Dielectric constant, commercial solutions

Another important group of oxide materials with a very low electrical conductivity is the oxide dielectrics. A number of these are based upon the perovskites, MXO3 or M0 X02. The archetype of these materials is BaTiC>3, which has a high dielectric constant, or relative permittivity to vacuum, the value at room temperature being 1600, and commercial use is made of the isostructural PbTi(>3 and ZrTi03 which form solid solutions, the PZT dielectrics. These materials lose their dielectric properties as the temperature... 

The most commonly used LC/MS interfaces in pharmaceutical analysis are ESI and APCI. An ESI interface on the majority of commercial mass spectrometers utilizes both heat and nebulization to achieve conditions in favor of solvent evaporation over analyte decomposition. While ionization in APCI occurs in the gas phase, ionization using ESI occurs in solution. Attributes of a mobile phase such as surface tension, conductivity, viscosity, dielectric constant, flow rate and pFi, all determine the ionization efficiency. They therefore need to be taken into consideration and controlled. 

Liquid solubilizers act by reducing the dielectric constant properties of the solvent system, thereby reducing the electrical conductance capabilities of the solvent and increasing the solubility of hydrophobic or non-polar drugs. Lanoxin, Valium, and Nembutal are examples of commercially available sterile solutions containing cosolvent solubilizers. A popular combination consists of 40% propylene glycol and 10 /o ethanol in water. 

It must also be recognized that, because of the usually much finer subdivision of natural ion exchangers in soils as compared with commercial synthetic resins, surface phenomena may play a more prominent role. Among these effects are water is more structured at the interface than in the bulk liquid ions and water molecules are less mobile at the interface because of stronger interactions the dielectric constant of water is lower than in the bulk solution and surface charges produce an electric double layer (Horst, 1990). 

Efforts at loading titanium oxide nanoparticles in PVA (commercially available from Nanophase) have been reported [65], In this report, titanium nanoparticles are dispersed in an aqueous solution of PVA with poly(melamine-co-formaldehyde). The solution is spun onto substrate and heated to generate a cross-Unked polymer-nanoparticle dielectric. A modest enhancement of dielectric constant is achieved for 600-nm thick films. Thin-film transistors using this composite show excellent pen-tacene mobility (> 0.2 cm V s ) and reasonable on/off ratios 10. Vj- —TV is reasonably high, suggesting static charge at the dielectric-semiconductor interface. 

The properties of ionic polymers in nonaqueous media have only recently become the subject of systematic studies. In solvents of low dielectric constant, salt groups resist dissociation and are poorly solvated. Thus, ionic moieties promote intra- and inter-polymer association in organic solvents. The tendency of ionic groups to aggregate or cluster resembles the coalescence of such groups in reversed micelles. Similar considerations underly the formation of ionic "cross-links" that modify the behavior of ionomers in the solid state. Solutions of polyions in nonaqueous media thus provide systems in which a powerful array of experimental techniques can be used to probe phenomena that are important to the bulk properties of a commercially important group of materials. The article by Teyssie and Varoqui in Part IV describe significant explorations in this novel field. 

One of the first on-line liquid chromatography detectors to be developed in the early forties was, in fact, a bulk property detector, the refractive index detector (1). Bulk property detectors continuously monitor some physical property of the column eluent and by the use of a suitable transducer provide a voltage - time output that is either proportional to the physical property being measured, or made proportional to the concentrations of the solute eluted. The properties of the mobile pheuse that are most commonly monitored in commercially available bulk property detectors are refractive index, electrical conductivity, and dielectric constant, the dielectric constant detector being the least popular of the three. 

Control of the Sample Environment High-vacuum conditions or constant inert gas flow are necessary for studying intrinsic relaxation phenomena in hygroscopic materials. A chemically inert atmosphere is also desirable in studies of polymer solutions, biomacromolecules, and other biological substances, where precise control of the hydration levels is vital. In isothermal scans fluctuations of the sample temperature should be as small as possible (below 0.1 K), such fluctuations are usually controlled by most commercial temperature stabilization/regulation systems (see Section 6.4.1.2). The same recommendation applies for the heating rates typically used in nonisothermal dielectric techniques. 

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