Synchrotron Radiation X-ray Diffraction Study of Liquid Crystal Formation and Polymorphic Crystallization of SOS ( sn -1,3-Distearoyl-2-oleoyl Glycerol)

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Polymorphic crystallization of SOS ( sn -1,3-distearoyl-2-oleoyl glycerol) has been studied with a time-resolved synchrotron radiation X-ray diffraction method (SR-XRD) using pure samples. SOS has five polymorphs: α, γ, β‘, β 2, and β 1 . Emphasis was placed on analyzing the events of melt-mediated transformation, in which the crystallization of more stable forms (γ, β‘, and β 2 ) was induced by rapid melting of the less stable forms. This mode of crystallization is closely related to a long running dispute concerning a “memory effect” possibly occurring in the polymorphic crystallization of triacylglycerols from liquid. Two types of thermal processes were applied: (a) a rapid heating soon after the least stable α form was crystallized from liquid and (b) a rapid heating after thermal annealing of the α form for 20 min before its melting. The SR-XRD measurements were monitored at a time interval of 10 s. The main results are as follows: (1) the formation of lamellar ordering of SOS occurred more rapidly than that of subcell packing, as exhibited in the earlier occurrence of SR-XRD long spacing spectra in comparison to the short spacing spectra, (2) the α-melt mediation without thermal annealing unveiled the formation of two types of liquid crystalline structures having long spacing values of 5.1 and 4.6 nm, (3) the γ form directly occurred in the α-melt mediation involving the thermal annealing, which generated γ-embryos in the solid phase of α to serve as seed materials, and (4) the formation of the second most stable β 2 form of SOS was obtained after quite long incubation of the β‘-melt mediation, yet the most stable β 1 did not crystallize. The present study has given deeper insights of the polymorphism of SOS in comparison to previous studies with optical, calorimetric, and conventional XRD techniques: in particular the presence of the liquid crystalline form and the formation of γ-embryos were directly observed for the first time.