The temperature dependence and thermodynamics of partitioning of phenols in the n-octanol-water system

Abstract

The temperature dependency of the n-octanol—water partition coefficient and the hydrophobic substituent constant, π, for a series of substituted phenols has been determined. The thermodynamics of transfer of p-alkylphenols revealed both a favorable enthalpic and entropic contribution whereas xylenols having an ortho substituted methyl group reduced the enthalpic and increased the entropic contributions. Polar substituents in the para position of the ring, such as halogen, nitro or methyl ester, increased the free energy of transfer of phenol through increased enthalpy and in spite of an unfavorable entropy. Para-methoxy or ethoxy groups caused a reversal of the enthalpy of transfer from negative to positive and at the same time increased the entropie contribution. A comparison of the n-octanol—water system results with those obtained using artificial membranes suggests that the bulk organic phase—water system can serve as a good model for distribution only when specific polar group interaction between the compound and the phospholipid bilayer is minimal or absent.