In an article published in the journal Applied Sciences, researchers investigated the influence of refractive index matching of double emulsion phases by the Raman signal strength of the inner water phase measurement for different refractive index combinations.
Emulsions are often utilized in the food, pharmaceutical, and chemical industries. Emulsions are more complicated in terms of their structure and subsequent qualities as compared to homogeneous systems. Their greater complexity is caused by several factors, including the compositions of each phase and the interfacial characteristics and size distribution of the inner droplet phase.
In recent years, Raman spectroscopy has been utilized in the pharmaceutical and food industries to research novel drugs and monitor the emulsion polymerization processes. One of its main advantages is Raman spectroscopy's strong selectivity for molecular bonds. Additionally, there is a correlation between the sample concentration and the Raman signal's strength. These features suggest that Raman spectroscopy is particularly suited for analyzing heterogeneous systems like emulsions.
Challenges in Emulsions Quantification
Quantifying emulsions presents a unique challenge. The incident light is elastically scattered at the border between the continuous and dispersed phases. Depending on the measuring apparatus, this impact may cause significant perturbations in the measurement process and can make it more challenging to interpret the data. When double emulsions' microstructure is assessed, this difficulty is further enhanced.
Double emulsions, or complicated multiphase systems in which both water-in-oil (W/O) and oil-in-water (O/W) emulsions occur concurrently, are emulsions inside emulsions. Multiple scattering occurs at the interface of double emulsion when light is irradiated on them. The capacity to encapsulate active or sensitive compounds, such as crop protection agents, vitamins or enzymes, is a significant benefit of the double emulsion structure.
Many researchers are now concentrating on increasing the stability of double emulsions. However, their study is constrained since it is complicated to evaluate structure-related metrics like inner and outer droplet size distributions and their filling levels in real-time. Typically, non-real-time measuring techniques, including dynamic and static scattered light methods, differential scanning calometry, and nuclear magnetic resonance NMR spectroscopy, are utilized to measure such characteristics. These procedures often involve offline execution and sample preparation.
Additionally, the measured emulsion sample is often useless for further analyses. Therefore, a novel measuring approach is required for double emulsions to offer real-time data without sample preparation and without affecting the double emulsion. These prerequisites are met by Raman spectroscopy.
The Impact of Refractive Index Differences
The focus of this study is to determine whether the refractive index matching method allows it to suppress the cross-influences caused by phase boundaries in Raman measurements of double emulsions and to assess the impact of refractive index differences between the three double emulsion phases on the Raman signal strength. The research also examines whether refractive index variations may be corrected mathematically using simple functional connections derived from Fresnel's formula.
The Raman measurements are performed using the RNX1532 Raman spectrometer. It has a laser with a 532 nm wavelength and a 150 mW laser output. The related backscatter probe illuminates the material using a half-inch lens with a 500-nanometer focal length and a 55-nanometer focal diameter. Each experimental set is monitored for 400 seconds to measure many droplets. This time is broken down into four accumulations of 20 seconds and five separate measurements, each lasting 80 seconds. Each experimental set yields five spectra in the end, merged into a single data point.
Double Emulsion Production
Different emulsification methods were used in a two-step procedure to create the double emulsion. First, a two-row gear ring was used to create five separate W1/O emulsions utilizing a rotor-stator system for 90 seconds at 20,000 per minute of rotating speed. These inner emulsions were emulsified in the outer water phase using a microfluidic glass capillary device to produce monodispersed W1/O droplets in a double emulsion. The droplet size distribution of the emulsions containing 16%, 24%, 33%, and 61% ammonium nitrate was determined using a particle analyzer and static laser light scattering.
The inner emulsions' Sauter means diameters vary from 2.67 m to 2.07 m. Due to the identical refractive indices, the emulsion with 49% ammonium nitrate in the aqueous phase could not be measured. Rectification results are determined using a simple 2-parametric function for emulsions whose refractive indices cannot be changed. As a result, a spreadsheet was used to do a multiple linear regression.
Conclusion and Future Outlooks
The findings show that it is desirable to use optical spectroscopic measuring techniques that match the inner phase's refractive indices to produce an inner phase beam of light.
This study demonstrates how Raman spectroscopy may be used to in situ measure microstructural characteristics in double emulsions. In the future, it will be fast and straightforward to explore the effects of formulation or process factors on the structure and stability of emulsions.
Thomas Hufnagel, Matthias Rädle and Heike P. Karbstein (2022) Influence of Refractive Index Differences on the Signal Strength for Raman‐Spectroscopic Measurements of Double Emulsion Droplets. Applied Sciences. https://www.mdpi.com/2076-3417/12/18/9056