Scale-dependent anomalous behavior of confined water between Al₂O₃ layers

Confined water exhibits anomalous behavior distinct from bulk water, fundamentally influencing chemical reactions at the nanoscale. However, the scale-dependent nature of water properties remains poorly understood, particularly regarding the respective contributions of spatial confinement and surface interactions. Here, we comprehensively investigate the mechanisms and scale-dependent behavior of confined water between Al₂O₃ layers across a confinement range of 1 to 50 nm. Our findings reveal that surface interactions primarily induce abnormal behavior in interfacial water, characterized by ordered structure, anisotropic and highly connected hydrogen bond network, reduced dielectric profiles, and suppressed self-diffusion. In contrast, spatial confinement selectively extends certain anomalous properties from the interfacial layer to the entire confined region following theoretical predictions. Such extension results in confined water exhibiting an extremely low dielectric response, high surface potential, and unexpectedly enhanced in-plane diffusion. Notably, we identify a confinement range of 10 to 20 nm as a threshold marking the transition between confined and bulk water behavior. We also elucidate the specific effects of ionic concentration, pH levels, surface functional groups, and surface polarity on the behavior of confined water. This work highlights the critical role of spatial confinement in determining the properties of confined water, advances our understanding of confined water in metal oxide systems, and informs the rational design of nanoconfined systems for applications in mass transport and chemical reactions.