Impact of polymorphism in oleogels of N-palmitoyl-L-phenylalanine.

Gels of edible oils, also called oleogels, are developed as alternative products of solid fats to limit the uptake of saturated and trans-unsaturated fats and lower the associated risk of coronary disease. The gelation of oils can be achieved with a low molecular weight organogelator (LMWO), a compound that self-assembles at low concentrations in a solid 3D network and provides the mixture its solid-like behavior. We have studied N-palmitoyl-L-phenylalanine (Palm-Phe), an endogenous compound (i.e. naturally present in the human body) as a model LMWO of rapeseed oil. Palm-Phe forms gels at a concentration of 1 wt% in rapeseed oil. We have studied the thermodynamic and mechanical behavior of the corresponding gels. As evidenced by DSC and rheology, this system exhibits two transitions upon heating, in addition to the sol-gel transition, a gel-gel transition between two polymorphic gels. The structural differences between both polymorphs were revealed using cryo-SEM, X-rays scattering, and FTIR experiments. The metastability of one of the polymorphs was proven by ageing and annealing experiments.

Beyond Sol-Gel: Molecular Gels with Different Transitions.

The existence of sol-gel transitions is one of the most manifest properties of molecular gels. These transitions reflect their nature since they correspond to the association or dissociation of low weight molecules through non-covalent interactions to form the network constitutive of the gel. Most described molecular gels undergo only one gel-to-sol transition upon heating, and the reverse sol-to-gel transition upon cooling. It has been long observed that different conditions of formation could lead to gels with different morphologies, and that gels can undergo a transition from gel to crystals. However, more recent publications report molecular gels which exhibit additional transitions, for instance gel-to-gel transitions. This review surveys the molecular gels for which, in addition to sol-gel transitions, transitions of different nature have been reported: gel-to-gel transitions, gel-to-crystal transition, liquid-liquid phase separations, eutectic transformations, and synereses.

Palmitoylethanolamide gels edible oils.

Palmitoylethanolamide (PEA) is an endogenous compound with no adverse effect for oral intakes of a gram per day. We show that PEA gels edible oils at concentrations as low as 0.5 wt%. The elastic moduli values of the formed gels are 1400 Pa at 1 wt% and 9000 Pa at 2 wt%. The study of the gels by cryo-SEM, optical microscopy and WAXS show that PEA forms lamellar solid aggregates with widths of several tens of micrometers. Upon heating, the sample shows two transitions. The first one is the gel-to-sol transition, observed by rheology and defined by the switch from a solid to a liquid behavior. During this transition, the solid particles remain but do no longer form a network. The second transition, observed at higher temperature by DSC corresponds to the melting of the solid particles.

Investigation of an Organogel by Micro-Differential Scanning Calorimetry: Quantitative Relationship between the Shapes of the Thermograms and the Phase Diagram.

The phase diagrams of organogels are necessary for applications and fundamental aspects, for instance to understand their thermodynamics. Differential scanning calorimetry is one of the techniques implemented to map these diagrams. The thermograms of organogels upon heating show broad endotherms, increasing gradually to a maximum, at a temperature Tmax, and decreasing back to the baseline, sometimes 10 °C above. This broadening can lead to uncertainty in determining the molar enthalpies and the melting temperatures Tm of the gels. Herein, we have measured the thermograms of the 12-hydroxystearic acid/nitrobenzene gels for weight fractions ranging from 0.0015 to 0.04. Compared with transition temperatures measured by other techniques, the inflection points of the thermograms provide a measurement of Tm with less bias than Tmax. The phase diagram explains why the molar melting enthalpies derived from the thermograms for samples of low concentration are lower than expected. The shapes of the heat flows below the peak correlate quantitatively with the diagrams: after suitable correction and normalization, the integral curves superimpose with the phase diagram in their ascending branch and reach a plateau when the gel is fully melted. The shape of the thermograms upon cooling is also qualitatively explained within the frame of the diagrams.

Gel-to-gel non-variant transition of an organogel caused by polymorphism from nanotubes to crystallites.

An amide based gelator forms gels in trans-decalin. Below concentrations of 1 wt% the gels melt at temperatures varying with concentration. Above a concentration of 1 wt%, upon heating, the gel transforms into an opaque gel at an invariant temperature, and melts at higher temperature. The gel-to-gel transition is evidenced by several techniques: DSC, rheology, NMR, OM and turbidimetry. The phase diagram with the domain of the existence of both morphs was mapped by these techniques. Optical and electronic microscopy studies show that the first gel corresponds to the self-assembled nanotubes while the second gel is formed by crystalline fibers. The fibers are crystalline, as shown by the presence of Bragg peaks in the scattering curves. Both morphs correspond to a different H-bonding pattern as shown by FTIR. The first gel forms at a higher cooling rate, is metastable and transforms slowly into the second one. The second gel is stable. It forms at a low cooling rate, or by thermal annealing or aging of the first gel.

Structure of Nanotubes Self-Assembled from a Monoamide Organogelator.

Some organic compounds are known to self-assemble into nanotubes in solutions, but the packing of the molecules into the walls of the tubes is known only in a very few cases. Herein, we study two compounds forming nanotubes in alkanes. They bear a secondary alkanamide chain linked to a benzoic acid propyl ester (HUB-3) or to a butyl ester (HUB-4). They gel alkanes for concentrations above 0.2 wt.%. The structures of these gels, studied by freeze fracture electron microscopy, exhibit nanotubes: for HUB-3 their external diameters are polydisperse with a mean value of 33.3 nm; for HUB-4, they are less disperse with a mean value of 25.6 nm. The structure of the gel was investigated by small- and wide-angle X-ray scattering. The evolution of the intensities show that the tubes are metastable and transit slowly toward crystals. The intensities of the tubes of HUB-4 feature up to six oscillations. The shape of the intensities proves the tubular structure of the aggregates, and gives a measurement of 20.6 nm for the outer diameters and 11.0 nm for the inner diameters. It also shows that the electron density in the wall of the tubes is heterogeneous and is well described by a model with three layers.

Impact of Chirality on Hydrogen-Bonded Supramolecular Assemblies and Photoconductivity of Diketopyrrolopyrrole Derivatives.

Hydrogen bonds can efficiently guide the self-assembly of organic materials, enabling to tune the properties of the aggregation processes. In the case of π-conjugated materials, several parameters such as temperature, concentration and solvent can be used to modify the aggregation state while tuning the optoelectronic properties. Chirality can be included within the impacting parameters due to the differences in molecular packing. Here, chiral and achiral thiophene-capped diketopyrrolopyrrole derivatives were designed and synthesized containing amide bonds, with the aim to study the interplay between chiral assemblies and their stabilization through hydrogen-bonding. Differences in aggregation properties were observed with spectroscopy and microscopy, and a contactless microwave-based technique was used to study their intrinsic charge carrier mobility. The positive role of hydrogen-bonding has been highlighted and the differences between chiral and achiral compounds have been elucidated.