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

Abstract

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.

Effect of unsaturated acyl chains on structural transformations in triacylglycerols

“…4), which can be described by a mixed type of TAG packing -the a 2 -phase (Fig. 1c) [15]. In spite of the very flexible triunsaturated linolenoyl acid residue present in this symmetrical mixed saturated/unsaturated TAG (which may cause significant structural imperfections), the observed scattering pattern with five orders of diffraction peaks corresponding to the longitudinal period of TAG packing suggests a very well-ordered system along the layer normal ( Fig.…”

“…This structure produces two quasi-Bragg peaks (similar to the firstand the second-order peaks corresponding to the long period of the sub-a 2 -phase) and a broad diffuse scattering peak generated by local defects formed by a mismatch in the packing of polar heads of the molecules within the lamellae. Following this analogy, it has been suggested [15] that after the initial formation of the a 2 -phase (Fig. 1c) some TAG molecules undertake a further relocation in the sub-a 2 -phase driven by the packing incompatibility of saturated and unsaturated acyl chains that has its origin in the difference in their rotational isomeric states.…”

“…1a, b, d) and a 2 for the phases with the mixed packing and a hexagonal subcell (Fig. 1c, d)] [11,15]. The a 2 -phase, which demonstrates a continuous set of irreversible transformations during isothermal crystallization [11,14], can be characterized as a transient structure.…”

“…A reconstruction of the electron density distribution along the layer normal leads to a structural model of TAG packing in this phase that is different from both 2L-and 3L-packing, but one that still contains their motives (Fig. 1c) [15]. The fact that this phase appears first during quenching from the melt and has a hexagonal subcell has allowed this phase to be designated as the a-phase [13,14].…”

Mitigating crystallization of saturated FAMES (fatty acid methyl esters) in biodiesel. 3. The binary phase behavior of 1,3-dioleoyl-2-palmitoyl glycerol – Methyl palmitate – A multi-length scale structural elucidation of mechanism responsible for inhibiting FAME crystallization

“…The hydrocarbon chain layering is responsible for the characteristic smallangle (long-spacing) reflections. The period of layers normally observed for TAG (Triacylglycerol) structures is usually proportional to the acyl chain lengths by a factor of two or three, suggesting a double-chain length (DCL) or a triple-chain length (TCL) packing [33,34].…”