Effect of the Molecular Structure on the Hierarchical Self-Assembly of Semifluorinated Alkanes at the Air/Water Interface

Abstract

Semifluorinated alkanes (C(n)F(2n+1)C(m)H(2m+1)), short FnHm display local phase separation of mutually incompatible hydrocarbon and fluorocarbon chain moieties, which has been utilized as a structure-forming motif in supramolecular architectures. The packing of semifluorinated alkanes, nominally based on dodecyl subunits, such as perfluoro(dodecyl)dodecane (F12H12) and perfluoro(dodecyl)eicosane (F12H20), as well as a core extended analogue, 1,4-dibromo-2-((perfluoroundecyl)methoxy)-5-(dodecyloxy)benzene) (F11H1-core-H12), was studied at the air/water interface. Langmuir monolayers were investigated by means of neutron reflectivity directly at the air/water interface and scanning force microscopy after transfer to silicon wafers. Narrowly disperse surface micelles formed in all three cases; however, they were found to bear different morphologies with respect to molecular orientation and assembly dimensionality, which gives rise to different hierarchical aggregate topologies. For F12H12, micelles of ca. 30 nm in diameter, composed of several circular or "spherical cap" substructures, were observed and a monolayer model with the fluorocarbon block oriented toward air is proposed. F12H20 molecules formed larger (ca. 50 nm diameter) hexagonally shaped surface micelles that were hexagonally, densely packed, besides more elongated but tightly interlocked wormlike structures. Conversely, F11H1-core-H12 films organized into linear rows of elongated surface micelles with comparable width, but an average length of ca. 400 nm, apparently formed by antiparallel molecular packing.

Spontaneous self-assembly and structure of perfluoroalkylalkane surfactant hemimicelles by molecular dynamics simulations

“…Several factors contribute to the observed reorganization of orientations: hydrogenated chains are less hydrophobic and thus have lower interfacial tension with water than fluorinated chains; perfluorocarbons have lower surface tensions than hydrocarbons; the interactions between hydrocarbons and perfluorocarbons are weaker than expected, which induces segregation. Experimental X-ray (24,25) and neutron reflectivity (53) and Kelvin surface potential (54) results are in agreement with the full fluorocarbon chain arrangement toward the air interface.…”

Size, Shape, and Lateral Correlation of Highly Uniform, Mesoscopic, Self‐Assembled Domains of Fluorocarbon–Hydrocarbon Diblocks at the Air/Water Interface: A GISAXS Study

“…The formation of these mesoscopic domains was first revealed by atomic force microscopy (AFM)o n transferred monolayers [6] and later grazing incidence smallangle X-ray scattering (GISAXS)a tt he air/water interface. [7] Xray and neutron reflectivity studies demonstrated that the FnHm moleculesa re oriented vertically with the F-chains up while the H-chains are down and in contact with water, [6,[8][9][10][11] which is consistentw ith the higher hydrophobicity of fluoroThe shapea nd size of self-assembled mesoscopic surface domains of fluorocarbon-hydrocarbon (FnHm)d iblocks andt he lateral correlation between these domains were quantitatively determined from grazing incidence small-angle X-ray scattering (GISAXS). The full calculation of structure and form factors unravelst he influence of fluorocarbon and hydrocarbon block lengths on the diameter and heighto ft he domains, and provides the inter-domain correlation length.…”

Dewetting behavior of Langmuir−Blodgett films of polystyrene‐b‐poly(methyl methacrylate) induced by solvent vapor annealing

“…It is well known that block copolymers (BCPs) can self‐assemble into nanometer‐sized domains due to thermodynamic instability . One important method by which such self‐assembly structures can be further adjusted is solvent vapor annealing (SVA) .…”