From Pure Isotherms to Confined Reactive Equilibria.

Ideal adsorbed solution theory (IAST) predicts multicomponent adsorption from pure-component isotherms, but it assumes chemically fixed species and therefore becomes underconstrained for confined systems with rapid reversible interconversion. Here, we introduce Reaction-IAST, a reaction-aware extension of IAST that couples the equal-spreading-pressure construction to a single scalar descriptor, Kconf, representing the confinement-shifted equilibrium constant. For canonical confined systems, the coupled adsorption-reaction problem collapses to a one-dimensional closure in spreading pressure, preserving IAST-level computational simplicity. Validation against reactive canonical Monte Carlo reference data in slit nanopores identifies both accurate single-phase regimes and systematic error regions. We further examine transferability beyond slit pores, analyze practical acquisition strategies for Kconf, and use confined NO dimerization as a real-system stress test. Reaction-IAST thus provides a practical, failure-aware baseline closure for single-equilibrium confined interconversions, enabling rapid screening while making its applicability limits explicit.