Thermal behaviour of mixtures of 1-alkylpyridinium halides with and without a common ion

Abstract The thermal behaviour of mixtures of 1-alkylpyridinium halide salts, comprising either three or four different types of ions, was investigated in order to develop a better understanding of this class of multi-ion systems. A total of six systems was examined, derived from all possible binary combinations of the following four salts: 1-butylpyridinium bromide ([C 4 Py]Br), 1-butylpyridinium chloride ([C 4 Py]Cl), 1-ethylpyridinium bromide ([C 2 Py]Br), and 1-ethylpyridinium chloride ([C 2 Py]Cl). The systems constituted by two of these salts with a common anion and therefore differing only in the length of the alkyl substituent of the cation (namely [C 2 Py]Br + [C 4 Py]Br and [C 2 Py]Cl + [C 4 Py]Cl) displayed a eutectic behaviour, with the eutectic compositions qualifying as ionic liquids (melting temperature below 373 K). In contrast, the systems having in common the cation and differing in the halide anion (namely [C 2 Py]Br + [C 2 Py]Cl and [C 4 Py]Br + [C 4 Py]Cl) showed the formation of solid solutions. The two remaining combinations ([C 2 Py]Br + [C 4 Py]Cl and [C 2 Py]Cl + [C 4 Py]Br), involving four different types of ions, were also found to display a eutectic behaviour. The melting point depressions of the eutectic points with respect to the parent salts were of about 30–70 K. This was also the liquid range increase achieved with the eutectic compositions, since their thermal stability was nearly the same as the similar thermal stabilities of their parent salts. All six binary combinations of the four 1-alkylpyridinium halides were also interpreted as subsystems of the general 4-ion system [C 2 Py] x [C 4 Py] 1-x Cl y Br 1-y (where x is the cationic fraction of [C 2 Py] + and y is the anionic fraction of Cl − ), and an analysis of the melting temperature throughout the domain of this system was carried out.