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 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 resolved kinetic analysis and a phase-resolved speciation/partitioning model to show that the flux of surface-active species is gated by the acid-base equilibrium. Two kinetic regimes emerge across the series: long-chain acids relax monotonically toward equilibrium, whereas short-chain acids display pronounced nonmonotonic behavior, consistent with initial interfacial activity followed by depletion through organic-to-aqueous transfer and dissociation in water. Our framework, which explicitly couples partitioning (log P) and dissociation (pK_a) parameters, illustrates the time-dependent interfacial tension, including nonmonotonic behavior driven by mass transfer, and the corresponding pH. Our approach 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 provides a predictive basis for designing responsive liquid-liquid interfaces from empirical trial-and-error to a predictive, thermodynamically guided approach, in which pH and molecular structure (pK_a and log P) serve as explicit design levers to control interfacial energy and dynamics.