Investigation of the Gel to Coagel Phase Transition in Monoglyceride−Water Systems

Abstract

The gel to coagel phase transition of monoglyceride−water systems has been studied by nuclear magnetic resonance and differential scanning calorimetry. It is shown that the molecular arrangements within the bulk β-crystal of monoglyceride and the coagel phase are identical. The mobility of the glycerol backbone is high in the gel phase. On a decrease of temperature the mobility drops and then crystallization of the gel into a coagel phase takes place where hydrogen bonds among the glycerol groups are formed. A prerequisite for this is that d and l isomers of monoglyceride rearrange within the bilayers through chiral discrimination. The gel to coagel transition can be discussed in the frame of the Avrami theory of crystallization kinetics. The values of the Avrami exponent indicate a transition from a 2-dimensional to a 3-dimensional crystal growth on increasing the monoglyceride concentration. In the case of a 2-dimensional crystal growth the phase transition is diffusion controlled. Moreover, on applying shear to the gel phase, the phase transition evolves from a random to a spontaneous nucleation mechanism.

Rheological Characterization, Crystallization, and Gelation Behavior of Monoglyceride Gels

“…The transitions are similar to what is found for the bulk crystallization behavior of monoglycerides: melt → α-crystalline → β -crystalline (12,13). The molecular nature of these states has been elaborately described before (9,11).…”

“…The extent of α-gel to coagel transition was expressed as the so-called coagel index (9). This was measured by DSC, using a Perkin-Elmer DSC-7 differential scanning calorimeter.…”

“…These plates can form a firm but brittle network, capable of holding up to 50 times its own weight on water. The molecular details of these phase transitions have been studied by DSC and NMR (9)(10)(11).…”

Aging and Metastability of Monoglycerides in Hydrophobic Solutions

“…At a lower temperature, the lamellar phase exists over a wide range of concentrations. 1 According to the literature, below the Krafft temperature, T K , the lamellar phase transforms into the alpha-crystalline phase which contained the hexagonal surfactant packing, characterized by a single X-ray short spacing at 4.18Å. 17,18 Results of our recent work 5 indicate that the same structural feature is seen in the inverse-lamellar phase of MG in the oil matrix, where the characteristic 4.17Åspacing arises from the hexagonal ordering of glycerol heads in the tightly compressed mid-bilayer planes (while the aliphatic tails remained disordered, although laterally densely packed).…”

Self-Organization of Gelator Molecules Inside an Organogel

“…In most of the cases reported in the literature, the gelator NMR signals completely disappear in the gel state while they are present in the liquid state. 8 This can be explained by the fact that the gel fiber can be considered as a crystal of the gelator in which the molecular motion is very limited and the solvent molecules are excluded from the fiber. Therefore, disappearance of the 1 H NMR peaks has been described to be one criterion for "dry gels".…”