Theory of capillary-induced self-coacervation in zwitterionic polymer solutions

Abstract

This study presents a mean-field theory for zwitterionic polymer solutions confined within attractive slit-like nanopores. Building upon our prior model for self-coacervation [Budkov et al., Soft Matter 19, 3281–3289 (2023)], we demonstrate that strong electrostatic dipole–dipole correlations between monomeric units, coupled with attraction to pore walls, can induce capillary-driven liquid–liquid phase separation inside the pore—a phenomenon we term capillary-induced self-coacervation. Using a thermomechanical approach (Budkov et al., J. Stat. Mech.: Theory Exp. 2024, 123201), we calculate the disjoining pressure, revealing a pronounced non-monotonic dependence on pore width linked to coacervate film formation and rupture. Remarkably, we identify a crossover between two film formation mechanisms controlled by wall attraction strength. For weak adsorption, film formation is bridging-dominated, with critical pore width scaling as Hc ∼ N1/2. For strong adsorption, the system crosses over to a cohesion-dominated regime where Hc saturates to an N-independent plateau, indicating collective phase behavior governed by a confinement-induced shift of the local binodal. This duality reveals the interplay between single-chain connectivity and collective thermodynamics in confined polyzwitterions. The theory offers new insights into coacervate film formation on mineral surfaces, with implications for prebiotic compartmentalization.