Selecting Solvents Solubility Parameter

A convenient practical measure of the intermolecular forces between polymer chains (hat might be used in selecting solvents for specific polymers is the solubility parameter. Solubility parameter 8 is related to the heat of vaporization, A//vap (cal/cm ), also a measure of the same intermolecular attractive force. The cohesive energy density c of a liquid is the energy needed to overcome intermolecular forces to separate the molecules from each other and is given by the expression 

These contributions are empirically determined, and the corresponding values of Sr obtained should in theory be die same as the Hilderbrand parameter S. Table 2.1 hsts solubihty parameters for conunon solvents and for selected polymers. Although it is an improvement on the single-parameter values of S, the Hansen Sp also fails to accurately and completely describe the solution thermodynamics of a significant number of the polymer-solvent systems. 

TABLE 2.1 Hansen solubility parameters of representative polymers and solvents 

The plots (or their simpler two-dimensional form, which is a plot of /h vs /p, in effect assuming/ to be invariant) have considerable practical utility in solvent selection (Burke 1984). The solubility of a polymer is often achieved using a mixture of solvents. As a general rule, a mixture of solvents will dissolve a polymer if the solubility parameter of that mixture lies close to that of a known good solvent for the polymer. In designing mixed solvent systems the Teas plot allows the solubility characteristics of solvent mixtures to be predicted to some extent. In Fig. 2.3, the solvents carbon tetrachloride (CCI4) and methanol (CH3OH) are clearly nonsolvents for poly(methyl methacrylate) 

High-surface-area nanofibers are sometimes made via selective dissolution of one polymer component from a bicomponent polymer nanofiber mat (Li and Hsieh 2006 You et al. 2(X)6b Zhang, Y. Z., et al. 2006a) (see Chapter 9). Selecting a solvent mixture that dissolves away just the one polymer component 

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The radiius of interaction parameter expresses the degree of mutual solubility. Reasonable compatibility between solvent and resin/polymer is determined if the value of R li < 10. Solubility comparisons can be made by using computer-based spreadsheets. By using the resin solubility data and solvent solubility parameters to determine the degree of resin solubility in the solvent, one can develop a list of likely solvent blends that can be verified by laboratory scale testing. Figure 2 illustrates the idealized spherical solubility envelope for a polymer. The total solubility parameter 5, of a material is a point in the three-dimensional space where the three partial Hansen solubility parameters intersect. If the calculated radius of interaction of the solvent and resin combination is less than the radius of the resin solubility sphere, then the solvent will most likely dissolve the resin and the solvent s solubility point will lie within the solubility sphere of the resin. A poor resin solvent combination will have a solubility point outside of the resin envelope. Application of the Hansen solubility theory will shorten the time and effort needed for solvent selection and substitution. 

In the development of a SE-HPLC method the variables that may be manipulated and optimized are the column (matrix type, particle and pore size, and physical dimension), buffer system (type and ionic strength), pH, and solubility additives (e.g., organic solvents, detergents). Once a column and mobile phase system have been selected the system parameters of protein load (amount of material and volume) and flow rate should also be optimized. A beneficial approach to the development of a SE-HPLC method is to optimize the multiple variables by the use of statistical experimental design. Also, information about the physical and chemical properties such as pH or ionic strength, solubility, and especially conditions that promote aggregation can be applied to the development of a SE-HPLC assay. Typical problems encountered during the development of a SE-HPLC assay are protein insolubility and column stationary phase... 

Solid-Fluid Equilibria The solubility of the solid is very sensitive to pressure and temperature in compressible regions, where the solvent s density and solubility parameter are highly variable. In contrast, plots of the log of the solubility versus density at constant temperature often exhibit fairly simple linear behavior (Fig. 20-19). To understand the role of solute-solvent interactions on sofubilities and selectivities, it is instructive to define an enhancement factor E as the actual solubihty divided by the solubility in an ideal gas, so that E = ysP/Pf, where P is the vapor pressure. The solubilities in CO2 are governed primarily by vapor pressures, a property of the solid... 

A number of models have been proposed to describe the solution formation process [505-509], some of which can be extended to Include chromatographic processes and other solvent-dependent phenomena. In terms of chromatographic aiqplications the most useful are the solubility parameter concept, solvatochromic parameters and Snyder s solvent strength and selectivity... 

Solubility parameters can be a useful guide to solvent selection, but precise quantitative relationships between solvent properties and extraction rates are not yet possible [37]. As an illustrative example we mention extraction of Irganox 1010 from PP [37]. Freeze-ground PP was extracted at 120 °C with 2-propanol,... 

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 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... 

To illustrate the influence of the solubility parameter on the phase separation behavior, a series of solvents has been selected which have either different values... 

The above discussion demonstrates that the solubility parameter concept in combination with the gradient oven is a useful tool to select a convenient solvent, which could undergo a phase separation during the crosslinking reaction. 

The selection of a solvent for a new separation problem, even today, is a matter of trial and error. The application of theory (2) with the additional application of the solubility parameters (6J-65) makes it possible to estimate the composition of appropriate solvent mixtures for the separation of relatively simple compounds. In order to calculate the necessary solvent strength, however, a set of experimental data concerning the behavior of the sample components, the adsorbent, and the elution strength of the eluents with the specific adsorbent are necessary. Others (J5) recommend a graphical method as a time-saving alternative to bi th calculation and the trial-and-error approach to obtain a first approximation of the eluent composition appropriate for the separation of a givin sample. 

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