Molecular Arrangement in Glycerides

Sustainable tandem acylation/Diels-Alder reaction toward versatile tricyclic epoxyisoindole-7-carboxylic acids in renewable green solvents

Tandem Diels-Alder reactions are often used for the straightforward formation of complex natural compounds and the fused polycyclic systems contained in their precursors. In the second step of this reaction, regio- and stereochemically controlled intramolecular cyclization leads to the formation of versatile nitrogen-containing tricyclic systems. However, these useful organic transformations are usually carried out in highly toxic organic solvents such as benzene, toluene, chloroform, etc. Despite recent efforts by 'green chemists', synthetic chemists still use these traditional toxic organic solvents in many of their reactions, even though safer alternatives are available. However, in addition to the harmful effects of these petrochemical solvents on the environment, the prediction that their resources will run out in the near future has led 'green chemists' to explore solvents that can be derived from renewable resources and used effectively in various organic transformations. In this context, we have shown for the first time that the 100% atom-economical tandem Diels-Alder reaction between aminofuranes and maleic anhydride can be carried out successfully in vegetable oils and waxes. The reaction was successfully carried out in sunflower seed oil, olive oil, oleic acid and lauryl myristate under mild reaction conditions. A series of epoxyisoindole-7-carboxylic acid and bisepoxyisoindole-7-carboxylic acids were obtained in good yields after a practical isolation procedure. The results obtained in this study demonstrate the potential of vegetable oils and their renewable materials to provide a reaction medium that is more sustainable than conventional organic solvents in cascade Diels-Alder reactions and can be used repeatedly without significant degradation. These materials also allow the reaction to be completed in less time, with less energy consumption and higher yields.

Colloidal Stability of Emulsifier-free Triolein-in-Water Emulsions: Effects of Temperature

Herein, we report the colloidal stability of emulsifier-free (EF-) triolein-in-water (TO/W) emulsions prepared by mixing TO and water using a high-powered bath-type ultrasonicator (HPBath-US; 28 kHz, 300 W) in the absence of emulsifiers such as surfactants. In particular, the effect of the temperature (15-60℃) on the colloidal stability of EF-TO/W emulsions was examined because this is important for the practical application of EF-TO/W emulsions, for example, in foods, pharmaceuticals, and cosmetics. We found that the colloidal stability of the EF-TO/W emulsions decreased with increase in the temperature from 15 to 25°C, whereas it increased with increase in temperature from 25 to 40°C, and the high colloidal stability of the EF-TO/W emulsions was maintained above 40°C. The reduction in the colloidal stability of EF-TO/W emulsions between 15 and 25°C is likely a result of the TO droplets formed by thermal motion, as well as enhanced Ostwald ripening at higher temperatures. On the other hand, the increase in the colloidal stability of the EF-TO/W emulsions from 25 to 40°C and their high colloidal stability above 40℃ is attributed to the reduction in the interfacial tension between TO and water at higher temperatures. This decrease in the interfacial tension between TO and water with temperature increase is related to the transformation of short-range ordered domains (clusters) of TO molecules in the liquid state, which increases the colloidal stability of the EF-TO/W emulsions.

Interplay of Aging and Lot-to-Lot Variability on the Physical and Chemical Properties of Excipients: A Case Study of Mono- and Diglycerides

The present study investigates the chemical composition governing the physical properties of mono- and diglycerides (MDGs) at the microstructural level, as a function of aging and lot-to-lot variability. The physical structure of the MDG plays a vital role in ameliorating the emulsion stability and is widely explored in diverse research horizons related to the pharmaceutical, cosmetic, and food industries. In an effort to understand the mechanism of emulsion stabilization, physical properties were extensively evaluated in selective commercial lots to determine if there is a correlation between the chemical composition of MDG and physical properties. The solid state of the MDG samples with different aging profiles was characterized using X-ray scattering, differential scanning calorimetry, attenuated total reflection-Fourier transform infrared spectroscopy, and NMR relaxometry. Moreover, the kinetic aspect of solid-state transformation was also evaluated via treating MDG samples with a heat-cool cycle. The chemical composition of MDGs was quantified using a quantitative NMR (qNMR) method. Interestingly, the X-ray scattering results demonstrated a change in the MDG polymorphic form and an increase in the %β content as a function of aging. The increase in the %β content led to the formation of rigid crystal structures of MDG, as evident from the NMR relaxometry. Chemical quantification of isomeric composition revealed chemical composition change as a potentially critical factor responsible for the altered physical structures of MDG with respect to aging and lot-to-lot variability. The findings correlated the solid-state transformation with the change in the chemical composition of the MDG as a combined effect of aging and lot-to-lot variability. This work serves as a basis to better understand the interdependency of the physicochemical properties of MDG. Furthermore, the present work can also be used as guidance for setting up the specifications of MDG, as per the required polymorphic form for a multitude of applications.

