Journal of the American Chemical Society
Article
compartmentalized in the water droplets and should not get in
contact with each other avoiding destruction. Meanwhile, the
reactant A in the continuous (oil) phase can freely move
through autodiffusion, and is converted to the intermediate B
upon meeting the acid at the droplet interface, which is
subsequently transformed to the final product C after meeting
the base-contained droplets located in the neighboring layer. In
this scenario, the whole Pickering emulsion reaction system is
just like a living system, which can spatially position diverse
cells in different regions, compartmentalize the mutually
destructive enzymes or molecules in the different cells but
allow for the free diffusion of other molecules located outside
the cells for biochemical reactions if needed.
emulsions remained virtually the same, and there was still a
clear boundary between the layers without color fading (Figure
2, a5). Impressively, after this layered Pickering emulsion stood
for 144 h, the zebra stripes of alternate colors were still well
maintained indicating the survival of acid and base in the same
system. Such a period of time is sufficiently long for most
chemical reactions themselves to take place. In contrast, the
surfactant-stabilized emulsions have poor ability to achieve the
survival of acid and base in a single system (Figure S4).
In order to investigate these results further, optical
microscopy, SEM and TEM were employed to observe the
microstructures of the Pickering emulsions. Before mixing, the
Pickering emulsions formulated with HCl and NaOH consist of
droplets with diameters ranging from around 10 to 250 μm
(Figure 2, b1 and b2). After mixing via a lamination procedure
and further standing for 24 h, the morphology and size
distribution of the emulsion droplets sampled from close to a
layer boundary show no apparent changes (Figure 2, b3). This
indicates that the layered Pickering emulsions have high
stability against droplet coalescence and Ostwald ripening,
and thereby have excellent ability to create stable micro-
compartments. The microcompartments created by emulsion
droplets were further confirmed by SEM. After the layered
Pickering emulsion (close to a layer boundary) was treated by
freeze-drying, microspheres were clearly observed (Figure 2,
c1), which originated from precursor emulsion droplets. In the
more magnified SEM image (Figure 2, upper inset of c1), silica
nanospheres are observed to be closely packed on the surface of
these microspheres. As expected, the microspheres are hollow
(Figure 2, lower inset of c1) since freeze-drying involves
removal of the inner water directly from a solid to a gas. This is
direct evidence for the pronounced microcompartments.
Similar to the SEM image, the TEM image obtained at low
temperature further confirmed the droplet microstructure
(Figure 2, c2). The excellent ability to prevent acid−base
neutralizations can be attributed to the microcompartments
within Pickering emulsions.
2. RESULTS AND DISCUSSION
2.1. Compartmentalization Effects of Water Droplets.
Although surfactant-stabilized emulsions were reported to have
the ability to compartmentalize opposing reagents, the trapping
time is only several minutes, which is too short to carry out
chemical reactions.50 The dynamic exchanges of molecular
surfactant between emulsion droplets and the presence of free
surfactant in the continuous phase probably accelerate mass
transport of the molecules between different droplets.51,52
Moreover, after the completion of reaction, the separation of
surfactants from products is relatively difficult. These obstacles
may be overcome with particle-stabilized emulsions. Partially
hydrophobic silica nanospheres with diameters of 130−200 nm
were used as emulsifier, which were easily prepared from bare
silica particles through a one-step modification with methyl-
trimethoxysilane [transmission electron microscopy (TEM)
images, scanning electron microscopy (SEM) images and
particle size distribution are included in Figure S1 (Supporting
Information); the methyl group loading is estimated to be
0.072 mmol g−1 (ca. 3 methyl groups per square nm) on the
basis of elemental analysis; the thermogravimetric curves are
displayed in Figure S2; the air−water contact angle of the
particle surfaces is 91°, as shown in Figure S3].
To further confirm the compartmentalization effects of
droplets, we used fluorescence microscopy to observe whether
reagent molecules transfer between droplets. Two parent
Pickering emulsions were formulated in the presence of
fluorescent dyes (there is no acid or base in the water in
these experiments). The first was prepared with water
containing FITC-dextran (green) and toluene containing Nile
red (red). Judging by the colors of Figure 2, d1 (green inside
droplets and red outside droplets), this Pickering emulsion is of
the water-in-oil type since FITC-dextran is a water-soluble dye
while Nile red is an oil-soluble one. The second was prepared
with pure water (without FITC-dextran) and toluene
containing Nile red. As seen in Figure 2, d2, the continuous
phase is red while the interior of the droplets is black because of
the absence of fluorescent dye molecules. These two Pickering
emulsions were also put in contact via a lamination procedure.
After standing for 4 h, the emulsion droplets close to a layer
boundary were withdrawn and observed with fluorescence
microscopy. Notably, green droplets and black ones were both
observed (Figure 2, d3). That is to say, the FITC-dextran dye
molecules did not enter the initially dye-free droplets over this
time scale. These findings further confirm that the formulated
Pickering emulsion has a good ability to compartmentalize
molecules within water droplets and to prevent the transfer of
these molecules to other droplets despite the large concen-
tration gradient.
We first checked the feasibility of the coexistence of opposing
reagents, e.g., HCl and NaOH, in a single vessel with the
proposed Pickering emulsion strategy. Congo Red was used as
indicator to visualize the pH changes of water droplets because
it is exclusively water-soluble (not oil-soluble) and its color
varies in response to pH changes (azure at pH < 3.0 and red at
pH > 5.0). Two parent water-in-toluene Pickering emulsions
were formulated with acidic or basic solutions of Congo Red
(the water volume fraction of each Pickering emulsion is ca.
70%). The use of a solution of HCl (0.01 M) led to an azure-
colored Pickering emulsion (Figure 2, a1), while the use of a
solution of NaOH (0.01 M) resulted in a red-colored Pickering
emulsion (Figure 2, a2). Mixing these two Pickering emulsions
through a lamination procedure yielded a new Pickering
emulsion, which exhibited zebra stripes with alternate azure and
red colors (Figure 2, a3). The layered architecture was
successfully achieved since no agitation is implemented during
the course of mixing. However, in a control experiment, mixing
two mixtures that were the same as a1 and a2 Pickering
emulsions in composition (including toluene, water, particle
emulsifier, acid or base, and indicator) but not emulsified, led to
a suspension that rapidly changed to red color (Figure 2, a4),
which is a result of acid−base reactions. These comparisons
underline that Pickering emulsions are crucial to obtain an
acid/base-coexisting system. More importantly, after standing
for 1, 3, and 24 h, the appearance of layered Pickering
C
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX