A R T I C L E S
George et al.
17
liquids (including quasi-liquids such as supercritical CO2 ) and
water18 has been gelated. The unusual structural and diffusional
properties of organogels have led to several interesting
Scheme 1
1
9-24
applications,
and they can be used as structure-directing
agents for inorganic nanoparticles.2
4a,25
Here, we report that low concentrations of structurally simple
N-alkyl perfluoroalkanamides, F(CF2)nCONH(CH2)mH, form
very stable gels with a wide variety of organic liquids, including
the first example of the gelation of a n-perfluoroalkane and other
liquids that have been gelated by few other gelators. The
molecular structures of these LMOGs contain “incompatible”
segments: fluorocarbons and hydrocarbons are immiscible
below a certain temperature, and the cross-sectional area of the
2
26
(
CH2)n and (CF2)n segments are 18.5 and 28.3 Å , respectively.
We reasoned that insertion of an amide group between the
alkyl and perfluoroalkyl portions of a (perfluoroalkyl)alkane
would increase the strength of intermolecular interactions via
They are joined by an amide group whose intermolecular
H-bonding interactions may enforce additional packing con-
straints.
Several other fluorinated gelators are known, but they are
either more complex structurally or are much less efficient than
the F(CF2)nCONH(CH2)mH. For instance, relatively high con-
centrations of related diblock molecules, (perfluoroalkyl)alkanes,
34
H bonding (since the secondary amide groups have a carbonyl
group for H-bond acceptance and retain one H atom for
donation) while retaining the segregation of packing between
the molecular fragments. In fact, the use of amide groups is a
common strategy to enhance the gelating ability of mol-
are known to gelate several liquids,2 including hydrocarbons
7
28
1
6c,18a,35
ecules.
It has allowed very low concentrations of some
29
and mixtures of perfluorooctane and isooctane, but not neat
perfluorooctane.2 Hexafluorobenzene has been gelated by 12-
hydroxyoctadecanoic acid,30 and perfluorodecaline has been
gelated by pentacosafluorodocosane (F(CF2)12(CH2)10H). In
the presence of ∼1-20% water, some partially fluorinated
neutral and charged surfactants gelate a variety of liquids,
including functionalized perfluoroalkanes. Finally, some struc-
turally complex, partially fluorinated aryl-alkyl amides and
sugar derivatives have been shown to gelate a limited number
of liquids.33
of the N-alkyl perfluoroalkanamides (3-5) in Scheme 1 to gelate
a wide variety of organic liquids that includes one that has not
been gelated previously (n-perfluorooctane), others that are
gelated by only a few LMOGs (CCl4 and DMSO), as well as
aromatic molecules, alkanes, and alcohols. Here, we correlate
the gelation temperatures and periods of stability of the gels
with data from DSC, IR, optical microscopy, X-ray diffraction,
and small-angle neutron scattering (SANS) to identify the factors
responsible for efficient gelation. In this way, the network
structures of the gels are investigated at several length scales
and the gelating properties of 3-5 are compared with those of
the corresponding alkyl perfluoroalkanoates (6), whose ester
groups can act only as H-bond acceptors.
8a
3
1
32
(
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