Thermodynamics of Partitioning System with Dimerization Taking Place in Both Phases

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Abstract

The partition coefficients of benzoic acid in partially miscible two phase mixtures of (methanol + cyclohexane) were measured at the temperature of 293.15, 303.15, and 313.15 K. The experimental results show that the benzoic acid is more soluble in methanol than in cyclohexane and thus it makes the partition coefficients, defined as the ratio of the total molar concentration of benzoic acid in methanol phase to cyclohexane phase, be greater than unity. In addition, the partition coefficient at each temperature is not a constant but increases with the increase of benzoic acid concentration in each phase even at very low concentration. This seeming deviation to the Nernst law can be explained by chemical theory of strong dimerization reactions taking place in both phases. Accordingly, a new generalized equilibrium model was proposed to describe the partition system with solute self-dimerization in both phases. The new model predictions agree well with the experimental data and the equilibrium constants for all the proposed association and partition reactions are obtained simultaneously by fitting the experimental data with nonlinear least-squares method. The results show that there is much stronger dimerization reaction for benzoic acid in methanol phase than in cycolhexane phase, which results in that the dimerization in cycolhexane phases can be totally neglected. In the methanol phase alone, the calculation results further show that dimerization of benzoic acid is so strong that almost all the solute molecules form dimers within the concentration investigated. The standard thermodynamics functions of Gibbs energy, enthalpy and entropy changes for the proposed reactions were estimated with the obtained parameters at different temperature.

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