Molecular Solid Solutions with Steric Complementary Pairing from the Binary Mixtures of 1-Naphthylmethylammonium Alkanoates

We report the formation of molecular solid solutions from the 1:1 binary mixtures of 1-naphthylmethylammonium n-alkanoates with different alkyl chain lengths. Systematic investigation of all the combinations from acetate to nonadecanoate concluded that a 1-naphthylmethylammonium n-alkanoate formed the molecular solid solutions with six or seven alkanoates due to similarity of their molecular structures. X-ray diffractions revealed that the molecular solid solutions had bilayered structures similar to those of pure alkanoates and that the two alkanoates formed steric complementary pairs and distributed randomly in the alkyl bilayers. When the differences of the numbers of the carbon atoms were less than three, the mixtures yielded selectively single-phased molecular solid solutions. When the differences became more than six, the mixtures yielded two-phased eutectic mixtures. Therefore, this size difference of the molecular structures (ΔV = ca. 100 Å3 for six carbon atoms) should be critical to form the molecular solid solutions. The numbers of carbon atoms in the mixed alkanoates play a crucial role in the formation of the binary solid solutions or eutectic mixtures.