(256i) Nano-Structured Films of Semifluorinated Alkanes “Primitive Surfactants”: Formation and Size of Hemi-Micelles By Molecular Dynamics

Nano-structured films of semifluorinated alkanes Òprimitive surfactantsÓ: formation and
size of hemi-micelles by Molecular Dynamics

Gonalo M. C. Silva, Pedro Morgado, Michel Goldmann
and Eduardo J. M. Filipe

Instituto Superior TŽcnico,
Universidade de Lisboa,
1049-001 Lisboa, Portugal

Perfluorinated chains (-CF₂CF₂CF₂-)
are not only highly hydrophobic but also lyophobic, i.e. they segregate
hydrogenated chains. Mixtures of alkanes and perfluoroalkanes
are known to be highly non-ideal systems, exhibiting large regions of
liquid–liquid immiscibility and large positive excess properties.

Semifluorinated alkanes (C_nF_2n+1C_mH_{2m+1 ,}FnHm) are diblock molecules in
which a perfluorinated and a hydrogenated segment are
covalently bonded together to form a single chain. Although not possessing a
hydrophilic group, the simultaneous presence of the antagonist segments gives FnHm the required amphiphilicity to
form various supramolecular structures. These are
potential candidates for numerous applications, from medicine to smart-materials
and tailored interfaces. Among them, the nano-structuration
exhibited by Langmuir films of these so-called Òprimitive surfactantsÓ is particularly
striking
[1,2,3]. A hexagonal 2D packing of domains (~30 nm diameter) is observed at the surface of water or an
adequate solid substrate (Figure). In spite of all efforts,
the origin and structure of the nano-domains have remained
un-understood. The non-coalescence of the domains, even under compression, is
particularly puzzling.

In
this work Molecular dynamics simulations have been successfully used to model
the formation, structure and size of the FnHm nano-domains. The simulated structures strikingly resemble
the experimental. The ultimate aim is to understand, and thus control, how the
simultaneous presence of mutually phobic hydrogenated and perfluorinated
chains induces organization.

[1] P.Fontaine, MC.Fauré, L.Bardin, E.J.M. Filipe, M.Goldmann,
Langmuir, 30, 15193 (2014).

[2] L.Bardin, MC.FaurŽ, E.J.M. Filipe, P.Fontaine,
M.Goldmann, Thin Solid Films, 519, 414 (2010).

[3] A. L. Sim›es Gamboa, E. J. M. Filipe,
P. Brogueira, Nano
Letters, 2, 1083 (2002).