Molecular Insights into the Eutectic Tripalmitin/Tristearin Binary System

Abstract

A molecular interpretation of the eutectic behavior of a binary mixture of tristearin (SSS) and tripalmitin (PPP) triglycerides was formulated using computer simulations and experimental techniques (calorimetry and X-ray scattering). A eutectic composition was identified using both experimental and computer simulation techniques at a composition of 70% PPP and 30% SSS, in agreement with previous findings in the literature. The decrease in the melting temperature at the eutectic composition can be ascribed to an interplay between enthalpic and entropic effects. In particular, a lower global melting enthalpy at the eutectic composition was detected here, caused by a less efficient packing of the triglycerides in the crystal. On the other hand, a higher crystalline disorder is reflected in a lower change in the entropy of melting. The simultaneous decrease in global enthalpy and entropy has a contrasting effect on the melting temperature, with a slight melting point depression found in both experiment and simulations, resulting from a combination of enthalpic and entropic factors. Computer simulations showed, in fact, that the eutectic effect can be ascribed to the reduction of crystalline order when SSS molecules are incorporated into the PPP crystal structure. This decrease of the crystalline order is due to the protrusion of SSS end-chains (last three carbons of each alkyl chain) into the interlamellar space between adjacent lamella. These end-chains disturb the orderly stacking of the lamella, as evidenced by low-density regions in the interlamellar space. Thus, the greater disorder of the last atoms of the SSS alkyl chains is consequently due to the greater conformational freedom. At molecular level, in fact, the conformational freedom of terminal atoms of SSS surrounded by shorter PPP molecules is larger than the conformational freedom of longer SSS in the neighborhood of short PPP.

Avocado-derived polyols for use as novel co-surfactants in low energy self-emulsifying microemulsions

“…1D). The relationship between small domain sizes and lower melting points has widely been reported for both eutectic 25,26 and monotectic systems 27 . Furthermore, our XRD analysis on structurally related odd-numbered carbon lipids (i.e., 1-heptadecanol, heptadecanoic acid and 16-heptadecynoic acid) showed larger lamellae (SAXS diffraction peaks), greater domain sizes, and different lateral packing of hydrocarbon chains (WAXS diffraction peaks) compared to avocado polyols ( Supplementary Fig.…”

“…In contrast, avocadene and avocadyne are two structurally similar natural compounds that form a eutectic mixture at a molar ratio of 1:1 which translates to their efficient incorporation into a microemulsion-based delivery system. Mechanisms of eutectic formation are generally attributed to differences in i) molecular size and geometry, ii) polymorphic state, and iii) external fields (cooling and shear rates), which collectively leads to the formation of a more disordered crystal packing 25 . The XRD data presented here confirmed that molecular size and potentially polymorphic state contribute to the eutectic formation seen in AVO.…”

Surface-tailoring of emulsomes for boosting brain delivery of vinpocetine via intranasal route: in vitro optimization and in vivo pharmacokinetic assessment

“…Concerning SL type, emulsomes prepared using TS exhibited higher size compared to those prepared suing TP. TP and TS have very different melting points but very similar molecular structures (the fatty acid chains on tripalmitin (C16) differ from the fatty acid chains on tristearin (C18) by only two carbons) (Seetapan et al., 2010 ; Pizzirusso et al., 2018 ); the different size of these two triglycerides along with their different physicochemical properties could be responsible of the smaller vesicles TP-based compared to that with TS (Seetapan et al., 2010 ). Vyas et al.…”

An Insight into the Solid-State Miscibility of Triacylglycerol Crystals

“…A different case is the triclinic parallel subcell (β), in which the dense molecular packing prevents from the crystallization of non-isomorphic TAGs in the same crystal lattice. In this connection, a recent study, based on experimental data and computer simulations, associated the β immiscibility of PPP/SSS mixtures with the steric effect induced by the protrusion of large SSS end-chains into the interface between adjacent lamellas [ 85 ]. By taking this into account, it becomes evident the strict requirements of molecular resemblance and thermal behavior of solid solution-forming TAGs, which might be alleviated to some extent in the less stable crystalline packing.…”