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

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--Overview-- Precisely controlled containment and release of chemicals is a critical aspect of many important industries and scientific areas including the pharmaceutical, biomedical, and food industries. One convenient way to both capture and release species of interest is by using self-assembled colloidal particles to form a shell around the material. 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) and divinyl-benzene (DVB) , and carboxylate particles were added to different emulsions and mixed via sonication. The colloidal particles spontaneously migrated to the droplet/continuous phase interface and self-assembled into highly ordered structures. In all cases, no thermal desorption from the droplet surface was observed. After assembly, the particulate shell was stabilized by way of polymer adsortption and sintered. 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 disordered shells dominated the surface morphology. In contrast, more stabilizing solvents such as toluene led to ordered, defect free monolayers.

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, typically sintering is performed. During sintering, the shells are heated to ~105oC. (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.

Shell Permeability Once the shells are stabilized, the interface of the emulsion droplet is removed from the interior of the shell. By removing the droplet interface, 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 stresses the shell interior as the emulsion drop/shell interfacial layer is removed. If stabilization is not adequate, the shell will be destroyed. Table 1 shows the effects of interface removal as a function of removal strategy for various stabilization regimes.

Shell Mechanical Response All capsules were tested mechanically via a compressive axisymmetric loading regime using calibrated microcantilevers. The mechanical properties were directly relatable to the 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 provides a readily tunable method to adjust particle mechanical properties. The longer a shell is sintered the higher the breaking force.

In addition to effecting mechanical properties, sintering time also effecting 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.

Stabilization is especially critical for shell performance. Without proper stabilization, the self-assembled shells will be destroyed during interfacial removal.