Crystal Memory near Discontinuous Triacylglycerol Phase Transitions: Models, Metastable Regimes, and Critical Points

It is proposed that "crystal memory", observed in a discontinuous solid-liquid phase transition of saturated triacylglycerol (TAG) molecules, is due to the coexistence of solid TAG crystalline phases and a liquid TAG phase, in a superheated metastable regime. Such a coexistence has been detected. Solid crystals can act as heterogeneous nuclei onto which molecules can condense as the temperature is lowered. We outlined a mathematical model, with a single phase transition, that shows how the time-temperature observations can be explained, makes predictions, and relates them to recent experimental data. A modified Vogel-Fulcher-Tammann (VFT) equation is used to predict time-temperature relations for the observation of "crystal memory" and to show boundaries beyond which "crystal memory" is not observed. A plot of the lifetime of a metastable state versus temperature, using the modified VFT equation, agrees with recent time-temperature data. The model can be falsified through its predictions: the model possesses a critical point and we outline a procedure describing how it could be observed by changing the hydrocarbon chain length. We make predictions about how thermodynamic functions will change as the critical point is reached and as the system enters a crossover regime. The model predicts that the phenomenon of "crystal memory" will not be observed unless the system is cooled from a superheated metastable regime associated with a discontinuous phase transition.

Global Small-Angle X-ray Scattering Data Analysis of Triacylglycerols in the Molten State (Part I)

The study of triacylglycerols (TAGs) in their molten state is of fundamental importance for a deeper understanding of the TAG crystallization processes, being highly relevant for both manufacturing and medical applications. Although different models have been proposed to explain the nanostructured nature of the fluid state of TAGs, none of them are fully satisfactory. In this paper, we propose a new model consisting of positionally uncorrelated lamellar TAG assemblies embedded in an isotropic medium that assist as prenucleating structures. This model was validated by applying a novel global fitting method, resulting in an excellent agreement with the small-angle X-ray scattering data. A deeper analysis of the scattering patterns at different temperatures, both in cooling and heating directions, allowed us further to detect the crystalline traces of TAGs even after heating to 40 °C and record, on cooling, the onset of crystallization at 30-25 °C. The application of the presented novel model not only explains the outstandingly structured fluid of molten TAGs, but also lays the basis for analyzing first the crystallization steps in greater detail, which is outlined in our follow-up paper "Global Small-Angle X-ray Scattering Data Analysis of Triacylglycerols in the α-Phase (Part II)".

Understanding the interactions between sebum triglycerides and water: a molecular dynamics simulation study

Sebum triglycerides actively contribute to the water transport across the sebum layerviathe percolating network of triglyceride head groups.

α-Melt Structure of 1,3-Dipalmitoyl-2-oleoyl-sn-glycerol (POP) under a Thermal Thawing Process Studied by Infrared Spectroscopy

The temperature thawing, as called tempering, of triacylglycerols (TAGs) is an important processing method in food productions, such as chocolates, cream, confections, and spreads. Especially, melt-mediation by temperature thawing is famous in chocolate production for controlling the polymorphic crystalline forms and accelerating crystallization. In the present study, we investigated the α-melt structure of 1,3-dipalmitoyl-2-oleoyl-sn-glycerol (POP), one of the major continuants of cacao butter, under a phase transition from its melt to γ-crystal with in-situ attenuated total reflection-infrared (ATR-IR) spectroscopy. The differential IR spectrum between α-melt via temperature thawing (α-melt mediation) and melt via simple cooling revealed that crystal-like local ordered structures remained in part in the α-melt, and that they acted as nuclei for a rapid phase transition to the γ-crystal. The changes to the γ-crystal occur in the local ordered structures at first from the glycerol moiety to the acyl chains in the crystallization, providing an important suggestion concerning the mechanism for the acceleration of crystallization to the γ-form via α-melt mediation.

