Water-like behavior of 1,2-ethanediol in binary mixtures with pyridine and its methyl derivatives: Thermodynamic excesses and the O–HN bonds energy Anna Przybyła, Piotr Kubica, Szymon Bacior, Piotr Lodowski, Wojciech Marczak ⇑ Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, Poland article info Article history: Received 16 May 2011 In final form 4 July 2011 Available online 7 July 2011 abstract The molecule of 1,2-ethanediol, like that of water, is capable of forming two hydrogen bonds as a donor of protons. Consequently, the complexes of pyridine and its methyl derivatives with ethanediol may associate in the liquid phase in a similar way as those with water. The association contributes to the excess molar expansion making its isotherms W-shaped for the mixtures with 2-methylpyridine and 2,6-dimethylpyridine. Negative excesses of volume and compression are correlated with the association energies of the 1:1 amine–ethanediol complexes. They increase in the order: pyridine < 2-methylpyri- dine < 2,6-dimethylpyridine. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Aqueous solutions have been extensively studied for more than a century, while water aroused interest since antiquity [1]. However, there are still fundamental questions concerning the structure of liquid water and the hydration phenomena. A compar- ison of the solvation of the same molecules in water and other solvents leads to better understanding of various modes of hydration. Pyridine and its methyl derivatives belong to the class of simple heterocyclic compounds. The electron pair localized at the nitrogen atom of the ring makes them typical Lewis bases capable of form- ing the hydrogen bonds as acceptors of protons. Moreover, the p electrons of the ring can participate in the hydrogen bonding as well. Although the C–H group is rather weak donor of proton, the arrangement of molecules in the crystals of pure 2-methylpyridine and 2,6-dimethylpyridine suggests that the C–HN bonds [2,3] or even the C–Hp ones arise [4]. No such bonds were postulated to explain the crystal structure of pyridine [5]. It is at least probable, that the weak H-bonds break in the liquid phase due to molecular motions. In spite of that, some authors argued for the self-associa- tion of pyridines even in aqueous solutions. However, the sug- gested mechanisms either seemed unreliable, as the association thanks the C–HN bonds [6], or they were not satisfactorily ex- plained [7,8]. In the recent publications, we took into consideration that water was not an inert solvent for the pyridines. On the con- trary, the association consists in the aggregation of the amine hy- drates through the O–HO bonds between water molecules, that leads to the amine-rich clusters (RNHOH) n in dilute aqueous solutions [9,10]. Similar bonding has been observed in the solid tri- hydrates of pyridine and 4-methylpyridine [5,11]. In this work, we report thermodynamic excesses of volume, expansion, and isentropic compression for three binary liquid mixtures: pyridine, 2-methylpyridine and 2,6-dimethylpyridine with 1,2-ethanediol, as well as the association energies of the 1:1 amine–ethanediol complexes calculated theoretically. Previous studies have shown that the bigger is the O–HN bond energy, the larger is the negative excess volume of binary aqueous and methanolic mixtures with pyridines [9,10]. Similarly as water and contrary to methanol, 1,2-ethanediol forms three-dimensional lattice in the crystalline state [12–14]. Thus, the amine–ethanediol complexes would be prone to association that should be evident in the excess expansion. The latter was positive for aqueous systems and negative for the methanolic ones [9,10]. 2. Experimental 2.1. Chemicals 1,2-Ethanediol (Fluka, min. 99.5%) and pyridine (POCH, >99.5%) were used as supplied by the manufacturers. 2-Methylpyridine (Merck, min. 99%) and 2,6-dimethylpyridine (Aldrich, 98%) were purified before the use by fractional distillation under argon. All the liquids were stored over molecular sieves 4 Å in dark glass bottles. Since pyridine and its methyl derivatives are chemically rather unstable, they were kept in a refrigerator. The Karl Fischer titration evidenced low content of water in the chemicals, while their overall purity was confirmed by the measured densities close to the literature values (Table 1). The mixtures were prepared by mass using analytical balances of the accuracy of ±1  10 3 g. Before measurements, every sample 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.07.007 ⇑ Corresponding author. Fax: +48 32 2599978. E-mail address: marczak@ich.us.edu.pl (W. Marczak). Chemical Physics Letters 512 (2011) 199–203 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett