Origin of the central pit in hemimicelles of semifluorinated alkanes: How molecular dipoles and substrate deformation can determine supra-molecular morphology.

HYPOTHESIS:Semifluorinated alkanes amphiphiles spontaneously form highly monodispersed hemimicelles at the surface of water. The origin of the formation and complex structure of these surprising supramolecular aggregates were only recently clarified using molecular dynamics simulations (MD). The existence of a pit at the center of these aggregates made up of almost 3000 molecules was indeed reproduced by the MD simulations, but not understood. METHOD:A careful strategy of atomistic MD simulations comparing non-electrostatic force fields with force fields that include electrostatic forces, thus bearing an implicit or explicit dipole, allowed demonstrating the roles of dipolar interactions and interactions with the liquid subphase on the morphology of the aggregates. FINDINGS:The simulation results clearly show that within the hemimicelles the strong molecular dipoles located at the CH2-CF2 junctions tend to align, leading to a collective shift of the PFAA molecules relatively to each other. This shift is responsible for the curvature of the hemimicelles and originates the central pit, provided the possibility of deforming the surface of the water sub-phase. Comparisons with non-electrostatic force field results further contribute to understand the origin of the self-assembling process. The results directly connect for the first time a molecular property with a mesoscopic structural feature. Given the molecular simplicity of these "primitive" amphiphiles compared to the common hydrophilic/hydrophobic surfactants, the results contribute to understand self-assembly in general.