Cosolvency of Partially Miscible Organic Solvents on the Solubility of Hydrophobic Organic Chemicals

The cosolvency of completely miscible organic solvents (CMOSs) and partially miscible organic solvents (PMOSs) on the solubility of hydrophobic organic chemicals (HOCs) was examined, with an emphasis on PMOSs. Measured solubilities were compared with predictions from the log-linear model and the UNIFAC model. In ternary mixed solvents, nonpolar PMOSs (e.g., toluene and trichloroethylene) did not appreciably increase HOC solubility. However, polar PMOSs (e.g., o-cresol and nitrobenzene) did significantly enhance HOC solubility. Polar PMOSs have greater cosolvent effect, not because they are stronger solvents, but because they are present in greater concentrations as a result of their higher aqueous solubilities (ca. 10⁴ mg/L or 1% volume fraction). The cosolvency of CMOSs increases with decreasing solvent polarity, whereas the opposite is true for PMOSs. Preliminary evidence is presented to suggest that the cosolvent effect of polar PMOSs is greater than that of less polar PMOSs; this is attributed, in part, to specific water-PMOS interactions arising from the presence of polar moieties (e.g., NO₂ and OH).