Nanocompartmentalization of soft materials with three mutually immiscible solvents: Synthesis and self-assembly of three-arm star-polyphiles

Article abstract of DOI:10.1021/cm503981t

We report synthesis, thermotropic, and lyotropic mesomorphism of a family of novel "star-polyphiles": small star-shaped molecules bearing three mutually immiscible arms, hydrophilic (oligoethylene glycol), oleophilic (alkanes), and fluorophilic (perfluorinated alkanes), attached to a central 1,3,5-trihydroxybenzene core. A facile, flexible multistep synthesis giving high-purity yields of 14 distinct members of the family is described, with variable arm lengths, forming crystalline, or various mesostructured gel phases in their neat state at room temperature. Furthermore, we show that specific members of these star-polyphiles can be simultaneously loaded with up to three mutually immiscible solvents, dodecane, perfluoro-nonane, and water, to form liquid crystals and microemulsions with multiple chemically distinct mesoscale compartments. The mesoscale chemical polyfunctionality of these polyphiles, coupled with thermotropic and lyotropic polymorphism, make them promising potential soft materials for a variety of applications, including host matrices for multiple mutually immiscible chemicals and drug delivery.

Full text of DOI:10.1021/cm503981t

Article
pubs.acs.org/cm
Nanocompartmentalization of Soft Materials with Three Mutually
Immiscible Solvents: Synthesis and Self-Assembly of Three-Arm Star-
Polyphiles
Liliana de Campo,*,†,∥ Minoo J. Moghaddam,‡ Trond Varslot,† Nigel Kirby,§ Rainer Mittelbach,†
Tim Sawkins,† and Stephen T. Hyde*,†
†Australian National University, Applied Maths, Research School of Physics & Engineering, Canberra, ACT 0200, Australia
‡CSIRO, Manufacturing Flagship, P.O. Box 52, North Ryde, NSW 2113, Australia
§Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australia
∥Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee, NSW 2232, Australia
S
* Supporting Information
ABSTRACT: We report synthesis, thermotropic, and lyo-
tropic mesomorphism of a family of novel “star-polyphiles”:
small star-shaped molecules bearing three mutually immiscible
arms, hydrophilic (oligoethylene glycol), oleophilic (alkanes),
and fluorophilic (perfluorinated alkanes), attached to a central
1,3,5-trihydroxybenzene core. A facile, flexible multistep
synthesis giving high-purity yields of 14 distinct members of
the family is described, with variable arm lengths, forming
crystalline, or various mesostructured gel phases in their neat
state at room temperature. Furthermore, we show that specific
members of these star-polyphiles can be simultaneously loaded
with up to three mutually immiscible solvents, dodecane, perfluoro-nonane, and water, to form liquid crystals and
microemulsions with multiple chemically distinct mesoscale compartments. The mesoscale chemical polyfunctionality of these
polyphiles, coupled with thermotropic and lyotropic polymorphism, make them promising potential soft materials for a variety of
applications, including host matrices for multiple mutually immiscible chemicals and drug delivery.
with potential to form far more structural and physical
complexity.
INTRODUCTION
■
Molecular self-assembly is a fascinating field of research.
Amphiphilic molecules, such as lipids and surfactants,
consisting of two covalently linked domains of opposing
affinity (usually hydrophilic and hydrophobic), are well-known
to exhibit rich phase diagrams and play an important role in
nature,1−3 as well as in a variety of nanotechnological
applications in fields such as pharmaceutical,4,5 medical,6 and
food sciences.7
As an extension, we have been developing small (oligomeric)
molecules with three domains of opposing affinities in a star-
shaped geometry, so-called “star-polyphiles”, consisting of a
central core bearing a hydrocarbon, fluorocarbon, and hydro-
philic chain;8 see Figure 1a. The star-shaped architecture of
such molecules imposes strong constraints on their self-
assembly behavior, leading to the formation of nanostructured
materials with increased complexity. In particular, their self-
assembly gives domains separated by three distinct interfaces
(hydrocarbon-fluorocarbon, fluorocarbon-polar, and hydro-
carbon-polar) necessarily merged along three-phase lines.
They are therefore very different materials from the wealth of
amphiphilic materials found in biological and synthetic systems,
This concept was first explored in the field of ABC miktoarm
star-copolymers,9−12 which like star-polyphiles have multiple
arms linked to a common core. Miktoarm copolymers have
polymeric arms; star-polyphiles have shorter oligomeric arms,
ideally just long enough to ensure mutual immiscibility of the
arms, yet short enough to accommodate lyotropic meso-
morphism in the presence of liquid solvents. The exploration of
self-assembly of miktoarm copolymeric materials continues to
bloom. A large variety of nanostructures have been found in
both experiments9−14 and simulations,15−17 including columns
of different chemical species arranged in various 2D arrays (as
in Figure 1b), zinc-blende type structures,18 quasi-crystalline
materials,13 and decorated bicontinuous cubic gyroid struc-
tures.9,10 Polyphilicity in combination with branching has also
been successfully employed in the field of thermotropic liquid
crystals.19−21 In particular, polyphilic X-shaped and T-shaped
mesogens have the capability to form structures of extra-
Received: October 28, 2014
Revised: December 30, 2014
Published: January 6, 2015
© 2015 American Chemical Society
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Figure 1. (a) Star-polyphilic molecules bearing three mutually immiscible arms, (b) a hexagonal columnar liquid crystalline phase based on columns
of different chemical species, whose cross section is 12.6.4. planar tiling, found experimentally for ABC miktoarm copolymers,9,37 X-shaped polyphilic
thermotropic liquid crystals38 and star-polyphiles,8 (c) a hypothetical tricontinuous structure consisting of three diamond networks composed of
three distinct chemical species.
Figure 2. Molecular structure of the star-polyphiles synthesized, abbreviated as B-Oa-Hb-Fc, where B denotes the aromatic molecular core and O/H/
F the methyl-capped oligo-ethylene glycol, hydrocarbon, and fluorocarbon chains of different lengths (a, b, and c), respectively (left). Various
precursor chains used (right).
ordinary complexity such as axial bundle phases.22,23 Also,
many hybrid hydrocarbon fluorocarbon surfactants24−27 share the
feature of different chemical arms attached to a common center,
and these molecules are most closely related to star-polyphiles.
