Dielectric constant, and solubility

The magnitude of the dissociation constant A plays an important role in the response characteristics of the sensor. For a weakly dissociated gas (e.g., CO2, K = 4.4 x 10-7), the sensor can reach its equilibrium value in less than 100 s and no accumulation of CO2 takes place in the interior layer. On the other hand, SO2, which is a much stronger acid (K = 1.3 x 10-2), accumulates inside the sensor and its rep-sonse time is in minutes. The detection limit and sensitivity of the conductometric gas sensors also depend on the value of the dissociation constant, on the solubility of the gas in the internal filling solution, and, to some extent, on the equivalent ionic conductances of the ions involved. Although an aqueous filling solution has been used in all conductometric gas sensors described to date, it is possible, in principle, to use any liquid for that purpose. The choice of the dielectric constant and solubility would then provide additional experimental parameters that could be optimized in order to obtain higher selectivity and/or a lower detection limit. 

Correlation between dielectric constant and solubility parameter... 

As electrical forces due to polarisability and polar moment determine the cohesive energy, a certain correlation between dielectric constant and solubility parameter may be expected. Darby et al. (1967) suggested such a correlation for organic compounds. It appeared that a surprisingly simple correlation holds for polymers, viz. ... 

Table 1 list solvents and cosolvents used in parenteral products. Water for injection is the most common solvent but may be combined or substituted with a cosolvent to improve the solubility or stability of drugs.f The dielectric constant and solubility parameters are among the most common polarity indices used for solvent blending.f Ethanol and propylene glycol are used either alone or in combination with other... 

Finally, there must be significant interaction between the solvent and solute to favor solubilization. This is related to the relative polarities of the solvent and solute (partition coefficients, dielectric constants and solubility parameters). Solubilization is improved by the formation of hydrogen bonds (e.g. hydroxyacids) or the electrostatic effects that occur with small, highly charged ions (e.g. sodium). 

When control of structural parameters of the polymer is achieved, properties of the material will also depend on processing routes. Processing routes will have an inq)act on polymer chain conformation and then on localization of charge carriers. Polymers are mainly processed from solutions or from their molten state. We will show here the extreme importance of the conformation of polymers in solution when one is concerned with final electrical properties. The conformation of polymer in solution can be tailored by a careful choice of the solvent type, i.e. dielectric constant and solubility parameters or some added salts in the solution. Even in the case of extrusion, when the polymer is processed in the molten state, the prior conformation in solution plays an important role on the final properties of conductive polymer based blends. 

Prediction of solubility in the inert solvents such as carbon disulfide, carbon tetrachloride, chloroform, etc., is somewhat more difficult. In these instances it is sometimes convenient to consult the following table of dielectric constants. No definite relationship between dielectric constants and solubilities has been developed since unknown factors are involved nevertheless, the dielectric constants may be used where they do not conflict with the more basic generalization given in Rule I. 

Polyimides containing C—F bonds have been receiving strong attention (96—98). Fluorine-containing polyimides possess lower dielectric constant and dielectric loss because of reduced water absorption and lower electronic polarization of C—F bonds vs the corresponding C—H bonds. Fluorine-containing polyimides are often more soluble and readily processible without sacrificing thermal stabilities. The materials are appHed primarily iu... 

Water in its supercritical state has fascinating properties as a reaction medium and behaves very differently from water under standard conditions [771]. The density of SC-H2O as well as its viscosity, dielectric constant and the solubility of various materials can be changed continuously between gas-like and liquid-like values by varying the pressure over a range of a few bars. At ordinary temperatures this is not possible. For instance, the dielectric constant of water at the critical temperature has a value similar to that of toluene. Under these conditions, apolar compounds such as alkanes may be completely miscible with sc-H2O which behaves almost like a non-aqueous fluid. 

For hydrophobic, (virtually) nonionizable substances [i.e., those that show no ionic species of significance in the pH range 1 to 10 (e.g., diazepam)], solubility can usually be improved by addition of nonpolar solvents. Aside from solubility, stability is also affected by solvents in either a favorable or a nonfavorable direction [6], Theoretical equations for solubility in water [7] and in binary solvents [8] have been reported in literature, but in general the approach in preformulation is pseudoempirical. Most often the solubility changes as the concentration of nonpolar solvent C2, increases. For binary systems it may simply be a monotonely changing function [9], as shown in Fig. 2. The solubility is usually tied to the dielectric constant, and in a case such as that shown by the squares, the solubility is often log-linear when plotted as a function of inverse dielectric constant, E, that is,... 

Both the dielectric constant and dipole moment are comparable to those of water, indicating that HF is a good solvent for inorganic compounds, but many organic compounds are also soluble. In general, the fluorides of +1 metals are much more soluble than those of +2 or +3 metals. At 11 °C, the solubility of NaF is approximately 30 g per 100 g of liquid HF, that of MgF2 is only 0.025 g, and that of A1F3 is 0.002 g. 

