We encounter emulsions of phase-separated polyelectrolytes more commonly than we might think. Why, our cells rely on these phase-separated droplets of biopolyelectrolytes to carry out complex functions. These droplets are usually 'membrane-less' - unlike many organelles, they do not have a layer of lipids on their interface. This absence of an interface provides them with far superior transport properties compared to membrane-bound organelles. However, from the standpoint of colloid science, these droplets (or MLO, for membrane-less organelles) should be prone to coalesce due to the absence of any stabilizing layer. Yet, our cells work perfectly well without any interfacial membranes in many of these MLOs. Using model bio-polymers and nucleotides, we found a mechanism by which phase-separated droplets of polyelectrolytes, called coacervates, can be stabilized against coalescence. In this talk, I will present a tool by which polyelectrolyte coacervates can form stable droplets by ironically cross-linking their oppositely charged chains, preferentially at the droplet-solvent interface. This technique has the added advantages over those involving artificial membranization of droplets using nanoparticles or block co-polymers - the droplets are still membrane-less (they do not have a layer of foreign molecules), and the transport properties of the droplets are largely maintained. We used these stable droplets of polyelectrolytes to transport biochemicals in a 2D space using spatially programmed external electric fields. As there is no complicated chemistry involved, this stabilization mechanism can pave a path for active colloidal stabilization for coacervate emulsions required in biochemical industries.

Interfacial crosslinking of polyelectrolyte complex coacervate droplets

ACS Spring 2022

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