Free energy of mixing of pyridine and its methyl-substituted derivatives with water, as seen from computer simulations.
The free energies of mixing of pyridine, 2-methylpyridine, and 2,6-dimethylpyridine with water have been calculated at 298 K for systems containing 1, 5, and 50 mol % pyridine or its derivative by the method of thermodynamic integration. The obtained results show that the trend of the free energy of mixing of these compounds with water, observed previously at infinite dilution, clearly vanishes at 1 mol % and reverses at higher pyridine concentrations. Thus, in the 5 and 50 mol % systems, the free energy of mixing increases with increasing number of methyl groups. A similar trend is observed for the entropic component of the free energy of mixing at any composition, indicating that, above 1 mol % pyridine content, the entropic term becomes dominant and determines the order of the free energies of mixing of these compounds with water. The fact that this order is in clear accordance with the experimentally observed order of the ease of miscibility of these compounds with water at concentrations above 1 mol % suggests that the entropy of mixing plays a decisive role in the mixing behavior of methyl-substituted pyridines with water.
Laboratory of Interfaces and Nanosize Systems, Institute of Chemistry, Eotvos Lorand University, Pazmany P. Stny 1/A, H-1117 Budapest, Hungary.
Mária
M
Darvas
Pál
P
Jedlovszky
Gábor
G
Jancsó