Emergent Dynamics at Liquid-Liquid Interfaces: Beyond Single Component Predictions

Interfaces between immiscible liquids control essential processes in chemical engineering, environmental systems, and biological transport. Yet experimental characterization remains challenging because fluid interfaces undergo continuous thermal fluctuations while spectroscopic techniques average over large areas, obscuring molecular-level organization. We use molecular dynamics simulations with instantaneous interface analysis to investigate water and toluene interfaces. At their respective vapor interfaces, water and toluene display vastly different surface behaviors. Water shows rapid, localized fluctuations with strong molecular ordering, while toluene exhibits slower, longer-range undulations reflecting its distinct molecular characteristics. However, at a water/toluene interface, both liquids experience dramatically slower interfacial dynamics compared to their vapor interfaces, indicating strong coupling between the two phases. Surface roughness characteristics also transform, with water becoming rougher while toluene becomes smoother at the shared interface. These findings demonstrate that liquid-liquid interfaces develop unique properties through mutual molecular interactions rather than simple combination of individual behaviors.