Self-assembled hemimicelles of perfluoroalkylalkanes: How chain length, shape, and dipole determine internal structure. A new (geometrical + electrostatic) model

Abstract

Perfluoroalkylalkanes form nanostructured Langmuir films comprising discrete surface aggregates or hemimicelles. The aggregates are formed by a large number of molecules (∼2000–3000), are highly monodisperse and round-shaped, and display a characteristic pit in their center. In this study, the influence of the hydrogenated and the perfluorinated chain lengths on the size of the hemimicelles was probed by a systematic atomistic molecular dynamics simulation study and further rationalized in terms of a model that describes the internal structure of the hemimicelles. The model, while set on geometrical considerations, was developed using quantitative information obtained from the molecular dynamics simulations performed using an atomistic force field that includes detailed electrostatic and dispersive intramolecular and intermolecular interactions. These interactions are therefore intrinsically incorporated in the model, which is consequently designated geometrical + electrostatic (G+E model). Eleven PFAA molecules (F8Hm: F8H14, F8H16, F8H18, F8H20; FnH16: F6H16, F8H16, F10H16, F12H16; F10Hm: F10H14, F10H18, F10H20) and the corresponding hemimicelles were studied, covering a representative range of molecular structures. The results of both the molecular dynamics simulations and the new model reproduce the available experimental data within the respective uncertainties. The results provide a rational basis for a complete understanding of the self-assembly process of PFAA molecules into discrete hemimicelles, well-founded in physical principles and molecular properties.