Lamellar structures in fluorinated phosphonium ionic liquids: The roles of fluorination and chain length

Article abstract of DOI:10.1039/c7cp04814a

Ionic liquids (ILs) exhibit tunable behaviour and properties that are due to their supramolecular structure. We synthesized a series of alkylated and fluorinated phosphonium dicyanamide ILs to study the relation between molecular structure and assembly with a focus on the roles of cation chain length and fluorination. Small angle X-ray scattering indicated a lamellar structure with long-range order for all fluorinated ILs, while alkylated ILs showed only the general structures of ILs, i.e., alternating a polar ionic-zone and a nonpolar alkyl-zone. "Fluorophobic" interactions caused microphase segregation between perfluorinated and other molecular segments, "fluorophilic" interactions among the perfluorinated segments stabilized the microphase structure, and the coupling of "fluorophobic" and "fluorophilic" interactions resulted in a stable mesophase structure. The perfluorinated segments packed more densely than the alkylated analogues; the fluorinated versions (except for F2) liquefied at temperatures considerably above that of alkylated ILs. The lamellar structures strongly affected the rheology of the ILs. Fluorinated ILs had higher viscosities and exhibited non-Newtonian shear thinning; the alkylated ILs of the same length had an order of magnitude lower viscosities and were purely Newtonian. We propose that the disruption of lamellar structure in the shear flow causes the non-Newtonian flow behaviour.

Full text of DOI:10.1039/c7cp04814a

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Lamellar Structures in Fluorinated Phosphonium Ionic Liquids:
the Roles of Fluorination and Chain Length
Daniel Rauber,‡,a Peng Zhang,‡,b Volker Huch,a Tobias Kraus*,b and Rolf Hempelmanna, c
a Saarland University, Campus B2 2, 66123 Saarbrücken, Germany
b INM-Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany
c Transfercentre Sustainable Electrochemistry, Saarland University and KIST Europe, Campus Dudweiler Zeile 3, 66125 Saarbrücken, Germany
† Electronic supplementary informaꢀon (ESI) available: IL synthesis, NMR spectra, Molecular interaction in ILs, TGA analysis, Obtaining peak positions of SAXS data, F8 single
crystal data, rheological measurements.
‡ These authors contributed equally to this work. * Corresponding author: Prof. Dr. T. Kraus, email: tobias.kraus@leibniz-inm.de
This submission was created using the RSC Article Template (DO NOT DELETE THIS TEXT)
(LINE INCLUDED FOR SPACING ONLY - DO NOT DELETE THIS TEXT)
Ionic liquids (ILs) exhibit tunable behaviour and properties that are due to supramolecular structure. We synthesized a series of alkylated and
fluorinated phosphonium dicyanamide ILs to study the relation between molecular structure and assembly with a focus on the roles of cation chain
length and fluorination. Small angle X-ray scattering indicated a lamellar structure with long-range order for all fluorinated ILs, while the alkylated ILs
showed only general structures of ILs, i.e., alternating of polar ionic-zone and nonpolar alkyl-zone. “Fluorophobic” interactions caused microphase
segregation between perfluorinated and other molecular segments, “fluorophilic” interactions among the perfluorinated segments stabilized the
microphase structure, and the coupling of “fluorphobic” and “fluorophilic” interactions resulted in a stable mesophase structure. The perfluorinated
segments packed more densely than the alkylated analogues; the fluorinated versions (except for F2) liquefied at temperatures considerable above
that of alkylated ILs. The lamellar structures strongly affected the rheology of the ILs. Fluorinated ILs had higher viscosities and exhibited non-
Newtonian shear thinning; the alkylated ILs of the same length had an order of magnitude lower viscosities and were purely Newtonian. We propose
that the disruption of lamellar structure in the shear flow causes the non-Newtonian flow behaviour.
were found to have low viscosities.34 In addition, such ILs show
1. Introduction
no molecular interactions like hydrogen-bonds as found in
protic ILs35,36 or π-interactions as in ILs based on imidazolium or
Ionic liquids (ILs) are a heterogeneous class of functional fluids
with very low vapour pressures that enable new processes1.
They have more persistent structures and dynamic
heterogeneity2 at different length scales3,4 than other liquids
due to the interplay of Coulomb interaction, hydrogen-bonding,
and van der Waals interaction.5 Their bulk structure at different
length scales are affected by the coordination of ion pairs,6 the
formation of clusters7,8 through hydrogen-bonding9 and the
segregation of polar and nonpolar parts,10,11 sometimes into
ordered structures such as liquid crystals.12 Segregation follows
the rule of “like dissolves like”12 and enables ILs to modify
pyridinium systems.37 A series of tributylphosphonium cations
was prepared with 4 to 12 carbons and varying levels of
fluorination in the longest alkyl chain. We were interested to
see how fluorination of the alkyl chain changes bulk structure
and properties.38
Using the designed IL series, we studied the dependence of
liquid bulk structure of ILs on the chain-length and fluorination
through small angle X-ray scattering, and the relationship
between nanostructure and phase transitions, thermal stability,
and rheological properties of the increasingly fluorinated ILs.
reaction kinetics,13 dynamics,14 and electrical conductivity15
Alkylated ILs, for example, are currently used as solvents/fluids
in electrochemistry16 synthesis,17 lubricants18 and in the
preparation of advanced materials.19
.
2. Experimental
,
.
