Perturbing Å-Scale Interfacial Clusters in Functionalized Nanoporous Graphene for Selective Evaporative Methanol–Water Separations

Graphene-based 2D nanoporous materials with specific edge functionalization can enhance the evaporative flux of water [ACS Nano <b>16</b>, 15328, 2022]. Here, molecular dynamics simulations are used to explore the potential for evaporative separation of liquid mixtures. Evaporation from bare methanol-water interfaces provides molecular insights into the dynamics of evaporation by carrying out a detailed cluster analysis of the size distributions of methanol and water clusters. Methanol exists predominantly as a solitary molecule with evaporation occurring from both methanol-rich and methanol-lean regions of the interface, with a preference for evaporation from the methanol-rich regions. In contrast, larger clusters are observed for water, and evaporation requires a water molecule to be released by breakage from strongly hydrogen-bonded interfacial water clusters. With 2D graphene nanopores placed at the air-water interface, the evaporative flux of both methanol and water is enhanced by a factor of 2.5-6 at lower methanol concentrations when compared with the bare interface, with greater enhancements observed for -OH functionalized pores. A strong selectivity preference for methanol occurs at higher methanol compositions. Enhanced local demixing of methanol and water, coupled with retardation of local interfacial water cluster dynamics, leads to a substantial reduction in water evaporative flux for select conditions. This yields significantly higher selectivities (35-600) and two orders greater evaporative flux enhancements when compared with traditional pervaporation-based methods. The nanopore edge retards interfacial water cluster dynamics, with relaxation times increasing 3 to 4-fold at higher methanol fractions. Our study provides molecular insights into the structure and dynamics at the bare methanol-water interface and how interfacial structures are modified in the presence of functionalized nanoporous graphene. Specific edge functionalization can be used to suppress water evaporation to potentially create energy efficient liquid mixture separation systems. Our computational study can be extended to explore the potential of different classes of 2D materials for efficient evaporation-based separation processes.