Effects of surfactants on water transport in W1/O/W2 emulsions

The effects of oil-soluble and water-soluble surfactants on water transport rates in W-1/O/W-2 emulsions under osmotic pressure were studied at the single-globule level by using capillary video-microscopy technique. By changing the constitution of the internal aqueous compartment W-1, the intervening oil O, and the outer aqueous suspending phase Wa, these effects were further studied for different controlling water transport mechanisms. Experiments were conducted at visual contact, when water is mainly transported via hydrated surfactants, and at no visual contact, in which case water migration occurs primarily in the forms of spontaneously emulsified droplets and reverse micelles. Water transport rates were found to rise linearly with increasing oil-soluble surfactant concentration in the oil phase over a significant range, irrespective of the transport mechanism, though the effects are more pronounced when water is transported via hydrated surfactants. In contrast to the system-stabilizing effect of the oil-soluble surfactant, water-soluble surfactants in both W-1 and W-2 phases always weaken the stability of the emulsion globules. Water-soluble surfactants in W-1 exhibited different effects on water transport than those in W-2. Although the eventual tendency is that the water transport rates increase with increasing water-soluble surfactant concentration in W-1, a small amount of water-soluble surfactants in W-1 will retard transport from W-1 to W-2 for all transport mechanisms as compared to when there is no surfactant in W-1. Water-soluble surfactants in W-2 phase always accelerate the water transport regardless of their concentrations.