Involvement of Hypervalent Nitrogen in the Unique Interfacial Reactivity of Gemini Cationic Surfactants: An Interface-Sensitive Mass Spectrometry Study

Hypervalent nitrogen species have previously been observed in the gas phase with lifetimes in the μs range. We have, for the first time, observed a hypervalent nitrogen species in the condensed phase with a lower-bound lifetime in the minute range. It results from a gemini surfactant that consists of a diammonium functionality counterbalanced to neutrality by two bromide ions and CH₃(CH₂)_n(CH₂CH₂O)_m alternating hydrophobic-hydrophilic terminations. The ammonium salt homolytically cleaves an N-Br bond, resulting in a bromine atom and a hypervalent nitrogen. The reaction takes place in the air-liquid interface involving a range of solvents and is enabled by an intricate interfacial structure that is generated by the “entanglement” of the alternating hydrophobic-hydrophilic terminations. The structure resembles a porous mesh, similar to what is encountered in the case of polymeric membranes. This structure facilitates a release of Br radicals (to relieve charge stress) rather than bromide ions because of the lack of sufficient water and space at the interface. The hypervalent species does ultimately decompose at the interface, but the reaction is slow enough for simultaneous observation of the parent molecule and the associated products. The findings pertaining to the interfacial membrane structure will enable the design of slow-release systems for highly reactive hypervalent species going forward.