Role of Ion Size and Hydration in Competitive Adsorption of Alkaline Earth Metals on TiO2 Nanoparticles: Experimental and Molecular Dynamics Insights.

Interactions between alkaline earth metal cations (M2+) and titanium dioxide nanoparticles (TiNPs) critically influence the TiNP surface charge and colloidal stability in biological and environmental systems. Here, we systematically investigate how the cation size, hydration, and concentration affect the interfacial adsorption of Mg2+, Ca2+, Sr2+, and Ba2+ on TiNPs. X-ray photoelectron spectroscopy and transmission electron microscopy provide direct evidence of cation adsorption on the TiNP surface and reveal ion-specific differences in binding extent and distribution. Zeta potential measurements across a broad pH and concentration range further demonstrate that adsorption increases with cation molar mass and leads to pronounced overcharging of the TiNP surfacean effect that is strongly ion-specific and concentration-dependent. To interpret these experimental findings at the molecular level, molecular dynamics simulations were used to identify binding sites, compute potential of mean force (PMF) profiles, and estimate adsorption free energies, providing insight into ion-specific cation-surface interactions. We find that elevated concentrations of background monovalent salt significantly reduce the level of M2+ adsorption, highlighting the importance of competitive adsorption at the nanoparticle-water interface. These results elucidate key factors governing TiNP surface chemistry in complex aqueous environments and inform the design of nanomaterials for biological and ecological applications.