2.1 Materials
The understanding of “structure-property” relationships is
crucial in the molecular design of ILs for applications.20 Adding
new chemical functionalities is a way of tuning segregation, and
thus bulk properties.21 Here, we study the fluorination of the
cation that affects IL structure22 via fluorophilic interactions.23,24
Fluorinated molecules are hydro- and lipophobic at the same
time.25 Fluorinated ILs were reported to possess self-assembled
domains of aggregated perfluoroalkyl-groups that occur in
addition to the familiar ordered structures in the polar and
nonpolar alkyl regions.26 Fluorination generally lowers surface
tension and enhances biochemical inertness compared to
alkylated ILs27 that makes fluorinated ILs promising as
Tributylphosphine (99%) was obtained from Strem Chemicals
(Kehl, Germany) and distilled prior to use. 1H,1H,2H,2H-
perfluorobutyl iodide (no further specification given),
1H,1H,2H,2H-perfluorohexyl iodide (95%) and 1H,1H,2H,2H-
perfluorodecyl iodide (97%), 1H,1H,2H,2H-perfluorododecyl
iodide (97%) were purchased from Apollo Scientific (Bredbury,
UK) and distilled or sublimed respectively. 1H,1H,2H,2H-
perfluorooctyl iodide (95%) was obtained from Fluorochem
(Hadfield, UK) and was distilled prior to use. The alkylbromides
1-bromobutane (99%) and 1-bromododecane (97%) from Sigma
Aldrich (St. Louis, USA), 1-bromohexane (99%) and 1-
bromooctane (98%+) from Alfa Aesar (Ward Hill, USA) and 1-
bromodecane from Abcr (Karlsruhe, Germany) were distilled
prior to use. Acetonitrile (>99.9%, Chromasolv Grad-HPLC),
dichloromethane (≥99.9%) from Sigma Aldrich and aceton
(99.9%, p.a.) from Thermo Fisher Scientific (Waltham, USA)
were dried over molecular sieves. Methanol (>99.8% Grad-
HPLC) was purchased from VWR International (Radnor, United
Kingdom) and water was purified with an Milli-Q® Type 1
surfactants,28 as lubricants,29,30 and for biomedical uses 26
.
So far, the effect of fluorination in alkylated ILs has not been
systematically studied. We synthesized a homologous series of
increasingly fluorinated ILs to assess the effect of perfluorinated
molecular segments on the bulk structure of the IL and its effect
on bulk properties such as liquefaction temperatures and
viscosities of the ILs. Quaternary phosphonium as cation lends
the high chemical,31 electrochemical32 and thermal stabilities33
.
The dicyanamide anion (DCA) was used for its relative high
charge delocalization and small size and most DCA-based ILs
1

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ultrapure water system from Merck (Darmstadt, Germany).
Sodium dicyanamide (97%) from IoLiTec-Ionic Liquids
Technologies (Heilbronn, Germany), silver nitrate (99.5%) from
Carbolution Chemicals (Saarbrücken, Germany) and dry
magnesium sulfate from Grüssing (Filsum, Germany) were used
as received. CDCl3 (99.8% D) from Sigma Aldrich was stored over
molecular sieves.
sodium bromide), removed the solvent by rotary evaporation,
and dissolved the residues in dichloromethane. For further
purification, the IL solution was washed four times with distilled
water, dried over MgSO4, filtered and the organic solvent
removed by rotary evaporation. To remove the volatiles in the
IL samples, we treated them at 60 °C in oil-pump vacuum at
1x10-2 mbar for two days to obtain the pure ILs in nearly
quantitative yields. We checked the purities of as-prepared ILs
by multinuclear NMR-spectroscopy (AVANCE II 400, Bruker,
Billerica, USA). More details for the synthesis process and NMR
data are given in the Supporting Information.
Exemplary NMR resonances for F8:
1H-NMR: (400 MHz, CDCl3): δ[ppm] = 2.56 – 2.52 (m, 4H,
P-(CH2)2-(CF2)8-F), 2.37 – 2.30 (m, 6H, P-CH2-(CH2)3-H), 1.60 –
2
1.53 (m, 12H, P-CH2-(CH2)2-CH3), 1.00 (t, JH/H = 7.0 Hz, 9H,
P-(CH2)3-CH3).
13C{1H}-NMR (101 MHz, CDCl3): δ[ppm] = 120.1 (s, N-(CN)2),
2
24.0 (d, 3JC/P = 15.6 Hz, P-(CH2)2-CH2-CH3), 23.6 (d, JC/P = 4.9 Hz,
1
P-CH2-CH2-(CH2)2-H), 18.7 (d, JC/P = 47.0 Hz, P-CH2-(CH2)3-H),
13.4 (s, P-(CH2)3-CH3).
19F{1H}-NMR (376 MHz, CDCl3): δ[ppm]
= -80.92 (3F,
P-(CH2)2-(CF2)7-CF3), -114.69 (2F, P-(CH2)2-CF2-(CF2)7-F), -121.88 –
-122.93 (10F, P-(CH2)2-CF2-(CF2)5-(CF2)2-F), -126.21 (2F,
P-(CH2)2-(CF2)6-CF2-CF3).
31P{1H}-NMR (162 MHz, CDCl3): δ[ppm] = 34.84 (s).
In the following, we refer to tributyl-alkyl-phosphonium
dicyanamide
ILs
and
tributyl(1H,1H,2H,2H-
perfluoroalkyl)phosphonium dicyanamide ILs as Fx and Cy,
where x and y indicate the numbers of alkyl- and perfluoro-
carbon numbers in the longest molecular chain. Note that the
non-fluorinated linker carbons are not counted for Fx (Figure
1b).
