The Structure of Nanoconfined THF-water Mixtures: From Local Hydrogen Bonding to Microphase Separation and Crystallization

Liquid behaviour can be significantly different when confined in nanopores compared to the bulk, for example miscibility and phase transitions. A central, application-relevant, regime in confined-liquids study is water and its mixtures with organic aprotic solvents. Yet it remains unclear how nanoconfinement affects the interfacial molecular structure and properties of such solutions. Here we combine total neutron scattering with data-driven atomistic simulations to analyse the liquid structure and crystallization of tetrahydrofuran (THF)-water mixtures in hydrophilic mesoporous silicas, with cylindrical pore-diameters of ~4 and 10 nm. We focus on the low-concentration regime, 5 mol % THF, which in the bulk is fully miscible and crystallizes to form a clathrate-hydrate. Using a multiscale model refined simultaneously to small- and wide-angle scattering data, we find a shell region of tetrahydrofuran-rich domains close to the silanol surfaces, leaving a core of pure water. In spite of this dramatic microphase separation, water’s nearest-neighbour tetrahedral motif is largely undisturbed, while longer-range interactions are reshaped by confinement. On freezing the confined liquids, we observe the formation of pure ice, with the anticipated clathrate-hydrate phase being completely absent. These findings demonstrate the profound impact of nanoconfinement on the structure and crystallization of aqueous-organic solutions, with far-reaching implications for fields as diverse as green chemistry, chromatography and pharmaceuticals.