Ion-Specific Interfacial Electric Fields on Water Microdroplets for Tuning Menshutkin Reactions

High interfacial electric fields (EFs) on water microdroplets can drastically accelerate chemical reactions; however, how dissolved ions can alter these fields and associated reaction kinetics remain elusive. By combining microdroplet experiments, molecular dynamics (MD) simulations, and quantum chemical calculations, here we studied the ion-specific effects in the pyridine-iodomethane Menshutkin reaction, a reaction that can be accelerated by the EF at the air-water interface of microdroplets. Experimental results reveal that monovalent anions regulate reaction yields in accordance with the Hofmeister series (HCOO^- > F^- > Cl^- > NO₃^- > BF₄^- ≈ SCN^- > ClO₄^- > PF₆^-). Theoretical analyses show that the strength of the EF depends on the interplay between the interfacial affinity of ions and hydration energy: cations with high interfacial affinity and strong hydration capacity amplify interfacial EFs and accelerate reaction rates, whereas anions adsorb to the interface competitively, weakening EFs and suppressing reactivity. This work deciphers how ion-specific interfacial microenvironments govern chemical reactivity in microdroplet systems, providing a basis for the rational design of new EF accelerated microdroplet reactions.