Improved Prediction of Octanol -Water Partition Coefficients from Liquid-Solute Water Solubilities and Molar Volumes CARY T. CHIOU,* ,† DAVID W. SCHMEDDING, ‡ AND MILTON MANES § U.S. Geological Survey, Box 25046, Mail Stop 408, Federal Center, Denver, Colorado 80225, Department of Environmental and Molecular Toxicology, Oregon State University, Corvallis, Oregon 97331, and Department of Chemistry, Kent State University, Kent, Ohio 44242 A volume-fraction-based solvent-water partition model for dilute solutes, in which the partition coefficient shows a dependence on solute molar volume ( V), is adapted to predict the octanol-water partition coefficient (K ow ) from the liquid or supercooled-liquid solute water solubility (S w ), or vice versa. The established correlation is tested for a wide range of industrial compounds and pesticides (e.g., halogenated aliphatic hydrocarbons, alkylbenzenes, halogenated benzenes, ethers, esters, PAHs, PCBs, organochlorines, organophosphates, carbamates, and amides- ureas-triazines), which comprise a total of 215 test compounds spanning about 10 orders of magnitude in S w and 8.5 orders of magnitude in K ow . Except for phenols and alcohols, which require special considerations of the K ow data, the correlation predicts the K ow within 0.1 log units for most compounds, much independent of the compound type or the magnitude in K ow . With reliable S w and V data for compounds of interest, the correlation provides an effective means for either predicting the unavailable log K ow values or verifying the reliability of the reported log K ow data. Introduction Octanol-water partition coefficient (Kow) and water solubility are two fundamental descriptors used to assess the transport and fate of organic compounds in environmental systems (1, 2). Kow serves not only as a general indicator for a compound’s tendency to partition into an organic phase, this coefficient is practically the same as the compound’s lipid (triolein)-water partition coefficient (Ktw), the latter accounting directly for the fish bioconcentration factor on a lipid-weight basis (3). Despite the usefulness of Kow in environmental research, accurate Kow values remain un- available for numerous compounds. On the other hand, for many compounds with available Kow data, the reported Kow values for a given compound are often vastly inconsistent. Indeed, it is not uncommon to see the discrepancy in reported Kow for a compound from different laboratories or by different analytical methods exceed 1-2 orders of magnitude. Such a wide spread in Kow poses an immense burden on the user to select reliable Kow values for the transport and fate assessment. Water solubility (Sw) of a partially soluble liquid or supercooled liquid may be thought of as a special form of the solute partition coefficient, namely that between its own excess liquid phase and water (4); the supercooled-liquid Sw of a solid may be obtained via a thermodynamic conversion method from the solid Sw, to be addressed later. Therefore, the liquid Sw is also a good general indicator of a compound’s partition tendency with an organic medium. As noted, the magnitude of Kow depends largely on liquid Sw (5-7). Compared to Kow, the Sw data for conventional chemicals are usually more available and accurate because of traditional interests in their solution properties. Nonetheless, for spar- ingly soluble compounds, such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), chlori- nated dioxins, phthalates, and many pesticides, accurate Sw data remain sparse. Similar to the case with Kow, available Sw data for given low-solubility compounds spread widely, subject to further resolution. Generally, accurate Kow and Sw values are obtained with careful steps to minimize measurement errors, such as those caused by the impurities of test compounds and solvents, the equilibration and separation methods, and the equipment sensitivity for detecting target compounds (4). Determination of accurate Kow and Sw for highly insoluble compounds becomes challenging because of the limitation in detecting trace solutes in water and the stringent effects of emulsions and phase separation on solute concentrations. Currently, the most consistent, and seemingly most reliable, analytical methods for Kow and Sw are the shake-flask (usually coupled with centrifugation) (4, 8) and the generator-column equili- bration techniques (9, 10). Indirect experimental methods, e.g., those by HPLC retention times (11, 12), and empirical computation models, e.g., those by fragment constants (13, 14), have been developed for predicting Kow and/or Sw but their uses are usually confined to relatively simple molecules or those within a homologous series (15). Correlations between log Kow and log Sw have been reported (5-7). Such correlations, however, usually do not cover a wide diversity of compounds and a wide range of Kow and Sw with the same accuracy for resolving the inconsistency of multiple Kow and Sw values of given compounds. Presently, accurate estimation of log Kow, with a standard deviation (SD) ) 0.12, could be achieved via polyparameter linear- solvation-energy relationships (pp-LSERs) (16, 17); the needed parameter values, however, are not available for many environmental compounds, especially the pesticides (17, 18). We here present a significantly improved log Kow-log Sw correlation that takes into account the solute-size effect on the partition coefficient. This new correlation is tested for a wide variety of industrial chemicals and pesticides that span some 8.5 orders of magnitude in Kow and 10 orders of magnitude in Sw. As illustrated later, it enables one to predict log Kow from reliable log Sw, or vice versa, with exceptional accuracies and thus may be used as a rapid tool to identify credible Kow and/or Sw data. Theory The partition coefficients for (dilute) solutes of limited water solubility in a two-phase solvent-water mixture are governed * Corresponding author phone: (303) 236-3967; fax: (303) 236- 3934; e-mail: ctchiou@usgs.gov. † U.S. Geological Survey. ‡ Oregon State University. Current Address: 29479 Beaver Creek Road, Corvallis, Oregon 97333. § Kent State University. Current Address: Amberson Towers #412, 5 Bayard Road, Pittsburgh, Pennsylvania 15213. Environ. Sci. Technol. 2005, 39, 8840-8846 8840 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 22, 2005 10.1021/es050729d CCC: $30.25  2005 American Chemical Society Published on Web 10/08/2005