Properties of β-carotene and retinoic acid in mixed monolayers with dipalmitoylphosphatidylcholine (DPPC) and Solutol

The liposoluble vitamins and carotenoids are sensitive molecules and their antioxidant properties can be dramatically affected when included in membrane environment or in encapsulated carriers as liposomes or lipid nanocapsules (LNCs). The formation and state of mixed monolayers at air–water interface of β-carotene (βC) or retinoic acid (RA) with dipalmitoylphosphatidylcholine (DPPC) as simplest membrane model or with Solutol (Sol) which forms the soft layer covering the LNCs were studied by means of surface pressure (π) and surface potential (ΔV) measurements. The mutual miscibility and interfacial organization of components in studied mixed monolayers was found to strongly depend on their composition. It was suggested a better inclusion of βC small quantities in pure DPPC monolayer at low surface pressure than for films with more βC at higher surface pressure. The measurements of surface properties were complemented by Atomic Force Microscopy (AFM) as a suitable technique for direct visualization at nanoscale level of the monolayer microheterogeneity and the presence of interfacial structures. It was also found that the addition of bigger amounts of βC led to the appearance of stripe like phase which is more deeply formed at higher surface pressure.