Stability of the Metastable α-Polymorph in Solid Triglyceride Drug-Carrier Nanoparticles

Colloidal dispersions of crystalline nonpolar lipids are under intensive investigation as carrier systems in pharmaceutics and nutrition. In this context, the controlled preparation of particles in a metastable polymorphic state is of some interest for the delivery of active substances. In the present study, tristearin particles stabilized with three α-polymorph-preserving emulsifier regimes ((I) sodium glycocholate/saturated long-chain phospholipids, (II) sodium glycocholate, and (III) poly(vinyl alcohol) (PVA)) were investigated concerning the stability of the metastable α-polymorph after controlled crystallization of the particles from the melt. Upon long-term storage, the α-polymorph was preserved best in PVA-stabilized dispersions, followed by those stabilized with the glycocholate/phospholipid mixture and finally those stabilized solely with the bile salt. In particular for rapidly crystallized nanoparticles, the formation of an α-polymorph with highly reduced lamellarity was observed. According to time-/temperature-resolved synchrotron X-ray diffraction analysis with simultaneous DSC (differential scanning calorimetry) studies, this less-ordered α-polymorph transformed into the common, lamellar α-form upon heating. Although the presence of the less-ordered form is probably related to the extraordinarily high stability of the metastable α-polymorph observed in some of the dispersions, it could not completely prevent the transition into the stable β-polymorph. The higher the transition temperature of the less-ordered α-form to the ordered one, the slower was the polymorphic transition to the stable β-polymorph. To estimate the polymorphic stability of the differently stabilized particles upon isothermal long-term storage, standard DSC measurements on samples stored at 23 °C for 4 weeks seem to be of predictive value.

Study of liquid-phase molecular packing interactions and morphology of fatty acid methyl esters (biodiesel)

BACKGROUND

(1)H low field nuclear magnetic resonance (LF-NMR) relaxometry has been suggested as a tool to distinguish between different molecular ensembles in complex systems with differential segmental or whole molecular motion and/or different morphologies. In biodiesel applications the molecular structure versus liquid-phase packing morphologies of fatty acid methyl esters (FAMEs) influences physico-chemical characteristics of the fuel, including flow properties, operability during cold weather, blending, and more. Still, their liquid morphological structures have scarcely been studied. It was therefore the objective of this work to explore the potential of this technology for characterizing the molecular organization of FAMEs in the liquid phase. This was accomplished by using a combination of supporting advanced technologies.

RESULTS

We show that pure oleic acid (OA) and methyl oleate (MO) standards exhibited both similarities and differences in the (1)H LF-NMR relaxation times (T2s) and peak areas, for a range of temperatures. Based on X-ray measurements, both molecules were found to possess a liquid crystal-like order, although a larger fluidity was found for MO, because as the temperature is increased, MO molecules separate both longitudinally and transversely from one another. In addition, both molecules exhibited a preferred direction of diffusion based on the apparent hydrodynamic radius. The close molecular packing arrangement and interactions were found to affect the translational and segmental motions of the molecules, as a result of dimerization of the head group in OA as opposed to weaker polar interactions in MO.

CONCLUSIONS

A comprehensive model for the liquid crystal-like arrangement of FAMEs in the liquid phase is suggested. The differences in translational and segmental motions of the molecules were rationalized by the differences in the (1)H LF-NMR T2 distributions of OA and MO, which was further supported by (13)C high field (HF)-NMR spectra and (1)H HF-NMR relaxation. The proposed assignment allows for material characterization based on parameters that contribute to properties in applications such as biodiesel fuels.

Oil binding capacities of triacylglycerol crystalline nanoplatelets: nanoscale models of tristearin solids in liquid triolein

Polycrystalline particles composed of triacylglycerol (TAG) molecules, and their networks, in anhydrous TAG oils find extensive use as edible oils in the food industry.

