Diffusion through colloidosome shells

Colloidosomes are aqueous cores surrounded by a shell composed of packed colloidal particles. Recent studies suggest that these colloidal shells reduce, or even inhibit, the transport of molecular species (diffusants). However, the effect of the colloidal shell on transport is unclear: In some cases, the reduction in transport of diffusants through the shell was found to be independent of the size of the colloidal particles composing the shell. Other studies find, however, that shells composed of small colloidal particles of order 100nm or less hindered transport of diffusants more than those composed of micro-scale colloidal particles. In this paper we present a simple diffusion model that accounts for three processes that reduce diffusant transport through the shell: (i) a reduction in the penetrable volume available for transport, which also increases the tortuousity of the diffusional path, (ii) narrow pore size which may hinder transport for larger diffusants through size exclusion, and (iii) a reduction in interfacial area due to 'blocking' of the surface by the adsorbed particles. We find that the colloidal particle size does not affect the reduction in transport through the colloidal shell when the shell is a monolayer. However, in closely packed, thick layers where the thickness of the multi-layer shell is fixed, the rate of transport decreases significantly with colloidal particle dimensions. These results are in excellent agreement with previously published experimental results.