Classical liquid–liquid phase separation predicts that droplets grow continuously via Brownian coalescence or Ostwald ripening, yet many nanoscale biomolecular condensates remain stable for hours or days without active regulation. Such condensates often arise through complex coacervation between oppositely charged macromolecules, making this interaction motif broadly relevant in biology and soft matter. Here we combine experiments, theory, and simulations to identify a merging-limited coarsening (MLC) regime in which merging of condensates decreases sharply below a critical droplet size. Chain-length asymmetry between oppositely charged polymers drives entropic interfacial charge separation even at globally neutral stoichiometry, imparting net droplet charges and generating long-range electrostatic repulsion. These size-dependent barriers lead to exponential rather than classical power-law growth and trap droplets in long-lived metastable states. Our framework unifies suppressed MLC and classical Brownian coalescence within a single predictive model and provides a general mechanism for condensate stability in both synthetic systems and living cells.

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• Biomolecular self-assembly
• Phase separation
• Phase transitions in biological systems
• Polymer behavior
• Spinodal decomposition
• Supramolecular phase separation
• Polyelectrolytes
• Polymer solutions
• Coarse graining
• Electron microscopy
• Light scattering
• Monte Carlo methods
• Optical microscopy