Solubility parameter of mixed solvents

Figure 5. Solvent uptake vs. solubility parameter of mixed solvent (3). Key X, isopropanol/water O, cellosolve/water , diglyme/water.

Attempts were made to relate intrinsic viscosity [T)] to solubility parameters of mixed solvents. 82 of polymer was calculated from the equation [T)] = f(8i). The authors assumed that the maximum value of [11] is when 5j = 8j of polymer. However, studying [t)] for polymethylmethacrylate in fourteen liquids, the authors found a large scatter of experimental points through which they have drawn a curve with a diffusion maximum. Thus the precision of 8j values was affected by 10% scatter in experimental data. 

Finding an appropriate mixed solvent system should not be done on a strictly trial and error basis. It should be examined systematically based on the binary solubility behavior of the solute in solvents of interest. It is important to remember that the mixed solvent system with the solute present must be miscible at the conditions of interest. The observed maximum in the solubility of solutes in mixtures is predicted by Scatchard-Hildebrand theory. Looking at Eq. (1.50) we see that when the solubility parameter of the solvent is the same as that of the subcooled liquid solute, the activity coefficient will be 1. This is the minimum value of the activity coefficient possible employing this relation. When the activity coefficient is equal to 1, the solubility of the solute is at a maximum. This then tells us that by picking two solvents with solubility parameters that are greater than and less than the solubility parameter of the solute, we can prepare a solvent mixture in which the solubility will be a maximum. As an example, let us look at the solute anthracene. Its solubility parameter is 9.9 (cal/cm ). Looking at Table 1.8, which lists solubility parameters for a number of common solvents, we see that ethanol and toluene have solubility parameters that bracket the value of anthracene. If we define a mean solubility parameter by the relation... 

Three-component systems. Crowley et. al. proposed the three-dimensional solubility volumes (Figure 4.1.3). Better known are Hansen s three-dimensional solubility volumes (Figure 4.1.4). In Hansen s approach, the components of solubility parameters for mixed solvents are calculated from Eq. [4.1.56] ... 

Hildebrand and Scatchard proposed a relationship between the internal energy of mixing and the solubility parameters of a solvent and a solute ... 

An alternative approach for choice of a suitable solvent was taken by Hildebrand [7]. He related the solvent contribution to the heat of mixing, AHi, to the solubility parameters of the solvent and polymer, and 82... 

Another type of nonideal SEC behavior, which will not be covered in this chapter, is related to the use of mixed mobile phases (multiple solvents). Because solute-solvent interactions play a critical role in controlling the hydrodynamic volume of a macromolecule, the use of mixed mobile phases may lead to deviations from ideal behavior. Depending on the solubility parameter differences of the solvents and the solubility parameter of the packing, the mobile phase composition within the pores of the packing may be different from that in the interstitial volume. As a result, the hydrodynamic volume of the polymer may change when it enters the packing leading to unexpected elution results. Preferential solvation of the polymer in mixed solvent systems may also lead to deviations from ideal behavior (11). 

Of course, the practical result of gel formation is an inability to mix the reagents (10. 52. 57. 58). However, it has recently been reported that this tendency to form gel can be greatly reduced by use of special solvents with solubility parameters of 7.2 e.g.,... 

Charles M. Hansen (4) was working in the area of paint technology. He was aware oT the Hildebrand/Scott solubility parameter, and explored the use of the solubility parameter in polymer-solvent interactions. He began his research with the consideration of the thermodynamic equation for the energy of mixing... 

While Eqs. (11)-(13) are useful in understanding the general medium effects on reaction rates, their ability to quantitatively predict reaction rates will most likely suffer from inaccuracies similar to those experienced with solubility prediction in mixed solvents. Dielectric constant as a single parameter is not capable of quantitating all interactions among solvent and solute molecules. LePree and Connors reported on the use of a phenomenological approach to predict reaction rates in mixed solvents. The approach is similar to the solubility studies reported by Khossravi and Connors and uses complex equilibria to characterize solvent-solute interactions. 

The static mode uses both organic solvents such as toluene [27], methanol [28] or acetone [29] and solvent mixtures (usually in a 1 1 ratio) including dichloromethane-acetone [20,28], acetone-hexane [30,31], heptane-acetone [31], acetone-isohexane [32] or methanol-water [33], The use of mixed solvents as extractants provides improved extraction in terms of expeditiousness and recovery [20,28,30-35] as a result of the solubility parameter for a binary mixture being roughly proportional volumewise to the parameters of its components [36], Thus, in the extraction of Irganox 1010 from polypropylene, the addition of 20% of cyclohexane to 2-propanol doubles the extraction... 

Volume fraction is frequently used to define the composition of mixed solvent systems, or to express the solubility of one solvent in another. However, since the volumes of solutions exhibit a dependence on temperature, the expression of concentrations in terms of volume fraction requires a simultaneous specification of the temperature. In addition, since volume defects may occur during the mixing of the solvents, and since these will alter the final obtained volume, defining the solubility of a solution in terms of volume fraction can lead to inaccuracies that can be avoided through the use of other concentration parameters. 

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