Effects of molecular and lattice structure on the thermal behaviours of some long chain length potassium(I) n-alkanoates

Abstract

Lattice structures and thermal behaviours for some long chain potassium carboxylates (nc=8–18, inclusive) are investigated using Fourier Transform Infrared spectroscopy, X-ray Powder Diffraction, Solid State spin decoupled 13C NMR spectroscopy, Differential Scanning Calorimetry and Thermogravimetry. The measurements show that the carboxyl groups are coordinated to potassium atoms via asymmetric chelating bidentate bonding, with extensive carboxyl intermolecular interactions to yield tetrahedral metal centers, irrespective of chain length. Furthermore, the hydrocarbon chains are crystallized in the fully extended all-trans configuration and are arranged as non-overlapping lamellar bilayer structures with closely packed methyl groups from opposite layers. Additionally, odd–even alternation, observed in density and methyl group chemical shift, is ascribed to the relative vertical distances between layers in the bilayer, that are not in the same plane. Therefore, for even chain homologues, where this distances is less than for odd chain adducts, more intimate packing is indicated. The phase sequences for all compounds show several reversible crystal–crystal transition associated with kinetically controlled gauche–trans isomerism of the polymethylene chains which undergo incomplete fusion when heated to the melt. The compounds degrade above 785K to yield carbon dioxide, water, potassium oxide and an alkene.