Internal coalescence as a mechanism of instability in water-in-oil-in-water double-emulsion globules

Using capillary microscopy, internal coalescence between the internal aqueous compartments in a single water-in-oil-in-water (W-1/O/W-2) double-emulsion globule was investigated, Globules were prepared using either sodium dodecyl sulfate (SDS) (ionic) or Tween 80 (nonionic) as the water-soluble surfactants and Span 80 as the oil-soluble surfactant. Concentrations of both the hydrophobic and hydrophilic surfactants, were varied until coalescence between the internal aqueous droplets was observed. Internal coalescence was observed when the concentration of SDS or Tween 80 in the internal aqueous phase was 1-3 CMC (critical micelle concentration) or 50-100 CMC, respectively, and occurred preferentially when the concentration in the internal phase was significantly greater than in the external phase. Internal coalescene always occurred in conjunction with coalescence between the internal W-1 and external W-2 aqueous phases (external coalescence). When W-1 and W-2 were not too dissimilar in their overall surfactant concentrations, only external coalescence occurred. These phenomena were shown to also depend on the concentration of the Span 80 in the oil phase, the stabilizing ability of which was confirmed. Spontaneous emulsification was observed at low concentrations (<25 CMC Tween 80, <0.5 CMC SDS) of the hydrophilic surfactants in the internal aqueous phase. Deformation of W-1 occurred at high concentrations (>100 CMC Tween 80, >3 CMC SDS).