Influence of interfacial rheological properties of mixed emulsifier films on the stability of water-in-oil-in-water emulsions

The purpose of this study was to investigate the influence of mixtures of t he emulsifiers Span 80, 83 and 85 and Tween 80 on multiple emulsion stabili ty. An oscillatory ring-surface rheometer was used to measure interfacial e lasticity at the oil-aqueous interface. Multiple emulsions were prepared vi a a two-step emulsification process and stability was evaluated by investig ation of drug transport from freshly prepared and eight-day-old emulsions b y use of a dialysis method. Photomicrography and droplet-size analysis of m ultiple emulsions were also conducted. Spans 80 and 83 were appreciably elastic (683.10 +/- 29.13 mN m(-1) and 112 8.09 +/- 14.81 mN m(-1), respectively at 5% w/v) when present at the minera l oil-aqueous interface whereas Span 85 and Tween 80 were not (11.10 +/- 3. 88 mNm(-1) (5% w/v) and 0 (0.1-5% w/v) respectively). The interfacial elast icities of Spans 80 and 83 decreased in the presence of Tween 80 in the aqu eous phase; this was attributed to co-adsorption of Tween 80 at the interfa ce or aqueous-phase solubilization of the Spans within mixed micelles, or b oth. Drug-transport studies indicated that drug release on storage was lowe r from water-in-oil-in-water (w/o/w) emulsions prepared with 5% w/v Span 80 or 83 and 0.1% w/v Tween 80 than from emulsions prepared with 5% w/v Spans 80 or 83 and 1% w/v Tween 80. Photomicrography and droplet-size analysis i ndicated the same trend-emulsions containing a higher percentage of Tween 8 0 were less stable. The relatively stable w/o/w emulsions (e.g. 5% w/v Span s 80 or 83 and 0 1% w/v Tween 80) contained a large number of multiple drop lets for up to eight weeks of storage whereas the relatively unstable w/o/w emulsions (e.g., 5% w/v Span 85 and 0.1% w/v Tween 80 and 5% w/v Spans and 1% w/v Tween 80) contained mostly simple droplets after only one week of s torage. The mean volume/weight droplet size decreased on storage with break down of these w/o/w emulsions to simple oil-in-water emulsions. There was a positive correlation between the interfacial elasticity and emu lsion stability data. Mixed emulsifiers giving higher film strength, as qua ntified by interfacial elasticity measurements, resulted in more stable w/o /w emulsions.