Enhanced partition model of 4-nitrophenol in water – octanol system. Effects of association/dissociation processes

Abstract

An enhanced partition model is proposed for the distribution of 4-nitrophenol between polar quasi-immiscible solvents (water and 1-octanol). Monitoring both phases over an extend wavelengths range (200–450 nm), the presence of 4-nitrophenol and 4-nitrophenolate species in the aqueous and organic layers was emphasized. A genetic algorithm has been used to minimize the sum of squared residuals model-experiment using the non-linear equations system model resulted from mass and charge balances. Thus, improved values for the thermodynamic constants associated to partition, dissociation and dimerization equilibria occurring in both phases were found: 1-octanol dissociation constant in the aqueous phase, KOw = 3.2 × 10−13 mol∙L−1, 4-nitrophenol dissociation constant in aqueous phase, KΦw = 1.45 × 10−8 mol∙L−1, water dissociation constant in the organic phase, Kwo = 3.5 × 10−16 mol∙L−1, 1-octanol dissociation constant in the organic phase, KOo = 1.1 × 10−16 mol∙L−1, 4-nitrophenol dissociation constant in organic phase, KΦo = 2.9 × 10−10mol∙L−1, 1-octanol dimerization constant in the organic phase, KDo = 3 × 10−9 L∙mol−1 and 4-nitrophenol dimerization constant in organic phase, KDΦ = 1.4 × 10−8 L∙mol−1. Although the logarithms of calculated partition coefficients are rather similar to the values reported in the literature, in the 1.86–2.07 range, the collected experimental evidence demonstrates that the process had been oversimplified and the polar characteristics of the organic solvent had been neglected in the former studies. The enhanced partition model emphasized the nonlinear dependency of the partition coefficient upon the analyte concentrations.