Thermodynamics of mixtures containing amines. IX. Application of the concentration–concentration structure factor to the study of binary mixtures containing pyridines

Binary mixtures formed by a pyridine base and an alkane, or an aromatic hydrocarbon, or a 1-alkanol have been studied in the framework of the concentration–concentration structure factor, SCC(0), formalism. Deviations between experimental data and those provided by the DISQUAC model are discussed. Systems containing alkanes are characterized by homocoordination. In pyridine + alkane mixtures, SCC(0) decreases with the chain length of the longer alkanes, due to size effects. For a given alkane, SCC(0) also decreases with the number of CH3– groups in the pyridine base. This has been interpreted assuming that the number of amine–amine interactions available to be broken upon mixing also decreases similarly, probably as steric hindrances exerted by the methyl groups of the aromatic amine increase with the number of these groups. Homocoordination is higher in mixtures with 3,5-dimethylpyridine than in those with 2,6-dimethylpyridine. That is, steric effects exerted by methyl groups in positions 3 and 5 are stronger than when they are in positions 2 and 6. Similarly, from the application of the DISQUAC (dispersive–quasichemical) model, it is possible to conclude that homocoordination is higher in systems with 3- or 4-methylpyridine than in those involving 2-methylpyridine.Systems including aromatic hydrocarbons are nearly ideal, which seems to indicate that there is no specific interaction in such solutions. Mixtures with 1-alkanols show heterocoordination. This reveals the existence of interactions between unlike molecules, characteristic of alkanol + amine mixtures. Methanol systems show the lowest SCC(0) values due, partially, to size effects. This explains the observed decrease of homocoordination in such solutions in the order: pyridine > 2-methylpyridine > 2,6-dimethylpyridine. Moreover, as the energies of the OH–N hydrogen bonds are practically independent of the pyridine base considered when mixed with methanol, it suggests that size effects are predominant over steric hindrances to the creation of the OH–N hydrogen bonds, which are expected to increase with the number of methyl groups in the aromatic amine. For a given 1-alkanol (≠methanol), SCC(0) varies in the sequence: pyridine > methyl pyridine ≈ 2,6-dimethylpyridine. For alkyl pyridines, stability seems to be independent of position and number of alkyl groups attached to the aromatic ring of the amine. Mixtures with isomeric 2-alkanols show lower heterocoordination, as the hydroxyl group is more sterically hindered than in 1-alkanols.