Enhanced partition model of 4-nitrophenol in water e octanol system. Effects of association/dissociation processes Adriana Monica Radu, Ana Maria Josceanu * , Daniel Dinculescu, Vasile Lavric University Politehnica of Bucharest, RO-011061, Polizu 1-7, Bucharest, Romania article info Article history: Received 9 June 2016 Received in revised form 1 August 2016 Accepted 11 August 2016 Available online 13 August 2016 Keywords: 4-Nitrophenol partition Thermodynamic model Complex interactions Polar solvents Associations Dissociations 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 (200e450 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 bal- ances. 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, K Ow ¼ 3.2  10 13 mol∙L 1 , 4-nitrophenol dissociation constant in aqueous phase, K Fw ¼ 1.45  10 8 mol∙L 1 , water dissociation constant in the organic phase, K wo ¼ 3.5  10 16 mol∙L 1 , 1-octanol dissociation constant in the organic phase, K Oo ¼ 1.1  10 16 mol∙L 1 , 4-nitrophenol dissoci- ation constant in organic phase, K Fo ¼ 2.9  10 10 mol∙L 1 , 1-octanol dimerization constant in the organic phase, K Do ¼ 3  10 9 L∙mol 1 and 4-nitrophenol dimerization constant in organic phase, K DF ¼ 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 1.86e2.07 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. © 2016 Elsevier B.V. All rights reserved. 1. Introduction As early as 1872, Berthelot and Jungfleisch [1] had addressed systematically the distribution of a substrate between two immis- cible liquids and pointed out the fact that the ratio of the substrate concentrations did not depend on the relative volumes of the so- lutions employed, being a constant, and referred to since as the partition coefficient, P . It was Nernst who in 1891 made the second significant contribution in the field, stressing that the partition coefficient is truly constant only if a single molecular species were considered partitioned between the two phases [2]. This approach led to treating partitioning as an equilibrium process, in which the tendency of a solute species to leave a solvent and move into another would be ‘a measure of its activity in that solvent and would be related to other commonly measured activity functions such as partial pressure, osmotic pressure, and chemical potential’ [3]. Thus the 1-octanol - water partition coefficients started to be seen as embedding quantitative information concerning the lipo- philic/hydrophilic nature of the partitioned substances, being the most popular descriptor for quantitative structure-activity rela- tionship studies [4e6]. Apart from biochemical and drugs manufacturing industries, correct evaluation of partition co- efficients is of uttermost importance in fundamental chemistry (for investigation of complex equilibria formation), chemical engi- neering (in connection to liquid-liquid separation, purification and carrier extraction), and food industry (for purification and extrac- tion of caffeine, for example) [7]. Over the years, log P has been measured experimentally by many methods and techniques, like the shake-flask test [8], generator column (adapting a common HPLC column) [9], NMR [10], capillary electrophoresis [7,11] and counter-current chroma- tography [12]. * Corresponding author. University Politehnica of Bucharest, 1-7 Gheorghe Polizu Street, Sector 1, RO-011061, Bucharest, Romania. E-mail address: a_josceanu@chim.upb.ro (A.M. Josceanu). Contents lists available at ScienceDirect Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid http://dx.doi.org/10.1016/j.fluid.2016.08.021 0378-3812/© 2016 Elsevier B.V. All rights reserved. Fluid Phase Equilibria 427 (2016) 575e582