Programmable Spatial Demixing in Prebiotic Coacervates

Stratified vertical positioning of protocells offers evolutionary opportunities in paleo-oceans yet remains challenging achieve under prebiotic resource scarcity. Physical encounters among protocell populations are essential for molecular exchange and chemical complexity. Here, we demonstrate minimal structural variations of amphiphiles and small molecules-including isomers, alkyl-chain length, and counterion-dictate coacervate buoyancy, thereby governing vertical migration and stratification in water columns. Systematic parameter tuning produces coacervates residing in either upper or lower phases, while environmental cues like concentration and temperature dynamically modulate droplet positioning. Structural analysis reveals buoyancy arises from a sponge-like architecture composed of hydrated, flexible multilayers, with vertical positioning determined by hydration layer thickness and hydrophobic-to-hydrated layer ratio. Temperature-driven migration further induces collisions and fusion between coacervate populations, promoting molecular exchange and generating new substances. These findings suggest a physicochemical mechanism by which subtle molecular variations can encode protocell-like spatial organization, interaction frequency, and access to external energy in stratified aqueous environments.