Self-assembled Shells Composed of Colloidal Particles: Fabrication and Characterization

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Authorship

Title of Original Work: Self-assembled Shells Composed of Colloidal Particles: Fabrication and Characterization

Journal of Original Work: Langmuir, 2005, 21, 2963-2970

Authors: Ming F. Hsu, Michael G. Nikolaides, Anthony D. Dinsmore, Andreas R. Bausch, Vernita D. Gordon, Xi Chen, John W. Hutchinson, and David A. Weitz*

Author of Review: Joseph Muth - AP 225 - Fall 2012 - 10/14/2012

Soft Matter Keywords

Surface Tension, Self-Assembly, Colloids, Elasticity

Overview

Precisely controlled containment and release of chemicals is a critical aspect of many important scientific areas including the pharmaceutical, biomedical, and food industries. One convenient way to both capture and release a species of interest is by using self-assembled colloidal particles to form a shell around the material. The shell acts as a protective containment layer while mediating flux to and from the species of interest via size sieving. This study explores the morphology and mechanical properties of self-assembled particle coatings around both aqueous and organic materials.

Experimental

The authors created two types of emulsions - water in oil (w/o) and oil in water (o/w). Colloidal polystyrene (PS) was added to different emulsions with various solvents and mixed via sonication. The colloidal particles spontaneously migrated to the droplet/continuous phase interface, and self-assembled into highly ordered structures in order to minimize the interfacial surface energy of the emulsion system. In all cases, no thermal desorption from the droplet surface was observed. After assembly, the particulate shell was stabilized by way of polymer adsorption or sintering. Morphology was analyzed via scanning electron microscopy and optical microscopy. Mechanical tests were performed using calibrated microcantilevers.

Results

Determinants of Shell Morphology

Different solvents and stabilization methods led to different surface morphologies. For solvents promoting high degrees of aggregation (dodecane, vegetable oil), less ordered multilayers formed. Van der Waals attractive forces are not neutralized by these solvents, as a result, particles aggregate either in solution and migrate as an aggregate to the emulsion interface, or aggregate on the surface of the emulsion. Shells formed in these solvents were characterized by disordered surface morphology. In contrast, more stabilizing solvents such as toluene led to ordered, defect free monolayers. Figure 1 shows a comparison of surface morphologies between shells formed with different solvents.

Figure 1. Top - PS particles packed in a defect free monolayer surrounding a water droplet in toluene. Bottom - PS particles packed in a disordered arrangement around a water droplet in dodecane. (Scale bars are 10 microns).

Effects of Stabilization Strategies

To stabilize the shells, polymer was added to the water phase in each emulsion type. When an aqueous polymer is added, the polymer adsorbs to the particles from the side of the shell nearest to the water phase. The polymer acts as a binder, locking the particles into place. Thus polymer adsorption helps solidify the self-assembled network of colloidal particles.

In addition to stabilization by polymer addition, sintering was also performed. (Glycerol is added to the water phase to prevent it from boiling). Sintering not only increases the stabilization of the shells but also provides a pathway for tuning the mechanical properties and porosity of the shell. As sintering time increases, particle contact area increases, and porosity decreases. The effect of sintering on shell density is shown in Figure 2.

Figure 2. SEM micrographs of PS particles surrounding a drop of vegetable oil after A) 5 min, B) 20 min, and C) 2 hours after sintering. D) PS particles after interface removal without sintering stabilization.

Shell Permeability

Once the shells were stabilized, the interface of the emulsion droplet was removed from the interior of the shell. When the droplet interface is removed, the shell serves as the selective membrane for passage to and from the droplet rather than the surface tension of the encapsulated droplet. In this manner, discrete control can be exerted on the surface flux of the capsule by tuning the pore size of the shell. Two methods are typically used to remove the droplet interface - centrifugation and addition of a cosolvent. Each of these methods stresses the shell interior as the emulsion drop/shell interfacial layer is removed. If stabilization is not adequate, the shell will be destroyed.

Shell Mechanical Response

All capsules were tested mechanically via a compressive axisymmetric loading regime using calibrated microcantilevers. The mechanical properties were directly correlated to stabilization method. Shells stabilized via polymer adsorption demonstrated the greatest resilience to deformation, and the greatest failure strain as a result of the energy dissipative nature of the polymer binder. Sintered particles were much stiffer and more brittle compared to polymer stabilized shells. However, sintering provided a readily tunable method to adjust particle mechanical properties. The longer a shell was sintered the higher the breaking force was.

In addition to effecting mechanical properties, sintering time also effected failure mode. For short sintering times, failure was local and discrete at the loading point. However, for long sintering times, the shells behaved in a continuum manner and failure encompassed most if not all of the shell. SEM micrographs of typical failure modes are shown in Figure 3.

Figure 3. Typical failure modes of sintered shells for short sintering times (20 min) (left) and long sintering times (150 min) (right)

Summary

PS shells formed around emulsified liquids can provide an extremely tailorable method for material encapsulation. Surface morphology is readily alterable via solvent selection, stabilization method directly effects mechanical integrity, and degree of sintering strongly effects porosity and mechanical properties. By optimizing each of the these parameters, self-assembled colloidal shells allow for precise engineering control over material containment and release.