Morphology and Composition of Structured, Phase-Separated Behenic Acid–Perfluorotetradecanoic Acid Monolayer Films

Abstract

The phase separation of immiscible surfactants in mixed monolayer films provides an approach to physically manipulate important properties of thin films, including surface morphology, microscale composition, and mechanical properties. In this work, we predict, based upon existing miscibility studies and their thermodynamic underpinnings described in the literature, the miscibility and film morphology of mixed monolayers comprised of behenic acid (C21H43COOH) and perfluorotetradecanoic acid (C13F27COOH) in various molar ratios. Predictions are tested using a combination of experimental surface characterization methods for probing miscibility and film morphology at the solid/air and air/water interfaces. Film components were immiscible and phase-separated into chemically well-defined domains under a variety of experimental conditions, with monolayer morphology consistent with initial predictions. The extensibility of these basic predictions to other systems is discussed in the context of using these works for different perfluorinated surfactant molecules.

Pattern Formation in Phase-Separated Langmuir and Langmuir Monolayer Films

“…In the BAM, hydrocarbon-rich domains contrast well with fluorocarbon-rich regions in the mixed films, however, the main challenges are the unambiguous identification of fluorocarbon domains, which cannot be distinguished from the water region, and the microscope resolution. Nevertheless, as highlighted by the arrow in Figure A, the polygonal-shaped domain in excellent agreement with AFM studies for the film deposited on mica (Figure B).Under the AFM, differences in domain height can be measured, and by comparing which with the film thickness (obtained by “scratch test” ,,, ) and the molecular structures of each components (calculated using molecular mechanics), we can confirm that in general, the hydrocarbons form hexagonal/polygonal domains with fluorocarbons being the surrounding matrix.…”

“…Compression isotherms for C21H (behenic acid, labeled BA), C13F (perfluorotetradecanoic acid, labeled PF), and their mixed monolayers at various mole fraction mixtures. Adapted with permission from ref . Copyright 2016 American Chemical Society.…”

“…Unfortunately, examples abound where this is not the case: Gleiche et al demonstrated that, because of dynamic wetting instability during film transfer, deposition of a single-component phospholipid film the monolayer onto a mica surface may cause formation of nanoscale patterns . To assess the role of the deposition process on film morphology in mixed hydrocarbon–fluorocarbon monolayer, and hence the reliability of morphological studies on such systems using AFM, our group has also conducted a comparison of film morphology with two different hydrocarbon–fluorocarbon mixtures (C13F with C19H and C17F with C15H) at the air–water (examined under BAM) and air–solid (examined under AFM) interfaces. ,, Results have generally shown excellent consistency between BAM and AFM results, suggesting deposition effects in these systems are minimal. Similar observations have been reported for the SFA systems .…”

“…AFM image of phase-separated domain structures formed in various mixed-monolayer systems with H:F ratio = 2:1: (A) C21H–C13F (image scale 15 μm × 15 μm), (B) C19H–C13F (image scale 20 μm × 20 μm), (C) C17H–C13F (image scale 11 μm × 11 μm) . Adapted with permission from corresponding references.…”

Langmuir Films of Perfluorinated Fatty Alcohols: Evidence of Spontaneous Formation of Solid Aggregates at Zero Surface Pressure and Very Low Surface Density

“…This intrinsic "antipathy" between hydrogenated and perfluorinated substances has been the basis for a number of studies on the segregation and 2D phase-separation in Langmuir films of mixtures of hydrogenated and perfluorinated carboxylic acids [36][37][38][39][40][41][42] and alcohols [43,44].…”