Investigations on the relationship between interfacial and single droplet experiments to describe instability mechanisms in double emulsions

Due to their multicomponent structure, water-in-oil-in-water (WOW) double emulsions are of great interest for the encapsulation of hydrophilic substances. Although promising applications are reported, few double emulsion products are available on the market because their stabilization is still challenging. To stabilize all internal interfaces, at least two interfacial active substances (IAS), a lipophilic and a hydrophilic one, have to be added. These IAS can interact with each other at the interfaces and thus might impair the double emulsion stability. Especially the impact of these interactions on the loss of inner water droplets due to coalescence with the outer water phase is largely unknown.In this study, we investigated if interfacial tension measurements can be related to the loss of inner water caused by coalescence. We determined the outer oil in water interfacial tension of a double emulsion, when a hydrophilic or a lipophilic IAS is used individually as well as in combination. Furthermore, the coalescence process between a water droplet encapsulated in oil and an adjacent continuous water phase was observed in single droplet experiments.As model system polyvinyl-alcohol (PVOH) was used as hydrophilic and polyglycerol-polyricinoleate (PGPR) was used as lipophilic IAS. For the sole use of PVOH, the behavior of the coalescence process can be related to the interfacial tension, which was changed by varying the PVOH type. It could be shown that the lower the interfacial tension is, the longer is the time until the coalescence process takes place. In addition, the duration of the coalescence process itself increases by decreasing the interfacial tension for the system under study.In case of the combined use of PVOH and PGPR, the observations differ in dependency on the PVOH type. In case of PVOH with a low hydrophobicity, the simultaneous presence of PGPR caused only a minor change of the coalescence behavior compared to the sole use of PVOH. In case of a PVOH type with higher hydrophobicity, the coalescence behavior changed clearly by the presence of PGPR. However, no correlation to the interfacial tension was found.