These surfactants have been largely explored with different
applications in mind (e.g., suitable surfactants for supercritical
CO2 microemulsions or stabilizers for fluorocarbon oil
emulsifiers as oxygen transporters). It seems that spatial
nanosegregation into hydrophilic, oleophilic, and fluorophilic
domains has not been reported in detail yet. For the purpose of
exploring exotic self-assembled architectures, spatial segregation
into three distinct nanodomains is of the highest importance.
Though hydrocarbons and fluorocarbons are known to have
nonideal mixing,28 full hydrocarbon fluorocarbon segregation in
triphilic systems cannot be assumed in general molecular
systems, since the surface tension between hydrocarbon and
fluorocarbon domains is much lower than the surface tension
between either of them with water.28 Using systematic SANS
contrast variation experiments, we have shown previously that,
for the star-polyphilic molecules discussed here, a fluorocarbon
chain length of 8 units in combination with a hydrocarbon
chain length of 14 units is sufficient to provide full segregation
in a hexagonal liquid crystal structure.8 Based on multiple
experiments we proposed a columnar pattern whose cross
section is a 12.6.4. tiling of the plane for a range of water
contents in these systems (Figure 1b), although other
hexagonal patterns are likely to form at low water contents,
to be explored in detail elsewhere.
We are most interested in the potential formation of
structures in which the different chemical species form
interpenetrating networks creating tri- and polycontinuous
assemblies, as exemplified in Figure 1c. Such assemblies have
emerged from theoretical considerations29−32 based on hints
from experimental data on miktoarm copolymers.14 These
patterns can be regarded as an extension of the well-known
amphiphilic bicontinuous cubic phases. Recently, an self
consistent field theory study postulated that the tricontinuous
so-called 3ths(5) phase is an equilibrium phase for triblock star-
copolymers when an appropriate molecular core is intro-
duced.33 The first experimental tricontinuous structure, the
3etc(193) structure, has been discovered in mesoporous silica34
and most recently in a Gemini surfactant phase.35 In contrast to
our aim of building interpenetrating networks of different
chemical species, the three networks in these cases consist of
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a
the same material (air for mesoporous silica, and water for the
Gemini surfactant), separated by a wall (silica wall and
surfactant membrane, respectively). The formation of this and
other tri- and polycontinuous assemblies in amphiphilic lipids
has also been postulated on theoretical grounds.36
Scheme 1. General Synthesis Scheme for Star-Polyphiles
In this article we focus on the synthesis, physicochemical
characterization, and self-assembly of star-polyphiles containing
a central 1,3,5-trihydroxybenzene core (Figure 1a and 2a). We
developed a modular synthesis method for ease of chain
variation, able to be scaled up readily to multigram levels. The
final compounds are achiral and monodisperse, and possess
high chemical stability, allowing for phase behavior studies and
applications within a broad temperature range. The general
synthesis scheme may also be useful for the asymmetric
synthesis of discotic liquid crystals or dendritic molecules.
Further, we demonstrate that star-polyphiles can be loaded
simultaneously with up to three mutually immiscible solvents,
namely dodecane, perfluoro-nonane, and water, to form
thermodynamically stable lyotropic mesophases, including
microemulsions and liquid crystals. This novel cosolubility,
coupled with mesoscale compartmentalization into three
distinct domains renders these star-polyphiles attractive for a
range of applications, including multicomponent-molecular
storage and drug delivery of mutually insoluble drugs and
other chemical species, as well as a number of other
applications that have been suggested for ABC miktoarm
copolymeric materials.39
RESULTS AND DISCUSSION
a
■
Steps: (a) triple protection of 1,3,5-trihydroxybenzene with
1. Synthesis. Our synthesis of triphilic star-polyphiles is
based on sequential conjugation of three chemically different
arms onto the three equivalent hydroxyl groups of 1,3,5-
trihydroxybenzene via O-alkylation (Williamson ether syn-
thesis). Sequential etherification cannot be achieved using the
nonprotected center, since the attachment of the first chain by
O-alkylation is not selective to one hydroxyl group, and the
attraction between chains of the same kind is expected to
reduce the yield of molecules with three mutually repelling
arms even further. Potential selective mono- and dialkylation
using zinc as a catalyst has been reported,40 but despite
considerable efforts we were unable to successfully reproduce
this reaction. Therefore, we developed an alternative method,
as shown in Scheme 1. First, the three hydroxyl groups in the
core module, 1,3,5-trihydroxybenzene, were protected with
tosyl-groups, which were then selectively and consecutively
deprotected in increasingly alkaline medium following a
modified procedure developed by Kampouris.41,42 The three
different chains were consecutively conjugated to the respective
unprotected hydroxyl groups in a classical etherification
reaction. A similar asymmetric synthesis approach but using
two different types of protection has been developed by
Lehmann and Jahr,43 designed for the sequential attachment of
chains via ester bonds as opposed to the ether bonds used here.
A suite of star-polyphiles was synthesized with a range of
chain lengths in the three arms listed in Figure 2. The resulting
molecules are summarized in Table 1. Most of the functional
hydrocarbon chains and fluorocarbon chains were commercially
available, except for the tosyl-activated phytanol chain and
monodisperse methyl-capped oligoethylene glycol chains with
various lengths. The former compound was obtained by
hydrogenation of phytol in the presence of Reney catalyst44,45
followed by the solid phase tosylation reaction developed by
Kazemi et al.46 The latter compounds were synthesized by an
tosylchloride, (b) removal of the first (protecting) tosyl group, (c)
attachment of the first chain (hydrocarbon chain Hb), (d) removal of
the second tosyl group, (e) attachment of the second chain (methyl-
capped oligoethyleneglycol chain Oa), (f) removal of the third tosyl
group, and (g) attachment of the third chain (fluorocarbon chain Fc).
