Structural and rheological investigation of Fd3m inverse micellar cubic phases
M. Pouzot, R. Mezzenga, M. Leser and L. Sagalowicz
Nestlé Research Center, CH
food liquid crystals, cubic phases, Fd3m bulk mesophase, self-assembly, shear rheology, phase transition, relaxation spectrum, order-disorder transition, order-order transition
Liquid crystalline phases (bicontinuous cubic, reversed hexagonal, lamellar) can be made easily from food emulsifiers such as monoglycerides. Recently we explored in detail the formation of an inverse discrete micellar Fd3m cubic phase, which usually forms between reversed hexagonal and L2 microemulsion phases. In the present study we demonstrate that Fd3m cubic phase can be obtained by adding a hydrophobic component, such as the food grade limonene, to the binary system monolinolein (i.e. classical food emulsifier) / water in a well defined composition. This phase does not exist in the pure binary mixture, which only exhibits inverse cubic phases of a bicontinuous type such as Pn3m and Ia3d. The Fd3m studied in this work has very slow kinetics of formation, as a consequence of partitioning of water into two type of micelle population with different size. The Fd3m formed at a = 40%wt is stable in bulk up to a maximum hydration of 12.68%wt water, beyond which it starts to coexist with dispersed water. At full hydration SAXS measurements as well as topological models have shown that the Fd3m phase is constituted by 16 smaller micelles and 8 larger micelles per cubic lattice cell (Q227 group), having radii for the micellar polar cores ranging between 1 and 3 nm, with average size of 2.3nm, and 170 monolinolein molecules per micelle. Temperature dependence of the structural and rheological properties of the Fd3m mesophase has been investigated using SAXS, rheology and turbidimetry. It appears that the Fd3m phase undergoes crystallization below 18°C and begins to melt in an inverse microemulsion (L2 phase) probably with water in excess above 28.5°C with complete melting obtained at 40-45°C, as evidenced by SAXS and rheology. Macroscopic phase separation between the L2 phase and excess water is observed with time at higher temperature. Investigation of the viscoelastic properties of Fd3m inverse discrete micellar cubic phase has shown a rheological signature similar to the bicontinuous cubic phases Pn3m and Ia3d observed in homologue binary systems. However Fd3m presents a complex set of slower relaxation mechanisms which lead, compared to inverse bicontinuous cubic phases, to a shift by one order of magnitude of the dominant relaxation times and whole relaxation spectrum. These findings have been tentatively explained by (i) the multiple relaxation micelles undergo upon deformation, (ii) the small hydration level of the Fd3m phase and (iii) by the low temperature at which this phase can be observed.
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