DETERMINATION OF THE HYDROPHOBICITY PARAMETER R(MW) BY REVERSED-PHASE THIN-LAYER CHROMATOGRAPHY

Experimental conditions were established that permit the determination of chromatographic lipophilicity parameters on the basis of thermodynamically true R(M) values. The superiority of methanol as compared with other modifiers (e.g., acetonitrile) is substantiated; physico-chemical reasons are discussed. Solvent pH influences only the silanophilic effect; hence, with highly ionized structures, determination at neutral pH is to be preferred. Extrapolation to modifier-free conditions (R(Mw) value) is essential to diminish the contribution of polar interactions. Advantages and disadvantages of non-linear extrapolation procedures are compared with linear regression. Correctly estimated R(Mw) values coincide numerically not only with log k(w), values, as theoretically expected, but also with partitioning data from the octanol-water system.