2.3 Thermal properties
The differential scanning calorimetry (DSC) measurements were
carried out in a DSC 1 setup (Mettler Toledo GmbH, Gießen,
Germany), equipped with a liquid nitrogen cooling system. IL
samples (about 15 mg) were sealed hermetically in aluminum
crucibles with lids and dry nitrogen was used as a purge gas. For
the DSC measurements, any IL sample was first heated from
25 °C to 120 °C at 5 °C/min and held for 30 min to remove its
thermal history. The sample was then cooled to -120 °C
at -1 °C/min and held at -120 °C for 30 min. Finally, the sample
was heated to 100 °C at 1 °C/min. We repeated all DSC
measurements for at least three times with fresh IL samples to
confirm the extracted liquefaction temperatures (Figure 2a).
Fig. 1 a) Reaction scheme for the synthesis of the fluorinated
and alkylated tributylphosphonium dicyanamide ILs by
quaternization reaction of the phosphine and subsequent anion
metathesis. b) Molecular structures and nomenclature of
synthesized cations and anions.
Thermogravimetric analysis (TGA) was performed with
a
TGA/DSC 1 setup (Mettler Toledo GmbH, Gießen, Germany).
10 mg IL sample sealed in an alumina crucible was heated from
30 °C to 550 °C at a rate of 10 °C/min, purged with nitrogen.
2.2 Synthesis of the ionic liquids
The ILs were synthesized by quaternization of the phosphine
and subsequent anion metathesis (Figure 1a). The synthesis of
the quaternary phosphonium salts was undertaken using
Schlenk techniques in carefully dried glassware under argon
atmosphere to prevent the oxidation of tributylphosphine.
Silver dicyanamide (freshly prepared and stored in dark) in
methanol was used for the anion exchange of the iodides. Anion
metathesis reactions of the bromides were performed using
sodium dicyanamide in acetone. To separate the ILs from the
solution, we filtered off the precipitates (silver iodide and
2.4 Small angle X-ray scattering (SAXS)
We injected the bulk IL samples into the quartz capillary tubes
(wall thickness 0.01 mm, Hilgenberg GmbH, Malsfeld, Germany)
in liquid state and sealed them with epoxy glue in an argon
atmosphere. Those samples that were crystallized at room
temperature were melted at 90 °C in an oil bath to facilitate
transfer.
2

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Small angle X-ray scattering (SAXS) measurements were
conducted using a Xeuss 2.0 set-up (Xenocs, Sassenage, France),
equipped with a GeniX Low Divergence Cu-Kα source. The X-ray
wavelength was 0.154 nm. A Pilatus 1M detector (Dectris Ltd.,
Switzerland) collected SAXS patterns and the integration time
was 1800 seconds. The radial integrations of the 2D SAXS
patterns were undertaken with Foxtrot (version 3.2.7, SOLEIL,
France). The sample-to-detector distance was calibrated with
silver behenate powder to be 335 mm. All the samples were
measured in liquid status and samples that were crystallized at
room temperature were heated to 120 °C in an oven and
investigated as a melt. A heating stage with the Xeuss 2.0 set-up
was used to observe structural changes as a function of
temperature in the range of -20 °C -120 °C with a stability of ±1
°C.
Figure 2a shows the liquefaction temperatures as a function of
alkyl and fluorinated chain length. The fluorinated samples F2,
F8 and F10, and the alkylated samples C4 and C12 had clear
melting points, the others had only glass transitions. Fluorinated
ILs melted at higher temperatures than their alkylated
analogues except for C4 (Tm = 62 °C) and F2 (Tm = 26 °C). The
lower liquefaction temperature of F2 may be due to the
disruption of the symmetry of the tetrabutylphosphonium
cation by fluorination as reported for other ILs.43 The
liquefaction behaviour of fluorinated ILs with long chain such as
F6 and F8 was dominated by fluorophilic interactions and the
enhanced charge localization of the phosphonium-core
indicated by a high-field shift in 31P-NMR (see Supporting
Information). Alkylated ILs of similar lengths liquefied at far
lower temperatures, indicating that fluorophilic interactions
influenced strongly the structure formation and phase
transitions in ILs.
2.5 Single-crystal X-ray diffraction
We prepared the single crystals of F8 by drying its
dichloromethane/heptane (with vol/vol = 1/1) solution slowly at
25 °C, purged with a slight steam of dry nitrogen. The F8 single-
crystals (colourless sheets) were measured at 142 K with an AXS
X8 Apex II diffractometer (Bruker AXS, Karlsruhe, Germany)
equipped with a Mo-Kα radiation source (λ = 0.71073 Å); 39865
reflexes were collected in the Theta range from 0.840° to
26.722°. The acquired data was refined by full-matrix least-
square on F2 using anisotropic refinements for the hydrogen
atoms resulting in 12493 independent reflections, 257
restraints, 897 parameters, R[F2 > 2σ(F2)] = 0.0593, wR(F2) =
0.1470, and the fitted crystal data are: Empirical formula =
C24H31F17N3P, Mr
b = 12.6081(10) Å,
=
715.49, triclinic, P-1, a = 10.1936(7) Å,
c = 25.0746(17) Å, α = 102.184(4)°,
β = 97.398(4)°, γ = 94.832(5)°, V = 3103.1(4) Å3, Z = 4, ρr = 1.531
g cm-3. A complete F8 single-crystal data can be found in
CCDC1503513 ([P(bu)3(CH2)2(CF2)8F][DCA]) that is published by
us in the Cambridge Crystallographic Database.
