Confined Phase Separation of Aqueous-Organic Nanodroplets

Nano-confined supercooled water often occurs in aqueous-organic aerosol nanodroplets. We used classical molecular dynamics (MD) simulations to study the structures of binary SPC/E water-TraPPE butanol nanodroplets, [PCCP 19, 26839 (2017)]. Water-butanol cross-interaction LJ parameters were adjusted to reproduce the experimentally observed mutual solubilities of water-butanol at 295 K. Simulations at 250 K show three different nanodroplet structures depending on the butanol concentration. At low concentrations, core-shell (CS) structures occur in which a butanol shell completely wets a water-rich core. At high concentrations, well-mixed (WM) structures occur as the water and butanol become fully miscible. At intermediate concentrations, phase-separated Russian Doll-Shell (RDS) structures occur. The RDS structure consists of a water-rich droplet partially wetted by a well-mixed water/butanol lens (a RD), and this lens-droplet structure is coated by a thin shell of butanol. At 295 K the RDS structure transforms into a WM droplet due to the increased mutual solubility of water and butanol. A classical DFT model produces structures similar to those found using MD.