Thermotropic Mesomorphic and Odd-Even Melting Behavior of a Homologous Series of Zinc(II) n-Alkanoates

The thermotropic phase behavior of a series of odd-chain polymeric zinc(II) n-alkanoates, Zn(CnH2n-1O2)(2) [ZnC5-19], is studied by differential scanning calorimetry (DSC) and temperature variation polarizing light microscopy and compared with data of even-chain homologues. The compounds exhibit both enantiotropic and monotropic phase behavior on heating to the isotropic liquid and on cooling back to the room-temperature solid: ZnC5-11; Lamellar crystal <-> Crystal I <-> Crystal II <-> Isotropic melt; ZnC13-15; Lamellar crystal <-> Crystal I <-> Isotropic liquid; ZnC17-19; Lamellar crystal -> Isotropic liquid (heating) "Smectic C-like" Lamellar crystal <- Smectic C <- Isotropic liquid (cooling) Odd-even behavior is observed in the thermodynamic data and is attributed to the difference in the packing efficiency of odd- and even-numbered hydrocarbon chains within the lattice. The thermodynamic data along with solid-state infrared (IR) and X-ray diffraction data suggest that electrostatic interactions play a prominent role in the melting process in addition to fusion of the alkyl chains. An equilibrium model is proposed as the structure for the isotropic melt.