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

## TL;DR

This study explores how different alkaline earth metal ions interact with titanium dioxide nanoparticles, revealing how ion size and hydration affect surface adsorption and stability.

## Contribution

The study combines experimental and molecular dynamics approaches to reveal ion-specific adsorption mechanisms and competitive effects on TiO2 nanoparticle surfaces.

## Key findings

- Adsorption of alkaline earth metal cations on TiO2 increases with cation molar mass and causes surface overcharging.
- Molecular dynamics simulations reveal ion-specific binding sites and adsorption free energies.
- High concentrations of monovalent salt reduce M2+ adsorption due to competitive effects at the nanoparticle-water interface.

## Abstract

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.