Encapsulating fragrance in tiny capsules can control how quickly the contents escape and how the capsules break under force. What happens to their fragrance-encapsulation performance if another material is added as a second shell layer?
A study published by Daniele Baiocco and colleagues in 2024 investigated adding a silica layer to a shell made from fungal chitosan. While exploring alternatives to conventional capsules that rely on synthetic resins or animal-derived ingredients, the researchers sought to combine strength with fragrance retention.
Combining a fungal material with an inorganic one
The capsules contained a fragrance ingredient called hexyl salicylate. Rather than using a perfume blend or an essential oil, the researchers examined shell performance with a single model compound.
They stabilized tiny fragrance-oil droplets dispersed in water using a surfactant and surface-modified silica nanoparticles. Fungal chitosan was then added, forming the first shell through electrostatic attraction.
Next, silica was generated from sodium silicate to form another layer. The comparison was between capsules with this additional layer and capsules made only through the first shell-forming stage. Since silica particles were already used to stabilize the oil droplets in the initial stage, this was not a comparison between the presence and complete absence of silica.
Microscopy confirmed roughly spherical capsules with space inside for the payload. Elemental analysis also suggested that the added material may have passed through the chitosan layer and formed silica on the inside. The process may therefore involve more than simply applying a uniform outer coating. Understanding how the layers form also offers clues for improving the material.
Compressing individual capsules until they break
For the strength test, 30 capsules were taken from each sample, dried, and compressed individually with a small, flat-ended probe. The researchers measured the force and deformation at shell rupture.
The nominal rupture stress, which expresses force while accounting for particle size, averaged 1.7 megapascals for the initial shell alone and 3.0 megapascals for the dual shell. The dual-shell capsules showed higher values and also underwent greater deformation before breaking.
The authors suggest that silica reinforced the relatively flexible chitosan structure. These values, however, describe the performance of these particular capsules when dried and compressed. Whether they break at the right moment under friction during laundering or forces during application to skin requires separate testing.
Less fragrance escaped into water
To investigate fragrance retention, capsules were placed in bags made from dialysis membrane and immersed in neutral water. The hexyl salicylate entering the surrounding water was measured over 20 days. A control containing only unencapsulated fragrance was included to check whether the bag membrane itself was the main barrier.
After 20 days, about 10% of the fragrance had escaped from capsules with the initial shell alone, compared with about 2.5% from dual-shell capsules. Under these conditions, adding the silica layer reduced leakage of the contents into water.
The authors suggest that the chitosan layer acted as the main barrier to fragrance transport, while the additional silica may have slowed it further. Because silica has a porous structure, the design combines the functions of different materials rather than relying entirely on one.
However, the rate at which a compound enters water is different from its evaporation rate into air or the length of time a person perceives its scent. The study did not show that fragrance lasts for 20 days on skin or that the same retention rates occur in commercial detergents.
More to adjust than shell strength
The dual shell brought other changes too. Fragrance accounted for about 36% of capsule weight with the initial shell alone and about 28% with the dual shell. When considering a product with an additional protective layer, the amount of fragrance carried per unit weight matters alongside resistance to rupture and leakage.
Only one fragrance ingredient, hexyl salicylate, was used. The authors note that changing the contents can alter the properties of the oil–water interface, potentially changing performance even with the same shell materials. These results cannot simply be extended to all essential oils or fragrance blends.
Although environmental aims informed the material selection, this study did not measure how extensively the finished dual shell biodegrades in natural environments or the environmental impact of the entire product. Skin safety was not evaluated either. The authors themselves identify replacing the surfactant SDS with candidates more suitable for skin contact as a future task.
Summary
In experiments using this model fragrance ingredient, the dual shell combining fungal chitosan and silica improved resistance to compression and retention of the contents in water. Capsule performance can be adjusted not only by replacing materials, but also by changing how different layers are combined.
The authors propose producing a denser, more uniform silica layer and reconsidering the choice of surfactant as the next improvements. Progress toward real products will require testing the encapsulated fragrance, surrounding liquid, and desired rupture behavior together, while also evaluating environmental performance and safety.
References
Baiocco, D., Al-Sharabi, M., Lobel, B. T., Cayre, O. J., Routh, A. F., & Zhang, Z.(2024). Eco-Friendly Fungal Chitosan-Silica Dual-Shell Microcapsules with Tailored Mechanical and Barrier Properties for Potential Consumer Product Applications. ACS Omega, 9(26), 28385–28396. DOI: 10.1021/acsomega.4c02287 / Full original article.
The research received support from the UK EPSRC. The authors declared no competing financial interests. Original article: © 2024 The Authors, CC BY 4.0.
