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Evaluation of alternative approaches for measuring n-octanol/water partition coefficients for methodologically challenging chemicals (MCCs)
Burkhard, L., D. Hoff, T. Lahren, Dave Mount, AND A. Squillace. Evaluation of alternative approaches for measuring n-octanol/water partition coefficients for methodologically challenging chemicals (MCCs). SETAC North America, Salt Lake City, UT, November 01 - 05, 2015.
Measurements of n-octanol/water partition coefficients (KOW) for highly hydrophobic chemicals, i.e., greater than 108, are extremely difficult and are rarely made, in part because the vanishingly small concentrations in the water phase require extraordinary analytical sensitivity and/or equilibration of very large volumes of water. We are evaluating the efficacy of four alternative approaches to estimate KOW designed to circumvent these problems. One alternative involves measuring n-alcohol/water partition coefficients for a series of shorter-chain, more water-soluble n-alcohols (n-butanol, n-pentanol, n-hexanol, n-heptanol), then extrapolating to the n-octanol/water partition coefficient The increased water solubility of these alcohols is predicted to lower the partition coefficient and increase the amount of chemical in the water phase at equilibrium, thereby reducing the sensitivity/sample size needed for analysis of the aqueous phase. The second approach involves developing a correlative relationship between Kbutanol/water and KOW over chemicals with a range of KOW, then extrapolating KOW from Kbutanol/water for chemicals with very high KOW. A third approach is to develop a correlative relationship between KOW and aqueous solubility, wherein aqueous solubilities are measured using a scaled up generator-column technique to generate large volume samples. The fourth approach scales up the generator-column KOW measurement technique to much larger volumes, i.e., bigger columns and high flow rates, and thus, enabling direct measurement of the chemical’s KOW. The likely efficacy of these alternatives based on initial experimentation is discussed.