On the sub‐α‐form and the α‐form in monoacylglycerols

Abstract

Abstract The concentration of 2‐monostearoyl‐glycerol (2‐MS) in 1‐MS kept at 100°C reaches a plateau concentration of about 12% (w/w) according to 1 H nuclear magnetic resonance. The phase diagram of these two components, made by differential scanning calorimetry (DSC), showed that the temperature of the transition from the sub‐α 1 ‐form to the α‐form and the melting point of the α‐form decreased when the content of 2‐MS increased above 9%. The temperature of transition from the sub‐α 2 ‐form to the sub‐α 1 ‐form, however, decreased at even lower contents of 2‐MS. The reversible phase transition of mixtures of 2‐MS and 1‐MS from the sub‐α‐form to the α‐form has been followed by DSC and X‐ray diffraction. Concentrations of 2‐MS above 9% change the tilt of the molecules in the mixture abruptly during the transition from the sub‐α‐form to the α‐form. At higher temperature, the conformational disorder in the α‐form of the mixture changes continuously. This conformational disorder is, however, unstable enough to change gradually into a more stable one. Even for pure 1‐ rac ‐MS, small changes were observed. In addition, monobehenoyl‐glycerol and monopalmitoyl‐glycerol, which also contained 2‐monoacyl‐glycerol, were also investigated. The longer fatty acid chains tend to give a higher degree of conformational disorder. The transition behavior has been analyzed on the basis of the crystal structures. Water swelling behavior has also been studied and compared for the sub‐α‐form and the α‐form. The α‐form can incorporate more water, probably because it has more disordered polar groups.