Thermodynamics of organic mixtures containing amines.: VII.: Study of systems containing pyridines in terms of the Kirkwood-Buff formalism

Mixtures formed by a pyridine base and alkane, benzene, or I-alkanol have been investigated in the framework of the Kirkwood-Buff theory. In this work, the derivatives of the molar excess Gibbs energies (G(E)), relative to the mole fraction, needed for the determination of the Kirkwood-Buff integrals (G(ij)) and related quantities were calculated using the DISQUAC group contribution model. Systems with alkanes are characterized by interactions between similar molecules (G(12) < 0). Mixtures with pyridine or 4-methylpyridine show G(ii) curves with a maximum at high amine concentrations, which indicates that amine-amine interactions are stronger than in solutions with dimethyl or trimethylpyridines. For solutions with a given alkane, the increase of methyl groups in the aromatic ring of the pyridine leads to a decrease of G(11) in the following sequences: pyridine > 4-methylpyridine and pyridine > 2-methylpyridine > 2,4-dimethylpyridine > 2,4,6-trimethylpyridine. The same variations are observed for the molar excess enthalpies (H-E), and volumes (V-E). Systems with benzene or 1-alkanols show lower vertical bar G(12)vertical bar values than those with alkanes. This has been ascribed to the existence of interactions between dissimilar molecules, which are of dipolar type. The comparison of G(12) for 1-alkanol + pyridine base (or + benzene or + toluene) mixtures clearly reveals that the interactions between dissimilar molecules are much more predominant in amine systems. However, their local mole fractions are scarcely dependent on the intermolecular interactions and the distribution of the molecules in the solution is practically random, even in methanol systems.