Molecular-Level Structure and Packing in Phase-Separated Arachidic Acid–Perfluorotetradecanoic Acid Monolayer Films

Abstract

Synchrotron-based X-ray scattering measurements of phase-separated surfactant monolayers at the air-water interface provide molecular-level structural information about the packing and ordering of film components. In this work, grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR) measurements were used to collect crystallographic structural information for binary mixed monolayers of arachidic acid (AA, CHCOOH) with perfluorotetradecanoic acid (PA, CFCOOH), a system that has previously been investigated using a variety of thermodynamic and micron-scale structural characterization methods. GIXD measurements at surface pressures of π = 5, 15, and 30 mN/m indicated that AA in pure and mixed films forms a rectangular lattice at π = 5 and 15 mN/m but a hexagonal lattice at π = 30 mN/m. PA formed hexagonal lattices under all conditions, with films being highly ordered and crystalline (as determined by Bragg peak width) at even the lowest surface pressures investigated. Phase separation occurred for all mixed monolayer film compositions and surface pressures, manifesting as diffraction peaks characteristic of the individual components appearing at different in-plane scattering vector q. For both pure and mixed films, the molecular tilt angle of the AA hydrocarbon chain toward the nearest-neighbor was substantial at low pressures but decreased with increasing pressure. The PA fluorocarbon chain showed negligible molecular tilt under all conditions, and was oriented normal to the subphase surface regardless of surface pressure or the presence of AA in the films. In all cases, the two components in the mixed film behaved entirely independently of film composition, which is exactly the expected result for a fully phase-separated, immiscible system. XR measurements of film thickness at the air-water interface supported these results; overall film thickness approached the calculated ideal surfactant tail lengths with increasing surface pressure, indicating nearly normal oriented surfactants. The overall surfactant packing and crystallographic features of the mixed monolayers are discussed in terms of the lipophobic nature of the perfluorinated surfactant as well as in context of thermodynamic miscibility and domain structure formation reported elsewhere in the literature for these mixed monolayer systems.

Pattern Formation in Phase-Separated Langmuir and Langmuir Monolayer Films

“…The driving force behind forming the distinctive polygonal domains in the mixed monolayers remains up for debate. The recent works by Silva et al support the concept of the rigid perfluorocarbon matrix “templating” polygonal void regions, which are then occupied by the hydrocarbons; further advances in this area can be realized by additional MD simulations on mixed fatty acid systems to support experimental GIXD results reported previously . The kinetics and detailed mechanism of domain formation and growth in these systems is also under explored; whereas static AFM “snapshot” experiments of film structure indicate domain growth occurs through diffusion-limited Ostwald ripening, and support for this can be found in some BAM experiments, a more in-depth exploration of this mechanism is needed.…”

“…Typical GIXD data from a mixed-monolayer system is illustrated in Figure , in which GIXD contour plots (scattering intensity along q z vs q xy ) are shown for monolayers of arachidic acid (AA; C19H), perfluorotetradecanoic acid (PA, C13F), and their mixtures . For the pure C19H monolayer, the contour plot shows two Bragg Peaks at q xy ≈ 1.40 and 1.46 Å –1 , corresponding to a distorted hexagonal lattice.…”

“…GIXD contour plot shown intensity along q z vs intensity along q xz for (A) pure C18H (labeled AA), (B) pure C13F (labeled PA), and (C, D) their mixed film (H:F = 1:1 and 2:1) at π = 5 mN m –1 . Adapted with permission from ref . Copyright 2019 American Chemical Society.…”

“…The most recent hypothesis (shared by others; see below) as to the source of the polygonal structures is that it is a consequence of the rigidity and crystallographic packing of the perfluorcarbon matrix. We have recently investigated the formation of polygonal domains in a typical mixed fatty acid monolayer (C19H–C13F) at the molecular level using GIXD and XR . As shown earlier in Figure , C13F, either alone or in mixtures, forms a highly crystalline matrix with a hexagonal lattice, which is unaffected by the compression pressure, whereas the hydrogenated fatty acid (C19H) forms rectangular centered lattice with an NN-tilted phase at lower surface pressure, which becomes less tilted as the surface pressure increases, and eventually transforms into an untilted S-phase at π = 30 mN/m.…”

“…(A) BAM image of a C19H–C13F mixed monolayer (H:F = 2:1) at the air–water interface at π = 10 mN m –1 , and (B) AFM image of a phase-separated C19H–C13F (H:F = 2:1) mixed monolayer deposited on a mica substrate at π = 20 mN m –1 from an air–water interface (image scale 20 μm × 20 μm). Adapted with permission from ref . Copyright 2019 American Chemical Society.…”