Abstract
Double emulsions are a promising formulation for encapsulation and targeted release in pharmaceutics, cosmetics and food. An inner water phase is dispersed in an oil phase, which is again emulsified in a second water phase. The encapsulated inner water phase can be released via diffusion or via coalescence, neither of which is desired during storage but might be intended during application. The two interfaces in a double emulsion are stabilized by a hydrophilic and a lipophilic surfactant, to prevent the coalescence of the outer and the inner emulsion, respectively. This study focuses on the influence of the hydrophilic surfactant on the release of inner water or actives encapsulated therein via coalescence of the inner water droplet with the outer O–W2 interface. Since coalescence and diffusion are difficult to distinguish in double emulsions, single-droplet experiments were used to quantify differences in the stability of inner droplets. Different lipophilic (PGPH and PEG-30 dipolyhydroxylstearate) and hydrophilic surfactants (ethoxylates, SDS and polymeric) were used and resulted in huge differences in stability. A drastic decrease in stability was found for some combinations, while other combinations resulted in inner droplets that could withstand coalescence longer. The destabilization effect of some hydrophilic surfactants depended on their concentration, but was still present at very low concentrations. A huge spread of the coalescence time for multiple determinations was observed for all formulations and the necessary statistical analysis is discussed in this work. The measured stabilities of single droplets are in good accordance with the stability of double emulsions for similar surfactant combinations found in literature. Therefore, single droplet experiments are suggested for a fast evaluation of potentially suitable surfactant combinations for future studies on double-emulsion stability.
Highlights
The most commonly found type of multiple emulsions is the water-in-oil-in-water (W/O/W) double emulsion
The measurement of the coalescence times of the single droplets and the transfer of this measurement data to emulsion stabilities or the composition of an interfacial film was not carried out according to a standard method; some general limitations and possible interference factors are presented
It is common practice to purify vegetable oils for interfacial sensitive measurements [25,28]. For this reason the oil phase was purified before executing the coalescence time measurements, as other authors suggested [17]
Summary
The most commonly found type of multiple emulsions is the water-in-oil-in-water (W/O/W) double emulsion. As the inner and outer water phases typically differ in their composition, the outer phase is often referred to as W2 and the inner phase as W1, resulting in the abbreviation of W1/O/W2 double emulsion. These emulsion systems are suitable for the encapsulation of water-soluble ingredients for pharmaceuticals, cosmetics or food [1,2,3]. Over time or with specific triggers like changes in pH or temperature, the encapsulated substance can be released Another possible application would be the production of fat-reduced food emulsions [4,5]. Despite the advantages that double emulsions can offer for various applications, only a few double-emulsion-based products are available on the market today
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