Transmission X-ray scattering as a probe for complex liquid-surface structures

Abstract

The need for functional materials calls for increasing complexity in self-assembly systems. As a result, the ability to probe both local structure and heterogeneities, such as phase-coexistence and domain morphologies, has become increasingly important to controlling self-assembly processes, including those at liquid surfaces. The traditional X-ray scattering methods for liquid surfaces, such as specular reflectivity and grazing-incidence diffraction, are not well suited to spatially resolving lateral heterogeneities due to large illuminated footprint. A possible alternative approach is to use scanning transmission X-ray scattering to simultaneously probe local intermolecular structures and heterogeneous domain morphologies on liquid surfaces. To test the feasibility of this approach, transmission small- and wide-angle X-ray scattering (TSAXS/TWAXS) studies of Langmuir films formed on water meniscus against a vertically immersed hydrophilic Si substrate were recently carried out. First-order diffraction rings were observed in TSAXS patterns from a monolayer of hexagonally packed gold nanoparticles and in TWAXS patterns from a monolayer of fluorinated fatty acids, both as a Langmuir monolayer on water meniscus and as a Langmuir-Blodgett monolayer on the substrate. The patterns taken at multiple spots have been analyzed to extract the shape of the meniscus surface and the ordered-monolayer coverage as a function of spot position. These results, together with continual improvement in the brightness and spot size of X-ray beams available at synchrotron facilities, support the possibility of using scanning-probe TSAXS/TWAXS to characterize heterogeneous structures at liquid surfaces.

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

“…The difference in EDP of PA and AA indicates that AA and PA molecules orient differently at the air–water interface. The tail layer thicknesses for pure PA were 16.1 ± 0.1 Å at π = 5 mN/m, 16.1 ± 0.2 Å at π = 15 mN/m, and 17.1 ± 0.4 Å at π = 30 mN/m, consistent with the value reported by Fukuto et al These values are in excellent agreement with the untilted orientation observed in GIXD. Density functional theory (DFT) calculations of PA tail length (in vacuum using Spartan ’14, V.1.1.4) gave values of 16.8 Å, which supports the proposal that PA molecules are vertically oriented, fully extended, and highly ordered at the air–water interface.…”

“…Jacquemain et al expanded on this work, suggesting that the potential for conformational disorder in fluorocarbon chains is more limited than in corresponding hydrocarbon systems because of high backbone rigidity and stronger chain–chain van der Waals attraction. Additional results that support these models can be found in closely affiliated systems, including PA, CF 3 (CF 2 ) 10 COOH, and semifluorinated alkanes …”

“…For pure PA at all surface pressures, diffraction patterns consisted of a single, sharp Bragg peak appearing at q xy = 1.26 ± 0.01 Å −1 and with an out-of-plane diffuse scattering maximum at q z = 0 Å −1 . The PA peak position is in good agreement with reports by Barton et al 15 and Fukuto et al 16 for fluorinated amphiphiles. The PA diffraction peak was indexed as triple degenerate {1,0}, {0,1̅ }, {1,1} reflection.…”

“…Jacquemain et al 13 expanded on this work, suggesting that the potential for conformational disorder in fluorocarbon chains is more limited than in corresponding hydrocarbon systems because of high backbone rigidity and stronger chain−chain van der Waals attraction. Additional results that support these models can be found in closely affiliated systems, including PA, 16 CF 3 (CF 2 ) 10 COOH, 17 and semifluorinated alkanes. 18 Although thermodynamic and micron-scale structural investigations of phase-separated fatty acid, fluorocarbon mixed monolayers are plentiful in the literature, there appear to be very few reports of the use of X-ray scattering methods for structural analysis on these or affiliated systems.…”

Pattern Formation in Phase-Separated Langmuir and Langmuir Monolayer Films

“…Perfluorinated surfactant monolayers are particularly well-suited to diffraction experiments because of their large electron density and, in the case of perfluorinated fatty acids, of the nature of their crystalline packing and reflection degeneracy. GIXD experiments from a perfluorotetradecanoic acid typically shows a single, strong diffraction peak in a GIXD contour plot ( q z intensity vs q xy ), indicating a normal orientation of the perfluorinated molecules to the water subphase and a hexagonal lattice packing. ,,, For hydrocarbon fatty acids, both in-plane and out-of-plane scattering can be obtained, which shows that the hydrocarbon molecules are tilted to the water subphase at various angles depending on the surface pressure and composition in the subphase . On the other hand, in the case of semifluorinated alkane (SFA), two Brag Peaks can be observed at q xy , which were associated with hexagonal packing of the fluorocarbon and hydrocarbon blocks, respectively .…”