2.6 Rheological properties
We measured the rheological properties of the IL samples with
a MCR 301 Rheometer (Anton Paar GmbH, Graz, Austria),
equipped with a PP25 measuring plate (diameter: 25 mm) and a
DD41 dish (diameter: 41 mm). We collected the rheological data
at 25 °C and high temperature from 30 °C to 100 °C with a step
of 10 °C. For each measurement, we first equilibrated the IL
sample at set temperatures for at least 25 minutes to reach
stable temperature (maximum deviations of ± 0.1 °C). The shear
history was removed by applying a constant shear at 150 s-1 for
60 s, after which the shear rate was increased from 200 s-1 to
10000 s-1 in linear steps. For each IL, 30 viscosity values were
recorded. Zero-shear viscosities of the fluorinated ILs for the
Vogel-Fulcher-Tammann (VFT) fit were obtained by
extrapolation of the Newtonian flow region.39
Fig. 2 a) Liquefaction temperatures for fluorinated and alkylated
phosphonium dicyanamide ILs of different chain lengths from
DSC data. Dots denote melting points and stars glass transition
temperatures; lines are merely used to guide the eye. b) TGA
data showing the thermal stability of F8 and C10.
All alkylated ILs showed good thermal stability (see Supporting
Information) with no significant decomposition below 400 °C
(C10 in Figure 2b); they were considerably more stable than
other reported DCA-based ILs with imidazolium or pyrrolidinium
cations44 and were similar in stability to other DCA-based ILs
with tetraalkylphosphonium cations.45 The fluorinated ILs were
far less stable with significant decomposition occurring already
at 250 °C (F8 in Figure 2b) independent of the number of
perfluorinated carbons. This is in contrast to previous reports
3. Results and Discussion
The synthesized series of ILs with increasing alkyl chain length
and perfluorination allowed us to study the balance between
Coulomb and van der Waals interactions while avoiding other
molecular interactions such as hydrogen-bonding in protic
ILs40,41 or the π-π-interactions of imidazolium and pyridinium
ILs42 (see Supporting Information). We could thus isolate the
effect of fluorination.
-
-
on fluorinated phosphonium ILs with NTf2 , OTf-, or BF4 anions
that had good thermal stabilities.46,47 We believe that the poor
thermal stabilities of our fluorinated ILs are due to electron
withdrawal by the perfluorinated groups that destabilizes the
3

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proton in alpha position and makes it easy to be abstracted by
the slightly basic anion, an effect also reported for imidazolium-
based ILs.48
Fluorination affected the polarity peaks, too. No polarity peak
was discernible for F6 to F10. The presence of perfluorinated
nanodomains is known to change the electron density contrast
among different regions.58 Perfluoroalkyl groups have been
reported to aggregate and form fluorinated nanodomains in
fluorinated ILs.26,53 For polymers, Percec et al. reported that
fluorination of the alkyl chains enhanced the stability of
mesophase structure by forming supramolecular columns in
The homologous series of both alkylated- and fluorinated-ILs let
us conveniently separate different structural features of the ILs
simply by inspecting the change in X-ray scattering as a function
of molecular chain length and fluorination. SAXS was used to
analyse average cluster size, shape, and structural
ordering.3,4,49–53
dendrimers.59,60
A
similar
effect
is
found
in
fluorocarbon/hydrocarbon diblock amphilphiles, too.61 We
believe that the perfluorinated segments in our ILs segregate,
too, and form
a region with increased electron density.
Segregation reduces the electron density contrast between the
polar and nonpolar parts and the polarity peak becomes
weaker. This is in consistent with the solubility studies of Blesic
et al. who found that fluorinated alkanes phase-segregated
from tetraalkylphosphonium ILs while the nonfluorinated
alkanes were miscible with the ILs.62
All alkylated-ILs exhibited three peaks in SAXS (Figure 3) that
correspond to order from polarity, charge, and adjacency; they
are frequently found in ILs.53,54 The sizes of corresponding real-
space structures were calculated from peak positions (q) as
2π/q and summarized in Table 1. The “prepeak” or “polarity
peak” at lowest q indicates the structural heterogeneity from
alternating polar ionic and non-polar alkyl zones. At higher q
follows the charge peak that probes the alternation between
cations and anions in the charged region as in a molten salt.55 At
highest q, the adjacency peak probes the intra- and inter-
molecular packing of adjacent atoms;53 it was broad and strong
for all alkylated-ILs as shown in Figure 3a.
The polarity peaks shifted to lower q values for increasing alkyl
chain lengths, while the charge and adjacency peaks retained
their positions. The shift indicates increased spacing in the IL
packing; small variations in the other peak positions indicated
that the packing of charged ions or adjacent atoms remained
constant. This implies that changing alkyl chain lengths mainly
affects the structure of the nonpolar zone and merely shifts the
positions of ionic regions and adjacent atoms.
Fluorination strongly changed the structure of the ILs and thus
the scattering pattern. Two new peaks appeared at q below the
usual “prepeak” (Figure 3b). They were particularly strong for
long perfluorinated chains, e.g., F8 and F10. We believe that
these two additional peaks are due to 1st and 2nd order
scattering signals from ordered lamellar structures in the
fluorinated ILs. The lamellar structures differ from the
heterogeneity that has been reported for alkylated ILs in this
work and elsewhere53,54. Lamellar structures with long-range
correlations of this order have not been reported yet.
All fluorinated ILs had lamellar structures, even those with only
two perfluorinated carbons. This is in striking contrast to similar
ILs with alkyl chains. Lamellar structures and a smectic phase
were previously found only for long alkyl chains.56 A minimal
carbon number (z) of 14 was required for the formation of a
smectic phase in 1-alkyl-3-methylimidazolium [Czmim][NO3] ILs;
the formation of a smectic phase was demonstrated to lower
the free energy of long alkyl chains by forming a layered
structure.57 We are not aware of any report on the effect of
fluorination on the formation of lamellae and their
supramolecular structures in ILs.
