Quantitative liquid-state ¹H NMR as a tool to map phase diagrams of oleogels

Oleogelators are compounds able to gel edible oils at concentrations of a few weight percent. They are widely studied as texturing agents for edible oils, and the resulting oleogels are considered healthier alternatives to solid fats. Understanding the phase behavior of gelator/oil, systems and mapping their c-T phase diagrams allows control of melting and formation parameters and also reveal the possible presence of polymorphs, eutectics or phase separation. The purpose of this study is to show how variable-temperature liquid-state nuclear magnetic resonance (NMR) spectroscopy can be used to map the concentration-temperature (c-T) phase diagrams of oleogels. Three gelators were selected for this investigation - DL-12-hydroxystearic acid (12-HSA), N-palmitoylethanolamide (Palm-EA), and N-palmitoyl-L-phenylalanine (Palm-Phe) - because their phase diagrams have already been reported in the literature. In addition to a classical gel-sol transition, Palm-Phe exhibits a polymorphic gel-gel transition, providing a test case to evaluate the technique. Using deuterated triolein as the solvent and a single sample per system, the method yields the liquidus line with more than 20 data points across a concentration range spanning more than one order of magnitude (typically ∼ 0.1 to ∼ 5 wt%). The liquidus lines obtained by NMR match those derived from differential scanning calorimetry (DSC), with deviations typically under 3 °C. Rheological measurements report gel-sol transition temperatures a few degrees lower than those obtained by NMR and DSC, reflecting the mechanical rather than thermodynamic nature of the transition. With Palm-Phe gels, the liquidus of the stable polymorph was measured with the same precision after annealing. Before annealing, the measured curves showed a marked difference due to gel-gel transition. The phase diagrams measured in triolein are close to those measured in rapeseed oil, indicating that triolein reproduces the phase behavior of the studied gelators in rapeseed oil.