Thermodynamic study of the partitioning of methyl and propyl parabens in some organic solvent/buffer systems

Abstract

The thermodynamic quantities of partitioning of methyl paraben (MP) and propyl paraben (PP) were studied at five temperatures in several organic solvent/buffer systems, namely, 1-octanol (ROH/W), isopropyl myristate (IPM/W), chloroform (CLF/W) and cyclohexane (CH/W). In all cases, the values of the mole fraction partition coefficient (Ko/wx) were greater than unity; therefore, the standard Gibbs free energies of transfer are negative indicating a high affinity of MP and PP for all the organic media evaluated. The Ko/wx values were approximately 470-fold and 1700-fold higher in the ROH/W system with respect to the CH/W for MP and PP, thus indicating a high degree of hydrogen bonding contribution to partitioning. Otherwise, in the case of the IPM/W system, the Ko/wx values were in the orders of 0.48 or 0.30 of those observed in ROH/W, whereas, in the case of CLF/W system, the Ko/wx values were in the orders of 0.03 to 0.04 of those observed in ROH/W. On the other hand, enthalpies and entropies of transfer of PP from water to organic solvents were all positive but in the case of MP the quantities were variable, negative or positive as well. These results could indicate some degree of participation of the hydrophobic hydration on the MP and PP partitioning processes.