in-house method, which we developed to allow for large-scale
synthesis and purification of monodisperse methyl-capped
oligo-ethylene glycol without the use of column chromatog-
raphy. This method is based on a classical etherification
reaction, distillation, and the repeated application of the well
established tosylation procedure by Ouchi et al.47
Scheme 1 shows the seven steps of our modular synthetic
method. Step (a) involves the triple tosylation of 1,3,5-
trihydroxybenzene (phloroglucinol) suspended in water with
tosyl chloride in alkaline medium. High yields of pure triply
protected product 1 can be achieved on a large scale following
the procedure developed by Kampouris for the protection of
polyhydric phenols such as benzenesulfonic esters.41 Meth-
anolic KOH solution is used to hydrolyze the first protecting
group in step (b), resulting in one free hydroxyl group and
double-protected product 2. Step (c) is a standard mild
Williamson ether synthesis reaction for phenols to introduce
the hydrocarbon chain (Hb); we chose dimethylformamide
(DMF) as the solvent rather than tetrahydrofuran (THF), as
DMF reduces the possibility of C-alkylation.48 Further, we used
Cs2CO3 instead of K2CO3 as the base due to its higher
efficiency in deprotonation of hydroxyl groups. Step (d) is a
further pH-dependent reaction to remove the second protective
group, followed by step (e), another standard Williamson
reaction to attach the Oa-chain. The ultimate protecting tosyl
group is removed at high pH (ca. 14) in step (f), using solid
KOH and 18-Crown-6 to achieve such high pH in a THF-
solution. In the final step (g), the fluorocarbon (Fc) chain was
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Table 1. Physicochemical Properties of the Synthesised Star-Polyphiles
a
b
DSC phase transition temp
Structure at RT
Heating
(°C)
Cooling
dlam
a
b
γ
Samples
(J/g)
(°C)
(J/g)
(°C)
(J/g) (°C) (J/g) [nm] [nm] [nm] [deg]
B1
B-O5-H12-F8
B-O5-H12-F10
B-O7-H14-F8
B-O7-H14-F10
B-O5-H14-F8
B-O5-H14-F10
B-O3-H14-F10
B-O7-H16-F8
B-O7-H16-F10
B-O5-H16-F8
B-O5-H16-F10
B-O3-H16-F10
B-O8-H20-F10
B-O7-H20-F10
3.7
36.5
−36
−11
−38
−40
−78
−73
−66
−24
−48
−79
−77
−85
−8
−33.4
34.5
20
11
28
8
-
-
-
-
-
-
liquid
B2
−30.6
−68.5
−10.5
5
4.02
-
-
-
simple gel
liquid
B3
10.5
−15.9
9.6
0.4
10
-
4.72
3.91
4.15
?
-
2.27
1.17
1.17
?
B4
24.5 (d)
30.7 (d)
40.1 (d)
48.6 (d)
9.5
−49.1
−6.0
0.1
10
4.65
3.91
4.13
3.89
-
99.5 oblique gel
B5
−16.7
20.4
22
11
12
24
8
93.6 oblique crystal
95.4 oblique crystal
B6
4.0
0.5
18
B7
33.5
−3.7 (d)
?
-
crystal
liquid
B8
−4.5
10.4
-
-
B9
31.0
5.8
11
4.58
4.05
4.21
4.03
-
4.64
4.06
4.29
?
2.40
1.17
1.17
?
99.5 oblique gel
B10
B11
B12
B13
B14
36.4
0.3 (d)
22.1
28
9
92.0 oblique crystal
93.6 oblique crystal
45.0
9.6
14
24
59.1
37.3
13
8
21.9 (d)
?
-
-
crystal
liquid
liquid
−0.6
−12.0 (d)
−8.1
−15.0 (d)
-
-
−23
5
−44.0
4
-
-
-
a
DSC transition temperatures (peak positions) and associated enthalpies at a scan rate of 2.5 °C/min. The heating transitions listed correspond to
melting or clearing points. The cooling transitions correspond to formation of (multiple monotropic) liquid crystalline phases until −70 °C.
b
Structural data at room temperature established from SAXS/WAXS spectra. The lamellar repeat was calculated as dlam = 2π/q, and the unit cell
parameters of the oblique unit cells are a and b, and γ is the angle between them.
attached. In the end, the crude final products were purified
using column chromatography, and we achieved a purity of
more than 99%. More experimental details are in the
Experimental Section and Supporting Information sections S1
and S2.
2. Physico-chemical Characterization. Liquid Crystalline
Textures of Neat and Hydrated Polyphiles: Polarized Optical
Microscopy (POM). At room temperature the neat star-
polyphiles are either liquid, waxy, or solid powders, as
summarized in Table 1. The waxy materials are strongly
birefringent, and typical textures as observed by POM for B2
(B-O5-H12-F10) and B4 (B-O7-H14-F10) are shown in Figure 3a,b.
The waxy and liquid star-polyphiles with an O7- and O8-chain
show lyotropic mesophormism at room temperature with
addition of water. For example, the neat polyphile B3 (B-O7-
H14-F8) is an isotropic liquid and develops a strongly
birefringent fan-like texture at low water content, as well as a
viscous isotropic phase at higher water content, as shown in
Figure 3c.
Thermal Behavior: Thermogravimetric Analysis (TGA).
Thermogravimetric data of two representative samples B5 (B-
O5-H14-F8) and B6 (B-O5-H14-F10) are shown in the Supporting
Information section S3. Weight loss occurs in a single step
above 300 °C, indicating evaporation (with possible concom-
itant degradation) of the material. Below this temperature, the
weight loss is negligible, which demonstrates that there is no
degradation and suggests that these compounds are thermally
stable.
Thermal Behavior: Differential Scanning Calorimetry
(DSC). Most of these star-polyphiles show complex thermal
behavior that is in part strongly history-dependent. DSC-scans
reveal thermotropic mesomorphism (enantiotropic or mono-
tropic) in all samples. This is exemplified by three examples.
Figure 4a shows the first heating, first cooling, and second
heating DSC-scans of B2 (B-O5-H12-F10). This star-polyphile
has a waxy appearance at room temperature. In addition to
some minor rearrangements at low temperature (below −20
°C), it shows one major reversible phase transition upon
heating/cooling with minor hysteresis in the cooling scan (at
Figure 3. Polarizing microscopy images. (a) Simple gel phase (B2, B-
O5-H12-F10), (b) Oblique gel phase (B4, B−O7-H14-F10), (c) lyotropic
liquid crystalline textures in a water penetration scan of B3 (B-O7-H14-
F8) at room temperature. The scale bars correspond to 100 μm.
∼35 °C). This transition is associated with a phase transition
between a liquid crystal (lc) structure and the molten fluid
isotropic state, with an associated enthalpy change of ∼11 J/g.
The lc-structure, that we call a “simple gel phase”, is lamellar
and similar to a surfactant gel or rotator phase in which the stiff
fluorocarbon chains are packed in a hexagonal array. We discuss
this phase in more detail in the SAXS section below.
