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

Abstract

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.

Impact of polymorphism in oleogels of N-palmitoyl-l-phenylalanine

“…The melting temperatures were measured at the inflection point following the maximum of the endotherm, rather than at the maximum itself, as justified by our previous work. 49 The molar enthalpies were calculated by integration of the peaks and their normalizaton to the number of moles of the gelator.…”

Editorial on Special Issue “Organogels: State of the Art”

“…Michael A. Rogers et al [ 3 ] contribute to this Special Issue with a study of 1,3:2,4-Dibenzylidene sorbitol (DBS)-polyethylene glycol organogels and the influence of solvent viscosity on the SAFiN fractal growth, concluding that the solvent viscosity has a higher impact on the supramolecular fractality than the crystallization temperature, in agreement with gelation through a diffusion-limited aggregation process. The contribution of Philippe J. Mésini et al [ 4 ] reports on the quantitative relationship between the shapes of the thermograms and phase diagrams of organogels based on 12-hydroxystearic acid in nitrobenzene, studied by micro-differential scanning calorimetry. Compared to the transition temperatures measured by other techniques, the inflection points of the thermograms measured with micro-DSC provide a melting temperature value with less bias than the temperature at the maximum of the endotherm.…”