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

Abstract

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.