Gel phase (Lβ) formation by mixed saturated and unsaturated monoglycerides

Abstract

It is commonly thought that the α-gel phase (Lβ) consists of surfactant layers where the alkyl chains are mostly in the all-trans conformation and packed in a two dimensional hexagonal lattice with rotational disorder, but no translational mobility. We have examined the phase behaviour of mixed saturated and unsaturated monoglycerides [glycerol monostearate (GMS) and glycerol monooleate (GMO)] to investigate whether gel phases will be formed from unsaturated monoglycerides. In addition, this study examined the effect of aging on the phase behaviour of the mixture. An array of techniques was employed to investigate the phase behaviour as function of temperature: optical microscopy to observe optical textures, differential scanning calorimetry (DSC) to determine the transition enthalpies between phases, and nuclear magnetic resonance (NMR) to study surfactant mobility in the various phases. Additionally, we used small angle X-ray scattering (SAXS) to characterise the different mesomorphic phases at different temperatures. Finally, Fourier transform infrared spectroscopy (FTIR) was used to study the states of hydrogen bonding in the systems. Our results suggest that various phases for GMS/GMO mixture were observed α-gel (Lβ) phase, new α-gel (Lβ1) structure, β1 crystal phase, sub-α phase, β2-crystal, as well as an isotropic, L2, phase. Additionally there is some unsaturated monoglyceride incorporated into the α-gel phase of the saturated monoglyceride, which generated the new type of α-gel structure (Lβ1) characterised by a more disordered structural arrangement. GMO cannot pack in the same hexagonal structure as the α-gel phase of saturated lipids (Lβ). During storage, all monoglyceride structures transform to a highly ordered packing β crystal structure.