Critical role of chemical potential to assure effective encapsulation

In emulsification-polymerisation avoiding monomer escape from emulsion droplets is the key to successful encapsulation. So far, it is believed that (1) a hydrophobe needs to be included and (2) free-micelles of surfactant need to be depleted. However, these criteria do not always work. The paper explores the critical role of the chemical potential difference between the inside and outside of the emulsion droplet for successful encapsulation.

Crossflow membrane emulsification was used to produce uniform droplets of 1-2 µm of solutions of 3-iodoprop-2-yn-1-yl butylcarbamate (a biocide), castor oil (hydrophobe) in methyl 2-methylprop-2-enoate (monomer) into aqueous solutions with a large amount of free-micelles of surfactant. The encapsulation was followed by polymerisation. The size distribution of microcapsule from different formula were examined.

The biocide encapsulation depends on castor oil content: >12% (full); 6-12% (either full or partial); <6% (minor). Results show a critical molar fraction ratio of the monomer in the droplet to water in the aqueous phase that provides a definitive criterion to assure size retention and full encapsulation. This critical value corresponds to an energy barrier of 116 J/mol to prevent the monomer escaping. This finding is proposed to be used as an advanced rule to guide precision formulation for desired microencapsulation.