The viscoelastic properties of processed cheeses depend on their thermal history and fat polymorphism

Both the composition and the thermal kinetics that are applied to processed cheeses can affect their texture. This study investigated the effect of the storage conditions and thermal history on the viscoelastic properties of processed cheese and the physical properties of the fat phase. The microstructure of processed cheese has been characterized. Using a combination of physical techniques such as rheometry, differential scanning calorimetry, and X-ray diffraction, the partial crystallization of fat and the polymorphism of triacylglycerols (TG; main constituents of milk fat) were related to changes in the elastic modulus and tan δ as a function of temperature. In the small emulsion droplets (<1 μm) dispersed in processed cheeses, the solid fat phase was studied at a molecular level and showed differences as a function of the thermal history. Storage of processed cheese at 4 °C and its equilibration at 25 °C lead to partial crystallization of the fat phase, with the formation of a β' 2 L (40.9 Å) structure; on cooling at 2 °C min(-1), the formation of an α 3 L (65.8 Å) structure was characterized. The cooling of processed cheese from 60 to -10 °C leads to the formation of a single type of crystal: α 3 L (72 Å). Structural reorganizations of the solid fat phase characterized on heating allowed the interpretation of the elastic modulus evolution of processed cheese. This study evidenced polymorphism of TG in a complex food product such as processed cheese and allowed a better understanding of the viscoelastic properties as a function of the thermal history.

Crystallization and polymorphism of triacylglycerols contribute to the rheological properties of processed cheese

The thermal, rheological, and structural behaviors of a spreadable processed cheese were studied by complementary techniques including differential scanning calorimetry (DSC), rheology, and X-ray diffraction as a function of temperature. In this product, fat is present as a dispersed phase. Thermal and rheological properties were studied at different cooling rates between 0.5 and 10 degrees C min(-1) from 60 to 3 degrees C. Crystallization properties of fat were monitored at a cooling rate of -2 degrees C min(-1) from 60 to -10 degrees C. Fat triacylglycerols (TGs) crystallized at 15 degrees C in a triple-chain length 3Lalpha (72 A) structure correlated to exothermic events and to the sudden increase in the rheological moduli G' and G''. Upon heating at 2 degrees C min(-1), the polymorphic transition of TGs evidence the melting of the 3Lalpha structure and the formation of a 2Lbeta' (36.7-41.5 A) structure. Melting of the latter follows. These transformations coincide with thermal events observed by DSC and the decrease in two steps of the rheological moduli. The influence of fat crystallization, melting, and polymorphism upon the viscoelastic properties is clearly demonstrated upon both heating and cooling.

Triglycerides obtained by dry fractionation of milk fat: 2. Thermal properties and polymorphic evolutions on heating

The crystallographic and thermal properties of milk fat and fractions were investigated on heating using the coupling of synchrotron X-ray diffraction with differential scanning calorimetry. We showed that re-crystallisations occurred during the heating of the stearin and the olein fractions with the formation of a beta' 2L (41.1-42.6A) structure and a beta' 3L (66A) structure, respectively. By creating a quantified solid-liquid phase behaviour versus temperature diagram, the amount of the solid and liquid phases and the relative proportion of each of the crystalline structures within the solid phase were determined.

A case for discotic liquid crystals in molten triglycerides

To date, essentially only two structural models have been proposed and debated in detail for explaining the liquid state order of triglycerides, and both invoke a form of thermotropic liquid crystalline order in triglyceride melts. These are the paralamellar model of Larsson et al. (J. Am. Oil Chem. Soc. 1992, 69, 835) and the nematic model of Cebula et al. (J. Am. Oil Chem Soc. 1992, 69, 130). An alternative discotic model is proposed here that adequately accounts for the broad small-angle X-ray diffraction peak often observed in the liquid state of fats and oils. In this alternative model, triglyceride molecules exist in the liquid state with fully splayed chains, approximating "Y"-shapes (Y-conformers). These are loosely bound within discs that stack into flexible, relatively short cylindrical rods of colloidal dimension, which in turn assemble into rod-packings with short-range order akin to disordered versions of thermotropic discotic liquid crystalline phases in other lipidic systems.

