Caught while Dissolving: Revealing the Interfacial Solvation of the Mg²⁺ Ions on the MgO Surface

Abstract

Interfaces between water and materials are ubiquitous and are crucial in materials sciences and in biology, where investigating the interaction of water with the surface under ambient conditions is key to shedding light on the main processes occurring at the interface. Magnesium oxide is a popular model system to study the metal oxide–water interface, where, for sufficient water loadings, theoretical models have suggested that reconstructed surfaces involving hydrated Mg2+ metal ions may be energetically favored. In this work, by combining experimental and theoretical surface-selective ambient pressure X-ray absorption spectroscopy with multivariate curve resolution and molecular dynamics, we evidence in real time the occurrence of Mg2+ solvation at the interphase between MgO and solvating media such as water and methanol (MeOH). Further, we show that the Mg2+ surface ions undergo a reversible solvation process, we prove the dissolution/redeposition of the Mg2+ ions belonging to the MgO surface, and we demonstrate the formation of octahedral [Mg­(H2O)6]2+ and [Mg­(MeOH)6]2+ intermediate solvated species. The unique surface, electronic, and structural sensitivity of the developed technique may be beneficial to access often elusive properties of low-Z metal ion intermediates involved in interfacial processes of chemical and biological interest.

Investigating the High-Temperature Water/MgCl2 Interface through Ambient Pressure Soft X-ray Absorption Spectroscopy

“…Notably, the NEXAFS spectrum of the intermediate Mg 2+ species is markedly different from the XAS spectrum assigned to a fully dissolved octahedral Mg 2+ ion. 22,47 The body of evidence we have presented suggests that an interaction between the surface Mg 2+ ions and the water vapor molecules is established while excluding the occurrence of a surface dissolution process.…”

“…Notably, while performing a similar AP-NEXAFS experiment to investigate the water/MgO interface, we recently observed that the water/MgO interaction is entirely reversed after treating the MgO sample up to 525 K for about 20−30 min. 22 Conversely, the fact that in our experimental conditions the water vapor/MgCl 2 interaction is not fully reversed after ca. 70 min of thermal treatment may be ascribed to the significantly different MgCl 2 surface properties if compared to those of MgO, e.g., to the higher higroscopicity of the MgCl 2 material.…”

“…Such a contribution indicates that there is a direct interaction of the water molecules with these surface magnesium sites, with this distance also being close to the Mg−O one previously determined for the Mg 2+ cation in aqueous solution. 22,48 The average number of water molecules interacting with each magnesium center has been obtained by integrating the g(r)'s up to a cutoff distance chosen at the first minimum after the first peak. As a result, each Mg-3 site is surrounded by an average number of 3.0 water molecules, while the integration number for the Mg-5 sites resulted to be 1.0.…”

“…Further, we point out that in our experimental conditions the percentage of Mg 2+ ions dissolved at the water/MgCl 2 interface is expected to be negligible, a behavior that differs from that of the water/MgO interface, where the dissolution of Mg 2+ surface species at 313 K was recently found to occur. 22 The key role played by high temperature in our experiment is thus 2-fold: on the one hand, by keeping…”

“…Among the advanced experimental spectroscopic techniques that may be used to simultaneously access structural and detailed electronic information, X-ray absorption spectroscopy (XAS) stands out as a useful method to monitor the local modifications of a selected photoabsorber. 19−21 While the use of XAS in the high-energy region is now routine, 22,23 this technique has been less employed to investigate surfaces containing low-Z metal ions, due to the requirement of soft X-rays (∼400−2000 eV) and of special experimental set-ups. 24 In fact, XAS beamlines operating in the soft X-ray regime (soft-XAS) need high-vacuum conditions and only very recently specific cells allowing operando soft-XAS measurements at ambient pressures have been made available.…”