Large-scale, low-cost fabrication of janus-type emulsifiers by selective decoration of natural kaolinite platelets

Article abstract of DOI:10.1002/anie.201106710

Kaolinite platelets have inherent Janus character owing to a polar crystal structure. This character is, however, ineffective without selective modification of the two hydrophilic external basal surfaces. Amplification of the difference in the chemical nature of the surfaces is achieved by cation exchange on one side and by covalent grafting on the other. After adjustment of the surface tension, kaolinite is an effective emulsifier. Copyright

Full text of DOI:10.1002/anie.201106710

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DOI: 10.1002/anie.201106710
Kaolinite-Based Janus Particles
Large-Scale, Low-Cost Fabrication of Janus-Type Emulsifiers by
Selective Decoration of Natural Kaolinite Platelets**
Dunja Hirsemann, Sankaranarayanapillai Shylesh, Roger A. De Souza, Bashar Diar-Bakerly,
Bernhard Biersack, David N. Mueller, Manfred Martin, Rainer Schobert, and Josef Breu*
Bifunctional and/or anisometric Janus[1] particles have proven
to be superior to spherical and chemically isotropic colloids
for a wide range of promising applications, such as in
nanorobots,[2] physical sensors,[3] microrheology,[4] drug deliv-
ery,[5] magnetic storage,[6] electronic devices,[7] surfactants, and
compatibilizers.[8] Nevertheless, large-scale technical applica-
tions have been halted by the restricted accessibility of these
polar colloids. Established synthesis methods that allow for
breaking the symmetry are laborious and expensive.[9–11]
Previously, microphase separation of triblock terpolymers,[12]
the arrangement of symmetrical colloids at an interface
followed by modification of the exposed hemisphere by
microcontact printing,[9] and different masking tech-
niques[13,14] have been used. However, nature provides multi-
functional spherical nanoparticles, such as proteins. Specific
modifications at different functional sites of ferritin or
transthyretin have been reported.[15,16] Recently, Kotov
et al.[17,18] pointed out that semiconductor nanoparticles may
resemble such proteins inasmuch as the specific local packing
into truncated tetrahedrally shaped morphologies leads to
breakage of symmetry of these nanoparticles, which may in
turn trigger self-assembling of these nanoparticles on the
mesoscale by directional attraction that is mainly of electro-
static nature. Symmetry-breaking in solids and minerals is,
however, not limited to the mesoscale. Crystal structures that
are inherently polar at the atomistic level are frequently
encountered.[19] As a consequence of the polar crystal
structure, opposing crystal faces carry different functional
groups.
Herein, we show that the natural mineral kaolinite
[Al2Si2O5(OH)4], an abundant, ubiquitous, and inexpensive
(E200/tonne) mineral, possesses Janus character. Kaolinite is
found as anisometric platelets with large aspect ratios that are
typically in the range of 20:1–40:1. Consequently, polar basal
planes dominate the external surface area.
Kaolinite is a dioctahedral layered silicate (Figure 1a).[20]
The octahedral layer and tetrahedral layer are linked to a 1:1
lamella with the basal surfaces confined by differing func-
tional groups (Figure 1b,c). The lamellae are stacked in a
polar mode through strong hydrogen bonds (Figure 1a).
Furthermore, as no twinning has usually been observed,[21] the
two opposing external basal planes of the kaolinite platelets
consist of an Al2(OH)4 octahedral layer (octahedral surface,
OS), which is capped by m-hydroxy groups at the external
surface, and the other side is capped by a SiO4 tetrahedral
layer (tetrahedral surface, TS). Although even pristine
kaolinite displays a Janus character, the surface tension of
the two unmodified external basal surfaces is similar.
Although the idealized chemical formula would suggest
natural kaolinite to be charge-neutral, it nevertheless pos-
sesses a small cation-exchange capacity (CEC) owing to an
isomorphous substitution in the tetrahedral layers, which can
[*] D. Hirsemann, B. Diar-Bakerly, Prof. Dr. J. Breu
Lehrstuhl fꢀr Anorganische Chemie I
Universitꢁt Bayreuth, 95440 Bayreuth (Germany)
E-mail: josef.breu@uni-bayreuth.de
Homepage: http://www.ac1.uni-bayreuth.de/de/team/Breu_Josef/
index.html
Dr. S. Shylesh
Department of Chemical & Biomolecular Engineering
University of California Berkeley (USA)
Dr. R. A. De Souza, Dr. D. N. Mueller, Prof. Dr. M. Martin
Institut fꢀr Physikalische Chemie
RWTH Aachen University (Germany)
Dr. B. Biersack, Prof. Dr. R. Schobert
Lehrstuhl fꢀr Organische Chemie I
Universitꢁt Bayreuth (Germany)
[**] We thank the German Science Foundation (SFB 840) and the Elite
Study Program “Macromolecular Science” as well as the Interna-
tional Graduate School “Structure, Reactivity and Properties of
Oxide Materials” within the Elite Network Bavaria and Bayer
Material Science for financial support. The authors thank Prof. Dr.
