Portland State University. Department of Physics
Term of Graduation
Date of Publication
Master of Science (M.S.) in Physics
1 online resource (ii, viii, 82 pages)
Life supporting biological membranes are barriers to toxic chemicals. One of the factors determining the toxicity of chemical compounds is their distribution between membranes and their an environment. An octanol-water system is frequently used as a model for biological membranes to estimate the toxic potency of environmental pollutants. The distribution of a chemical between the octanol and the water phase is described by the octanol-water partition coefficient KOW. This study is concerned with the relationship between KOW and the molecular structure of the toxic chemical.
In the study the following trihalophenols were included: 2,4,6-trifluorophenol (TriFP), 2,4,6-trichlorophenol (TriCP); 2,4,6-tribromophenol (TriBP) and 2,4,6-triiodophenol (TrilP). The distribution of halophenols between octanol and water was measured as a function of the pH. Experimental results were analyzed in terms of a two compartment distribution model which accounts for the pH dependent dissociation of the trihalophenol. We showed that, with the exception of TriIP, the pH dependence of the distribution coefficient of 2,4,6 trihalophenols can be understood with this model.
From the fit of the distribution model to the experimental results, the following log(KOW) of the neutral molecules were determined: 1.96 (TriFP), 3.65 (TriCP), 4.11 (TriBP) and approximately 4.42 (TrilP). For the ionized species the log(KOW) are 1.38 (TriFP), 0.15 (TriCP), 0.08 (TriBP) and 1.16 (TriIP).
In relation with these distribution studies, the following values for the dissociation constants pKa were obtained: 7.12 (TriFP), 6.15 (TriCP), 5.9 (TriBP) and 6.6 (TriIP). We also found that octanol dissolved in the water phase does not significantly affect the dissociation of TriIP.
The relationship between KOW and trihalophenol molecular structure was studied in two models. Linear relationships were found between log(KOW) and the surface area as well as between log(KOW) and molar volume for both the neutral and the ionized halophenols. It was not possible to discriminate between the two models.
Distribution coefficients of the ionized and unionized 2,4,6-trihalophenols are proportional to the each other. The proportionality factor, (3.54±1.49)x10-4, is a measure of the effect of electric charge on the transfer across the water-octanol interface.
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Hensler, Sven, "Distribution Coefficients of Ionized and Un-ionized Halogenated Phenols in an Octanol-water System and their Relationship to Molecular Structure" (1996). Dissertations and Theses. Paper 5169.