Solubility Considerations

It is generally more difficult to find a solvent that will dissolve a polymer than it is to find a solvent for a small molecule such as, for example, the monomer from which the 

The above conclusion is unfortunate for the case of polymeric solutes, because then-entropies of dissolution are unusually small. The repeat units can not become as disordered as can the corresponding monomer molecules since they are constrained to be part of a chain-like structure. Such disordering is particularly difficult if the chain is stiff. Thus, in this situation dissolution is even less likely. Crystalline polymers are also more difficult to dissolve than are their amorphous counterparts since the enthalpy of dissolution also contains a large, positive contribution from the latent heat of fusion. 

The above equation also explains why two different polymers are seldom miscible. Both solute and solvent are now polymeric and thus both suffer from the entropy decreases described above. It also explains why it is necessary to heat a mixture in which the solute does not dissolve at room temperature. This increase in T increases the magnitude of the last term in equation (23), and this is the term that generally makes the free energy change negative. 

As already mentioned, interactions between unlike species are generally unfavorable. This observation is, in fact, the basis for the rule of thumb that like dissolves like. This rule is obviously very qualitative, but it has now been sucessfully extended to provide a quantitative basis for finding potential solvents for a polymer. In the theory of Hildebrand, it is acknowledged that AHiis is almost certainly going to be positive. The goal is then to find a means to make AHiis as small as possible. Specifically, it is given by the equation 

It is relatively easy to determine the solubility parameter of a solvent. The molar volume can be obtained from pycnometry, or a value can possibly be found in the literature. Also, since most solvents of interest have significant volatility, their heats of vaporization can be determined calorimetrically. The experimentally determined heat of vaporization can be converted into the desired energy of vaporization through a conversion term that is simply the change in pressure-volume product for the process. Specifically, AH = AE + A(pV). At constant pressure this is pAV and, to the adequate approximation that the vapor is an ideal gas, the conversion term is thus simply RT (where R is the usual gas constant). 

---

Solubility Considerations An accurate precipitation gravimetric method requires that the precipitate s solubility be minimal. Many total analysis techniques can routinely be performed with an accuracy of better than 0.1%. To obtain this level of accuracy, the isolated precipitate must account for at least 99.9% of the analyte. By extending this requirement to 99.99% we ensure that accuracy is not limited by the precipitate s solubility. 

Finally, it is necessary to observe that the values of activities and fugacities calculated are thermodynamic quantities that cannot always be realised in practice, e.g. very high activities of metal ions cannot be attained because of solubility consideration and very low activities have no physical significance. 

Huang et al. (7) suggested that the brevetoxin binding site lies in the hydro-phobic portion of the channel, and since PbTx-1 and PbTx-7 are also the most hydrophobic of the toxins, their potency may also be in part due to solubility considerations. In general, the more hydrophobic the toxin, the higher is its potency and ability to displace tritiated PbTx-3 from its specific binding site. It is our contention that substituent character on each toxin s K (type-1) or J (type-2) ring... 

K. S. Lin, J. Anschel, and C. J. Swartz, Parenteral formulations IY solubility considerations in developing a parenteral dosage form, Bull. Parenter. Drug Assoc., 25, 40-50 (1971). 

It should be noted that at the excitation wavelength employed, the absorbance of MDI-PUE is 1.1 while the absorbance of the solvent THF is 0.3. This is a condition dictated by polymer solubility considerations and choice of excitation wavelength. We are confident that the spectral results for the photolysis at 248 nm are derived from radicals generated by direct excitation, as opposed to radical abstraction by solvent radicals, since the kinetic curves indicate no delay in radical formation of the transients. 

The doso-clusters B Hn2-, B10H102-, and B6H62 are stable entities. Their alkali salts are very water soluble. Cesium as a counter-ion reduces the water solubility considerably, and ammonium ions (especially quaternary ammonium ions) precipitate the cluster anions quantitatively from aqueous solutions. The resulting tri- and tetraalkylammonium salts are usually soluble in organic solvents. This allows chemistry to be performed under conditions which are standard for organic... 

Aromatic polyimides are synthesized by the reactions of dianhydrides with diamines, for example, the polymerization of pyromellitic anhydride with p-phenylenediamine to form poly(pyromeUitimido-l,4-phenylene) (XLV) [de Abajo, 1988, 1999 Hergenrother, 1987 Johnston et al., 1987 Maier, 2001]. Solubility considerations sometimes result in using the half acid-half ester of the dianhydride instead of the dianhydride. 

A notable feature of carbohydrate crystals is the absence (or small proportion) of water of hydration. As monosaccharides are hydrophilic and water-soluble, considerable deposition of water (association) in a sugar crystal may be expected. The absence of water from the crystal lattice of a sugar must, therefore, be explained by supposing that the sugar is able to form sufficiently strong and numerous hydrogen bonds without the involvement of molecules of water this has been observed, although there are some exceptions. 

The reactivity (or availability for reaction) can be influenced by solvation. To illustrate, if a salt with a large organic cation, for example, tetraethylammonium, rather than a small cation, for example, sodium (see below), can be used, the importance of the solubility consideration becomes smaller (Miller and Parker, 1961). A balance must be struck between the solubility of the reactant salt and the availability of the anion for the reaction. 

