Influence of electrolyte concentration on microchannel oil-in-water emulsification using differently charged surfactants

The purpose of this study was to investigate the formation characteristics of oil-in-water (O/W) emulsion droplets in the presence of electrolyte by microchannel (MC) emulsification using differently charged surfactants. Droplet formation was conducted by pressurizing a dispersed phase (refined soybean oil) via 23,348 asymmetric through-holes micro fabricated on a 24 mm × 24-mm silicon chip into a continuous phase in the presence of a surfactant (1.0 wt%) and an electrolyte (NaCl) (0–1.0 mol/L). Monodisperse O/W emulsions with an average droplet diameter (dav,drop) of 26 μm and a coefficient of variation (CV) below 5% were produced from the asymmetric through-holes when the NaCl concentration was lower than a threshold level that is 0.3 mol/L for sodium dodecyl sulfate (SDS) and 0.5 mol/L for polyoxyethylene (20) sorbitan monolaurate (Tween 20). Further increase in the NaCl concentration resulted in increase of the dav,drop and CV of the resultant O/W emulsions, which is due to unstable and unsmooth motion of the oil–water interface during droplet formation. In the presence of SDS, it was found that interfacial tension must be higher than a threshold level (c.a. 0.9 mN/m) in order to achieve stable droplet formation by MC emulsification. The ζ-potential on the droplet and chip surfaces was negative, and their absolute value decreased with increasing the NaCl concentration. The ζ-potential results also indicated that repulsive electrostatic interaction between the droplet and chip surfaces in the presence of NaCl is high for SDS and negligible for Tween 20. The difference in their electrostatic interaction is assumed to affect the smoothness of droplet formation and the behavior of the droplets formed.