Solid lipid nanoparticles and nanostructured lipid carriers of dual functionality at emulsion interfaces. Part I: Pickering stabilisation functionality

Solid lipid nanoparticles and nanostructured lipid carriers are two types of lipid nanoparticulate systems, that have been primarily studied for their capability to function as active carriers, and only more recently utilised in Pickering emulsion stabilisation. Unveiling the factors that impact upon the lipid particle characteristics related to their Pickering functionality could enable the development of a liquid formulation with tailored microstructure and potentially the capacity to display a two-fold performance. In part I, this work investigates how certain formulation characteristics, namely solid-to-liquid lipid mass ratio and presence of unadsorbed surfactant in the aqueous carrier phase, affect the structural properties of the lipid particles, and in turn how these influence their Pickering stabilisation capacity.

Highlights

• The lipid matrix composition affects the interfacial tension reduction ability.
• The affinity of the lipid matrix components influences the lipid particle hydrophilicity.
• Both SLNs and NLCs can provide Pickering emulsion stabilisation.
• The lack of unadsorbed surfactant in the continuous phase affects the emulsion droplet size.
• The particles maintained close association with the emulsion interface for up to 12 weeks.

The effect of the formulation parameters was assessed in terms of the wettability and physicochemical properties of the lipid particles, including particle size, crystallinity and interfacial behaviour. Lipid particles fabricated with higher liquid lipid content (70% w/w) were shown to be more hydrophilic and have lower surfactant decoration at their surface compared to particles containing lower or no liquid lipid in their crystalline matrix. The emulsion stabilisation ability through a Pickering mechanism was confirmed for all types of lipid particles using polarised microscopy. Increasing liquid lipid content and removal of excess surfactant did not compromise the particle stabilisation capacity, though emulsion droplets of larger sizes were initially acquired in the latter case. The particle-stabilised emulsions maintained their physical integrity, with particles retaining close association with the emulsion interface over a storage period of 12 weeks.

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Materials

Glyceryl behenate (Compritol® 888 ATO) was kindly provided from Gattefossé (Saint-Priest, France). Medium chain triglycerides (MCTs) (Miglyol® 812) was a kind gift from IOI Oleo (IOI Oleochemicals GmbH, Germany). Polyoxyethylene sorbitan monooleate (Tween® 80), perylene and Nile Red were purchased from Sigma-Aldrich (Sigma-Aldrich, UK). Sunflower oil was purchased from a local supermarket and stored in a closed container at ambient temperature in the dark. The consistency of the oil used in this work was monitored through interfacial tension measurements between distilled water and the used commercial oil (at least on a weekly basis). No significant deviation in the equilibrium value of the interfacial tension was observed (21.8 ± 0.3 mN/m). All chemicals were used without further purification. Double distilled water from Milli-Q systems (Millipore, Watford, UK) was used during all sample preparation processes and characterisation measurements.

Georgia I. Sakellari, Ioanna Zafeiri, Hannah Batchelor, Fotis Spyropoulos, Solid lipid nanoparticles and nanostructured lipid carriers of dual functionality at emulsion interfaces. Part I: Pickering stabilisation functionality, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2022, 130135, ISSN 0927-7757,
https://doi.org/10.1016/j.colsurfa.2022.130135.