  Phase separation and transitions among different molecular states are
ubiquitous in living cells. Such transitions can be governed by local
equilibrium thermodynamics or by active processes controlled by biological
fuel. It remains largely unexplored how the behavior of phase-separating
systems with molecular transitions differs between thermodynamic equilibrium
and cases where detailed balance of the molecular transition rates is broken
due to the presence of fuel. Here, we present a model of a phase-separating
ternary mixture where two components can convert into each other. At
thermodynamic equilibrium, we find that molecular transitions can give rise to
a lower dissolution temperature and thus reentrant phase behavior. Moreover, we
find a discontinuous thermodynamic phase transition in the composition of the
droplet phase if both converting molecules attract themselves with similar
interaction strength. Breaking detailed-balance of the molecular transition
leads to quasi-discontinuous changes in droplet composition by varying the fuel
amount for a larger range of inter-molecular interactions. Our findings
showcase that phase separation with molecular transitions provides a versatile
mechanism to control properties of intra-cellular and synthetic condensates via
discontinuous switches in droplet composition.
