Interactions in water in oil in water double emulsions: Systematical investigations on the interfacial properties and emulsion structure of the outer oil in water emulsion

Abstract

Due to their multicomponent structure, double emulsions are of great interest for different industrial areas. Nevertheless, there are only a few products on the market. Due to the enforced use of different stabilizing agents, production and stabilization of the double emulsions remains challenging. The stabilizing agents can interact with each other, which might cause a change of interfacial properties and emulsion structure.To gain a deeper understanding of occurring interactions at the oil water interface, interfacial properties were measured using the pendant and oscillating drop technique. The preparation and characterization of single oil in water emulsions containing all stabilizing agents of double emulsions enables the direct correlation between interfacial properties and emulsion structure. For the interfacial and emulsification experiments, polyvinyl-alcohol (PVOH) with varying molecular weight and degree of hydrolysis, polyglycerol-polyricinoleate (PGPR) and sodium chloride (NaCl) were added step-by-step. Furthermore, stability to coalescence in emulsifying processes was evaluated using different devices: a colloid mill (CM) and high-pressure homogenization (HPH).High molecular weight PVOHs interacted with PGPR at the interface but this did not influence the emulsion structure neither for efficient (HPH) or gentle (CM) droplet disruption processes. The low molecular weight PVOH of slight hydrophobic character interacted with PGPR at the interface and this lead to the breakage of the emulsions. At increased hydrophobicity of PVOH, no change in interfacial activity and no change in emulsion structure was found for the low molecular weight PVOH.Based on these findings it is recommended to use high molecular weight emulsifiers to stabilize the outer emulsion of double emulsions. Thereby, it is not necessary to select emulsifiers with a well defined interfacial activity. However, the viscosity of the product is high in this case. If a low product viscosity is required, the hydrophilic emulsifier has to be selected carefully due to a specific interfacial activity.