Molecular dynamics simulation of the cone-jet electrospray: Role of surfactants

Abstract

Electrospray in the cone-jet regime could produce highly charged and fine monodisperse drops, and further disintegrates into nanoscale drops, even ions due to synergism of Rayleigh instability and evaporation. The jet and its breakup instability into drops are significantly affected by liquid properties including surface tension, conductivity, density and others. Surfactants may play important roles on gas-liquid interface and are usually added in solution to vary liquid property and may further affect electrospray behaviors. In present work, the ethanol containing surfactants is atomized in the cone-jet to discuss the synergistic effect of surfactant types, concentrations and surfactant-salt on the jet evolution behaviors under direct current electric fields using molecular dynamics (MD) method. It is found that the jet length of ethanol solution with Span-80 is the longest (Span-80, SDS, CTAB and TX-100), while it has excellent dispersion. The addition of CTAB could make ethanol solution transform from continuous jet into dripping. SDS interacts most violently with ethanol due to the presence of sodium ions. As surfactant concentration increasing, the number of solution molecules sprayed remarkably increases, while the interaction generated from ethanol decreases. The number of molecular clusters (composed of three or more ethanol molecules) in the cone-jet electrospray increases with increasing Span-80 concentration. It is also found that the addition of Span-80 slightly changes the average bond angle of ethanol and bond angle distribution. The ethanol atomization morphology transforms to spindle becasue of the synergistic effect of surfactant and salt ions. The interaction generated between ethanol molecules plays a dominating role and decreases due to the generation of spindle-shaped drops. The hydrogen bond generated from ethanol decreases with increasing conductivity and surface tension. The conclusions of this paper will be latent value for the optimization of physical parameters in electrospray.