Structural models of metastable phases occurring during the crystallization process of saturated/unsaturated triacylglycerols

Isothermal crystallizations of saturated and mixed saturated/unsaturated triacylglycerols (TAGs) have been studied by simultaneous time‐resolved small‐angle and wide‐angle X‐ray scattering measurements (SAXS/WAXS). The projection of the electron density profile along the layer normal derived from the lamellar peaks of the α2‐phase can be assembled from two types of molecular dimers initially formed in the liquid state. One dimer corresponds to a typical two‐chain packing of TAGs and the other is formed by opposing molecules overlapping by their two‐acyl‐chain sides. This structure occurs only in triacylglycerols containing both saturated and unsaturated acyl chains. The structural organization of the α2‐phase is analogous to smectic A interdigitated phases. In the particular case of 1,3‐distearoyl‐2‐oleoyl‐sn‐glycerol, StOSt, the α2‐phase can evolve into a mixture of two phases (α1 and γ) with double and triple (2L and 3L) chain packing, respectively. A calculation of X‐ray scattering patterns for simulated structures of randomly packed three‐chain and two‐chain layers of StOSt (2L + 3L) reproduced the three diffuse scattering peaks observed in the experimental SAXS patterns.