arXiv subjects

Madeleine Djabourov

Publications and source records attributed to Madeleine Djabourov.

5 recordsLinked to original sources

arXiv2020

Effect of Laponite on the structure, thermal stability and barrier properties of nanocomposite gelatin films

The effect of Laponite (a synthetic clay) on the structure, thermal and water vapor barrier properties of nanocomposite gelatin films produced by casting with 0, 4.5 and 15% w Lap/w gelatin, was studied. X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and Fourier transform infrared spectroscopy measurements were reported. The X-ray diffraction patterns showed dual crystalline structure of the films with collagen-type bundles of triple helices, intercalated inside clay platelets, increasing interlayer distances. Depending on the renaturation of triple-helices and Laponite content, the glass transition temperatures substantially decreased. The amount of helices decreased with Lap concentration, affecting the enthalpy of melting. The nanocomposite gelatin films showed improved thermal stability. Changes of water vapor permeability could be related to the presence of larger free volume of the coils and intercalated structures, facilitating water transfer through the film.

cond-mat.soft↗

arXiv2018

Pickering emulsions with alpha-cyclodextrin inclusions: Structure and thermal stability

This paper explores structural, interfacial and thermal properties of two types of Pickering emulsions containing alpha-cyclodextrin inclusion complexes: on one hand, emulsions were obtained between aqueous solutions of alpha-cyclodextrin and different oils (fatty acids, olive oil, silicone oil) and on the other hand, emulsions were obtained between these oils, water and micro or nano-platelet suspensions with inclusion complexes of hydrophobically-modified polysaccharides. The emulsions exhibit versatile properties according to the molecular architecture of the oils. Experiments were performed by microcalorimetry, X-ray diffraction and confocal microscopy. The aptitude of oil molecules to be threaded in alpha-cyclodextrin cavity is a determining parameter in emulsification and thermal stability. The heat flow traces and images showed dissolution, cooperative melting and de- threading of inclusion complexes which take place progressively, ending at high temperatures, close or above 100°C. Another important feature observed in the emulsions with micro-platelets is the partial substitution of the guest molecules occurring at room temperature at the oil/water interfaces without dissolution, possibly by a diffusion mechanism of the oil. Accordingly, the dissolution and the cooperative melting temperatures of the inclusion crystals changed, showing marked differences upon the type of guest molecules. The enthalpies of dissolution of crystals were measured and compared with soluble inclusions.

cond-mat.soft↗

arXiv2018

Auto-assemblies of Alpha-Cyclodextrin and Grafted Polysaccharides: Crystal Structure and Specific Properties of the Platelets

Cyclodextrins (CD) are a family of oligosaccharides with a toroid shape, which exhibit a remarkable ability to include guest molecules in their internal cavity providing a hydrophobic environment for poorly soluble molecules. Recently new types of inclusions of alpha CD with alkyl grafted polysaccharide chains (pullulan, chitosan, dextran, amylopectin, chondroitin sulfate) have been prepared which are auto assembled into micro and nano platelets. We report in this paper an extensive investigation of platelets with different compositions, including their reversible hydration (Thermo Gravimetric Analysis), crystalline structure (Powder X Rays Diffraction), dimensions and shapes, (Scanning Electron Microscopy Field Emission Gun), thermal properties, solubility and melting (Micro Differential Scanning Calorimetry). The crystalline platelets exhibit layered structures intercalating the polysaccharide backbones and CD complexes hosting the grafted alkyl chains. The monoclinic symmetry of columnar type crystals suggests a head to tail arrangement of the CDs. The platelets have a preferentially hexagonal shape with sharp edges, variable sizes, and thicknesses and sometimes show incomplete layers (terraces). The crystal parameters change upon dehydration. Melting temperatures of platelets in aqueous solutions exceeds 100 degree. Finally, we discuss the potential relation between the platelet structure and applications for mucoadhesive devices.

cond-mat.soft↗

arXiv2018

Dehydration, Dissolution and Melting of Cyclodextrin Crystals

Cyclodextrins are a family of oligosaccharides with a toroid shape which exhibit a unique ability of entrapping guest molecules in their internal cavity. Water is the primary guest molecule and is omnipresent in the crystalline phases stabilizing the overall architecture. Despite the presence of water molecules inside the cavity, cyclodextrins provide a hydrophobic environment where poorly soluble molecules can easily fit. In this investigation we put in evidence different types of water in the hydrated α , \b{eta} and γ cyclodextrin crystals. Thermogravimetric measurements identify various binding sites of water and highlight the difference between the crystals equilibrated under various humid atmospheres. We establish by microcalorimetry the limit of solubility versus temperature and measure for the first time the melting temperatures of the hydrated crystals. Dissolution and melting enthalpies are derived and the solubility curves are compared to existing literature. The specific features of each cyclodextrin are underlined.

cond-mat.soft↗

arXiv1999

On the Viscosity of Emulsions

Combining direct computations with invariance arguments, Taylor's constitutive equation for an emulsion can be extrapolated to high shear rates. We show that the resulting expression is consistent with the rigorous limits of small drop deformation and that it bears a strong similarity to an a priori unrelated rheological quantity, namely the dynamic (frequency dependent) linear shear response. More precisely, within a large parameter region the nonlinear steady-state shear viscosity is obtained from the real part of the complex dynamic viscosity, while the first normal stress difference is obtained from its imaginary part. Our experiments with a droplet phase of a binary polymer solution (alginate/caseinate) can be interpreted by an emulsion analogy. They indicate that the predicted similarity rule generalizes to the case of moderately viscoelastic constituents that obey the Cox-Merz rule.

physics.flu-dyn↗