Timed formation and aging of complex coacervates using a volatile salt

Biomolecular condensates are liquid droplets formed by liquid-liquid phase separation, which play a role in a variety of cellular processes. In the last decade, there has been a growing interest in their study, often by using 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. Here, we present a simple protocol to trigger delayed formation and aging in coacervates. We use ammonium carbonate for this protocol, a volatile salt which decreases the ionic strength of the solution as it decomposes. Using this salt, we have been 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.