Reactivity of Nanoconfined Water Is Modulated by the Properties of Confining Materials

The autoionization of water, a fundamental process in aqueous chemistry, is profoundly altered under nanoconfinement. The properties of the confining material determine the layer dependence of autoionization, dictating whether reactions are stabilized at the interface or in the subsurface. Under weak water-wall interactions, hydroxide is destabilized at the interface, leading the reaction to proceed preferentially in the subsurface. Conversely, under strong water-wall interactions, the interfacial and subsurface states are nearly isoenergetic, thereby diminishing the selectivity between the two regions. This contrast arises from confinement-enforced coordination motifs where hydronium remains tri-coordinated across environments, while hydroxide is restricted to tetra-coordination at the interface but adopts hypercoordinated states in the subsurface. Mechanical flexibility of the confining framework further modulates the overall thermodynamics by reducing the entropic penalty, as water molecules can explore a broader configurational space compared to rigid pores. These findings establish how layer-specific solvation and wall flexibility govern confined-water reactivity, providing molecular-level design principles for engineering dynamic nanoscale interfaces in catalysis, energy storage, and molecular separations.