Jꢀrgen Senker for providing the NMR equipment.
Figure 1. a) Crystal structure of kaolinite viewed perpendicular to the
stacking direction (c*), which shows the polar nature of each individ-
ual 1:1 lamella and also of their stacking. b) Top view of the external
tetrahedral surface (TS), which is confined by basal oxygen atoms
from the tetrahedral layer. c) Top view of the external octahedral
surface (OS), which is capped by m-hydroxy groups.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106710.
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only be counterbalanced at the outer TS by hydrated
cations[22] that render the surface hydrophilic. The OS is
capped by hydrophilic m-Al2-OH groups. Consequently, the
Janus character inherent to the kaolinite structure is ineffec-
tive without selective modification of the external surfaces
that amplifies the difference in chemical nature of the
opposing basal surfaces (depicted in Figure 2). We used a
Whereas changing the surface tension of one external
surface already boosts the Janus character of kaolinite,
additional selective modification of the second external
surface is highly desirable for fine-tuning the Janus character.
Modification of the OS is, however, more difficult. Recently,
we showed that m-Al2-OH groups could be covalently grafted
by glycol,[26] demonstrating sufficient reactivity of the OS.
Catechols are known to form strong inner-sphere surface
complexes with titania surfaces that are chemically similar to
the OS of kaolinite.[27,28] These siderophiles are expected to
have a pronounced chemical affinity for the aluminol groups
of kaolinite. For analytical reasons, we selected a P-labeled
catechol, which allows for the detection of immobilization by
31P-MAS (magic-angle spinning) solid-state NMR spectros-
copy. The spectra from pristine kaolinite (Supporting Infor-
mation, Figure S3a) has one sharp 31P signal at À4.23 ppm,
which may be assigned to a well-known accessory phosphate
mineral called gorceixite [BaAl3(PO4)2(OH)6].[29] This trace
impurity (P content ca. 0.12% by weight) cannot be
selectively removed by physical or chemical treatment.
EDX (energy-dispersive X-ray spectroscopy) element map-
ping of phosphorus and barium showed that the impurity
phase is well-separated from the kaolinite particles and that
the basal surfaces were completely free from any phosphorus
and barium impurities (Supporting Information, Figure S4).
The 31P spectrum of modified kaolinite (Supporting
Information, Figure S3b) featured a signal at 34.0 ppm in
addition to the gorceixite signal at À4.2 ppm. The signal at
34.0 ppm is due to the phosphorus linker from the catechol
that was immobilized on the kaolinite. Neither the peak
position nor the intensity (normalized) of the 31P signal at
34.0 ppm (Supporting Information, Figure S3c) were affected
by subsequent modification of the TS by cation exchange,
which suggests that catechol immobilization and cation
exchange are independent from each other and most likely
occur on different surfaces. The chemical environment of the
phosphorus in the crystalline impurity is more rigorously
defined compared to the linker immobilized on the surface.
Consequently, the 31P signal of the linker is broader than that
of the impurity.
Figure 2. Amplification and fine-tuning of the Janus character: Selec-
tive modification of the TS and OS of kaolinite by covalent grafting
with catechol and cation exchange with [Ru(bpy)3]2+, respectively. The
molecular structures of modifiers are shown in the gray box.
natural coarse-grained kaolinite with typical dimensions of
the ideally hexagonal platelets that were 2 mm in diameter and
70 nm in height (Supporting Information, Figure S1). The
specific surface area was approximately 4 m2 gÀ1, and about
90% of this area could be attributed to the external basal
surfaces. Nevertheless, the detection of monolayer coverage
of the external surfaces required highly sensitive analytical
methods, and the proof of the selective modification is
inherently difficult.
Selective modification of the TS was achieved by simple
ion exchange[23] of the hydrated inorganic counterions (typ-
ically Na+) by organic or metal–organic cations that render
this surface more hydrophobic. For analytical reasons, we
selected the [Ru(bpy)3]2+ complex (bpy = 2,2’-bipyridine),
which is known to exhibit a high selectivity for clay surfaces
and for which emission properties adsorbed onto clays have
been intensively studied.[24] Successful cation exchange can
even be followed visually, as the color of the modified
kaolinite changes from white (pristine) to orange (modified).
The adsorption isotherm of [Ru(bpy)3]2+ was monitored by
UV/Vis spectroscopy (Supporting Information, Figure S2).
