Decoupling interfacial and nanoconfinement effects in charged nanopores

Determining the length scale at which nanoconfinement first alters the properties of water remains a longstanding challenge, with reported thresholds ranging from subnanometer to tens of nanometers. These discrepancies likely arise from differences in materials, surface chemistry, measured observables, and, critically, the choice of reference system, as separating interfacial from confinement effects is challenging. Here, we use molecular dynamics simulations of aqueous LiCl confined between positively charged Mg/Al layered double hydroxide (LDH) surfaces to disentangle interfacial and nanoconfinement effects. We show that the commonly used single-surface geometry is not a valid interfacial reference in charged systems due to asymmetric ion adsorption and additional structuring at water-air interface. Instead, a sufficiently wide slit pore with a bulk-like central region provides a consistent baseline. By comparing systems at matched ionic strength but different pore sizes, we isolate nanoconfinement effects from concentration changes. Nanoconfinement suppresses counterion accumulation in the Stern layer, with onset at ∼60 Å, and leads to undercharging at high ionic strength, in contrast to overcharging in the interfacial limit. Confinement is also property-dependent, with dipole orientation remaining sensitive up to ∼70 Å. Importantly, pore-averaged properties can misleadingly suggest persistent confinement effects even in large pores, whereas spatially resolved profiles provide a more reliable indicator. These results reveal long-range confinement effects in charged nanopores and provide a general framework for identifying nanoconfinement in aqueous systems.