Binary Phase Behavior of 1,3‐Dipalmitoyl‐2‐oleoyl‐<i>sn</i>‐glycerol and 1,2‐Dioleoyl‐3‐palmitoyl‐<i>rac</i>‐glycerol

Abstract

Abstract 1,3‐Dipalmitoyl‐2‐oleoyl‐ sn ‐glycerol (POP) and 1,2‐dioleoyl‐3‐palmitoyl‐ rac ‐glycerol (OOP) are two major molecular species that account for roughly half of the total triacylglycerols in palm oil. The binary phase behavior of a POP/OOP mixture plays an important role in the crystallization of palm oil. We conducted thermodynamic and kinetic studies of OOP and its mixtures with POP using differential scanning calorimetry and X‐ray diffraction with a conventional generator and synchrotron radiation. We found that OOP has two polymorphs, α as a metastable form and β′ as the most stable form, and that the two forms are stacked in a triple‐chain‐length structure. The POP/OOP mixtures exhibited immiscible eutectic natures in both their metastable and their most stable states, in contrast to POP/1,2‐dipalmitoyl‐3‐oleoyl‐ rac ‐glycerol and POP/1,3‐dioleoyl‐2‐palmitoyl‐ sn ‐glycerol mixtures, in which molecular compounds of a double‐chain‐length structure were formed. A time‐resolved synchrotron radiation X‐ray diffraction study undertaken during the cooling and heating processes indicated that the α and β′ forms of the POP and OOP fractions crystallized and melted in separate manners, and that crystallization of the β′ form and the polymorphic transformation from α to β′ of POP and OOP are promoted in the presence of another component. The absence of molecular compound crystals in the binary mixtures of POP/OOP is explained by taking into account the molecular interactions of acyl chain packing, glycerol conformation, and methyl end stacking, among which glycerol conformation appeared to be most influential.