Exotic Ethanol–Water Diffusion Mediated by Molecular Self-Assembly

Ethanol and water play important roles in daily life, yet there is no accurate physicochemical model for their mutual-diffusion process, making research on this process intriguing. Here, we in situ monitored the one-dimensional ethanol solution-water diffusion process by a homemade plasmonic microsensor with a high space-time resolution. The experimental results reveal two non-Fickian diffusion behaviors: the effective diffusion coefficient decreasing to a smaller constant and the diffusion layer center moving in a three-stage manner as diffusion proceeds. Moreover, the diffusion process was observed to be accompanied by the molecular self-assembly, which, coupled with the excess volume and interfacial tension effects, provides a plausible explanation for the exotic diffusion behaviors. The discovery of the exotic diffusion phenomena enriches the liquid-liquid diffusion theory and reveals the physicochemical features of the ethanol-water mixing process.