Transition Metal Ions in Solution: Complexed Ion Speciation at the Air-Liquid Interface

Creators: So̷lling, Theis I.¹; Andersson, Martin P.¹; Abdel-Azeim, Safwat¹; El Zohry, Ahmed M.¹; Beauchamp, Jesse Lee²

1. King Fahd University of Petroleum and Minerals
2. California Institute of Technology

Abstract

We have studied the interfacial properties of Zn²⁺ vs selected transition metal cations such as Fe²⁺, Cu²⁺, and Cr³⁺ in a water-ethanol mixture using field-induced droplet ionization. This was in particular inspired by the specific surface activity of Zn²⁺ that has been observed on several occasions and a desire to clarify the root cause for this behavior. We have found that Zn²⁺, due to its unique electronic configuration and atomic size, is the only ion of those under study that gives rise to specific speciation at the air-liquid interface with three ethanol molecules attached to the central atom for optimal polarity.

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Acknowledgement

T.I.S. gratefully acknowledges the award of an International Summer Program award (ISP) from the Deanship of Scientific Research, KFUPM. Without the ISP program this study would not have been possible. Helpful conversations with Prof. Jesper Bendix, Department of Chemistry, University of Copenhagen are also acknowledged. They will lead to the way for future studies of transition metals at liquid–liquid interfaces.

Supplemental Material

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.4c03115.

Table of the calculated hydration free energies using various force field sets from ref (41), DFT-optimized structures of the first coordination spheres extracted from MD snapshots at 100 ns, a zoom-in of the first coordination sphere for the two metal cations at the oil–brine interface, and the coordination numbers of the two metal cations from MD simulations calculated using radial distribution function in Figure 5 (PDF)

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