Exactly the same problem arises with the recent studies of NiO solubility by Tremaine and Leblanc (25) and again the thermodynamic data on the aqueous anionic species at 300 C are likely to be more reliable than on the Ni + ion. There is good spectroscopic evidence for complex formation in chlorides of nickel (II), (26) cobalt (II) (27), and copper (II) (28) at 300°C and above. Most of the work was done at rather high Cl concentrations but qualitatively the effects of dielectric constant and concentration are as expected. A noteworthy feature (which estimation procedures will have to allow for) is the change from 6 to 4 coordination at the lower pressures (150-300 bar) and the higher Cl concentrations. This change appears to take place with only 2 or 3 Cl ions coordinated to the metal (at least in the case of Ni(II)). 

Fluorinated poly(imide-ether-amide)s are readily soluble in organic solvents like dimethylformamide (DMF), N-methylpyrrolidone (NMP), pyridine or tetrahydrofu-ran (THF) and give flexible films by casting of such solutions. These polymers exhibit decomposition temperatures above 360°C, and glass transition temperatures in the 221-246° C range. The polymer films have a low dielectric constant and tough mechanical properties. 

FIGURE 3 2 Solvent extraction efficiencies (EF) as functions of dielectric constants (D), solubility parameters (6), and polarity parameters (P and E -). Solvents studied silicon tetrachloride, carbon disulfide, n pentane. Freon 113, cyclopentane, n-hexane, carbon tetradiloride, diethylether, cyclohexane, isooctane, benzene (reference, EF 100), toluene, trichloroethylene, diethylamine, chloroform, triethylamine, methylene, chloride, tetra-hydrofuran, l,4 dioxane, pyridine, 2 propanol, acetone, ethanol, methanol, dimethyl sulfoxide, and water. Reprinted with permission from Grosjean. ... 

When restrictions are not placed on the amount of time or the amount of material, different solvents or different quantities of acid or base are usually used as variables. It is not only important to know the solubility of the compounds in aqueous solutions but also in other solvents to which the compound might be exposed during synthesis and formulation. The solvents usually have a wide range of dielectric constants and the experimental results provide a solubility profile which can be utilized in the selection of appropriate solvents to use during the development of the compound. Since the compounds almost always selected for development are either weak acids or weak bases, the solubilities of the compounds will be pH-dependent. The use of different amounts of acid or base with an excess amount of compound permits the determination of a pH-solubility profile. 

At low temperature, reduce dielectric constant and enhance electrostatic interactions between protein molecules Removes protein-bound water molecules and promotes hydrophobic interactions between protein molecules Neutralize charge of protein, which decreases protein solubility in aqueous environment... 

Dielectric constant and cohesive energy density are determined by the same type of electrical force. Based on that observation van Krevelen reported that values of e at room Icrnpcralure (RT) correlate with the solubility parameter 8 ... 

PFMB can be used to prepare aromatic polyimides that display solubility in ketone, ether, and polar aprotic solvents. This unusual solubility can be utilized in die facile preparation of thin films that display anisotropy in their structures and properties. The anisotropy in the optical properties of the films makes them promising candidates for use as compensation layers in liquid-crystal displays. Their low dielectric constants and CTEs in combination with their outstanding thennal and thermooxidative stabilities make diem candidates for dielectric layers in microelectronics applications. 

Several other non salt-specific factors, such as pressure and temperature, influence crystallization. In SCWO, the solubility reduces as the temperature increases, owing to the reduction in dielectric constant. For example, sodium chloride s solubility in supercritical water is 824 mg/L at 400°C while at 500 °C it is only 299 mg/L [28], Pressure increase produces an increase in dielectric constant, and it is not infrequent for the precipitated salt to redissolve itself. For example, Foy et al. [24] oxidized chloro-compounds at temperatures around 550°C and 600 bar in these conditions, the dielectric constant is sufficiently high to avoid chloride deposition. 

Much attention has been paid to the synthesis of fluorine-containing condensation polymers because of their unique properties (43) and different classes of polymers including polyethers, polyesters, polycarbonates, polyamides, polyurethanes, polyimides, polybenzimidazoles, and epoxy prepolymers containing pendent or backbone-incorporated bis-trifluoromethyl groups have been developed. These polymers exhibit promise as film formers, gas separation membranes, seals, soluble polymers, coatings, adhesives, and in other high temperature applications (103,104). Such polymers show increased solubility, glass-transition temperature, flame resistance, thermal stability, oxidation and environmental stability, decreased color, crystallinity, dielectric constant, and water absorption. 

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