Structure of Langmuir Monolayers of Perfluorinated Fatty Acids: Evidence of a New 2D Smectic C Phase

Abstract

Due to the characteristic chain rigidity and weak intermolecular interactions of perfluorinated substances, the phase diagram of Langmuir monolayer formed by perfluorinated molecules has been interpreted so far as displaying only two phases, a 2D gas (G) and a liquid condensed (LC). However, in this work, we presented Grazing Incidence X-ray Diffraction measurements, which exhibit two diffraction peaks on the transition plateau: One is the signature of the hexagonal structure of the LC phase, the second one is associated to the low-density fluid phase and is thus more ordered than expected for a 2D gas or a typical fluid phase. Atomistic molecular dynamics simulations, performed on the transition plateau, revealed the existence of clusters in which domains of vertical molecules organized in a hexagonal lattice coexist with domains of parallel lines formed by tilted molecules, a new structure that could be described as a “2D smectic C” phase. Moreover, the diffraction spectrum calculated from the simulation trajectories compared favorably with the experimental spectra, fully validating the simulations and the proposed interpretation. The results were also in agreement with the thermodynamic analysis of the fluid phase and X-ray Reflectivity experiments performed before and after the transition between these two phases.

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

“…As in our previous study of fluorinated alcohols, the terminal –CF 2 CH 2 OH segment was modelled after the work of Duffy on trifluoroethanol [ 57 , 58 ], adjusting the partial charge of the fluorinated carbon to keep the electrical neutrality of the molecule. The model for the remainder of the perfluorinated tail is based on the parameters described by Watkins and Jorgensen [ 59 ], with adjustments to the atomic partial charges and to the dispersive energy parameter ( ε ) of fluorine atoms to account for the long CF chains, as described in previous work [ 12 ]. The rigid 3-site SPC/E model was used for the water molecules [ 60 ].…”

“…On the contrary, studies of Langmuir films of fluorinated amphiphiles have been less systematic and scarcer. Nonetheless, there have been important efforts to describe and interpret phase transitions and molecular organization in Langmuir films of several perfluorinated acids, alcohols and phospholipids [ 10 , 11 , 12 , 13 , 14 ], as well as amphiphiles with more complex molecular structure such as fluorinated bolaamphiphiles [ 15 , 16 ].…”

“…The main purpose of this work is to contribute to understanding the self-assembling of perfluorinated fatty alcohols, in particular in the very low surface density region. Following our recent paper on perfluorinated fatty acids [ 12 ], a combined experimental and computational methodology is used, bringing together experimental data (thermodynamic, atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXD)) and atomistic molecular dynamics simulations results.…”

Ice Nucleation Activity of Perfluorinated Organic Acids

“…The critical micelle concentration for PFOA also falls below the solubility limit, and X-ray reflectivity measurements showed that the thickness of a perfluorinated carboxylic acid layer corresponds to a monolayer state of the film. 23 …”

“…In fact, grazing incidence X-ray diffraction studies of monolayers of perfluorinated carboxylic acids have revealed tight hexagonal packing of molecules with their long axes nearly perpendicular to the water surface and the coexistence of crystalline and dilute disordered phases. 23 , 31 …”