Chromatographic quantification of hydrophobicity using micellar mobile phases

In micellar liquid chromatography (MLC), the hydrophobicity of a compound is the predominant effect on its retention and interaction with micelles; however, some contradictory results have been obtained concerning whether k or log k best correlates with the logarithm of partition coefficients (log P) in the biphasic solvent system octanol-water. An empirical model which describes the relationship between retention in MLC and log P is presented. The retention data for series of neutral compounds eluted with different pure and mixed mobile phases and alkyl-bonded stationary phases were used to test the model. The results indicate that non-linear relationships between k or log k and log P are to be expected and only in particular circumstances can linear relationships be obtained. In contrast, in all the series studied, excellent correlations between the logarithm of the retention factor at zero micellar concentration, k(m), and log P, were found (r(2) in the range 0.965-0.987 and F in the range 277-608). Log k(m) is proposed as the best chromatographic index for the quantification of the hydrophobicity of solutes using micellar mobile phases.