Timed Formation and Aging of Complex Coacervates Using a Volatile Salt

Biomolecular condensates are liquid droplets formed by liquid–liquid phase separation and play a role in a variety of cellular processes. In the past decade, there has been a growing interest in their study, often through the use of complex coacervates as models. Despite their similar properties, a limitation of complex coacervates is their inability to show time-dependent behavior, such as aging, as they are typically structures in thermodynamic equilibrium (or kinetically trapped). Here, we present a simple protocol to trigger the delayed formation and aging of coacervates. We use ammonium carbonate for this protocol, a volatile salt that decreases the ionic strength of the solution as it decomposes. Using this salt, we were able to program coacervate formation after delays ranging from hours to days. This process can be repeated multiple times, as the decomposition of ammonium carbonate leaves no waste products. The mechanical properties of the coacervate phase also change over time with this protocol, showing a steady increase in viscosity reminiscent of aging in condensates. Since the element that causes temporal evolution is the salt and not the coacervates, this protocol does not require any synthesis and can be easily adapted to multiple complex coacervates.