Upon increasing the H-chain length from 12 to 14 (B6 B-O5-
H14-F10), the sample is crystalline at room temperature and
melts upon heating at ∼40 °C with a (split) melting enthalpy of
73 J/g, as shown in Figure 4b. Upon cooling, multiple
metastable (so-called monotropic) liquid crystalline phases
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Figure 4. Typical DSC scans of neat polyphiles. Red: first heating, Blue: first cooling, and Orange: second heating scan. Samples were cooled to −75
°C at 10 °C/min and equilibrated for 10 min before the scans. Subsequent heating/cooling scans were performed at 2.5 °C/min. The scans are
displayed with an offset for clarity. (a) B2 (B-O5-H12-F10), (b) B6 (B-O5-H14-F10), and (c) B4 (B-O7-H14-F10).
occur, and only upon reheating, does the sample recrystallize
back to its original thermodynamically stable crystalline state, as
shown by the pronounced exothermic peak in the second
heating scan.
Extending the O-chain from 5 to 7 units (B4 B-O7-H14-F10)
results in a waxy material that is close to melting at room
temperature. The DSC heating scan reveals a split clearing peak
at ∼25 °C, as shown in Figure 4c. Its associated enthalpy
change of 40 J/g is between typical values obtained for the
melting enthalpy of a crystal (as in Figure 4b) and a simple gel
phase (as in Figure 4a). We call this phase a “complex gel
phase” (see the SAXS section for more details). Upon cooling,
this sample also forms multiple monotropic liquid crystalline
structures, marked by multiple phase transitions. Two of these
phase transitions are associated with significant enthalpies (−8
J/g at 9.5 °C and −10 J/g at −10.5 °C), and another very small
rearrangement is observed at lower temperature. During
Figure 5. Characteristic SWAXS patterns of star-polyphiles in liquid
cooling, the sample remains in a metastable statesimilar to
form (red, B8 B-O7-H16-F8), simple gels (black, B2 B-O5-H12-F10),
complex (oblique) gels (green, B9 B-O7-H16-F10), and crystalline forms
(blue, B11 B-O5-H16-F10).
the previous sample, and only upon heating does a rearrange-
ment to the thermodynamically stable phase take place, as
shown by the combined melting/crystallization peak at ∼ −5
°C that occurs in the second heating scan. For more details see
Supporting Information section S4.
Table 1 gives a summary of the melting/clearing points as
liquid samples, that for B8, is shown as a red curve in Figure 5.
In general, the broad small-angle peak stems from a correlation
observed in the first heating scan, and the phase transition
between the H−H, F−F, and O−O domains, and its exact
temperatures observed in the cooling scans together with the
position depends on the size of each of the chain lengths.
associated enthalpies for all star-polyphiles investigated here. In
b. Simple Gel Phase. The next simplest scattering pattern we
observed stems from sample B2 (B−-O5-H12-F10) and is shown
general, the melting/clearing points increase upon increasing
the H-chain and/or F-chain length and decreasing the O-chain
length. The only two samples with one single reversible clearing
transition and little hysteresis are B2 and B13. All other samples
show multiple monotropic liquid crystalline phases upon
cooling.
Small- and Wide-Angle Scattering (SAXS/WAXS) of the
Neat Star-Polyphiles. Figure 5 shows characteristic SAXS/
WAXS spectra of the neat star-polyphiles at room temperature,
recorded at the SWAXS beamline of the Australian
Synchrotron.49 Star-polyphiles can form either structured fluids
(red curve), thermotropic liquid crystals (simple and complex
gels, black and green curve), or solid crystals (blue curve).
a. Liquid Star-Polyphiles. All liquid samples show a single
correlation peak in the small-angle range (q ∼ 1.5 nm −1),
signifying the formation of structured fluids, and a broad peak
in the wide-angle range (q ∼ 13 nm−1), demonstrating that all
chains are fully molten. A characteristic scattering pattern for
as a black curve in Figure 5. It contains only one set of
equidistant lamellar peaks in the small angle range resulting in a
lamellar repeat of 4.02 nm, and one sharp peak in the wide-
angle range with a repeat distance of 0.498 nm. Measurement
over an extended wide-angle range (Figure 6) reveals four wide-
angle peaks that can be indexed to a 2D hexagonal lattice, with
lattice parameter 0.576 nm. This gives a 2D cell area of 28.7 Å2
(0.287 nm2), in very good agreement with the value found for
stiff hexagonally arranged fluorocarbon chains.28 A broad halo
around the first sharp wide-angle peak is consistent with
disordered (molten) H-chains and O-chains. Although a little
more complex, this scattering is similar to that of lipid or
surfactant gel phases with molten hydrophilic groups and stiff
hydrocarbon chains packed in a 2D hexagonal arrangement,
free to rotate around their long axis (known as surfactant gel or
rotator phases50,51). Therefore, we call this phase, formed by
star-polyphile B2 at room temperature, a simple gel phase.
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Figure 6. Measured SWAXS (blue) and extended WAXS (black) pattern for star polyphile B2 in the simple gel phase. The inset shows the merged
WAXS-range. The black arrows indicate the equidistant peaks in the small-angle region, corresponding to a lamellar structure as the molecular
arrangement of the molecules. The red arrows indicate the peaks in the wide-angle range and can be indexed on a 2D hexagonal lattice (√1, √3,
√4, √7). The resulting hexagonal lattice parameter is in very good agreement with that typically found for stiff fluorocarbon chains arranged on a
hexagonal lattice. (Note that the decreased intensity in the black extended spectrum at ∼12 nm−1 is due to a shadow in the experimental setup, and
the broad peak at 27 nm−1 is background scattering from the sample cell.) Right: Suggested structural model for this simple gel, made of stiff
fluorocarbon chains (green) aligned normal to the vertices of a 2D hexagonal array, separated by interdigitated hydrocarbon (H) and oligo-ethylene
glycol (O) chains (red and blue).
Figure 7. Measured SAXS pattern for sample B9 with 2D oblique indexing. Right: Proposed schematic model of complex gel phases. Note the
colocation of hydrocarbon (H, red) and fluorocarbon (F, green) chains, segregated from the hydrophilic (oligo-ethylene, O) chains (blue), in
contrast to the simple gel phase.
