Formation of giant colloidosomes by transfer of pendant water drops coated with latex particles through an oil–water interface

We have devised a new technique for producing and studying giant pendant colloidosomes based on transferring particle-coated pendant water drops in oil through a planar oil–water interface.

Colloidosomes are a novel class of microcapsules whose shell consists of smaller colloid particles.

Such structures were produced for the first time by Velev et al1. by templating latex particles adsorbed on the surface of octanol-in-water emulsion drops and subsequent removal of the oil after fusing the particle monolayers.

Caruso et al2,3. templated solid nanoparticles on the surface of solid sacrificial microparticles based on electrostatic attraction and layer-by-layer assembly of multilayer shells consisting of alternating positively and negatively charged nanoparticles or polyelectrolytes.

Dinsmore et al4. produced colloidosomes by assembly of polymer latex colloidal particles into shells around water-in-oil emulsion drops followed by partial fusion of the shell and centrifugal transfer into water to yield stable capsules in which the shell permeability can be controlled by adjustment of the partial fusion conditions.

Noble et al5. created “hairy” colloidosomes by templating microrod particles over aqueous gel micro-beads.

Despite the importance of colloidosomes as a novel and promising encapsulation vehicle, very little experimental work has been reported investigating the phenomena occurring as particle monolayers adsorbed on liquid templates (drops) transfer through the oil–water interface and how this process leads to the formation of a colloidosome membrane.

Very recently, we studied the bridging of water drops densely coated with latex particles in an oil phase with a planar oil–water interface.6

The liquid surfaces strongly adhere to each other but do not coalesce due to the formation of a stable oil film whose surfaces are bridged by the latex particle monolayer.

Here, we report formation of giant pendant colloidosomes produced by displacing pendant water drops coated with a particle monolayer through a planar oil–water interface.

We have been able to form colloidosome membranes directly attached to the tip of a capillary which opens new possibilities for studying their properties.

Our method is schematically illustrated in Fig. 1.

Initially a pendant drop of an aqueous suspension of latex particles is formed in an oil phase.

We produce a closely packed particle monolayer adsorbed on the drop surface by multiple infusion and withdrawal of the particle suspension through the capillary in the oil phase (see ref. 6).

Finally, the pendant water drop in oil, densely coated with adsorbed particles, is transferred through a planar oil–water interface (free of particles) to form a giant pendant colloidosome, which consists of a spherical water/oil/water film supported by latex particles possibly bridging both surfaces.6,7

In the present method no partial fusion of the particles is required, as the integrity of the particle monolayer is supported by the oil film.

Our technique also allows formation of pendant colloidosomes with fused particle monolayers.

With this methodology we can produce colloidosomes attached to a capillary which allows the properties of the colloidosome membrane to be studied as a function of the number of adsorbed particles.

The latter can be easily controlled by changing the volume of the inner aqueous phase.

In addition, our approach allows us to study the curvature effect on the stability of colloidosome membranes.

These factors are very difficult to control by other methods for producing such films bridged by colloid particles.6,7

Single pendant drops of an aqueous suspension of 220 nm monodisperse sulfate PS latex particles (home made) were produced in tricaprylin oil (from Sigma) by using a Krüss DSA-10 Drop Shape Analysis system with syringe needles ranging from 1.0 to 1.5 mm in diameter.

In a similar experiment we produced drops of an aqueous suspension of 9.6 μm monodisperse sulfate PS latex particles (from IDC) in n-decane.

Tricaprylin and decane were purified by passing through chromatographic alumina.

The concentration of NaCl in the latex suspensions was adjusted to 0.5 mM to facilitate particle adsorption to the oil–water interface.

To obtain a planar tricaprylin–water interface the external rim of an internal cell (a rectangular cuvette from Helma) was coated with ∼5 μm of SU-8, a hydrophobic negative resist material, to pin the oil–water–solid contact line.

For this purpose, a clean microscope slide was spin-coated with SU-8 and then the inverted cuvette was pressed onto the slide before the SU-8 layer was photopolymerised in UV light.

