Interfacial Organization of β-Carotene Loaded Lipid Nanocapsules Modelized by 2D Rheological Measurements and Electrophoretic Mobility

Lipid nanocapsules (LNCs) were developed according to phase inversion temperature (TIP) process with polyoxyethylene 40 stearate (POE 40 stearate) or polyethyleneglycol 660 12-hydroxystearate (PEG 15 stearate), since they are able to change from hydrophilic to hydrophobic as function of temperature, in order to encapsulate lipophilic pro-vitamin (beta-carotene (beta C)). It has been previously shown that PEG 15 stearate LNCs were stable during storage unlike POE 40 stearate LNCs whose mean diameters increased accordingly to Lifshitz-Slyozov-Wagner theory. POE 40 stearate LNCs are thus destabilized by Ostwald ripening. The purpose of this study was to understand the role of electrostatic interaction and LNCs excipients at the interface during particle maturation process. Electrophoretic mobility of both types of LNCs did not evolve during storage. As capsules shells and their electrical characteristics were unchanged over time, LNCs electrostatic properties could not explain discrepancies between behaviour of particles during storage. Adsorption kinetics of POE 40 stearate, PEG 15 stearate, phospholipids and beta C at oil/water interface were studied using a drop tensiometer as well as the rheological properties of resulting interfacial films. Interfacial behaviour was interpreted using a model corresponding to a two-dimensional viscoelastic solid. Interfacial layers were characterised first by equilibrium tension, the rheological parameters (storage and loss dilational moduli and relaxation time) were then calculated for each system. The encapsulated molecule (beta C) strongly adsorbed at oil/water interface and exerted lateral interactions with phospholipids and the macromolecular surfactant (POE 40 stearate). Phospholipids/POE 40 stearate film stability was not affected by the presence of beta C. POE 40 stearate pure and mixed films were found less stable than PEG 15 stearate pure and mixed films. Low cohesive properties of interfacial molecule in PEO 40 stearate mixed films might explain why POE 40 stearate based LNCs are less stable during storage than PEG 15 stearate LNCs.