Solvent Hildebrand parameter

Even though Hildebrand theory should not apply to solvent systems having considerable solvent-solvent or solute-solvent interactions, the solubility of compounds in co-solvent systems have been found to correlate with the Hildebrand parameter and dielectric constant of the solvent mixture. Often the solubility exhibits a maximum when plotting the solubility versus either the mixed solvent Hildebrand parameter or the solvent dielectric constant. When comparing different solvent systems of similar solvents, such as a series of alcohols and water, the maximum solubility occurs at approximately the same dielectric constant or Hildebrand parameter. This does not mean that the solubilities exhibit the same maximum solubility. 

The minimum in fractal dimension is mainly due to the polar contribution of the solubility Hildebrand parameter. This can be proved by the reproducibility of the minimum when the polar solubility parameter of Hansen is used, while no correlation between fractal dimension and the dispersive or hydrogen bond Hansen parameters can be found [69]. Moreover a similar tendency, but oppositely now showing a maximum, can hold for the glass transition temperature of the resulting membrane [67], as shown in Figure 5.9 with a very similar extremal Hildebrand parameter. The maximum glass transition temperature, or minimum fractal dimension, appears at a solvent Hildebrand parameter that very finely estimates that of the polymer. 

Solubility Parameter. CompatibiHty between hydrocarbon resins and other components in an appHcation can be estimated by the Hildebrand solubiHty parameter (2). In order for materials to be mutually soluble, the free energy of mixing must be negative (3). The solubiHty of a hydrocarbon resin with other polymers or components in a system can be approximated by the similarities in the solubiHty parameters of the resin and the other materials. Tme solubiHty parameters are only available for simple compounds and solvents. However, parameters for more complex materials can be approximated by relative solubiHty comparisons with substances of known solubiHty parameter. 

Solvent Solubility parameter (S) (Hildebrands) Hydrogen bonding index (y) Relative evaporation rate (mPa s) Viscosity at 20 C... 

Vandenburg et al. [37,489] have described the use of Hildebrand solubility parameters in a simple and fast solvent selection procedure for PFE of a variety of polymers. Hildebrand parameters for several common solvents and polymers are presented in Tables 3.2 and 3.34, respectively. When the proper solvent mixture for the polymer was determined, PFE resulted in essentially the same recoveries as the traditional extraction methods, but used much less time and solvent. PFE can be used to give very fast extractions and appears to offer the greatest flexibility of solvents and solvent mixtures. The method is ideal for a laboratory which analyses a large number of different polymers. 

The Hildebrand parameter as the unit of solubility of non-electrolytes in organic solvents/ reaction products is high... 

Many solvent properties are related to density and vary with pressure in a SCF. These include the dielectric constant (er), the Hildebrand parameter (S) and n [5], The amount a parameter varies with pressure is different for each substance. So, for example, for scC02, which is very nonpolar, there is very little variation in the dielectric constant with pressure. However, the dielectric constants of both water and fluoroform vary considerably with pressure (Figure 6.3). This variation leads to the concept of tunable solvent parameters. If a property shows a strong pressure dependence, then it is possible to tune the parameter to that required for a particular process simply by altering the pressure [6], This may be useful in selectively extracting natural products or even in varying the chemical potential of reactants and catalysts in a reaction to alter the rate or product distributions of the reaction. 

As a possibility to classify the mixing behaviour of solvents, Hildebrand and Scott [26,27] developed the theory of the solubility parameter (S). It is... 

Because the entropy of formation in Hildebrand theory is ideal, this approach should be restricted to those systems in which there are no structure effects due to solute-solvent and solvent-solvent interactions. The implication of this is that the solute should be non-ionic and not have functional groups which can interact with the solvent. According to Equation (4.8), the maximum solubility occurs when the Hildebrand parameter of the solvent is equal to the Hildebrand parameter of the solute. That is, when plotting the solubility versus the Hildebrand parameter, the solubility exhibits a maximum when the solubility parameter of the solvent is equal to the solubility parameter of the solute. 

The Hildebrand parameter for the solvent in Equation (4.8), 61, needs to be replaced by the value for the mixture determined by multiplying the pure solvent values by their volume fractions as given below for a two-solvent system. 

Lin and Nash (1993) have proposed an equation to estimate the Hildebrand solubility parameter of a solute strictly from its mole fraction solubilities and their respective solvent solubility parameters ... 

The other two parameters are defined similarly the sum of the three parameters is thus normalized to 1. Values for some common solvents are listed in Table 15 (along with the Hildebrand solubility parameters and the Snyder solvent strength parameters). 

Hildebrand parameter A parameter measuring the cohesion of a solvent (energy required to create a cavity in the solvent). 

High Hildebrand parameter as unit of solubility of nonelectrolytes in organic solvents... 

Figure 1.1-3 The density and the solvent power (as expressed by the Hildebrand parameter) of SCCO2 as a function of temperature and pressure [IS, 16].

Several experimental techniques have been used to determine values of the polymer/solvent interaction parameter, see for example Scott and Hildebrand [16] or Orwell [17] who describe these techniques in extensive reviews. 

On the other hand, the quality of the solvent or the solubility of the polymer in a solvent is determined by the solubility parameter ( ) and the Flory-Huggins polymer-solvent interaction parameter (j). Solvating potential of a solvent can be written by using Hildebrand theory [34, 63, 64]. 

Table 12.2 Dielectric constant e and Hildebrand parameter 8 for some useful solvents...

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