Negative Interfacial Tension Driven by Intrinsic Molecular Disorder

Intrinsically disordered molecular systems, such as random copolymers and intrinsically disordered proteins, exhibit scale-invariant, power-law cluster distributions that cannot be explained by conventional mean-field theories. A fundamental challenge is to understand how sequence randomness, which cannot be averaged into effective parameters, drives the formation of such polydisperse assemblies. Using a minimal coupled field theory that explicitly links a conserved density field to a nonconserved "junction" field representing stochastic binding site density, we show that the attraction between two fields χ, together with the fluctuations in the junction field Tϵ, renormalizes the effective interfacial tension to negative values: Δγ ∼ -Tϵχ2. This negative interfacial tension suppresses coalescence and stabilizes a scale-invariant ensemble of clusters with a power-law size distribution, as confirmed by both numerical calculations and coarse-grained simulations of sequence-disordered biomolecules, and are consistent with diverse experimental observations. Our work reveals a general physicochemical mechanism by which intrinsic molecular disorder and fluctuations jointly govern phase separation in systems with built-in randomness, providing a unifying, chemistry-oriented framework for understanding and engineering heterogeneous soft matter systems.