A likely model structure for this simple gel can be inferred
from the molecule’s architecture combined with the scattering
data. The lamellar repeat distance of 4.02 nm obtained from the
scattering is at the upper limit of a fully stretched crystalline
molecule (i.e., chains in all-trans configurations, see Supporting
Information section S5), which suggests an interdigitated
bilayer arrangement. The 2D hexagonal (close-packed) array is
strongly suggestive of stiff extended fluorocarbon chains stacked
along the long axis. A structure consistent with the scattering,
containing mixed O and H chains segregated from the F chains,
is shown on the right in Figure 6. This gel structure is built of
flat bilayers, containing extended fluorocarbon chains normal to
the layers, interdigitated in an antiparallel fashion to form stiff
layers, sandwiched between molten, mixed hydrocarbon and O-
chains, with a single star-polyphile molecule per unit cell. We
note that the proposed structure resembles the smectic B phase
found for branched and unbranched perfluoroalkyl-alkanes.52
Unlike all other observed liquid crystalline phases formed by
these star polyphiles, it is reasonable to assume that the H-
chains and O-chains are able to mix in the structure of this neat
polyphile, since the short dry O-chains (O5) are not very polar
in the absence of free OH-groups. This assumption is strongly
supported by the observation that this structure cannot be
swollen with water. We note, however, that the paucity of
scattering peaks makes unequivocal structural assignment
difficult, and we cannot rule out more complex disordered
structures. This structural model, with a 3D unit cell defined by
the lattice vectors of the 2D hexagonal lattice (giving the a and
b lattice vectors of the 3D cell) and the c axis equal to the
lamellar repeat distance, has a volume of 1.15 nm3. Our
estimate of the molecular volume based on bulk densities of the
constituent parts is between 1.3 and 1.4 nm3. As a consequence,
our proposed structure would be unusually tightly packed,
though this increased density can be ascribed to very closely
packed chains, consistent with the gel character of the phase
(see Supporting Information section S6).
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Figure 8. Scattering pattern of (a) dry star-polyphile B13 (B-O8-H20-F10) with either H-oil, F-oil, combined H/F-oil, or water; (b) a hexagonal liquid
crystal with 5% water with added H-oil, F-oil, or both at 22 °C; (c) same samples as in (b) at 50 °C. All percentages are given in V/wt % (volume of
added solvent/weight of star-polyphile*100).
c. Complex Gel Phases. Simultaneous extension of the H-
chains (from H12 to H14 or H16) and the O-chains (from O5 to
O7) results in the more complex gel phases at room
temperature (sample B4 and B9). In common with the simple
gels, these waxy samples also display a small-angle signal
consistent with a 1D lattice. However, the occurrence of
additional peaks in the small- and wide-angle range signifies a
more complex arrangement (Figure 5). Given the multitude of
(broadened) wide-angle peaks, these samples form phases
between more common gel and crystalline phases, which is also
in agreement with their DSC thermograms. They display
characteristics of disordered crystalline phases, though their
waxy appearance, as well as their capability to significantly swell
with water, is consistent with macroscopic gel phases. We
therefore call samples with these features “complex gels”.
All small-angle peaks can be indexed on a 2D oblique lattice,
as shown in Figure 7 for sample B9. The resulting lattice
parameters and angles are summarized in Table 1. A large
number of overlapping peaks of variable widths are observed in
the wide-angle range, arising from chain packing, as well as the
higher-order lattice peaks. Resolution of the individual chain−
chain structural contributions to the wide-angle scattering is
currently impossible, leaving an open question as to which of
the molecular moieties are molten and which are frozen or even
crystalline. Some information can nevertheless be inferred from
the swelling of these complex gels. In contrast to the simple gel
above, these liquid crystals can be swollen with water, retaining
a swollen oblique lattice (from SAXS). Clearly, this feature is
incompatible with the mixed H and O chains of the simple gel,
suggesting that they are segregated in the complex gel phase,
consistent with the formation of this phase on lengthening H
and O chains in the star polyphiles. The scattering and swelling
data suggest the presence of lamellae made up of layers of
molten O-chains, interleaved with layers of alternating patches
of H- and F-chains that cause in plane ordering as illustrated in
the right picture of Figure 7, resulting in the second repeat
distance. The formation of an obliqueand not rectangular
unit cell is likely caused by a tilt between the O-chains with H-/
F-chains mismatch in order to match cross-sectional areas (see
Figure 7).
d. Crystals. Crystals are obtained if the O-chains are small
(O5 or O3), and their crystalline character is retained when the
F-chain is shortened from F10 to F8. The observed scattering
peaks are much sharper for these samples, and once again the
lamellar spacing is retained. The samples with O5 chain can
again be indexed on a 2D oblique lattice, as summarized in
Table 1. The first peak that cannot be accounted for is already
in the wide-angle range; see Supporting Information section S6
for more details. The indexing of crystals with an O3-chain
remains ambiguous at this stage due to the formation of
coexisting metastable structures.
3. Lyotropic Phase Behavior: Formation of Multi-
compartments. The star-polyphiles with an O-chain length of
7 or 8 exhibit lyotropic mesomorphism, as already demon-
strated by the water penetration scan shown in Figure 3c. A
detailed study of their lyotropic phase behavior will be
published in the near future. Here we report only preliminary
data establishing that some polyphiles can be loaded
simultaneously with the three mutually immiscible solvents
dodecane, perfluoro-nonane, and water, to form single,
thermodynamically stable mesophases. The resulting materials
contain four distinct chemical species, intimately mixed at the
supramolecular level to form microemulsions and liquid crystals
with multiple chemically distinct compartments, as outlined
next.
At room temperature, the neat star-polyphile B13 (B-O8-H20-
F10) forms an isotropic, slightly viscous liquid phase. The liquid
is miscible with H-oil (dodecane) in any ratio, while the
maximum amount of incorporated F-oil (perfluoro-nonane) is
limited to 7.5 V/wt % at this temperature. This star-polyphile
can also be swollen with water, whereby only a small amount of
water (about 3 V/wt %) can be added at room temperature
before a transition from a liquid to a birefringent liquid crystal
takes place, which persists in excess water (at above 25 V/wt
%). These solubilities suggest that, in this particular liquid star-
polyphile phase, the H-chains can form a continuous matrix,
embedding the F- and O-domains.
