Solute-cosolvent-solvent systems, solid solubilities

Solute/Cosolvent/Solvent Systems. Solubilities of solids may be modified by adding a small concentration of a nonpolar hydrocarbon (e.g. propane, octane) or a polar molecule (e.g. acetone, methanol). CO2 has a small polarizability and no dipole moment, so additives increase the polarizability of the solvent (i.e. the refractive index of Equation 7) and the dielectric constant (Equation 3). Polar cosolvent molecules also interact with functional groups on the solutes. Cosolvents may increase solubilities up to an order of magnitude although the enhancement is dependent on cosolvent concentration. Unfortunately, relatively few fundamental cosolvent studies are published at this time. 

The objective of this paper is to propose a predictive method for the estimation of the change in the solubility of a solid in a supercritical solvent when another solute (entrainer) or a cosolvent is added to the system. To achieve this goal, the solubility equations were coupled with the Kirkwood-Buff (KB) theory of dilute ternary solutions. In this manner, the solubility of a solid in a supercritical fluid (SCF) in the presence of an entrainer or a cosolvent could be expressed in terms of only binary data. The obtained predictive method was applied to six ternary SCF-solute-cosolute and two SCF-solute-cosolvent systems. In the former case, the agreement with experiment was very good, whereas in the latter, the agreement was only satisfactory, because the data were not for the very dilute systems for which the present approach is valid. 2001 Elsevier Science B.V. All rights reserved. 

Solute/Solute/Solvent Systems. Solute/solute/solvent studies are presented by Kurnik and Reid, ( ), Kwiatkowski, et al., ( ), and Gopal, et al., ( ). Unfortunately, some studies have not proven clearly that solutes were always solids under experimental conditions. In general it is noted, for solid solutes, that the ternary solubility of a less soluble solid is greatly increased by the presence of a significantly more soluble solid ( ). For example, in the naphthalene/phenanthrene/C02 system, naphthalene increases the solubility of phenanthrene and the ternary system selectivity is substantially below the selectivity predicted from the ratio of binary solubilities. Thus, substantially soluble solids act as cosolvents for less soluble compounds. 

Schmitt (1984) verified the entrainer behavior reported by Kurnik and Reid. Schmitt and Reid (1984) show that very small amounts of an entrainer in the SCF-rich phase have very little effect on the solubility of a second component in that phase. This observation is consistent with the work of Kohn and Luks for ternary mixtures at cryogenic temperatures. The data of Kurnik and Reid have been corroborated for the naphthalene-phenanthrene-carbon dioxide system (Gopal et al., 1983). Lemert and Johnston (1989, 1990) also studied the solubility behavior of solids in pure and mixed solvents at conditions close to the upper critical end points. Johnston finds that adding a cosolvent can reduce the temperature and pressure of the UCEP while simultaneously increasing the selectivity of the solid in the SCF-rich phase. In these studies Johnston found the largest effects with a cosolvent capable of hydrogen bonding to the solute. 

The study by Van Alsten and coworkers compared the solubility of the five solids shown in table 5.1 in pure CO2 and in a solvent mixture of 95wt% CO2 and 5 wt% methanol. Only acridine showed any enhancement when methanol is added to CO2. Schmitt and Reid determined the solubility of two different solids, phenanthrene and benzoic acid, in two supercritical solvents, CO2 and ethane, with benzene, cyclohexane, acetone, and methylene chloride as cosolvents. In this case, the solubility of benzoic acid was greatly enhanced in supercritical ethane when acetone was the cosolvent. But no cosolvent effect was observed for all other combinations of solids, supercritical solvents, and cosolvents. Walsh and coworkers argue that the two systems that showed a large increase in solubility can be explained by assuming either the cosolvent hydrogen bonded to the solute or the cosolvent and the solute formed an acid-base complex. 

A third illustration of the use of FST for open systems involves the effects of cosolvents or additives on the solubility of a solute in a solvent. If one follows the solute solubility curve, at a fixed temperature and pressure, then the chemical potential of the solute at saturation remains constant as it is in equilibrium with the solid solute. Hence, the effect of an additive on the molar solute solubility can be expressed in terms of derivatives of this curve taken at constant T, p, and P2. Using these constraints in Equation 1.43 and taking the appropriate derivatives, one immediately finds (Smith and Mazo 2008)... 

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