Fig. 3 Radially integrated SAXS data for a) alkylated ILs and b)
fluorinated ILs in liquid status. Typical scattering peaks
attributed to lamellae, polarity, charge, and adjacency are
marked with coloured boxes.
The concept of Cohesive Energy Densities (CED) that quantify
the strength of intra- and inter-molecular interactions of
4

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nonfluorinated and fluorinated alkanes can be used to provide a
more detailed picture of structure formation. The CED of
fluorinated alkanes at 298 K are below those of the alkane
homologues (e.g., 0.16 kJ/cm3 for C8F18 and 0.24 kJ/cm3 for
C8H18).63 In the fluorinated ILs studied here, “fluorophobic”
interactions cause strong microphase segregation between the
longest (fluorinated) chain and the other (nonfluorinated)
segments. “Fluorophilic” interactions that favour close packing
of perfluorinated groups over heterogeneous packing support
microphase segregation and assist the formation of lamellae
and crystalline supramolecular structures in ILs; typical
examples are shown in Figure 2a (F8 and F10). Future molecular
dynamics simulation should provide more quantitative insights
into the origin of the long-range ordered lamellae structure.
shear rate using rheometry. Zero-shear viscosities (η0,
extrapolated from the viscosity in the Newtonian region) for
fluorinated and alkylated ILs are shown in Figure 5a and Figure
5b, respectively. All viscosities decreased exponentially with
increasing temperature and followed the Vogel-Fulcher-
Tammann (VFT) equation,65–67
ꢅ
ꢀꢁ ꢂ ꢀꢃ exp ꢄ
ꢉ
ꢆ ꢇ ꢆꢈ
with temperature dependent material parameters η∞ and B,
and the Vogel temperature T0 (see Supporting Information) as
expected for ILs.68,69
Single-crystal data on IL solids can be helpful to understand the
ILs’ liquid structure.64 We compared the lamellar structure
observed in the F8 melt to the structure of an F8 single crystal
prepared by slow solvent evaporation. X-ray diffraction of the
crystal at 142 K indicated a supramolecular structure with
interdigitated fluorinated segments (Figure 4a and single-crystal
structural parameters in the Supporting Information). The
lamellar periodicity along the c-axis was 25.07 Å, smaller than
the 28.56 Å in the liquid (Table 1, data from liquid phase SAXS).
The difference is probably due to thermal expansion and the
reduced packing density in the liquid. The sketch of the liquid
structure in Figure 4b is based on the following assumptions: a)
the ions’ arrangement in the polar regions of the liquid is similar
to that in the crystal; b) the lamellar structure of the non-polar
regions of the liquid has less positional (translational) and
orientational order than that of the crystal.
Table 1 Real-space length scales corresponding to the scattering
peak positions¶ in Figure 3.
Sample
Peak 1 Peak 2 Peak 3
Peak 4 Peak 5
(Å)
(Å)
(Å)
(Å)
(Å)
C4
C6
C8
C10
C12
F2
F4
F6
-
-
-
-
-
-
-
-
-
-
-
-
9.11
9.97
10.47
12.57
16.11
9.52
10.13
-
7.39
7.39
7.06
7.06
7.14
7.66
7.06
7.06
7.66
7.48
4.22
4.16
4.22
4.22
4.22
4.39
4.87
4.87
5.37
5.66
-
16.98
18.48
24.17
28.56
31.42
F8
F10
14.96
16.98
-
-
Fig. 4 a) Illustration showing the single-crystal XRD data of F8. b)
Molecular scheme showing the molecular assemblies of F8 melt
in lamellar and smectic phase with regions dominated by
¶We obtained the peak positions by fitting the peaks with Gauss profiles
or from the zero-crossing of the first derivative (see Supporting
Information). Real-space length scales calculated with 2π/q from peaks
in Figure 3a and b. Peaks 1 and 2 refer to 1st and 2nd order scattering
from the lamella formed by perfluorinated segments of adjacent ILs.
Peak 3 is due to alternating polar and nonpolar zones. Peak 4 is due to
cations and anions. Peak 5 is due to inter- and intra-molecular spacing.
Coulomb and fluorophilic interactions as marked. The length
scales of molecular assemblies calculated from typical
scattering peaks in Fig. 3 are marked with lines in panel b, i.e.,
dashed line for lamellae, dotted line for polarity, dash-dotted
line for charge and solid line for adjacency.
The strong effects of perfluoroalkyl groups on the structure of
the ILs, and in particular the emergence of a new ordered
lamellar structure, suggests that the flow behaviour of
fluorinated ILs should deviate from that of the corresponding
alkylated ILs. We studied the flow behaviour of F2 to F6 (Figure
5a) and C6 to C12 (Figure 5b) as functions of temperature and
The viscosities of all fluorinated ILs were consistently above
those of alkylated ILs (Figure 5a, b); they were approximately
one order of magnitude larger at 25 °C. The lamellar structure
of fluorinated side chains apparently leads to larger cohesive
aggregates in fluorinated ILs. Large aggregates restricted
5

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momentum transfer on microscopic scale and thus increase of
viscosity on macroscopic scale. To verify our hypothesis, we
studied the viscosities of ILs at shear rates from 200 to 10000 s-1
at different temperatures (Figure 5c, d). First, we considered
the viscosity of F6 that progresses, with increasing shear, from
Newtonian flow through a transition zone to linear shear
thinning in accordance with the Ostwald-de Waele
relationship,70
Newtonian behaviour for fluorinated ILs depended on the chain
length; it occurred at 50 °C for F2, 70 °C for F4, and 80 °C for F6.
The flow behaviour index n at 25 °C decreased with increasing
fluorine content, from 0.77 for F2 to 0.58 for F4.
