Single-Molecule Fluorescence Spectroscopy of Phase-Separated 10,12-Pentacosadynoic Acid Films

Phase-separated monolayers of 10,12-pentacosadiynoic acid and perfluorotetradecanoic acid can be photopolymerized to produce micrometer-sized, fluorescent polydiacetylene fibers at the air-solid interface. The photopolymer fibers were not uniformly fluorescent but rather showed a series of fluorescent spots along their lengths. The spots exhibited the classic properties of single-molecule fluorescence emission, including diffraction-limited size and fluorescence intermittency ("on-off blinking"). We have analyzed the fluorescence blinking dynamics of these spots using a variety of single-molecule analysis approaches, including fluorescence intensity histograms, autocorrelation analysis, as well as cross-correlation analysis as a function of distance between individual transition dipole moments, and propose a simple physical model for the fiber structure based on the observed blinking dynamics, in which the polymer fibers contain numerous structural defects. The model was supported by grazing incidence X-ray diffraction measurements of the mixed monolayer films at the air-water interface, in which it was observed that the presence of perfluorocarbon in the mixed monolayers significantly inhibited the ability of the 10,12-pentacosadiynoic acid to polymerize.

Immiscible Anionic Gemini Surfactant-Perfluorinated Fatty Acid Langmuir Monolayer Films

A new member of the N,N,N',N'-dialkyl-N,N'-diacetate ethylenediamine family of anionic gemini surfactants has been synthesized, and its miscibility with the model perfluorocarbon, perfluorotetradecanoic acid (PF), has been investigated in monolayer films at the air-water interface. Thermodynamics of mixing and the accompanying changes in the mixed film structure have been probed using a combination of compression isotherm measurements supported by Brewster angle microscope imaging and X-ray scattering measurements, and results have been compared with those collected for a previously studied, shorter tail chain variant of the surfactant. Thermodynamic measurements showed that the gemini surfactant and perfluorotetradecanoic acid were immiscible, with weak repulsive interactions, manifesting as small positive deviations from ideal mixing, observed between the two film components. Films were highly textured, with micrometer-scale, phase-separated domains readily detectable. Grazing incidence X-ray diffraction measurements showed that the gemini surfactant was disordered in the monolayers, whereas the perfluorocarbon formed discrete crystallites in the disordered matrix. Despite the small deviations from ideal mixing detected in the thermodynamic measurements, the X-ray measurements indicated that the presence of the gemini perturbs the PF crystal lattice from that of pure PF. Finally, X-ray reflectivity measurements showed that the addition of equimolar PF to the gemini monolayer induces a significant increase in the nominal head group thickness of the film, suggesting that interactions between the two surfactants can lead to structural rearrangements of gemini's head group near to the water surface.

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

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, C₁₉H₃₉COOH) with perfluorotetradecanoic acid (PA, C₁₃F₂₇COOH), 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_xy. 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.

Mixing Behavior in Binary Anionic Gemini Surfactant-Perfluorinated Fatty Acid Langmuir Monolayers

The miscibility and film structure of mixed Langmuir monolayer films composed of an anionic gemini N,N,N',N'-dialkyl-N,N'-diacetate ethylenediamine surfactant (Ace(12)-2-Ace(12)) with perfluorotetradecanoic acid (C₁₃F₂₇COOH; PF) have been investigated using a variety of thermodynamic and structural characterization methods. The two film components were found to be miscible in monolayers at the air-water interface over a range of compositions and at all but the lowest surface pressures, with attractive interactions occurring between the two components. While pure PF monolayers formed crystalline lattices with hexagonal symmetry and with the surfactant tails oriented normal to the underlying water subphase, the pure gemini surfactant formed amorphous films with little tendency to orient at the subphase. In mixed films with mole ratios of PF:Ace(12)-2-Ace(12) < 2.5, the miscibility of the two components resulted in a nearly complete loss of crystallinity of the PF, though films at higher mole fractions of PF showed some residual crystallinity, albeit with lattice structures that were significantly different from that of pure PF. Miscibility and film structure in this mixed system are discussed in comparison with other mixed gemini surfactant systems in the literature as well as binary mixtures of phospholipids or monomeric fatty acids with PF.