Density, polar cohesive

A nonpolar neutral species in a polar medium such as water experiences interfacial tension. Solvophobic theory is a general statement of hydrophobic theory, which has been developed to explain the tendency of neutral organic species to flee the water phase. It has been reported that the solution of nonelectrolytes in water is attended by a net decrease in entropy [65,158]. This has been attributed to an increased structuring of water molecules in the vicinity of the solute. The process may be conceptually rationalized by considering that a solute must occupy space in a cohesive medium. The solute must create a cavity in the water milieu and then occupy that cavity [19,65,158]. The very high cohesive density of water creates considerable interfacial tension in the... 

Rapid predictive and experimental techniques for screening extractive distillation solvents are reviewed. In preparing a list of potential solvents the method of Scheibel is recommended for non-hydrocarbon systems for hydrocarbon systems solvents of high polar cohesive density should be considered. For screening the potential solvents the method of Pierotti, Deal, and Derr is recommended. If it is not applicable, the method of Helpinstill and Van Winkle should be considered next. Finally, reliable screening is accomplished through a simple, rapid technique recently developed that uses gas-liquid chromatography. 

Figure 2. Variation of selectivity with solvent s polar cohesive density system hexane (l -benzene (2) at 25°C (12)...

The polarity of the polymer is important only ia mixtures having specific polar aprotic solvents. Many solvents of this general class solvate PVDC strongly enough to depress the melting temperature by more than 100°C. SolubiUty is normally correlated with cohesive energy densities or solubiUty parameters. For PVDC, a value of 20 0.6 (J/cm (10 0.3 (cal/cm ) has been estimated from solubiUty studies ia nonpolar solvents. The value... 

A more polar comonomer, eg, an AN comonomer, increases the water-vapor transmission more than VC when other factors are constant. For the same reason, AN copolymers are more resistant to penetrants of low cohesive energy density. AH VDC copolymers, however, are very impermeable to ahphatic hydrocarbons. Comonomers that lower T and increase the free volume in the amorphous phase increase permeability more than the polar comonomers higher acrylates are an example. Plasticizers increase permeabiUty for similar reasons. 

High degree of chain-to-chain attraction as a result of polarity (high cohesive energy density) results in chain stiffness and immobility... 

The flexibility and cohesive energy density or polarity of each group arc nearly independent of the other groups in the molecule to which they are attached (55 60).because of this, each group can be assigned an apparent Tg value, and the Tg value of a polymer becomes Che sum of the contri-... 

Chiou et al. (1998) attributed the enhanced partitioning of PAHs with respect to other HOCs to relatively high compatibility between the cohesive energy densities of PAHs and the aromatic components in SOM. However, the difference in Koc values between soils and sediments is related to the difference in polar group, rather than aromatic carbon, contents (Kile et al. 1999). The authors concluded that the content of polar groups (O-aryl and carboxyl C) has a large negative influence on Koc values, and hence on HOC sorption in soil and sediment. 

The properties of organic liquids relevant to their use as solvating agents have also been reviewed [76]. The ability of liquids to solvate a solute species depends mainly on their polarity and polarizability properties, ability to hydrogen bond, and cohesive electron density. These molecular properties are best measured by the Kamlet-Taft solvatochromic parameters, and the square of Hildebrand s solubility parameter. 

All of these models predict that the hydrophobic effect provides a significant driving force for the exclusion of even highly polar, charged peptides from an aqueous environment to the nonpolar environment of the RPC sorbent. According to the solvophobic model, in order to place a peptide into a mobile phase, a cavity of the same molecular dimensions must first be created. The energy required to create this cavity is related to the cohesive energy density or the surface tension of the mobile phase. Conceptually, each solvent-accessible unit... 

In the present context, the solvation of a solvent molecule in its own liquid (i.e., condensation from the vapor, the opposite of evaporation) is of interest (Ben-Naim and Marcus, 1984). The solvation properties of solvents (solvent effects) depend mainly on their polarity/polarizability (accounting also for dispersion interactions), hydrogen-bond donation and acceptance abilities, and cohesive energy density (Marcus, 1993). 

Whereas the polarity effect is ascribed to the dielectric constant, the hydrophobic effect is a consequence of the high cohesive energy density (c.e.d.) of water, resulting from a unique hydrogen-bonding network (Lubineau et al., 1994). Given table 6.5, which compares the cohesive energy density and the dielectric constant of selection of common solvents at 25°C, there is no correlation between the structuralization and the polarity of the solvents. 

A better estimate of all attractive forces surrounding a molecule was found in the use of the solubility parameter [32,33], Hancock et al. [34] has reviewed the use of solubility parameters in pharmaceutical dosage form design. The solubility parameter is used as a measure ofthe internal pressures ofthe solvent and solute in nonideal solutions. Cosolvents that are more polar have larger solubility parameters. The square root ofthe cohesive energy density, that is, the square root of the energy of vaporization per unit volume of substance, is known as the solubility parameter and was developed from Hildebrand s Regular Solution Theory in the Scatchard-Hildebrand... 

Since the unrelaxed bulk modulus, Ku e.g., determined by ultrasonic propagation velocity measurements, is a good measure of the cohesive energy density, CED (Ku 11 CED Chapter 10), and CED gives a good indication of the overall material s polarity, one can expect a correlation between Ku and W. This is shown in Fig. 14.3 for the amine-epoxy and styrene-vinyl ester networks. The following relationship is found ... 

The cohesion in crystals of electrically neutral atoms, typified in Figure 5.10 by Ar, is ascribed to van der Waals interaction, generated by zero-point fluctuation of the electron density and its polarization effect on neighbouring atoms. The energy of interaction is given by the London formula, as a function of zero-point vibration frequency, between atoms at a distance d apart, as ... 

Clearly, an objective and quantitative measure for the polarity of compounds of chromatographic interest is needed. Such a quantitative measure may be found in the solubility parameter introduced by Hildebrand [206]. The solubility parameter is defined as the square root of the cohesive energy density (c) ... 

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