All rheological data is consistent with the structure formation
discussed above. To the best of our knowledge, this is the first
report on a lamellar mesophase in ILs that induces a strong
effect on flow behaviour. Shear thinning was previously only
reported for ILs with strong interactions, e.g. protic ILs with
hydrogen-bond as in ref.73 and trioctylalkylammonium
τ
ꢂ k
(ꢊꢋ/ꢊꢌ)n
with shear stress τ (Pa), consistency index k (N s m-2), the shear
rate ∂v/∂y (s-1), and the dimensionless flow behaviour index n.
The greatest deviation of F6 from Newtonian flow (n = 1)
occurred at 25°C, where n = 0.46. We believe that this strong
shear thinning results from the disruption of the cohesive
lamellar structure. In contrast, only Newtonian flows were
observed for C8 at all temperatures. All other ILs showed similar
trends – strong shear thinning for fluorinated and purely
Newtonian flows for alkyl ILs. We also studied the flow
behaviour of F6 at elevated temperatures because previous
work on other fluorinated ILs reports that the fluorinated
aggregates formed by perfluorinated chains in ILs are less
thermally stable and would be dissociated at temperatures
higher than -25 °C.49 High temperatures provide the positive
excess enthalpy necessary for the mixing of fluorocarbons and
hydrocarbons.71 Figure 5c shows that F8 exhibits purely
Newtonian flow behaviour above 80 °C. The segregated lamellar
structure readily dissolves at this temperature through the
mixing of fluorinated and alkylated segments as observed in
macroscopic systems.72 The temperature of transition to
bis(salicylato)borate in ref.74
.
4. Conclusions
We synthesized two series of new phosphonium dicyanamide
ionic liquids, i.e., alkylated-ILs and fluorinated-ILs, and studied
their structure-property relationships by following the effect of
increasing fluorination. Small angle X-ray scattering indicated a
monotonic increase of the size of the non-polar region with
chain length while the polar regions did not change. The trends
were in accordance with previous reports on alkylated IL
structure. Fluorination strongly changed the ILs’ structure.
There was no regular polar structure visible in the scattering
data of F6 to F10. An additional lamellar structure with
surprisingly long-ranged order was found in all fluorinated ILs.
Fig. 5 Temperature dependent zero-shear viscosity, ƞ0, for a) the fluorinated and b) the alkylated phosphonium dicyanamide ILs.
c) Rheological behaviour of F6 at different temperatures and shear rates. A typical shear-thinning behaviour indicated by a
remarkable change from Newtonian to shear-thinning regions (marked with dashed lines) is shown in the 25 °C curve. d)
Rheological property of C8 at different temperatures and shear rates. Only Newtonian flow behaviour is observed at all
investigated temperatures and shear rates.
6

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We believe that the “fluorophobic” and “fluorophilic”
interactions caused the strong microphase segregation and
resulted in a lamellar mesophase structure. The perfluorinated
segments packed densely and caused the absence of polarity
peak in F6 to F10 because of the rearrangements of the
electrons accompanying with the formation of new lamellar
structures. The lamellar structure introduced non-Newtonian
behaviour that was entirely absent without fluorination. The
temperature dependence of shear thinning strongly suggests
that the lamellar structure disintegrates at increased
temperatures or shear rates. It provides a straightforward
strategy to tune the viscosity of fluorinated ionic liquids for
applications such as lubrication.75 Corrosion due to the
presence of fluoride that has recently been observed76,77 needs
to be addressed for such applications.
17
18
19
R. E. Morris, Chem. Commun., 2009, 2990.
I. Minami, Molecules, 2009, 14, 2286–2305.
S. Zhang, Q. Zhang, Y. Zhang, Z. Chen, M. Watanabe
and Y. Deng, Prog. Mater. Sci., 2016, 77, 80–124.
T. L. Greaves, D. F. Kennedy, Y. Shen, A.
Weerawardena, A. Hawley, G. Song and C. J.
Drummond, J. Mol. Liq., 2015, 210, 279–285.
C. Yue, D. Fang, L. Liu and T.-F. Yi, J. Mol. Liq., 2011,
163, 99–121.
20
21
22
T. L. Greaves, D. F. Kennedy, Y. Shen, A. Hawley, G.
Song and C. J. Drummond, Phys. Chem. Chem. Phys.,
2013, 15, 7592–7598.
23
24
J. J. Tindale, K. L. Mouland and P. J. Ragogna, J. Mol.
Liq., 2010, 152, 14–18.
O. Kysilka, M. Rybáčková, M. Skalický, M. Kvíčalová, J.
Cvačka and J. Kvíčala, J. Fluor. Chem., 2009, 130, 629–
639.
Conflicts of interest
There are no conflicts of interest to declare.
25
26
J. A. Gladysz, D. P. Curran and I. T. Horváth, Eds.,
Handbook of Fluorous Chemistry, Wiley-VCH Verlag
GmbH & Co. KGaA, Weinheim, FRG, 2004.
A. B. Pereiro, J. M. M. Araújo, S. Martinho, F. Alves, S.
Nunes, A. Matias, C. M. M. Duarte, L. P. N. Rebelo and
I. M. Marrucho, ACS Sustain. Chem. Eng., 2013, 1, 427–
439.
Acknowledgements
D. Rauber and R. Hempelmann thank the Deutsche
Bundesstiftung Umwelt (DBU) (31925-41) for financial support.
Prof. Eduard Arzt is acknowledged for his continuing support of
this project.
References
27
M. L. Ferreira, M. J. Pastoriza-Gallego, J. M. M. Araújo,
J. N. Canongia Lopes, L. P. N. Rebelo, M. M. Piñeiro, K.
Shimizu and A. B. Pereiro, J. Phys. Chem. C, 2017, 121,
5415–5427.
