Framework for Speciation-Gated Mass Transfer at Liquid–Liquid Interfaces

The behavior of ionizable surfactants at oil/water interfaces is central to applications from emulsification to liquid-liquid extraction, yet predictive control remains a challenge. Current models often decouple interfacial adsorption from the underlying acid-base chemistry that inevitably will influence surfactant availability. Here, we establish a unified thermodynamic framework demonstrating that interfacial phenomena are controlled by speciation-gated mass transfer. Using a homologous series of linear carboxylic acids (C5-C16), we combine dynamic interfacial tension measurements with kinetic phenotyping and a phase-resolved speciation/partitioning model to show that the flux of surface-active species is gated by the acidbase equilibrium. Two kinetic regimes emerge across the series: long-chain acids relax monotonically toward equilibrium, whereas short-chain acids display pronounced non-monotonic behavior, consistent with initial interfacial activity followed by depletion through organic-toaqueous transfer and dissociation in water. Our framework, which explicitly couples partitioning (logP) and dissociation (pKa) parameters, quantitatively predicts the time-dependent interfacial tension, including non-monotonic behavior driven by mass transfer, and the corresponding pH control of the aqueous phase. The model predicts interfacial-tension dynamics and captures the pH switching gate at which the system switches from oil-favored to water-favored behavior. We validate these predictions across concentrations, pHs, and chain lengths, as well as in weak bases (decylamine). This work transforms the design of responsive liquid-liquid interfaces from empirical trial-and-error to a predictive, thermodynamically guided approach, in which pH and molecular structure (pKa and logP) serve as explicit design levers to control interfacial energy and dynamics. Final version: https://doi.org/10.1021/jacs.6c04456