Figure 8a shows the evolution of the SAXS pattern upon
addition of the various solvents. The neat polyphile is a
structured liquid phase, characterized by a single broad
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correlation peak in the SAXS region (black curve). The
addition of 5 V/wt % H-oil dampens the peak and moves it to
slightly higher q-values (red curve), indicating decreased order
in the system at a slightly reduced correlation length. The
addition of 5 V/wt % F-oil, on the other hand, enhances the
correlation peak and moves it to lower q-values (green curve),
denoting swelling and increased order in this sample. Addition
of 5 V/wt % H-oil to this F-oil loaded sample (orange curve)
demonstrates a combination of both these effects. Water also
swells the structure (blue curve), in a similar fashion to the
addition of F-oil for small water fractions. Water dilution of 5
V/wt % and beyond is sufficient to induce a phase transition
from the molten phase to a hexagonal liquid crystalline
mesophase (cyan curve). We have shown elsewhere that the
star-polyphilic hexagonal phase consists of hexagonally arranged
aqueous domains, embedded in a hydrophobic matrix that is
segregated into hydrocarbon and fluorocarbon regions.8 Oil
dilution necessarily enlarges the size of the hydrocarbon and
fluorocarbon domains, and further promotes nanophase
segregation. Note that an F10-chain is required to allow for
the incorporation of F-oil, whereas an F8-chain is too short,
despite the fact that an F8-chain was shown to be sufficiently
long to induce full segregation between the fluorocarbon and
hydrocarbon nanodomains in the hexagonal phase.8
Figure 8b demonstrates the effect of oil addition to this
binary polyphile−water hexagonal liquid crystal at 22 °C. Once
again, H-oil and F-oil have different effects: 5 V/wt % H-oil
dampens the peaks and shrinks the hexagonal unit cell slightly
(red curve), while 5 V/wt % F-oil significantly swells this
structure (green curve). The addition of both oils at the same
time results in a structure that gives rise to the orange scattering
curve. The resulting structure shows the combined effect of
both oils: the material remains a well-ordered hexagonal liquid
crystal, indicating that the melting effect of the H-oil is annulled
by the addition of F-oil. Its lattice parameter is only slightly
larger than that of the starting binary sample, due to competing
swelling and shrinkage effects of the F- and H-oil, respectively.
At 50 °C, these hexagonal liquid crystals melt to form
microemulsions, whose scattering data are plotted in Figure
8c. The effects of the H- and F-oils are very similar to those at
lower temperature, namely shrinkage of characteristic distances
and reduced order in the presence of H-oil, and enhanced
ordering on addition of F-oil.
These phenomena are consistent with swelling of conven-
tional amphiphiles in the presence of oils. As a general rule, H-
oils whose chain lengths are less than those of the host
amphiphile intercalate between chains toward the hydrophilic
end of the host, splaying and shortening the chains, analogous
to the effect of increasing temperature. For example, Siegel et
al.53 report that addition of diglycerides and short-chain alkanes
to a type II hexagonal mesophase in excess water (curved
toward water) increases the curvature of the cylindrical micelles
within the phase, leading to reduced lattice parameters. In
contrast, large hydrophobic molecules, such as long-chain
hydrocarbons and squalene, intercalate between the hydro-
phobic chain ends and swell the structure.
The orange curves in Figure 8b and c denote single-phase
liquid crystals and microemulsions containing star-polyphile
and all three mutually immiscible solvents simultaneously,
demonstrating the formation of separate hydrophilic, hydro-
phobic, and fluorocarbon domains. A schematic illustration of
this three-phase miscibility in the presence of the polyphiles at
room temperature is shown in Figure 9. The neat star-polyphile
Figure 9. Sketch of quaternary phase tetrahedron at room temperature
in wt/wt. While the solvents dodecane (H-oil), perfluoro-nonane (F-
oil), and water are mutually immiscible, star-polyphile B can
incorporate a significant amount of them simultaneously, forming
macroscopic single phases of multicompartment microemulsions
(yellow) and liquid crystals (red). The remaining part of the phase
tetrahedron is multiphasic and shown as empty.
can incorporate significant amounts of H-oil, F-oil, and water
either separately or simultaneously to form multicompartment
microemulsions (yellow body) at lower water contents or liquid
crystals at higher water contents (red body). Generally,
increasing the amount of incorporated H-oil reduces the
maximum solubilization capacity of water. Preliminary tests
suggest that the solubilization capacity of FC-oil is around 7.5
V/wt % irrespective of the presence of water and/or H-oil.
CONCLUSION AND OUTLOOK
■
We hope that this report on the synthesis and rudiments of self-
assembly in the presence of up to three mutually immiscible
solvents demonstrates the extraordinary versatility of star-
polyphiles. The key feature of these star-polyphilic assemblies is
the spatial segregation into three distinct hydrophilic,
oleophilic, and fluorophilic nanodomains, enabling them to
simultaneously incorporate three mutually immiscible solvents
(dodecane/perfluoro-nonane/water). It is clear that they can
form crystals, structured melts, and at least two distinct
thermodynamically stable gel phases as neat chemicals
(thermotropic liquid crystals), dependent on chain lengths
and temperature. In addition, a variety of metastable
(monotropic) liquid crystals can form upon cooling neat
polyphiles from the melt. Star-polyphiles whose hydrophilic
methyl-capped oligo-ethylene glycol chain is sufficiently long
(more than 5 monomers) are able to incorporate water and
self-assemble to form reverse microemulsions and a variety of
lyotropic liquid crystals. Detailed exploration of those phases
will be reported in the near future. Their capability to
incorporate multiple immiscible solvents enriches their capacity
to form exotic nanostructures and also makes them very
attractive for a range of applications. The most obvious
applications lie in simultaneous segregated loading, storage, and
release of multiple immiscible cargos. Segregated loading has
previously been achieved in miktoarm polymeric micelles,54 and
in particular applications in fields such as simultaneous storage,
delivery and release of multiple incompatible cargos are being
explored.39
EXPERIMENTAL SECTION
■
Polarizing Light Microscopy (POM). Textures were observed
with an Olympus BX53F microscope using crossed polarizers and a
quarter wavelength retarder (λ = 530 nm), and images were taken with
a digital MicroPublisher 3.3 RTV camera.