According to simple solubility considerations, a precipitate will be formed when the product of the concentrations of anions and cations exceeds the solubility product. From another viewpoint, phase transformation occurs when the free energy of the new phase is lower than that of the initial (metastable) phase. However, there are many examples where the ion product exceeds K p, yet no precipitation occurs—the phenomenon of supersaturation. The solubility product also does not provide information on how the particles of the precipitate form—nucleation. Nucleation involves various physical processes, and both thermodynamic and kinetic aspects must be considered. 

Initial investigations of base-catalyzed imidization of polymeric systems, in particular PMDA/ODA based polyfamic alkyl esters), have been difficult due to the insolubility of the polyimide precursor at imidization levels exceeding 40%. Nevertheless, preliminary studies indicate that the base-catalyzed polymer imidization reaction appears to be significantly slower at ambient temperatures as compared to the phthalamide model compounds. It is yet unclear whether this is a direct result of the conformational aspects associated with the polymer chain or solubility considerations arising from the less soluble, partially imidized polymer chain. Since much of the initial work involved IR studies of supported... 

One of the limitations in using the xanthene dyes as photoinitiators is their relatively low solubility in nonpolar media. Solubility considerations become important in dye-sensitized photopolymerization of multifunctional acrylates in the absence of solvent. In the search for faster initiators for three-... 

A basic premise of solubility considerations is that a solution in contact with a solid can be in an equilibrium state with that solid so that no change occurs in the composition of solid or solution with time. It is possible from thermodynamics to predict what an equilibrium ion activity product should be for a given mineral for a set of specified conditions. As will be shown later in this chapter, however, it is not always possible to obtain a solution of the proper composition to produce the equilibrium conditions if other minerals of greater stability can form from the solution. It shall also be shown that while it is possible to calculate what mineral should form from a solution based on equilibrium thermodynamics, carbonate minerals usually behave in a manner inconsistent with such predictions. 

The inhomogeneous structure of a micelle (or inverse micelle) can influence the course of a photoinduced electron transfer. Such a micelle is biphasic, containing a hydrocarbon-like core and a water-like surface. If the photoinduced electron transfer produces a product which has lower solubility in the aqueous phase (a situation which might obtain if a cationic acceptor is reduced to a neutral product), this product will be directed by solubility considerations to move toward the hydrophobic center of the micelle, i.e., remote from the site of the forward electron transfer. This spatial separation, shown conceptually in Scheme 4, in turn will retard the rate of the back reaction compared with that of the forward reaction. 

This analysis demonstrates that when 9 < 1, dose-solubility considerations should be taken into account in accord with (5.18) for the calculation of MDT the MDT is infinite when 9 > 1. Equation (5.15) can be used to obtain an estimate for MDT only in the special case 9=1. Finally, (5.19) describes the MDTS of the fraction of dose dissolved when 9 > 1. 

There are a number of claims4 40 51,69,150,247,268-274 for the isolation of bimolecular ethers from various other heterocyclic cations although the structures of these products have rarely been unambiguously established. The reaction mechanism outlined for the formation of 114 probably does occur in other heterocyclic systems, particularly in those cases in which alkoxide ion formation from the pseudobase readily occurs. Solubility considerations may dictate the precipitation of the bimolecular ether rather than the pseudobase from basic aqueous solutions containing relatively high concentrations of the heterocycle. However, such bimolecular ether formation will usually be in direct competition with the pseudobase disproportionation reaction (Section V,D) which shows the same pH dependence. 

Jacques CHM, Wyzgoski MG (1979) Prediction of environmental stress cracking of polycarbonate from solubility considerations. J Appl Polym Sci 23(4) 1153—1166... 

The most popular system in which the effects of various salts have been investigated over the years has been that of ethanol-water. Other systems which have been studied are ethylene glycol-water, acetic acid-water, methanol-water, 1- and 2-propanol-water, nitric acid-water, acetone-methanol, 1-octane-propionic acid, phenol-water, and formic acid-water. Aqueous systems have been choices for such studies because of salt solubility considerations. 

Silica with a saturated layer of —OH groups on its sur ce has a surface tension measured [10] by solubility considerations of 46 eig/ cm. Without this layer of —OH groups on its surface it has a svirface energy [7] of 880 erg/cm. Will water with its svirface tension [11] of 72 erg/cm spread on a flat surface of silica ... 

Because of solubility considerations, the measurements were made in 50% (v/v) dioxane-water. 4,5-Dihydroxyacridine is essentially colorless in 50% (v/v) dioxane-water solution at pH 7 but becomes yellow as the pH is increased and the phenol groups ionize. UV-visible spectrophotometry revealed that the ionized compoimd absorbed at 450 nm. Solutions were prepared in volumetric flasks and the pH and A450 were measured, in 1.00-cm cells. A graph of absorbance vs. pH is shown in Figure 22-13. 

In addition to overall solubility, salts can have a significant impact on the dissolution rate of a solute. Hence, solubility considerations are important factors to be assessed when trying to make a rational choice of an appropriate pharmaceutical salt. ... 

---