Predictions of Micelle−Water Partition Coefficients and Retention in Micellar Electrokinetic Chromatography from Solute Structure. 2. Fragmental Constant Approach

The fragmental constant approach (FCA) was used to calculate water−sodium dodecyl sulfate (SDS) micelle partition coefficients, K_mw, for uncharged solutes from their structure. Subsequently, the availability of K_mw values allows prediction of retention factor, k, in micellar electrokinetic chromatography (MEKC) using the simple relationship k = K_mwφ, where φ is the phase ratio. The FCA model describes a micelle−water partition coefficient as the sum of the partition coefficients of the constituent atomic/molecular fragments, measured by fragmental constant values, f _i, as well as correction factors to account for various “intramolecular effects” that cause deviations from the predicted partition coefficients as, log K_mw = a_if _i + k_iC_m. The fragmental constants for a set of 41 fragments were determined using a training set of 229 aromatic solutes and 198 aliphatic compounds. The K_mw of the aromatic compounds in the training set were determined by MEKC, while the K_mw of the aliphatic solutes were estimated using the linear solvation energy relationship (LSER) for the SDS micelles. The fragments consisted of both aromatic fragments (i.e., directly attached to an aromatic ring) and aliphatic fragments. The FCA predictions agree nicely with the observed and LSER partition coefficient values, even for complex molecular structures such as β-blocker drugs. The results show the great potential of the FCA for a priori prediction of retention behavior in MEKC from solute structure.