Menthol Crystals

Yingngam, Bancha, et al. Journal of drug delivery science and technology, 2019, 53, 101138.

Menthol is a valuable cooling agent that creates a cooling sensation on the skin's surface and has anti-inflammatory and anti-itch properties that can relieve sunburn. To overcome the limitations of low water solubility and high volatility of menthol, a nanoprecipitation method was used to encapsulate menthol in lipid core nanocapsules. The developed nanocapsules exhibit rapid skin absorption properties as an alternative to cosmetic textiles.
Preparation of nanocapsules
· Nanocapsule suspensions containing menthol were prepared using a nanoprecipitation method with some adjustments. A mixture of poly(ε-caprolactone), menthol, sorbitan monooleate, and caprylic/capric triglycerides in acetone (20 mL) was added to a solution of polysorbate 80, poloxamer 188, and deionized water using a glass syringe at a flow rate of 4 mL/min. The process took place under stirring at 1000 rpm and a temperature of 25 ± 2 °C.
· Acetone was then eliminated from the mixture using a rotary evaporator at 35 °C until the final volume reached 35 mL. The nanocapsules were subjected to gentle N2 flow for 30 min to ensure complete removal of acetone, which was verified through headspace gas chromatography-mass spectrometry analysis.
· Furthermore, Nile red-containing nanocapsules were prepared to support rapid in vitro skin penetration by replacing menthol with 0.4 mg/mL Nile red in the same manner as mentioned previously.

Zhang, Jun, et al. Journal of Inclusion Phenomena and Macrocyclic Chemistry, 2011, 71, 593-602.

(-)-Menthol can be added to skin-contact pharmaceutical formulations and cosmetics such as lotions and creams due to its unique fragrance and cooling effect. This work proposes a nanoporous silica material-based adsorbent to achieve controlled release strategy of volatile (-)-menthol. The results show that vesicular silica materials that have thick walls and hydrophobic functional groups exhibit the most delayed release properties.
(-)-Menthol adsorption and isothermal release study
· The adsorption of (-)-menthol into silica samples (S1-S6) was conducted using a rotary evaporator. In a standard procedure, 0.5 g of each siliceous product (S1-S6 in six separate batches) was combined with 6.25 g of (-)-menthol-ethanol solution (8 wt%) in a long cylindrical flask. The flask was then connected to a rotary evaporator and subjected to evaporation at 45 °C under a vacuum with a residual pressure of 2.25 x 10-3 Torr until the solvent was completely removed.
· Isothermal release studies of S1-S6 after adsorption of (-)-menthol were carried out at a constant temperature of 60°C using a thermogravimetric analysis (TGA) station.
· The results show that the pore size, structure, wall thickness and surface functionality of the silica material are important parameters for fine-tuning the release behavior. The release rate of (-)-menthol depends on the specific surface area and pore size of the host matrix. The adsorbent with small pore size is beneficial to the slow release of (-)-menthol. Silica samples with hydrophobic walls showed two-step weight loss and much lower release of (-)-menthol.