Counterion-Controlled Nanoscale Morphogenesis in Interfacial Polymerization via Decoupled Interfacial Transport and Reaction Kinetics

Abstract

The morphology of thin-film composite membranes is encoded during interfacial polymerization (IP) by competing monomer transport and reaction kinetics, yet molecular control remains limited due to their intrinsic coupling. Herein, we show that counterions decouple solvation-controlled transport from transition-state energetics, enabling deterministic control over nonequilibrium thin-film morphogenesis. Using a model polyester IP system, we demonstrate that varying phenoxide counterions (Li+, Na+, K+, TBA+) reveals anomalous energetic trends, with transition-state energies (ΔG‡ ≈ 6.9–21.7 kcal mol–1) and interfacial partitioning energies (ΔGpartition ≈ 11.2–0.7 kcal mol–1) spanning more than an order of magnitude. Despite faster intrinsic kinetics, Li+ produces ultrathin, smooth films (≈9.4 nm thickness; Rq ≈ 1.9 nm), whereas TBA+ yields substantially thick films with rough architectures (≈32 nm; Rq ≈ 27–30 nm). This inversion is attributed to a kinetically locked regime, where confined ultrafast reactions suppress reaction–diffusion instabilities before morphological amplification. Consequently, resulting networks exhibit tunable surface roughness (∼1.9 to ∼27 nm). The controlled ∼14-fold increase in surface roughness from Poly-Li to Poly-TBA yields 75% enhanced water permeance at equivalent solute rejection. These findings establish a predictive energetic framework that connects interfacial energetics to reaction–diffusion instability, membrane morphology, and transport behavior.