Cohesive energy density, of water

An intruding hydrocarbon must compete with the tendency of water to re-form the original structure and is squeezed out of solution [23]. This hydrophobic effect is attributed to the high cohesive energy density of water because the interactions of water with a nonpolar solute are weaker than the interactions of water with itself [24]. Leo [22] notes that part of the energy cost of creating the cavity in each solvent is paid back when the solvent interacts favorably with parts of the solute surface. ... 

Figure 2. Dependence of diffusion constant, D, solubility constant, S, and permeability constant, P, of oxygen and water on the cohesive energy density of polymer.

The cohesive energy density of the solvent is higher at increased solvent density, which results in better surfactant tails-solvent interactions. Therefore, smaller particle size and narrower size distributions obtained at higher densities could be attributed to weaker intermicellar interactions and dependence of the water-core size on pressure and temperature (by increasing the pressure or temperature, the size of the water core wiU be deaeased). 

Multiparameter correlation analyses of the effect of solvent on rates and equilibria were introduced by Koppel and Palm [37], but these have been utilized extensively by Taft and his students who recognized the contribution of five factors [38]. These are (i) the abihty of the solvent to stabilize an ion or dipole by dielectric effects (n ) (ii) the polarizability of the solvent (iii) the hydrogen-bond donation (a) (iv) the acceptor ability (P) and (v) the cohesive energy density of the solvent (8 ). In the case of the solvolysis reaction of t-butyl chloride, Taft s group found that the logarithm of the rate constant in 21 solvents from water to diethyl ether could be correlated with four factors [39] ... 

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. 

The second important solvent effect on Lewis acid-Lewis base equilibria concerns the interactions with the Lewis base. Since water is also a good electron-pair acceptor129, Lewis-type interactions are competitive. This often seriously hampers the efficiency of Lewis acid catalysis in water. Thirdly, the intermolecular association of a solvent affects the Lewis acid-base equilibrium242. Upon complexation, one or more solvent molecules that were initially coordinated to the Lewis acid or the Lewis base are liberated into the bulk liquid phase, which is an entropically favourable process. This effect is more pronounced in aprotic than in protic solvents which usually have higher cohesive energy densities. The unfavourable entropy changes in protic solvents are somewhat counterbalanced by the formation of new hydrogen bonds in the bulk liquid. 

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. 

The first generation of research involving surfactants in SCFs addressed water/oil (w/o) microemulsions (Fulton and Smith, 1988 Johnston et al., 1989) and polymer latexes (Everett and Stageman, 1978) in ethane and propane (Bartscherer et al., 1995 Fulton, 1999 McFann and Johnston, 1999). This work provided a foundation for studies in C02, which has modestly weaker van der Waals forces (polarizability per volume) than ethane. Consequently, polymers with low cohesive energy densities and thus low surface tensions are the most soluble in C02 for example, fluor-oacrylates (DeSimone et al., 1992), fluorocarbons, fluoroethers (Singley et al., 1997), siloxanes, and to a lesser extent propylene oxide. Since C02 is... 

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