Charge-Dependent Interfacial and Optical Properties of Ionic Liquid Microdroplets Formed in Vacuum

Electrospray deposition of ionic liquids offers a versatile route to generate nano- and microscale architectures with tunable interfacial properties. Here, we investigate the formation and characterization of ionic liquid microdroplets (ILDs) of [EMIM][NTf₂] deposited on undoped silicon substrates by in vacuo electrospray emission operating with several different polarity configurations. Atomic force microscopy and scanning electron microscopy reveal dome-shaped ILDs with diameters of 2-8 μm, whose size, height, and number density depend strongly on the applied extraction bias. Cation-rich ILDs exhibit the highest contact angles and lowest coalescence tendencies, reflecting the combined effects of coulombic repulsion and steric hindrance from the alkyl-substituted imidazolium cations. Nano-FTIR spectroscopy confirms uniform chemical composition across ILDs, indicating that variations in wetting behavior arise from subtle cation-anion imbalances. Dark-field scattering measurements demonstrate that ILDs act as stable broadband scatterers. These findings establish electrospray bias as a means to control droplet geometry and surface interactions, providing new opportunities for tailoring wetting, patterning, and optical properties in ionic-liquid-based materials.