1
2
T. Welton, Chem. Rev., 1999, 99, 2071–2084.
Z.-P. Zheng, W.-H. Fan, S. Roy, K. Mazur, A. Nazet, R.
Buchner, M. Bonn and J. Hunger, Angew. Chemie Int.
Ed., 2014, 54, 687–690.
28
29
30
31
32
33
34
35
T. L. Merrigan, E. D. Bates, S. C. Dorman and J. H. Davis
Jr., Chem. Commun., 2000, 2051–2052.
3
4
5
6
R. Hayes, G. G. Warr and R. Atkin, Chem. Rev., 2015,
115, 6357–6426.
J. Qu, J. J. Truhan, S. Dai, H. Luo and P. J. Blau, Tribol.
Lett., 2006, 22, 207–214.
R. Caminiti and L. Gontrani, Eds., The Structure of Ionic
Liquids, Springer International Publishing, Cham, 2014.
T. L. Greaves and C. J. Drummond, Chem. Soc. Rev.,
2013, 42, 1096–1120.
I. Minami, M. Kita, T. Kubo, H. Nanao and S. Mori,
Tribol. Lett., 2008, 30, 215–223.
F. Atefi, M. T. Garcia, R. D. Singer and P. J. Scammells,
Green Chem., 2009, 11, 1595–1604.
B. Kirchner, F. Malberg, D. S. Firaha and O. Hollóczki, J.
Phys. Condens. Matter, 2015, 27, 463002.
R. Ludwig, J. Phys. Chem. B, 2009, 113, 15419–15422.
A. Strate, T. Niemann, D. Michalik and R. Ludwig,
Angew. Chemie Int. Ed., 2017, 56, 496–500.
P. A. Hunt, C. R. Ashworth and R. P. Matthews, Chem.
Soc. Rev., 2015, 44, 1257–1288.
K. Tsunashima and M. Sugiya, Electrochem. commun.,
2007, 9, 2353–2358.
7
8
K. Tsunashima and M. Sugiya, Electrochemistry, 2007,
75, 734–736.
K. Tsunashima, S. Kodama, M. Sugiya and Y. Kunugi,
Electrochim. Acta, 2010, 56, 762–766.
9
O. Russina, F. Lo Celso, M. Di Michiel, S. Passerini, G. B.
Appetecchi, F. Castiglione, A. Mele, R. Caminiti and A.
Triolo, Faraday Discuss., 2013, 167, 499–513.
Y. Shen, D. F. Kennedy, T. L. Greaves, A.
Weerawardena, R. J. Mulder, N. Kirby, G. Song and C. J.
Drummond, Phys. Chem. Chem. Phys., 2012, 14, 7981.
R. P. Matthews, T. Welton and P. A. Hunt, Phys. Chem.
Chem. Phys., 2014, 16, 3238–3253.
10
K. Shimizu, M. F. Costa Gomes, A. A. H. Pádua, L. P. N.
Rebelo and J. N. Canongia Lopes, J. Mol. Struct.
THEOCHEM, 2010, 946, 70–76.
36
11
M. A. A. Rocha, C. M. S. S. Neves, M. G. Freire, O.
Russina, A. Triolo, J. A. P. Coutinho and L. M. N. B. F.
Santos, J. Phys. Chem. B, 2013, 117, 10889–10897.
K. Binnemans, Chem. Rev., 2005, 105, 4148–4204.
J. P. Hallett and T. Welton, Chem. Rev., 2011, 111,
3508–3576.
37
38
12
13
A. Luís, K. Shimizu, J. M. M. Araújo, P. J. Carvalho, J. A.
Lopes-da-Silva, J. N. Canongia Lopes, L. P. N. Rebelo, J.
A. P. Coutinho, M. G. Freire and A. B. Pereiro,
Langmuir, 2016, 32, 6130–6139.
14
M. Kofu, M. Nagao, T. Ueki, Y. Kitazawa, Y. Nakamura,
S. Sawamura, M. Watanabe and O. Yamamuro, J. Phys.
Chem. B, 2013, 117, 2773–2781.
39
40
C. Patrascu, F. Gauffre, F. Nallet, R. Bordes, J.
Oberdisse, N. de Lauth-Viguerie and C. Mingotaud,
ChemPhysChem, 2006, 7, 99–101.
15
16
K. Ueno, H. Tokuda and M. Watanabe, Phys. Chem.
Chem. Phys., 2010, 12, 1649–1658.
P. Hapiot and C. Lagrost, Chem. Rev., 2008, 108, 2238–
2264.
R. Hayes, S. Imberti, G. G. Warr and R. Atkin, Angew.
Chemie Int. Ed., 2013, 52, 4623–4627.
7

Physical Chemistry Chemical Physics
Page 8 of 10
View Article Online
DOI: 10.1039/C7CP04814A
CREATED USING THE RSC ARTICLE TEMPLATE - SEE WWW.RSC.ORG/ELECTRONICFILES FOR FURTHER DETAILS
41
42
K. Dong, S. Zhang and J. Wang, Chem. Commun., 2016,
52, 6744–6764.
Rebelo, Phys. Chem. Chem. Phys., 2010, 12, 9685–
9692.
P. Stepnowski, J. Nichthauser, W. Mrozik and B.
Buszewski, Anal. Bioanal. Chem., 2006, 385, 1483–
1491.
63
64
G. Marchionni, G. Ajroldi, M. C. Righetti and G. Pezzin,
Macromolecules, 1993, 26, 1751–1757.
Y. Ji, R. Shi, Y. Wang and G. Saielli, J. Phys. Chem. B,
2013, 117, 1104–1109.
