Countercurrent chromatography for the measurement of the hydrophobicity of sulfonamide amphoteric compounds

Octanol-water partition coefficients (P-o/w) of 17 sulfonamides (SAs) were determined by countercurrent chromatography (CCC). The measured P-o/w values were in the 0.002-46 range (-2.65 < log P-o/w < 1.7). The lipophility of these compounds depends on the pH showing a maximum for intermediate values. The apparent P-o/w coefficients of SAs were obtained at 5 pH values: 2, 3, 5, 7 and 11, using 0.01 M ammonium phosphate octanol saturated buffers. A theoretical model linking these values with pH for amphoteric compounds was checked and verified. Often the P-o/w coefficients of the molecular forms obtained with the CCC method differ significantly from literature values obtained using softwares and/or from experimental values calculated with extrapolation. The CCC method allows also the determination of the P-o/w coefficients of the ionic forms of the SAs, cationic forms at low pH values and anionic forms at elevated pHs. The acidity constants were also estimated using the theoretical model. Relationships between SA retention factors obtained by RPLC and hydrophobicity were also discussed.