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

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.