The rim of the tip of the metal capillary was also coated with SU-8 to provide a hydrophobic “face” with hydrophilic inner and outer surfaces.

The latter was necessary to pin the oil film on the capillary tip when a colloidosome is formed.

The aqueous dispersion of particles and electrolyte was prepared and loaded into a syringe.

The internal cell was placed in a larger cuvette and was filled to the top with milli-Q water.

The external cell was then filled with the oil phase (tricaprylin or decane).

A pendant water-in-oil drop of diameter about 1–2 mm coated with 220 nm PS-sulfate particles was prepared by multiple infusion and withdrawal of the aqueous suspension with a syringe (see Fig. 2a) which allows more particles from the suspension to be adsorbed on the drop surface.

Further compression of the particle monolayer to near collapse resulted in an asymmetric shape of the pendant drop.

The stage supporting the external cuvette was raised while the capillary was kept stationary, therefore pushing the planar oil–water interface (on the top of the internal cuvette) above the water drop to create a colloidosome.

Fig. 2 presents typical images from the different stages of the described procedure.

We noticed that if the initial water drop was wider than the internal diameter of the capillary tip, part of it detached from the tip upon transfer and remained on top of the planar oil–water interface, where it stayed for hours without coalescing with the bottom water phase (see also ref. 6).

However, the remaining part of the particle-coated water drop transfers through the oil–water interface intact and forms a hemispherical oil film entrapping an adsorbed dense monolayer of latex particles.

Expansion of the colloidosome was then performed by gradually increasing the volume of the internal aqueous phase, in a similar way to the preparation of the initial water drop in oil.

Note that following this procedure we were able to increase the volume of the initial colloidosome “bubble” more than 10 times.

Such pendant colloidosomes showed remarkable stability against expansion and survived attached to the capillary for several hours before breaking up.

Our previous results6 indicate that a bridging film is formed when the particle stabilised drop touches the flat oil–water interface.

If the particle monolayer is bridging both surfaces of the oil film (e.g. Fig. 2e and 2f) this result implies that no dense particle monolayer is needed to keep the two film surfaces far apart.

A possible explanation is that individual particles bridging the oil film are acting as spacers and prevent the two oil–water interfaces from coalescing.

In addition, it is likely that if the colloidosome is expanded gradually, more particles will adsorb to the inner oil–water interface from the drop interior.

More research is needed to reveal whether the particles are uniformly distributed in the diluted bridging monolayer due to mutual repulsion8 or clustered in 2D aggregates.

Free giant colloidosomes were also prepared by detaching water-in-decane pendant drops coated with 9.6 μm PS sulfate latex particles from the capillary above the decane-water interface.

In this case, the water droplet falls on the flat decane-water interface and transfers into the water phase.

Fig. 3 shows such a giant w/o/w colloidosome transferred below the planar liquid interface.

Such colloidosomes were unstable, rupturing after a few minutes of contact with the decane-water interface.

The instability could possibly be due to fact that the latex particles in the colloidosome membrane are not fused and cannot effectively prevent its coalescence with the top decane phase.

In conclusion, we show that colloidosomes can be formed without fusing the colloidal particles.

We have developed a technique for producing giant pendant colloidosomes supported by an oil film bridged by a particle monolayer.

Such pendant colloidosomes allow expansion more than 10 times their initial volume.

We expect that this technique can become a useful tool for studying the properties of colloidosome membranes, like the membrane tension, elasticity, the effect of the particle surface concentration and the thickness of the oil film on the membrane permeability.

An interesting extension of this approach would be to transfer a particle-coated pendant drop through a particle-laden planar interface to produce a particle bilayer colloidosome membrane.

With minor modifications of this technique, the elasticity of colloidosome membranes can be determined from shape changes of colloidosomes attached to two capillaries and brought into contact.

In the case of non-fused particles, the membrane elasticity can also be determined as a function of the particle surface concentration.

The release kinetics of encapsulated materials in a pending colloidosome can also be studied as a function of the particle surface concentration.

Some of these avenues will be pursued in a follow-up publication.