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Differential Scanning Calorimetry (DSC) and Thermogravi-
metric Analysis (TGA). Phase transition temperatures and associated
enthalpies were measured with a Mettler Toledo DSC 822 system with
a Mettler TSO 801RO sample robot (Mettler Toledo; Melbourne,
Australia). Temperature and enthalpy calibrations were performed
using indium as the standard. The samples were run in aluminum
crucibles in a sealed furnace and were cooled to −75 °C at 10 °C/min,
before subsequent heating/cooling cycles at a rate of 2.5 °C/min
between −75 and 75 °C. All samples were thoroughly freeze-dried
using tert-butyl alcohol before the first DSC scan. Thermogravimetry
analyses were performed using a Mettler Toledo TGA SDTA851 with
a Mettler TSO801RO sample robot (Mettler Toledo; Melbourne,
Australia). Samples were run in aluminum crucibles in a sealed furnace
and in a stream of air at a scan rate of 10 °C/min from RT to 500 °C.
Synchrotron SAXS/WAXS. SAXS/WAXS measurements were
performed at the Australian synchrotron.47 Most samples were
measured in 0.7 mm borosilicate capillaries, using the temperature
controlled multicapillary sample holder designed and built at the
synchrotron. The measurements shown in Figure 7b were done in
homemade solvent tight temperature controlled paste cells. Measure-
ments were performed at a maximally offset detector to cover a wide q-
range, with the energy of 8 or 20 keV. For the measurement in Figure
6, an additional wide-angle detector was installed. The 2D spectra were
converted to 1D scattering curves, transmission and background
corrected using the software Scatterbrain.
Mass Spectroscopy (LC-MS). LC-MS spectra of all reaction
mixtures and compounds were monitored on a Thermo Fisher
Scientific LCQ Advantage MAX ion trap mass spectrometer (Thermo
Scientific, San Jose, CA, USA) equipped with an APCI interface. The
LC was equipped with a Phenomenex column Gemini-NX 5 μm C18-
110 Å (150 × 2.0 mm). For the final products, the mobile phases
consisted of (A) 100% water and (B) 100% methanol. The samples
were analyzed on a linear mobile phase gradient from 60% solvent A to
100% solvent B within 5 min and then 100% buffer B within the next
10 min. For analysis of the reaction mixtures, the mobile phase was
methanol at a constant rate of 0.5 mL/min.
High Performance Liquid Chromatography (HPLC). Analytical
HPLC was performed on Waters HPLC equipment (Waters
Corporation, Milford, MA, USA), comprising a 600 solvent delivery
system with a 600 automated gradient controller using a Phenomenex
Gemini C18 column (5 μM, 4.6 X150 mm) and a UV detector and
Altech 2000 evaporative light scattering detector (ELSD). The mobile
phases consisted of (A) 100% H2O, 0.05% (v/v) trifluoroacetic acid
(TFA) and (B) 60% THF, 40% AcCN with 0.05% TFA. The samples
were analyzed on a linear mobile phase gradient from 80% solvent A to
100% solvent B within 5 min followed by 100% solvent B within the
next 15 min. Flow rate was set to 1 mL/min. The ELSD detector
nebulizer temperature and nitrogen gas flow were set to 103 °C and
2.9 L/min, respectively.
Starting Materials. Most of the starting materials were purchased
from Sigma-Aldrich (Castle Hill, Sydney-Australia), unless otherwise
mentioned. All solvents were of analytical grade and purchased from
MERCK (Australia) and were used without further processing.
Cs2CO3 and DMF were anhydrous. The fluorocarbon-iodides and
FluoroFlash cartridges were obtained from Fluourous (Pittsburgh,
USA).
pestle. The precipitate was washed with copious amounts of water to
remove all nonreacted TsCl and TsOH. Yield: 92% (108.2 g). Mw:
588.67. Rf-values in 70/30 hexane/ethyl acetate on silica gel TLC
plates with fluorescent indicator 254 nm: TsCl: 0.76, B-Ts3 0.46, B-Ts2
0.32, B-Ts1 0.18
Step (b) B-Ts2 (2). B-Ts3 (41.1 g, 0.07 mol) was dispersed in 100
mL of methanol at 35 °C. Twenty-nine mL of an aqueous solution
containing water/methanol/KOH = 40/40/20 solution was added
dropwise within about 10 min. The dispersed material dissolved,
resulting in a brown solution accompanied by some salt. The solution
was heated to 45 °C for 5 min, diluted to 1.5 L with water (brown
solution), acidified with 1/2 conc HCl to ∼pH = 5 (thick emulsion),
and continued to stir for 1 h. A sticky, viscous precipitate formed (B-
Ts2 and some B-Ts1), which was separated from the aqueous phase.
We could not find a suitable solvent to recrystallize B-Ts2, and
proceeded with the crude sample that was redissolved in DCM,
washed with water, and dried over MgSO4. Yield: 85% (25.8 g). Mw:
434.48. LC-MS m/z: 434.48/452.02 at t = 1.22 min, HPLC: t = 9.2
min.
Step (c) B-Ts2-H16 (3). Crude product 2 (25.8 g, 0.06 mol) and
C16Br (16.8 g, 0.055 mol) were dissolved in 200 mL of anhydrous
DMF at ∼30 °C. Cs2CO3 (58 g, 0.18 mol) was added, and the
suspension was stirred overnight. DMF was evaporated under reduced
pressure. The residue was dissolved in ether and washed 3 times with
10% NaCl solution. Yield: 92.7% (36.4 g). Mw: 658.91. LC-MS m/z:
659.42/676.37 at t = 2.86 min, HPLC: t = 11.5 min.
Step (d) B-Ts-H16 (4). Crude product 3 (36.4 g, ∼0.055 mol) was
dispersed in 100 mL of methanol at 40 °C. 65 mL of a 20% KOH
solution was added dropwise, and the dispersion was stirred well until
all the material was dissolved and no more B-Ts2-H16 was left (∼45
min). The solution was diluted with water to 1.5 L (turbid solution),
acidified with 1/2 conc HCl to about pH = 5, and stirred for 1 h. The
residual oily material on top of the solution was separated from the
rest, dissolved in DCM, and washed with copious amounts of water.
The DCM phase was dried over Mg2SO4 and evaporated to dryness to
obtain the title compound. Yield: 85% (23.1 g). Mw: 504.72. LC-MS
m/z: 505.35 at t = 1.94 min, HPLC: t = 11.1 min.
