Influence of refractive index differences on the signal strength for Raman-spectroscopic measurements of double emulsion droplets

Double emulsions show great potential for encapsulating active substances and protecting them against external influences. However, due to their complex structure, double emulsions tend to become unstable during storage. Research on double emulsions therefore focuses on maintaining their microstructure during their shelf life. Optical measurement methods such as Raman spectroscopy have hardly been used to date to analyze the microstructure of double emulsions mainly due to multiple scattering effects. This study concentrates on reducing scattering effects by matching the refractive indices of the individual emulsion phases. Double emulsions with adapted refractive indices are investigated using Raman spectroscopy. The refractive indices of the inner and outer water phases are varied, while the refractive index of the oil phase is kept constant. In order to evaluate the signal of the inner water phase the same amount of tracer is present in all inner phases. For individual phase boundaries of single droplets, the refractive index matching plays a minor role. However, if there are many droplets with correspondingly numerous phase boundaries, which leads to multiple scattering during the measurement, the matching has a significant influence on the signal strength of the inner phase. When measuring double emulsions, the phases should always be matched if possible, as this results in higher signals. This in turn improves the sensitivity of the measurement.