Effect of glycerol on formation, stability, and properties of vitamin-E enriched nanoemulsions produced using spontaneous emulsification.

Oil-in-water nanoemulsions are finding increasing use as delivery systems to encapsulate lipophilic bioactive components in functional food, personal care, and pharmaceutical products. We investigated the influence of a water-soluble cosolvent (glycerol) on the formation, stability, and properties of vitamin E acetate-loaded nanoemulsions (VE-NEs) prepared by spontaneous emulsification. VE-NEs were formed by titration of a mixture of vitamin E acetate, carrier oil (MCT) and non-ionic surfactant (Tween 80) into an aqueous glycerol solution with continuous mixing. Cosolvent concentration had an appreciable effect on the particle size produced, with the smallest mean droplet diameters (d<50 nm) being formed at 40 and 50 wt% glycerol. Nanoemulsions (d<100 nm) containing 10% vitamin E acetate could be produced at relatively low surfactant concentrations (5%) using these high glycerol levels. The turbidity of the NEs decreased at high glycerol concentrations due to the reduction in droplet size and refractive index contrast. The long-term stability of the VE-NEs was strongly influenced by glycerol concentration and storage temperature. VE-NEs containing 40% glycerol were relatively stable to droplet growth when stored at 5 and 20°C, but a rapid increase in droplet size and turbidity occurred during storage at 37°C. Temperature scanning experiments (20-80-20°C) indicated that a steep and irreversible increase in turbidity occurred during heating, which was around 70°C in the absence of glycerol and 60°C in the presence of 40% glycerol. Droplet instability was attributed to an increase in the rate of Ostwald ripening and/or coalescence as the temperature was increased, associated with dehydration of the non-ionic surfactant head-group leading to a reduction in phase inversion temperature. Dilution (100×) of VE-NEs containing glycerol with water considerably improved their stability to droplet growth, especially at high storage temperatures. This study provides important information about the effect of glycerol on the formation, stability and physical properties of VE-enriched NEs suitable for food, personal care, and pharmaceutical products.
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