Step (e) B-Ts-O7-H16 (5). Crude product 4 (23.1 g, 0.047 mol) was
reacted with O7-Ts (23.3 g, 0.047 mol) similar to step (c) to obtain
the title compound. Yield: 88% (34.3 g). Mw: 827.12. LC-MS m/z:
828.32 at t = 2.1 min.
Step (f) B-O7-H16 (6). 34.3 g (∼0.041 mol) of crude product 5 and 1
g of 18 Crown 6 (=0.0038 mol) were dissolved in 150 mL of THF,
followed by addition of an excess amount of crushed KOH flakes (7
g). The dispersion was heated to 50 °C and stirred overnight while
covered with parafilm. 150 mL of water was added, forming a dark
brown solution, followed by neutralization with 1/4 conc HCl. The
reaction mixture was extracted with ether, and the ether phase washed
repeatedly with water. Yield: 90% (25.1 g). Mw: 672.93. LC-MS m/z:
673.58 at t = 1.81 min, HPLC: t = 10.4 min.
Step (g) B-O7-H16-F8 (7). Analogous to step (c), but react crude
product 6 (10.0 g, 0.015 mol) with C8F17C3H6I (6.1 g, 0.01 mol).
Yield: 89% (15g). Mw: 1133.06. LC-MS m/z: 1133.58 at t = 3.12 min,
HPLC: t = 11.5 min. 1H NMR in CDCl3: δ: 0.89 (t, 3H, J = 6.59 Hz,
Hb (CH3)), 1.19−1.39 (m, 20H, Hb (CH2)), 1.43 (quin, 2H, J = 7.20
Hz, Hb (γ-CH2)), 1.70−1.84 (quin, 2H, J = 8.10 Hz, Hb (β-CH2)),
2.08 (m, 2H, Fc (CF2-CH2-CH2)), 2.19−2.41 (m, 2H, Fc (CF2-CH2-
CH2)), 3.38 (s, 3H, Oa (CH3-O)), 3.52−3.59 (m, 2H, Oa (CH2-O−
CH3)), 3.60−3.77 (m, 22H, Oa (CH2-O)), 3.84 (t, 2H, J = 4.76 Hz,
Oa (β-CH2)), 3.90 (t, 2H, J = 6.59 Hz, Hb (α-CH2)), 3.99 (t, 2H, J =
5.85 Hz, Fc (α-CH2)), 4.09 (t, 2H, J = 4.94 Hz, Oa (α-CH2)), 6.00−
6.16 (m, 3H, b (CHC)).
Synthesis. The synthesis details are exemplified for B8 (B-O7-H16-
F8) in the following; all other syntheses were performed analogously. It
should be noted that there was no purification performed for
intermediate compounds, which means that the given yields after each
step are not a direct measure of pure sample, but include all
byproducts formed.
Step (a): 1,3,5-p-Toluenesulfonylbenzene B-Ts3 (1). To a stirred
dispersion of 1,3,5-trihydroxybenzene·2H2O (32.2 g, 0.2 mol) in 500
mL of water was added 0.63 mol of tosyl chloride TsCl (120.1 g). The
dispersion was heated to 35 °C and stirred vigorously while
concentrated KOH was continuously added. The addition was
continued until the pH remained permanently basic and no more
KOH was required to keep the solution basic. A sticky, white
precipitate formed that was crushed into a powder using a mortar and
The total yield of the 7 steps combined after purification (using
column chromatography and Fluoroflash cartridges) is between 10 and
15% with respect to the starting 1,3,5-trihydroxybenzene. More details
on the purification procedure and sample purity of the final
compounds by NMR, HPLC, and LC-MS are included in Supporting
Information sections S1 and S2.
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(19) Tschierske, C.; Nuernberger, C.; Ebert, H.; Glettner, B.; Prehm,
M.; Liu, F.; Zeng, X. B.; Ungar, G. Interface Focus 2012, 2 (5), 669−
680.
ASSOCIATED CONTENT
■
S
* Supporting Information
More details on sample purification, final sample purity,
thermogravimetric analysis data (TGA), differential scanning
calorimetry data (DSC), and indexing and interpretation of the
SAXS/WAXS data. This material is available free of charge via
the Internet at http://pubs.acs.org.
(20) Ungar, G.; Tschierske, C.; Abetz, V.; Holyst, R.; Bates, M. A.;
Liu, F.; Prehm, M.; Kieffer, R.; Zeng, X.; Walker, M.; Glettner, B.;
Zywocinski, A. Adv. Funct. Mater. 2011, 21 (7), 1296−1323.
(21) Lehmann, M. Chem.Eur. J. 2009, 15 (15), 3638−3651.
(22) Liu, F.; Prehm, M.; Zeng, X.; Ungar, G.; Tschierske, C. Angew.
Chem., Int. Ed. 2011, 50 (45), 10599−10602.
(23) Prehm, M.; Liu, F.; Zeng, X.; Ungar, G.; Tschierske, C. J. Am.
Chem. Soc. 2011, 133 (13), 4906−4916.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: liliana.decampo@icloud.com.
*E-mail: stephen.hyde@anu.edu.au.
(24) Kondo, Y.; Yoshino, N. Curr. Opin. Colloid Interface Sci. 2005, 10
(3−4), 88−93.
(25) Dupont, A.; Eastoe, J.; Murray, M.; Martin, L. Langmuir 2004,
20 (23), 9953−9959.
Notes
(26) Sanchez-Dominguez, M.; Benoit, N.; Krafft, M. P. Tetrahedron
2008, 64 (3), 522−528.
The authors declare no competing financial interest.
(27) Long, P.; Hao, J. Adv. Colloid Interface Sci. 2012, 171−172 (0),
66−76.
ACKNOWLEDGMENTS
■
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(29) Hyde, S. T.; de Campo, L.; Oguey, C. Soft Matter 2009, 5 (14),
2782−2794.
We gratefully acknowledge Zoltan Blum for synthesis advice,
and Ron Cruikshank for his contribution to the construction of
the new sample cells. Part of this research was undertaken on
the SAXS/WAXS beamline at the Australian Synchrotron,
Victoria, Australia. We thank Calum Drummond for supporting
the collaboration. S.T.H. and L.d.C. acknowledge support from
the Australian Research Council’s Discovery grant scheme.
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