Structure stability and crystallization behavior of water in oil in water (WOW) double emulsions during their characterization by differential scanning calorimetry (DSC)

Abstract

Water in oil in water (WOW) double emulsions are of great interest for the encapsulation of valuable substances. Nevertheless, the stabilization against inner water loss is still a challenging issue that impedes systematical research on the impact of the formulation. One common and applied technique to characterize the amount of inner water encapsulated in double emulsions is based on differential scanning calorimetry (DSC). Thereby, WOW double emulsions are frozen and thawed according to a defined method and afterward the amount of encapsulated water is calculated based on the DSC measurement result. For systematical investigations on the formulations’ impact on the encapsulation efficiency (EE), structure stability during this freezing and thawing procedure is required to be independent of formulation changes, whereby the EE is defined as the ratio between the amount of inner water after a specific time and the initial amount of water used for preparation of the double emulsion. In this study, we focused on the structural stability of double emulsions during their characterization by DSC. We prepared WOW double emulsions with varied formulations and characterized them considering inner and outer droplet size distribution (laser diffraction), microstructure (microscope), and EE (DSC). In some cases, the EE values determined by DSC were not reproducible and contradicted microscopic pictures and measured droplet size distributions. By using a polarizing microscope with temperature-controlled sample chamber, structure changes during the freezing and thawing procedure could be observed. Contact-induced crystallization, water diffusion as well as the spread of the phase transition temperature are discussed as reasons for the barely reproducible and sometimes contradictory results. Based on our findings, it is recommended for further studies on double emulsions to examine the structure stability during the DSC measurements.