Polymorphism, crystal–crystal and liquid crystalline thermotropic phase transition behaviour of even chain length zinc(II) n-alkanoates

Abstract

The thermotropic phase behaviour of a homologous series of even chain length polymeric zinc(II) n-alkanoates, Zn(C_nH_2n-1O₂)₂ (ZnC_4–20) has been studied by means of differential scanning calorimetry (DSC) and temperature variation polarising light microscopy. The compounds exhibit both enantiotropic and monotropic phase behaviour on heating to the isotropic liquid and on cooling back to the room temperature solid, as seen from the following phase sequences: ZnC_4–12, Lamellar Crystal ↔ Crystal I ↔ Crystal II ↔ Isotropic Melt; ZnC_14–16, Lamellar Crystal ↔ Crystal I ↔ Isotropic Liquid; ZnC_18–20, Lamellar Crystal → Isotropic Liquid, Lamellar Crystal ← Smectic C ← Isotropic Liquid. The total enthalpy and entropy change for melting increase linearly with chain length and are higher than expected for complete fusion of alkyl chains suggesting that the step-wise melting process involves fusion of alkyl chains and changes in electrostatic interactions in the polar region of the molecule. The phase diagrams show that phase sequences for first and second heating cycles are almost indistinguishable, where the lamellar crystal is most stable for the long chain length homologues. In addition, there is a stable mesomorphic region over a narrow temperature range, at high temperatures for the long chain length compounds. Polymorphism is exhibited and is dependent on the method of preparation.

Keywords:

• zinc(II) n-alkanoates
• thermotropic phase behaviour
• differential scanning calorimetry
• polarising light microscopy
• step-wise melting process
• smectic C mesophase
• polymorphism

Acknowledgement

The authors express thanks to the School for Graduate Studies and Research of the University of the West Indies, Mona, for funding this research.