Solubility method, Hildebrand

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. 

Limited method development (Hildebrand solubility parameters for solvent choice)... 

The use of the Hildebrand solubility parameter approach to aid solvent selection with a few simple experiments, starting from the liquid solvents used in traditional extraction methods, limits the efforts needed in method development. As for other extraction... 

Once the local parameters have been fitted to a limited set of data then solubilities can be calculated in a representative set of solvents. Plotting the experimental and predicted data against the Hildebrand solubility parameter of the solvent gives a veiy good indication of behaviour with solvent type, figure 19. The application of the SoluCalc method to Cimetidine is briefly presented in Section 6. 

Lin, H., and R. A. Nash. 1993. An experimental method for determining the Hildebrand solubility parameter of organic nonelectrolytesl. Pharm. Sci82 1018-1026. 

The Hildebrand solubility parameters may be calculated using group contribution methods, wherein the overall solubility parameter is the sum of contributions from Van derV feals dispersion forces, Sd dipole-dipole interaction and hydrogen bonding. 

A thermodynamic method, more fitting to this chapter, has been proposed by Nauman et al. They claim a process for the separation of a physically mixed solid polymers by selective dissolution. They rely on the different polymer solubility characteristics. Tables of this property have been reported and are based on regular solution theory and Hildebrand solubility parameters. The core of the Nauman invention is to find suitable solvents to dissolve particular polymers under defined temperature and pressure conditions. A mixture of polymers is first added to one solvent, at a given temperature, in order to dissolve a particular polymer. The remaining polymer mixture is then treated at a higher temperature with the same solvent or with a different solvent. For clarity, two examples are taken from the patent."... 

A number of methods based on regular solution theory also are available. Only pure-component parameters are needed to make estimates, so they may be applied when UNIFAC group-interaction parameters are not available. The Hansen solubility parameter model divides the Hildebrand solubility parameter into three parts to obtain parameters 8d, 5p, and 5 accounting for nonpolar (dispersion), polar, and hydrogenbonding effects [Hansen,/. Paint Technot, 39, pp. 104-117 (1967)]) An activity coefficient may be estimated by using an equation of the form... 

Hildebrand solubility parameter 101 Hittorff method 121 Homer-Wadsworth-Emmons reaction 189... 

The works of Giddings and coworkers in the late 1960s (1968, 1969) are, perhaps, the most well-known and most referenced papers on the extension of Hildebrand solubility parameters to supercritical fluids. We excerpt from Giddings et al. and we italicize parts of their statements and phrases to accent their feelings and intentions. We concentrate on the applicability of their methods for calculating solubility parameters of dense gases their telling statements are often not heeded. 

While the Hildebrand solubility parameter is justified on theoretical concepts, the Kauri butanol test is an empirical method of testing based on the solubility of a natural gum in hydrocarbon solvents. It is a good guide to the solubility of resins in paraffinic, naphthenic and aromatic hydrocarbons and some chlorohydrocarbons. Many of the hydrocarbons used in industry are mixtures, such as special boiling point spirits and white spirit, and it is easier to measure their solvent performance rather than to try to calculate it (Table 13.5). 

Simpler methods are also used. In the paint industry, Kauri butanol values are determined by establishing the tolerance of a standard solution of Kauri resin in n-butanol to the addition of diluents. This method is applicable to hydrocarbons (both aromatic and aliphatic) and CFCs. Figure 2.3.20 shows that there is a good correlation between the Kauri butanol number and the Hildebrand solubility parameter. The Kauri butanol number can be as high as 1000 (amyl ester of lactic acid) or 500 (Freon solvent M-162). 

Miscibility between the individual polymers is the most important factor to determine the performance characteristics of a polymer blend. Mutual solubility of the phases, the thickness and properties of the interphase formed during blending and the structure of the blend are mainly dependent on the miscibility of individual polymers within a polymer. As a result, a quantitative estimation of interactions is very much important for the prediction of blend properties. Comparison of solubility parameters of individual polymers is an effective method to predict the extent of miscibility within a blend. According to the Hildebrand solubility theory, a large difference in solubility parameters (6p) of individual matrices results in immiscibility between them in the absence of any interfacial compatibil-izer [222]. Jandas et al. have reported that PLA and PHB have Hildebrand solubility parameters (6p) of 23.5 J /cm and 19.8 J Vcm which can turn out to be partially miscibile blends in between them [35]. In case of partially miscible blends, the miscibility can be controlled by compatibili-zation using proper interactables. 

As stated before, the Hildebrand solubility parameter concept was developed for nonpolar, low molecular weight liquids at room temperature. For polar molecules, the method did not provide consistent information. To avoid trouble, initially all liquids were divided into three categories for poorly, moderately, and strongly interacting systems. Another route was taken by Hansen (1967) who postulated that all intermolecular forces ... 

Another useful tool is the Hildebrand solubility theory, which is applicable to apolar and moderately polar systems. For strongly polar systems, it is unable to correctly qualify the compatibility between components. However, the massive amount of interaction parameters data obtained in recent decades, and mainly Small s method, allowing to assess them, make this method quite efficient and readily applicable. The Hildebrand solubility parameter, 5, can be defined as the square root of the cohesive energy density (CED) and it is measured in (MJ m )° . This parameter indicates the polarity level of the component and goes from 12 (MJ m )° for nonpolar components to 23 (MJ m )° for water. The larger the difference... 

Calculation of solubility parameters (5) for a drug and a polymer have been used as a method for predicting miscibility in amorphous solid dispersions (Hancock et al. 1997), using, for example, the Hildebrand solubility parameter (Greenhalgh et al. 1999), which was calculated from the cohesive energy density (CED) by ... 

Any of the common laboratory solvents can be used in the extractor, but the final choice will depend on the analytes that are to be extracted. Fritzpatrick and Dean described a method for selecting the optimum solvent for the extraction of DDT (and metabolites) and pentachlorophenol (PCP) from soil when using ASE they broke the Hildebrand solubility parameter down into three components, to optimally select methylene chloride for DDT and a mixmre of methylene chloride and acetonitrile for PCP (61). 

Values of fAB and B can be estimated using a semiem-pirical approach, by a simple method based on Hildebrand solubility parameters [11] ... 

The Hildebrand solubility parameters of 18 RTILs were calculated from the inverse gas chromatography derived infinite dilution activity coefficients by Marciniak [194] who later added values for eight additional RTILs [195], and compared the values with those obtained by other methods. Gas chromatography... 

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