Improvement of encapsulation efficiency of water-in-oil-in-water emulsion with hypertonic inner aqueous phase

Water-in-oil-in-water (w/o/w) emulsions encapsulating tryptophan or theophylline were prepared where these compounds are regarded as model drugs. The effects of sodium chloride on the drug entrapment into the w/o/w emulsions and on the separation of aqueous phases were studied. The degree of encapsulation of tryptophan in the w/o/w emulsion increased with the concentration of sodium chloride added in the inner aqueous phase, while it decreased with that in the outer aqueous phase. As for theophylline, although the degree increased with a concentration of sodium chloride in the inner phase, the effect was smaller than that on tryptophan. The difference in the effects between on tryptophan and on theophylline was attributed to their partition coefficients. Theophylline was easily leaked out from the inner phase to the outer aqueous phase after its dissolution and diffusion in the oil phase due to a higher partition coefficient. More than 55% of the aqueous phase was separated from the w/o/w emulsion within 24 h, when sodium chloride was not added in the inner aqueous phase. However, the separation was not observed when more than 0.2M sodium chloride was added. To the contrary, sodium chloride added in the outer aqueous phase accelerated the separation. It was, therefore, concluded that sodium chloride in the inner aqueous phase plays an important role in suppression of the separation and in encapsulation of the drug which does not penetrate into the oil membrane.