Molecular dynamics of electrosprayedwater nanodroplets containing sodiumbis(2-ethylhexyl)sulfosuccinate

The behavior of aqueous solutions of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) subject to electrospray ionization (ESI)has been investigated by molecular dynamics (MD) simulations at three temperatures (350, 500 and 800 K). We considerseveral types of water nanodroplets containing AOTNa molecules and composed of a fixed number of water molecules(1000), N0AOT AOT anions (N0AOT = 0, 5, 10) and N0Na sodium ions (N0Na = 0, 5, 10, 15, 20): in a short time scale (less than 1 ns),the AOTNa molecules, initially forming direct micelles in the interior of the water nanodroplets, are observed in all casesto diffuse nearby the nanodroplet surface, so that the hydrophilic heads and sodium ions become surrounded by watermolecules, whereas the alkyl chains lay at the droplet surface. Meanwhile, evaporation of water molecules and of solvatedsodium ions occurs, leading to a decrease of the droplet size and charge. At 350 K, no ejection of neutral or charged surfactantmolecules is observed, whereas at 500 K, some fragmentation occurs, and at 800 K, this event becomes more frequent. Theinterplay of all these processes, which depend on the values of temperature, N0AOT and N0Na eventually leads to anhydrouscharged surfactant aggregates with prevalence of monocharged ones, in agreement with experimental results of ESI massspectrometry. The quantitative analysis of theMD trajectories allows to evidencemolecular details potentially useful in designingfuture ESI experimental conditions.