Mineral sun care emulsions: COSSMA

With the growing awareness of the importance of sun protection, the requirements for sun care products are growing as well. Sophie Rübel, Technical Marketing Lead at Croda GmbH, is on the trail of stable sun care emulsions with mineral UV filters.

More and more people are becoming aware of the effects of UV radiation on our skin – and the number continues to rise. Increasing cases of skin cancer and other skin damages caused by UV radiation result in consumers using sun protection more frequently.¹ Therefore, sun care cosmetics have to fulfil many different requirements. Various formats such as sticks, sprays, or lotions are in demand. The sensory feel of the formulation, sustainability aspects, transparency, and, of course, a very high sun protection factor (SPF) are relevant aspects for consumers.²

Sun care emulsions

In sun care, a desired high SPF can be challenging. This is because high amounts of mineral UV filters may cause instabilities like sedimentation or flocculation. In addition to the location of the UV filter in the emulsion, components such as oil thickeners or emulsifiers influence the stability and SPF value of the formulation. Depending on which emulsifiers were chosen, different emulsion systems result. Traditionally applied stearate derivatives, polymeric surfactants or (non-)ionic emulsifiers can be used as ingredients. Some of the latter can form liquid crystal network structures.
To better understand the effects of the different raw materials, their location, and the stability of the sun care formulations, detailed research is of intere

Cryo-SEM with EDS

Simple optical microscopy can help to predict instabilities of sun care formulations, for example by detecting particles that have agglomerated. Scanning electron cryomicroscopy (Cryo-SEM) with energy-dispersive X-ray spectroscopy (EDS) is an interesting additional tool for the investigation for the characterisation of sun care formulations. The aim of the following measurements is to observe the behaviour and microstructure of mineral UV filters in sun care formulations with a special focus on different emulsion systems and titanium dioxide dispersions (water and oil based). Cryo-SEM is an interdisciplinary tool suitable to characterise soft and hard materials at cryogen temperatures, meaning in frozen state. It gives the opportunity to receive information about the droplet size distribution of the dispersed water phase, surface topography, morphology, roughness, and interactions between the phases. Combined with EDS, the microstructure of the emulsions can also be investigated by identifying the element distribution and composition.^3,4

Table 1,2,3

Ingredients used for O/W emulsion with TiO2 based.

Material and method

Formulations with different emulsifiers, as well as mineral UV filters based on titanium dioxide containing oil and water-soluble carriers, were analysed and compared. The optical microscopy can provide information about the organisation of the micelles and potential agglomerations of the mineral filters, but information on the morphology of the micelles and where the titanium dioxide is located is missing. Cyro-SEM analysis was used to capture images to investigate this in more detail. Using EDS, it is possible to detect elements in the various phases of emulsions. Oxygen represents the water phase, the high carbon concentration indicates the emollients in the oil phase, and titanium reflects the presence of the titanium dioxide UV filter particles.
The first formulation (F1) includes an anionic phosphate O/W emulsifier with a single broad spectrum UV filter. This mineral UV filter consists of titanium dioxide with an oil-soluble carrier. The second formulation (F2) contains a mixture of organic and mineral UV filters and emulsifiers. The mineral UV filter has a water-soluble carrier. The third formulation (F3) was developed with a non-ionic O/W emulsifier, which can build liquid crystal networks, organic UV filters, and a mineral UV filter with a water-soluble carrier.
Samples of these formulations were analysed by Cryo-SEM with EDS associated technique and simple optical microscopy (Olympus) for correlation. For the Cryo-SEM analysis, a small volume of each sample was transferred to the cryogenic state, plunged into slushed liquid nitrogen, and images were taken. All samples were studied using a TESCAN FIB SEM S8000G.

Results

In the following, the optical microscopy image of formulation F1 (Figure 1 (A)) shows the structure of the organised micelles in the emulsion, but as mentioned before, not the location of the mineral UV filter with oil-soluble carrier. The optical microscopy is shown in addition to the CryoSEM images (Figure 1 (B)).

The images of the Cryo-SEM show a detailed morphology of those various micelles regarding the internal and external phase.
Moreover, the images associated with EDS lead to the identification of the water, oil, and mineral UV filter location (Figure 2).

Oxygen (A) is identified in the external phase which represents the water phase. The oil phase is shown as the internal phase. Therefore, the creation of an O/W formulation can be confirmed. The location of the mineral UV filter with an oil-soluble carrier is, as presumed, detected in the internal oil phase.
The organisation of the micelles for formulation F2 is shown with the optical microscopy (Figure 3 (A)) and the additional analysis of the morphology of the internal and external phase with the Cryo-SEM images (Figure 3 (B)).
Due to the various emulsifiers used in the formulation, different micelle sizes can be observed. The Cryo-SEM analysis can help to characterise the emulsion and the influence of emulsifiers regarding the micelle interface package

The use of EDS characterised that water and the mineral UV filter with a water-soluble carrier are, as expected, presented in the external phase (A) and (C) and the oil drops (B) are located in the internal phase.

The formulation F3 contains the liquid crystal emulsifier and a mineral UV filter with a water-soluble carrier, which are shown in the Cryo-SEM with EDS images of Figure 5.

Oxygen (B) was identified as the external water phase and carbon (C) as the internal oil phase. Based on this measurement and some complementary SAXS analysis, it concluded that the titanium dioxide with water-soluble carrier was not homogeneously distributed in the external water phase. Contrary to the expectations and different to thebehaviour of formulation F1, it was present within the oil phase and was much more concentrated on the water interphase of the emulsifier. That unique characteristic also represented an increase on the SPF values observed for this emulsifier.

Conclusion

Conclusion the microstructures with element distribution of O/W emulsions containing mineral UV filters. The results showed that mineral UV filters with oil-soluble carrier tend to be found in the oil phase and UV filters with water-soluble carrier have an affinity to be based in the water phase.
A different behaviour can be found for UV filters with water-soluble carrier combined with emulsifiers, which can build liquid crystal networks. The emulsifier used creates lyotropic mesophases with formation of hydrosomes. Thereby, a continuous phase with lamellar gel structure and oil droplets, which stabilise the visco-elastic gel network, can be found. In this case, titanium dioxide is not homogenously dispersed in the external water phase, but instead allocated in the trapped water of a hydrosome network emulsifier. Especially, the measurement of the SPF in the formulation 3 showed an increased SPF.
According to the results, Cryo-SEM with EDS is a promising and useful method to find out more about the behaviour of emulsions with UV filters. It can successfully provide information on what the microstructure of emulsions looks like and where the UV filter used is located. For further research, it is interesting to see what other advantages are offered by emulsifiers that form liquid crystal network structures and how the SPF can be increased for stable formulations through the choice of the emulsifier system.

Acknowledgements
I would like to express my gratitude to Aline Moreira de Souza for her support and providing data for the Cryo-SEM analysis.

References:
1 Strahlenschutzkommission, S3-Leitlinie Prävention von Hautkrebs S3-Leitlinie Prävention von Hautkrebs (leitlinienprogramm-onkologie. de) (17.01.2024 10:57)
2 Mintel GNPD, Suncreen formats and Top 4 growing claims in sun care from 2017-2022
3 Wightman, Raymond (2022) An Overview of Cryo-Scanning Electron Microscopy Techniques for Plant Imaging. Plants (Basel) 11(9):1113
4 Nishino,Yuri; Miyazaki, Kanako; Kaise, Mizuho; Miyazawa, Atsuo (2022) Fine cryo-SEM observation of the microstructure of emulsions frozen via high-pressure freezing. Microscopy 71(1), 60-65