The observed adsorption capacity (2.6 mval/100 g) corre-
sponded with the CEC of the pristine kaolinite sample, which
was determined using the Ba2+ method ((2.7 Æ 0.1) mval/
100 g).[25]
Luckily, the phosphorus-containing impurity may be used
as an internal phosphorus standard, which allowed the
determination of the minute adsorption capacity of catechol.
This adsorption capacity could not be determined by other
methods because of the small surface-to-volume ratio of the
kaolinite particles.
A comparison of the integral of the 31P signal of catechol
and that of the trace impurity (Supporting Information,
Figure S3b) yields the additional phosphorus content that was
introduced by catechol adsorption, which was approximately
0.03% by weight and corresponded to 1 mval/100 g (details of
the calculations are provided in the Supporting Information).
Assuming a uniform hexagonal morphology for the kaolinite
platelets (thickness 70 nm, diameter 2 mm), we calculated a
reasonably high grafting density of approximately one linker
per two unit cells (92 ꢀ2). Furthermore, the edges of the
kaolinite platelets expose hydroxy groups that are expected to
adsorb catechol as well. Because of the high aspect ratio of the
platelets, the specific surface area of the edges was estimated
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to be less than 16% of the OS; therefore, the adsorption
capacity of the edges may be neglected.
Although the orthogonality of the adsorption of catechol
and [Ru(bpy)3]2+ already suggested a selective adsorption of
the two modifiers at distinct surface sites, unequivocal
evidence for the selectivity requires a sensitive analytical
method with sufficient spatial resolution.
ToF-SIMS (time-of-flight secondary-ion mass spectrome-
try) combines high surface sensitivity with sufficient lateral
resolution (< 1 mm), thus allowing individual platelets depos-
ited from dilute suspensions onto an appropriate substrate to
be analyzed. The high aspect ratio ensures a textured
sedimentation from suspensions with the basal planes orien-
tated parallel to the substrate. We used two different
substrates: silicon wafers and gold-coated silicon wafers. As
it turned out, the drop-casted kaolinite platelets showed
different orientations on the different substrates. In addition,
the modifiers for the TS and OS yield distinctly different
secondary-ion fragments. Furthermore, the exposure time to
the primary ion beam (analysis time) was chosen so that not
only secondary ions from the modifiers were detected but also
secondary ions from the underlying surfaces. In total, we
estimated that 1–2 nm of material were removed during the
analysis; as this is much less than the typical thickness of the
kaolinite particles, we can be sure that the acquired data refer
solely to the exposed surfaces.
Secondary-ion images were acquired by rastering the ion
beam multiple times over a selected area and recording (at
high sensitivity but low mass resolution) a full mass spectrum
at every pixel. In Figure 3, only selected masses that
correspond to atomic or molecular secondary ion fragments
characteristic for the fragmentation of the OS and TS
modifiers are shown in the cumulative images. The low
mass resolution means that there are unresolved mass
interferences; for example, there is another secondary ion
present at m/z = 102 in addition to Ru+.
On both substrates, the kaolinite platelets can be easily
located by the silhouettes that become apparent in the
secondary ion images of Au+ and Si+. The location of the
platelets was further confirmed by the high intensities of the
silicon and aluminim masses, which are the main constituents
of kaolinite (Supporting Information, Figures S5 and S6).
We selected the characteristic mass image of the modifiers
of the TS (Ru+, Figure 3b) and of the OS (P+, Figure 3c and
Figure 3. Mass fractions determined using SIMS measurements on
A) a gold-coated silicon wafer (200 mmꢂ200 mm) and B) on a silicon
wafer (100 mmꢂ100 mm). Secondary-ion images of: a) Au+ (m/z=197;
(A)) Si+ (m/z=28; (B); wafer material), b) Ru+ (m/z=102) (TS modi-
fication), c) P+ (m/z=31) (OS-modification), and d) C3H5O2 (m/z=
+
73) (OS modification). Both scale bars for relative intensity of the
masses (right-hand side: black=low intensity, white=high intensity)
and size (d) refer to the complete set of four masses measured on
each substrate.
+
C3H5O2 , Figure 3d; additional masses are shown in the
Supporting Information, Figures S5 and S6). On the gold-
coated substrate (Figure 3Ab), the intensity of the Ru+ mass
on the kaolinite particles was below the level of significance,
and the intensity of the Ru+ mass recorded in the background
of the substrate was greater than that recorded on the
kaolinite particles (unresolved mass interference). However,
the intensities of the fragments related to the phosphorus
was oriented towards the wafer; the OS was exposed to the
analysis chamber and was selectively modified by the
phosphorus-labeled catechol.
