Semi-fluorinated phosphonic acids form stable nanoscale clusters in Langmuir–Blodgett and self-assembled monolayers

Abstract

We compared molecular monolayers of semifluorinated phosphonic acids (F8H11PO₃, F10H6PO₃, F8H8PO₃, and F6H10PO₃) on mica substrates prepared by two different methods: Langmuir–Blodgett (LB) transfer of pre-formed monolayers from the air–water interface, and self-assembled monolayers (SAMs) formed spontaneously at the interface between mica and solution (with tetrahydrofuran as the solvent). The films were investigated with atomic force microscopy (AFM) and contact angle measurements. Nanometer-scale two-dimensional (2D) clusters (20–30 nm in size) were observed in both the LB films and the SAMs. Time-dependent AFM images suggested that for SAMs derived from F8H11PO₃, F8H8PO₃, and F6H10PO₃, small clusters nucleated, but stopped growing once a stable size was reached. In situAFM images suggested that the clusters were dome-shaped. The LB monolayer structures were consistent with those of the SAMs, but with greater long-range order of the clusters in some cases. For SAMs derived from F10H6PO₃, however, long immersion times led to continued growth and coalescence of the 2D clusters and formation of a flat, untextured monolayer. Similarly, LB films generated from F10H6PO₃ exhibited cluster coalescence. The similarity of the observed structures in LB films and SAMs suggests that these nanostructured films represent an equilibrium state based on intrinsic self-organization of the semifluorinated molecules, even in SAMs, where mobility is significantly restricted at the solid–liquid interface. Based on these observations, we hypothesize that the packing incommensurability between the hydrocarbon and fluorocarbon blocks leads to splay of neighboring chains and spontaneous curvature. However, when intermolecular interactions are dominated by a long fluorocarbon block, there is a transition to a flat structure, where the packing mismatch is compensated for by increased disorder within the hydrocarbon block.