Moments and Dielectric Constants

Efforts have also been made to correlate solvent properties, namely hydrophobic-ity, dipole moment, and dielectric constant with enanhoselectivity, but such correlations have only been demonstrated in some systems [31-33]. 

The physicochemical properties of excipients used in pMDIs are different from most dosage forms and are a derivative of the propellant system that constitutes the bulk of the formulation. The transition from CFC-based formulations to HFA-based systems has been lengthened by the historically empirical formulation approach and the dissimilarity of the physicochemical properties of the replacement HFA propellants. Both HFA 134a and HFA 227 show an increased polarity, revealed in increased dipole moments and dielectric constant. The most significant practical change has therefore been a general change in the solvency properties. 

The formation of solvates is association between unlike species. Solvation is more frequent between substances of high polarity than those of low polarity. This is illustrated by the decrease in the tendency to form solvates with decrease in dipole moment and dielectric constant (shown in parentheses) for N-methylacetamide (3.59 172), to water (1.84 78.4), to ethanol (1.70 24.6) to ammonia (1.48 78.4) to ethanol (1.70 24.6) to ammonia (1.48 J7.8) to me thylcy clohexane (0 2.02) for which few associations are known. 

Solvent polarity the dipole moment and dielectric constant. The net... 

The dipole moments and dielectric constants of aprotic solvents range from near-zero values for the hydrocarbons to moderate values for solvents like... 

The first challenge encountered during the reformulation with HFA propellants was the altered physicochemical properties of HFA 134a and HFA 227 compared to the CFC propellants [124], Table 2 compares the physicochemical properties of the CFC and HFA propellants [124], The increased polarity of HFA 134a and HFA 227 is illustrated by the increased dipole moments and dielectric constant. From a practical point of view, the altered solvency properties of the HFA propellant for the drug, excipient, water, and, surprisingly, components of the MDI have been the... 

Several other treatments of solvent effects on solvolysis rates have been developed. The equations typically include several terms related to (a) macroscopic nonspecific solvent properties, such as the dipole moment and dielectric constant (b) empirical polarity criteria, such as Ej.(30) (c) solvent electrophilicity and nucle-ophilicity parameters and (d) terms related to solvent cohesivily. The last term accounts for the difference in work required to disrupt structure within the solvent, when, for example, there is expansion in volume between reactants and the TS. 

The product of dipole moment and dielectric constant is called the electrostatic factor and it is a means of classifying solvents according to their polarity. 

To complete the discussion of the relation between dipole moment and dielectric constant, we consider a material medium containing N molecules per cubic centimeter, all of the same kind and in the same state a (the extension to mixtures is obvious if more than one state need be considered, it must be weighted with the appropriate Boltzmann factor). The polarization of the medium will be P = ATiR. For an isotropic medium, we may use for the effective electric field the Lorentz field... 

XVII. Electric and Magnetic Phenomena Moments Induced by an Electromagnetic Field, 332. Dipole Moments and Dielectric Constant, 337. The Theory of Optical Rotatory Power, 342. Diamagnetism and Paramagnetism, 347. 

A liquid film on the fiber surface may increase the contact area and promote adhesion. It is well known that an oily film on fibers increases soiling. Kissa [19,23] found that liquids belonging to different classes of chemical compounds (hydrocarbon, fatty acid, alcohol, and ester) increased soiling of a hydrophilic fabric (cotton) and a hydrophobic fabric (polyester) with different types of particulate soil. Attempts to correlate soiling with the refractive index, viscosity, surface tension, dipole moment, and dielectric constant showed that soiling increases with the ratio of the viscosity to the dielectric constant of the liquid. Because hydrocarbons are nonpolar and have a low dielectric constant, a viscous hydrocarbon film is especially prone to capture particulate soil. 

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