Milk fat and primary fractions obtained by dry fractionation

The chemical composition and crystallisation properties of milk fat and its primary fractions, obtained by dry fractionation at 21 degrees C, were investigated. The solid fraction (stearin) and the liquid fraction (olein) displayed a different triacylglycerol (TG) composition. Stearin fraction was enriched in long-chain fatty acids, whereas olein fraction was enriched in short-chain and unsaturated fatty acids. Crystallisation properties of milk fat, and both the stearin and olein fractions were studied on cooling at |dT/dt|=1 degrees C min(-1) by differential scanning calorimetry and time-resolved synchrotron X-ray diffraction (XRD) at small and wide angles. Two main types of crystals corresponding to double chain length structures were characterised in the stearin fraction: alpha 2L(1) (47.5 Angstrom) and beta' 2L(2) (41.7 Angstrom). A triple chain length structure was formed in the olein fraction: alpha 3L (72.1 Angstrom). Crystallization of milk fat showed the formation of two 2L (47.3 and 41.6 Angstrom) and one 3L (72.1 Angstrom) lamellar structures with an hexagonal packing (alpha form). A schematic representation of the 3L packing of olein fraction was proposed to explain how a wide diversity of TG can accommodate to form a lamellar structure with a thickness of 72 Angstrom. Furthermore, the sharpness of the small-angle XRD lines associated to the alpha form was explained by the formation of liquid crystals of smectic type.

Flexible chocolate

Chocolate is a complex soft solid that undergoes a series of physical transformations in the mouth during consumption. Normally chocolate products are manufactured by liquid processing techniques that include 'melting and casting' liquid chocolate into moulds as well as enrobing sweet centres with a chocolate outer shell. There is however another method of shape forming chocolate and this review discusses the key elements of a so called 'cold extrusion' process. This process involves a combination of pressure and flow, which results in a product with a temporary flexibility. The review covers the basic aspects of the composition and microstructure of chocolate together with experimental observations on the cold extrusion process and the subsequent post extrusion flexibility of the extrudate. Finally some physical insight into the unusual behaviour of chocolate during and after cold extrusion is presented.

Thermal and Structural Behavior of Anhydrous Milk Fat. 3. Influence of Cooling Rate

The crystallization behavior of anhydrous milk fat has been examined with a new instrument coupling time-resolved synchrotron x-ray diffraction as a function of temperature (XRDT) at both small and wide angles and high-sensitivity differential scanning calorimetry. Crystallizations were monitored at cooling rates of 3 and 1 degrees C/ min from 60 to -10 degrees C to determine the triacylglycerol organizations formed. Simultaneous thermal analysis permitted the correlation of the formation/melting of the different crystalline species monitored by XRDT to the thermal events recorded by differential scanning calorimetry. At intermediate cooling rates, milk fat triacylglycerols sequentially crystallize in 3 different lamellar structures with double-chain length of 46 and 38.5 A and a triple-chain length of 72 A stackings of alpha type, which are correlated to 2 exothermic peaks at 17.2 and 13.7 degrees C, respectively. A time-dependent slow sub-alpha <--> alpha reversible transition is observed at -10 degrees C. Subsequent heating at 2 degrees C/min has shown numerous structural rearrangements of the alpha varieties into a single beta' form before final melting. This polymorphic evolution on heating, as well as the final melting point observed (approximately 39 degrees C), confirmed that cooling at 3 degrees C/min leads to the formation of crystalline varieties that are not at equilibrium. An overall comparison of the thermal and structural properties of the crystalline species formed as a function of the cooling rate and stabilization time is presented. The influence on crystal size of the cooling rates applied in situ using temperature-controlled polarized microscopy is also determined for comparison.

Dynamical dimer structure and liquid structure of fatty acids in their binary liquid mixture: dodecanoic and 3-phenylpropionic acids system

Dimer structure and liquid structure of fatty acids in the binary liquid mixture of dodecanoic (LA) and 3-phenylpropionic acids (PPA) were studied through the measurements of DSC, self-diffusion coefficient (D), density, viscosity, 13C NMR spin-lattice relaxation time, small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). The phase diagram of LA/PPA mixture exhibited a typical eutectic pattern, which means that LA and PPA are completely immiscible in solid phase. In the liquid phase of the LA/PPA mixture, D of LA always differed from that of PPA irrespective of their compositions. This exhibited that, in the liquid phase of the binary mixture of fatty acids giving a complete eutectic in the solid phase, the fatty acid dimers are composed of the same fatty acid species irrespective of their compositions. The liquid structure of the LA/PPA mixture was clarified through the SAXS and also the SANS measurements.