First-Principles Insights into Coverage-Dependent Water Adsorption Mechanisms on Representative Lunar Regolith Mineral Surfaces.

Water retention on the lunar surface is governed by water-mineral interactions, yet the atomic-scale transition from isolated adsorption to high-coverage water accumulation remains insufficiently understood. We perform spin-polarized first-principles calculations to investigate single- and multi-water adsorption on representative surfaces of four major lunar regolith minerals: CaAl2Si2O8, MgFeSi2O6, FeTiO3, and Mg3FeSi2O8. Single-water adsorption reveals that H2O preferentially anchors at exposed metal sites via O-M coordination, with Ti and Fe sites exhibiting stronger initial binding than Mg, Ca, or Al sites. The Hard-Soft Acid-Base (HSAB) principle provides a qualitative framework for this low-coverage site preference based on Lewis acidity. Specifically, the accessible d-orbitals and localized states of Ti/Fe centers introduce substantial covalent orbital coupling and interfacial polarization, which effectively reinforce the binding with the hard O-donor of water. However, as water coverage increases, the stabilization mechanism undergoes a fundamental transition. At low coverage, adsorption is localized and site-specific, governed by cation acidity. At high coverage, the formation of laterally connected hydrogen-bonded networks becomes the dominant stabilizing factor, and the overall adsorption behavior is increasingly dictated by surface topology and geometric compatibility for hydrogen-bond connectivity rather than by isolated cation acidity. This coverage-dependent evolution from electronic-driven anchoring to topology-driven network formation establishes a dual-stage cooperative mechanism for water accumulation on lunar mineral surfaces. Our findings suggest that models for volatile retention on airless bodies must account for both the electronic activity of surface cations and the structural topology of mineral surfaces.