Finite Volume Effects in Water Nanodroplets: A Molecular Level Investigation

Aqueous interfaces are responsible for a plethora of processes. At the nanoscale, interfaces are overwhelmingly influenced by finite volume effects that are thought to impact both molecular level and macroscopic properties. Here, finite volume effects are investigated using electrophoretic mobility and vibrational sum frequency scattering of water nanodroplets in oil and oil nanodroplets in water, made from the same chemicals. Notably, there is a substantial difference in the orientational ordering of water between the two aqueous interfaces. Isotope dilution studies reveal that water outside the oil droplets participates significantly in intramolecular coupling, while water inside the droplets predominantly exhibits intermolecular coupling. These spectral variations underlie different water structures, pointing to a larger heterogeneity inside water droplets, which are explained by finite volume effects that include a pronounced difference in electrostatics.