Equilibrium morphologies of nonionic lipid-nanoparticle mixtures in water: a self-consistent mean-field prediction

We model an aqueous system which comprises a mixture of lipid molecules and hydrophobic nanoparticles, by means of a self-consistent mean-field theory (SCMFT). Lipids are modeled as nonlinear/branched copolymers with a single hydrophilic head group and a double hydrophobic tail, whereas nanoparticles are modeled as hard-spheres of a particular size. The mixture of lipids and nanoparticles leads to a formation of core-shell micellar structures where the hydrophobic nanoparticles and lipid tails form the core of the micelle, and the hydrophilic lipid head groups form the shell. Different micellar morphologies are found depending on the total concentration of lipid molecules and nanoparticles, as well as the relative size of nanoparticles. There exist three distinct equilibrium morphologies of lipid-nanoparticle micelles: circular micelles (CM), ellipsoidal micelles (EM), and bilayer/lamellar structures (BL). We observed some smooth morphological transitions and phase coexistences by evaluating the excess free energy of micelles.