Crystallization and phase behavior of 1,3-propanediol esters

Polymorphic influences on the phase behavior of two types of binary mixtures of saturated monoacid 1,3-propanediol esters (PADEs), dipalmitate/distearate (PP/SS) and dimyristate/distearate (MM/SS) were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and by solid fat content (SFC), hardness and microscopy measurements. Three stacking modes have been found in the PP/SS binary system. Mixed SS-PP bilayers were detected in all mixtures, SS-SS bilayers in x(PP)=0.0-0.4 mixtures and PP-PP bilayers in x(PP)=0.6-0.1 mixtures. Two different but close beta polymorphs and one beta' polymorph were detected for this system. beta' was only detected in x(PP)=0.5-0.9 mixtures for the mixed bilayers. For the MM/SS binary system, only MM-MM and SS-SS bilayers were detected and both solid phases crystallized in two different beta forms. XRD data evidenced clearly that the MM and SS components were completely immiscible in the solid state. The phase diagrams constructed using DSC data, exhibited a typical eutectic-type phase boundary. The presence of eutectics, the shape of the solidus lines as well as the analysis of the individual enthalpies of melting indicated typical phase separation for both systems. A thermodynamic study based on the Hildebrand equation and using the Bragg-Williams approximation for non-ideality of mixing confirmed the phase separation in the solid phase and suggested that the PP and SS were miscible in the liquid phase and that SS formed an ideal mixing with MM. Avrami analysis of SFC vs. time curves indicated heterogeneous nucleation and spherulitic crystal development from sporadic nuclei, and suggested that the nucleation rate was higher for the mixture at the eutectic composition. The relative hardness was correlated with the enthalpies, the final SFC and the microscopy measurements.

Crystallization and phase behavior of fatty acid esters of 1,3-propanediol

Six pure fatty acid esters of 1,3-propanediol (PADE) molecules were investigated. A careful analysis of XRD, DSC as well as SFC results has allowed the determination of their structure and phase behavior. Two beta polymorphs were observed for C10-C18 and three beta polymorphs for C8. The same first polymorph (beta1) was observed for all the samples. The second polymorph (beta2) observed for C12-C18 was different from the second beta-form observed for C8 and C10. For all properties, the short chain length C8 and C10 samples were distinguished from the C12 to C18 samples and this explained much of the observed trends in behavior. Their lamellar packing was similar and has been explained by a simple addition of multiples of the length of a carbon bond to a primitive structure. The estimated long-range order highlighted a geometric effect that enabled the small chain molecules to better order than the longest molecules. The XRD results have been confirmed by DSC. The difference in property between the short and long chain molecules has also been clearly verified by the evolution of the energy of activation for nucleation as well as the enthalpy of melting and confirmed by microscopy measurements. For all the samples, the hardness which increased with increasing chain length is correlated with final %SFC. Avrami analysis of SFC versus time indicated heterogeneous nucleation and spherulitic crystal development from sporadic nuclei, and suggested that the rate of nucleation was higher for longer chain molecules.