Hydrophobicity, protolytic equilibrium and chromatographic behaviour of some monoazoic dyes

Abstract

Reverse-phase thin layer chromatography (RP-TLC), water:octanol partition studies and spectrophotometric determinations of p K-values are reported for 8 monoazo derivatives of sulfanilic acid (3 of them having aromatic amino groups). The results are compared to previously published p K values for 7 derivatives of aminoazobenzene and R M0-values for 11 azo dye derivatives of 4-aminobenzoic acid (5 of them having amino groups), in an attempt to study RP-TLC and partition results and to assess the relative importance in polar:lipophilic phase-partition processes of neutral, amphionic and ionic forms of these compounds. Calculated C log P and log P Suzuki values are also reported. No correlation was found between R M0 and the apparent water:octanol partition coefficient, log P exp-values for the first 8 azo dyes probably due to the different neutral and amphionic forms in the polar phases (water:methanol mixtures vs. water). Not only neutral, but amphionic and possibly ionic forms are important in assessing chromatographic R M0-values for lipophilicity characterizations. Acceptable correlation between R M0 and log P Suzuki was observed.