The mass images were different on the silicon substrate
(Figure 3B) compared to those on the gold-coated substrate.
At the positions where the particles were located, the mass
signal that was due to the phosporus-labeled catechol was
below the detection limit, while a high intensity was observed
for the mass signal from Ru+. This result suggests that the
kaolinite platelets were facing the ion beam with the TS, and
that the TS was selectively modified by [Ru(bpy)3]2+.
linker (P+, Figure 3Ac and C3H5O2 , Figure 3Ad) were
+
significantly greater on the kaolinite platelets compared to
the background. For unknown reasons, the intensities of these
two masses appeared patchy, which was most likely due to the
inhomogeneous reactivity of the OS towards catechol because
the more reactive sites (kinks) adsorbed more of the modifier.
On the gold-coated substrate, the TS of the kaolinite platelets
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The arrangement of the kaolinite particles on the
substrate was defined and not random. A comparison of the
sediments on the gold-coated and silicon wafers in Figure 3
indicates that either the catechol (P-labeled) or [Ru(bpy)3]2+
was detectable. This result demonstrates not only that a
selective modification of kaolinite platelets is feasible, but
that, surprisingly, a selective alignment on one specific basal
plane was achieved.
We hypothesize that for selective orientation, the platelets
must be far enough away from the surface of the substrate to
allow for rotation (> 2 mm) and that the specific interaction
with the substrate must therefore be long-range, which
suggests that electrostatics likely play a major role. Although
the zeta potential of the platelets was diminished after cation
exchange with [Ru(bpy)3]2+, the surface potential of the TS
remained negative (À36 mV and À18 mV for pristine and
[Ru(bpy)3]2+-exchanged kaolinite, respectively). The surface
potential of the gold-coated wafer was expected to be positive
at pH 5 because of oxidation during the cleaning proce-
dure.[30] In contrast, the surface potential of the silicon wafer
was negative.[31] If electrostatics does induce a selective
orientation, the negatively charged TS should be pushed
away from the silicon substrate and attracted to the gold-
coated substrate, which is in agreement with experimental
observations (Figure 3).
As a preliminary test to demonstrate the superiority of the
Janus-type particles as Pickering emulsifiers, we investigated
their potential as stabilizers for oil-in-water emulsions. For
this particular emulsion, one of the external basal surfaces of
kaolinite must remain hydrophilic, and therefore only the TS
was rendered more hydrophobic while the OS was left
unchanged and hydrophilic. Exchanging the hydrophilic
inorganic cations at the TS with hydrophobic hexylammo-
nium ions ensured a good wettability of the modified TS with
the hydrophobic oil phase. For comparison, emulsions were
prepared with pristine and TS-modified kaolinite, respec-
tively (Figure 4). The pristine kaolinite reflected the stabili-
zation of the emulsions through a simple Pickering effect. The
emulsions prepared with the pristine kaolinite quickly
coalesced, and oil droplets with sizes in the range of
millimetres were observed after 48 h. Some of the kaolinite
formed sediment on the bottom of the vessel. However, the
emulsions with the TS-modified kaolinite were stable for
months and yielded smaller droplet sizes. Typical droplet sizes
achieved with the modified kaolinite were in the range of
100 mm (Supporting Information, Figure S7).
The emulsifying power of the modified particles indicated
a high affinity of assembly at the oil–water interface. This
indirectly confirmed the selective nature of the modification
because platelets that were concomitantly modified at the TS
and OS sites were expected to be tracked into the oil phase
because of the purely organophilic nature of both the OS and
TS sites of dually modified platelets.
In summary, we have presented evidence for the Janus
character of inexpensive natural kaolinite platelets. Unfortu-
nately, the TS and OS sites of pristine kaolinite were
hydrophilic, and the Janus character was hidden. Fortunately,
the selective modification of the TS and OS was feasible, and
the modification was achieved by cation exchange and
covalent grafting of catechol ligands, respectively. The
modification is not limited to the model reactions selected
for the proof-of-concept experiments performed herein;
rather, a wide range of selective modifications are possible
and will allow the chemical nature of the Janus surfaces to be
fine-tuned. Moreover, the Janus character was emphasized by
the plate morphology and large aspect ratio. Because of the
facile modification, the environmentally benign nature of
kaolinite and its low-cost availability, the range of possible
applications is limitless. From an industrial point of view, the
unique surface activity of Janus particles that arise from the
combination of the Pickering effect and the amphiphilic
nature of the Janus particles holds the largest potential, such
as for strong emulsifiers that control the microstructure of
polymer blends.
Received: September 21, 2011
Published online: December 27, 2011
Keywords: emulsions · kaolinite · organic–
.
inorganic hybrid materials · polar platelets
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