Generation-fusion strategy in metal-earth ionosome investigation using single-entity electrochemistry at the micro-water/trifluorotoluene interface

Ionosomes are water clusters/droplets formed in an organic phase via the spontaneous assembly of ionic bilayers by hydrated cations or anions. The ionosomes formed by monovalent ions, i.e., Li⁺, Na⁺, K⁺, Cl⁻, etc., have been thoroughly investigated. Herein, single nanowater clusters (viz. ionosomes) in organic solutions, which are formed by the transfer of metal earth cations (i.e., Mg²⁺, Ca²⁺, Ba²⁺) into organic lipophilic electrolytes, were discovered using single-entity electrochemistry (SEE). The generation-fusion strategy involving two-step potentiostatic chronoamperometry was applied to investigate single Mg²⁺-ionosomes at the water/α,α,α-trifluorotoluene (w/TFT) interface. After an exciting potential forced the hydrophilic ion into the organic phase, a train of current spikes with information on the integrated charges, frequency, and duration were recorded to gain insight into the ionosomes. Variates were thoroughly investigated, including the exciting potential, exciting time, recording potential, size of the applied w/TFT interface, ion species, and concentration. These results suggest that: (1) the size of a single Mg²⁺-ionosome was revealed; (2) the fleshly formed ionosomes are mainly located on the organic side of the w/TFT interfacial surface instead of diffusing into the organic bulk; (3) when the interface decreases, fewer ionosomes form, and the size of the ionosomes decreases; (4) the mean charge carried by a single ionosome is positively related to the charge density of a single cation; (5) by alternating the recording potential, the negative zeta potential of Mg²⁺-ionosomes was revealed; (6) Mg²⁺-ionosomes can form when the aqueous solution contains only 2 μmol·L⁻¹ Mg²⁺. This work thoroughly investigated hydrated metal earth ion clusters in the organic phase via SEE and provides insights into the interface electric double layer from the electrochemistry perspective. In addition to promoting the understanding of ionosomes, this strategy can provide potential applications to identify high-purity water.