Nanoconfinement-Induced Phase Segregation of Binary Benzene–Cyclohexane Solutions within a Chemically Inert Matrix

Binary solutions provide a fertile arena to probe intermolecular and molecular/surface interactions under nanoconfinement. Here, the phase segregation of a solution comprising 0.80 mol fraction benzene and 0.20 mol fraction cyclohexane confined within SiO 2 nanopores was evaluated using small-angle neutron scattering with hydrogen–deuterium contrast matching. It is demonstrated that benzene and cyclohexane are fully miscible at 303 K (30 °C), yet they unambiguously phase segregate by 153 K (−120 °C), which is below their respective freezing points and below the cubic-to-monoclinic phase transition of cyclohexane. Specifically, the cyclohexane and benzene separate into a core|shell morphology with cyclohexane concentrated toward the nanopore centers. Additionally, pure benzene is shown to form a frozen core of bulk density with a thin shell of slightly reduced density immediately adjacent to the SiO 2 nanopore wall at 153 K. Because the SiO 2 matrix is chemically inert to cyclohexane and benzene, the observed radially dependent phase segregation is strong evidence for the effects of confinement alone, with minimal host–wall attraction.