43
44
45
46
47
48
49
N. V Plechkova and K. R. Seddon, Chem. Soc. Rev.,
2008, 37, 123–150.
65
66
67
H. Vogel, Phys. Z., 1921, 22, 645–646.
Y. Yoshida, O. Baba and G. Saito, J. Phys. Chem. B,
2007, 111, 4742–4749.
G. S. Fulcher, J. Am. Ceram. Soc., 1992, 75, 1043–1055.
G. Tammann and W. Hesse, Zeitschrift für Anorg. und
Allg. Chemie, 1926, 156, 245–257.
T. J. Wooster, K. M. Johanson, K. J. Fraser, D. R.
MacFarlane and J. L. Scott, Green Chem., 2006, 8, 691.
J. J. Tindale and P. J. Ragogna, Chem. Commun., 2009,
1831–1833.
68
69
H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan and
M. Watanabe, J. Phys. Chem. B, 2005, 109, 6103–6110.
A. Rupp, N. Roznyatovskaya, H. Scherer, W. Beichel, P.
Klose, C. Sturm, A. Hoffmann, J. Tübke, T. Koslowski
and I. Krossing, Chem. - A Eur. J., 2014, 20, 9794–9804.
G. L. Burrell, N. F. Dunlop and F. Separovic, Soft Matter,
2010, 6, 2080.
J. J. Tindale, C. Na, M. C. Jennings and P. J. Ragogna,
Can. J. Chem., 2007, 85, 660–667.
J. Liu, S. D. Chambreau and G. L. Vaghjiani, J. Phys.
Chem. A, 2014, 118, 11133–11144.
70
71
O. Russina, F. Lo Celso, M. Di Michiel, S. Passerini, G. B.
Appetecchi, F. Castiglione, A. Mele, R. Caminiti and A.
Triolo, Faraday Discuss., 2013, 167, 499–513.
P. Zhang, H. Huang, Y. Chen, S. Yu, C. Krywka, S. K.
Vayalil, S. V. Roth and T. He, Chinese J. Polym. Sci.,
2014, 32, 1188–1198.
P. Morgado, J. Ben Lewis, C. M. C. Laginhas, L. F. G.
Martins, C. McCabe, F. J. Blas and E. J. M. Filipe, J. Phys.
Chem. B, 2011, 115, 15013–15023.
50
72
73
74
I. T. Horvath and J. Rabai, Science (80-. )., 1994, 266,
72–75.
G. L. Burrell, N. F. Dunlop and F. Separovic, Soft Matter,
2010, 6, 2080–2086.
51
52
53
T. P. Russell, J. F. Rabolt, R. J. Twieg, R. L. Siemens and
B. L. Farmer, Macromolecules, 1986, 19, 1135–1143.
J. J. Hettige, J. C. Araque and C. J. Margulis, J. Phys.
Chem. B, 2014, 118, 12706–12716.
R. Gusain, P. S. Bakshi, S. Panda, O. P. Sharma, R.
Gardas and O. P. Khatri, Phys. Chem. Chem. Phys.,
2017, 19, 6433–6442.
H. V. R. Annapureddy, H. K. Kashyap, P. M. De Biase
and C. J. Margulis, J. Phys. Chem. B, 2010, 114, 16838–
16846.
75
76
77
Z. Zeng, B. S. Phillips, J.-C. Xiao and J. M. Shreeve,
Chem. Mater., 2008, 20, 2719–2726.
M. Itoga, S. Aoki, A. Suzuki, Y. Yoshida, Y. Fujinami and
M. Masuko, Tribol. Int., 2016, 93, 640–650.
A. Arcifa, A. Rossi and N. D. Spencer, J. Phys. Chem. C,
2017, 121, 7259–7275.
54
J. C. Araque, J. J. Hettige and C. J. Margulis, J. Phys.
Chem. B, 2015, 119, 12727–12740.
55
56
J. Dupont, Acc. Chem. Res., 2011, 44, 1223–1231.
K. Goossens, K. Lava, C. W. Bielawski and K. Binnemans,
Chem. Rev., 2016, 116, 4643–4807.
57
58
G. Saielli, A. Bagno and Y. Wang, J. Phys. Chem. B, 2015,
119, 3829–3836.
P. Zhang, The calculation of the scattering length
density (SLD) is based on the calculator in NIST centre
for neutron research,
https://www.ncnr.nist.gov/resources/activation/. For
SLD of perfluorinated nanodomain, the material C8F18
and density value 1.766 g/cm3 were used and the real
part value of SLD was 14.470×10-6/Å2; for the alkylated
nanodomain, the material C10H22 and density value
0.730 g/cm3 were used and the real part value of SLD
is 7.156×10-6/Å2; for F8, the material C24H31F17N3P and
density value 1.531 g/cm3 , extracted from the single-
crystal data, were used and the real part value of SLD
was 13.296×10-6/Å2. As a comparison, the real part
value of SLD of C8F18 is 109% times that of F8 single-
crystal and 202% times that of C10H22
.
59
60
61
62
V. Percec, G. Johansson, G. Ungar and J. Zhou, J. Am.
Chem. Soc., 1996, 118, 9855–9866.
G. Johansson, V. Percec, G. Ungar and J. P. Zhou,
Macromolecules, 1996, 29, 646–660.
M. P. Krafft and J. G. Riess, Chem. Rev., 2009, 109,
1714–1792.
M. Blesic, J. N. C. Lopes, M. F. C. Gomes and L. P. N.
8

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We report a new lamellar superstructure and non-Newtonian shear
thinning behavior in fluorinated phosphonium dicyanamide ILs.