The Effect of Salt on the Uptake and Arrangement of Dodine Fungicide at the Aqueous Interface: Vibrational Sum Frequency Generation Study

Dodine, a widely used fungicide, exhibits amphiphilic behavior that drives its accumulation at aqueous interfaces, where its structural organization can influence interactions with biomolecules and the overall surface activity. In agricultural systems, salts are commonly present and are expected to modulate the interfacial behavior of dodine. Here, we investigate the effect of varying salt concentrations on the interfacial enrichment and structural arrangement of different doses of dodine at the air/water interface by using vibrational sum frequency generation (VSFG) spectroscopy and surface pressure measurements. Dodine adopts disordered, ordered, and well-packed states at the aqueous interface depending on its concentration. The addition of monovalent salt to the subphase enhances the accumulation of dodine at the interface and modulates its packing, likely due to the greater stabilization of ion pairs between dodine cations and salt anions at the interface compared with the bulk solution. Divalent salts induce similar effects at lower concentrations than monovalent salts, highlighting the importance of ionic charge in this process. The VSFG and surface pressure data indicate that salt anions, depending on their charge and the initial ordering of dodine, can drive dodine molecules to the interface, transforming the air/dodine/water interface from a disordered state to an ordered and ultimately well-packed arrangement. The salt-anion-induced recovery of dodine packing at the aqueous interface may provide valuable insights into the interaction of different biomolecules with this fungicide at the aqueous interface.