Magnesium chloride
is a prototypical deliquescent material
whose
surface properties, although central for Ziegler–Natta cataysis,
have so far remained elusive to experimental characterization. In
this work, we use surface-selective X-ray absorption spectroscopy
(XAS) at ambient pressure in combination with multivariate curve resolution,
molecular dynamics, and XAS theoretical methods to track in real time
and accurately describe the interaction between water vapor and the
MgCl2surface. By exposing MgCl2to water vapor
at temperatures between 595 and 391 K, we show that water is preferentially
adsorbed on five-coordinated Mg2+sites in an octahedral
configuration, confirming previous theoretical predictions, and find
that MgCl2is capable of retaining a significant amount
of adsorbed water even under prolonged heating to 595 K. As a consequence,
our work provides first experimental insights into the unique surface
affinity of MgCl2for atmospheric water. The developed
technique is proven highly sensitive to the modifications induced
by adsorbates on a given low-Z metal based surface and may be useful
in the toolbox required to disentangle the mechanisms of interfacial
chemical processes.