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.

Chitosan-Starch Films with Natural Extracts: Physical, Chemical, Morphological and Thermal Properties.

The aim of this study is to analyze the properties of a series of polysaccharide composite films, such as apparent density, color, the presence of functional groups, morphology, and thermal stability, as well as the correlation between them and their antimicrobial and optical properties. Natural antioxidants such as anthocyanins (from cranberry; blueberry and pomegranate); betalains (from beetroot and pitaya); resveratrol (from grape); and thymol and carvacrol (from oregano) were added to the films. Few changes in the position and intensity of the FTIR spectra bands were observed despite the low content of extract added to the films. Due to this fact, the antioxidants were extracted and identified by spectroscopic analysis; and they were also quantified using the Folin-Denis method and a gallic acid calibration curve, which confirmed the presence of natural antioxidants in the films. According to the SEM analysis, the presence of natural antioxidants has no influence on the film morphology because the stretch marks and white points that were observed were related to starch presence. On the other hand, the TGA analysis showed that the type of extract influences the total weight loss. The overall interpretation of the results suggests that the use of natural antioxidants as additives for chitosan-starch film preparation has a prominent impact on most of the critical properties that are decisive in making them suitable for food-packing applications.