Nanoscale Surface Tension, Adsorption Isotherms and Separation of Water–Ethanol Mixtures Confined between Graphene-Based Slit Pores

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Confined water–ethanol mixtures between graphene-based slit pores are studied using grand-canonical Monte Carlo and molecular dynamics simulations to connect the adsorption isotherm with nanoscale surface tension. We highlighted preferential interactions between ethanol and graphene, indicating high selectivity and suggesting that the graphene pore is effective for separating water from ethanol. Ethanol molecules act as precursors to trigger water adsorption due to favorable hydrogen bond interactions. We shed light on a nonmonotonic behavior of surface tension that closely follows the adsorption isotherm. We demonstrate that the decrease in surface tension is a consequence of the negative ethanol–ethanol surface tension contribution, in line with a specific interfacial organization leading to an “intramolecular” phase separation. Additionally, we show the existence of local phase separation between water and ethanol close to the solid surface, where ethanol molecules are preferentially adsorbed, forming a first interfacial layer with less water and a second layer rich in water. This generates a multilayer organization and an interfacial microphase separation. By using a gravitational field mimicking the confinement effect, we numerically and experimentally highlight that the water–ethanol interface between two bulk phases is gravitationally stabilized.