Inorganic nanoparticles modify the phase behavior and viscoelastic properties of non-lamellar lipid mesophases

The inclusion of inorganic nanoparticles (NPs) within organized lipid assemblies combines the rich polymorphism of lipid phases with advanced functional properties provided by the NPs, expanding the applicative spectrum of these materials. In spite of the relevance of these hybrid systems, fundamental knowledge on the effects of NPs on the structure and physicochemical properties of lipid mesophases is still limited. This contribution combines Small-Angle X-ray Scattering (SAXS) and Rheology to connect the structural properties with the viscoelastic behavior of liquid crystalline mesophases of Phytantriol (Phyt) containing two kinds of hydrophobic NPs of similar size, i.e., gold NPs (AuNPs) and Superparamagnetic Iron Oxide NPs (SPIONs). Both types of NPs spontaneously embed in the hydrophobic domains of the liquid crystalline mesophase, deeply affecting its phase behavior, as SAXS results disclose. We propose a general model to interpret and predict the structure of cubic mesophases doped with hydrophobic NPs, where the effects on lipid phase behavior depend only on NPs' size and volume fraction but not on chemical identity. The rheological measurements reveal that NPs increase the solid-like behavior of the hybrid and, surprisingly, this effect depends on the chemical nature of the NPs. We interpret these results by suggesting that the long-range dipolar interactions of SPIONs affect the viscoelastic response of the material and provide an additional control parameter on mechanical properties. Overall, this study discloses new fundamental insights into hybrid liquid crystalline mesophases doped with hydrophobic NPs, highly relevant for future applications, e.g. in the biomedical field as smart materials for drug delivery.