Transport through self-assembled colloidal shells (colloidosomes)

Colloidosomes, namely, microcapsules coated by a colloidal shell, have been widely studied as potential carriers of active compounds for various applications. The colloidal shell differs from the shells of other ‘somes’ (liposomes, polymersomes) since it is a composite material with an impenetrable phase—the particles, and a penetrable one—the voids or pores between them. Recent analysis shows that in the shells composed of monodisperse and charged particles, the maximal volume fraction of colloids in the self-assembled layer depends on the size ratio between the particleʹs hard-sphere radius and the effective radius, which includes the range of repulsive electrostatic interactions. Thus, somewhat counter-intuitively, the density of particles in the shell increases with increasing particle radius. However, mixing particle sizes can lead to highly packed shells where the impenetrable phase volume fraction approaches 100%. The diffusional flux through the colloidal shell is highly sensitive to the packing density (or particle volume fraction); this parameter sets the average size of the pores, their distribution through the shell, and their tortuosity. However, while in thick multi-layer shells the flux increases with increasing particle size, in the case of monolayer-thick shells there is no apparent dependence of the flux on the colloid dimensions.