 98 (1993) 8929-45 B. M. Ladanyi and M. S. Skaf, Wave vector-dependent dielectric relaxation of methanol-water mixtures, J. Phys. Chem., 100 (1996) 1368-80 M. S. Skaf, Molecular dynamics simulations of dielectric properties of dimethyl sulfoxide Comparison between available potentials, J. Chem. Phys., 107 (1997) 7996-8003. 

The dielectric properties of water and oil differ radically. A high water concentration in food systems greatly increases its dielectric properties. Oil, however, contributes relatively little to the dielectric behavior of a food system (1). Consequently, in the 90/10 oil/water mixture, the microwave energy was directed primarily at the 10% aqueous phase. Acids added to this 90/10 mixture will partition into this aqueous phase to the extent of their relative solubility in the two phases. Greatest losses were observed for acetic acid which exhibits the greatest solubility in water and was concentrated in the aqueous phase. Losses of the more nonpolar acids, i.e. caproic, were also much greater in microwave samples. Losses of the relatively... 

Salt Addition to the Microwave Medium. Addition of 3% sodium chloride to the 90/10 oil/water mixture had a significant impact on the loss of volatile acids. Data is summarized in Table III. Loss of acids was much greater in systems with added salt. The increased loss is attributed to the change in dielectric properties because of dissolved salt in the water. 

In spite of the fairly high densities used for the experiments on ethyl ether and propyl ether (at the lowest temperature used in the experiment the vapour pressure of ethyl ether is nearly equal to the saturation pressure), the pressure is found to obey a purely linear law. This difference of behaviour between the ethers and the chlorides is in complete agreement with K. L. Wolf s recent deductions from the dielectric properties of mixtures of liquids. In his paper Wolf attempts to explain the occurrence of association as being chiefly due to the presence of dipoles. If, as in the case of the ethers, the definitely polar parts of the molecule are sheltered by neutral alkyl groups on either side—and in this his statements are in complete agreement with our results —association phenomena will only occur to a trifling extent, whereas if the alkyl groups are terminal, as in the chlorides, marked association phenomena must occur. 

The mixture theories were originally developed for dielectric properties, but can be applied to other properties that are governed on a macroscopic level by Laplace s equation ( 10). Consequently, the generalized conductivity in the above equations can be the ionic conductivity o, thermal conductivity K, or complex electrical conductivity o defined by ... 

The basic advantages of using binary mixtures with the critical CP instead of the GL critical point can be found in the history of critical phenomena, namely establishing the basic universal parameters appeared to be much simpler for binary mixtures with CP than for GL systems. Firstly, CP investigations can be carried out under atmospheric pressure. Secondly, one can select a binary mixture with CP close to room temperature. " Finally, it is possible to select a mixture which emphasizes the desired specific feature, for instance (a) methanol - cyclohexane mixture can simulate weightless conditions since densities of both components are almost equal (b) there are almost no critical opalescence for isooctane - cyclohexane mixture since their refractive indices are almost the same (c) one can considerably change the concentration of the dipole component of the mixture. The latter feature can strongly influence both dielectric properties and solvency. 

Impedance is the ratio of the voltage across a system to the current passing through the system. It measures the dielectric properties (permittivity and conductivity) of the system. The dielectric behavior of colloidal particles in suspension is generally described by Maxwell s mixture theory [26]. This relates the complex permittivity of the suspension to the complex permittivity of the particle, the suspending medium and the volume fraction. Based-on Maxwell s mixture theory, shelled-models have been widely used to model the dielectric properties of particles in suspension [35-40]. A single shelled spherical model is shown in Fig. la. 

Food is a complex mixture of different components and its dielectric properties are highly dependent on its compositions. Moisture content and salt concentration usually play a major role in determining the dielectric properties of foods. Figure 3.1 shows the relationship between the dielectric loss factor components and temperature. 

Erie, U., M. Regier, C. Persch, and H. Schubert. 2000. Dielectric properties of emulsions and suspensions Mixture equations and measurement comparisons. The Journal of Microwave Power and Electromagnetic Energy 35 185-190. 

The energy quantum (0.0016 eV) of the microwave irradiation is totally inadequate for exciting atom-atom bonds or specific parts of a molecule and hence cannot induce chemical reactions, as opposed to ultraviolet or infrared radiation (Table 25.1). When molecules rotate in a matrix, they generate heat by friction. The amount of heat generated by a given reaction mixture is a complex function of its dielectric property, volume, geometry, concentration, viscosity, and temperature. Thus, two samples irradiated at the same power level for the same period of time will most likely end up with rather different final temperatures. 

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