Difference in inclusion behavior of β-cyclodextrin and hydroxypropyl-β-cyclodextrin with menthol

Volatile oils are important active components in traditional Chinese medicine, but their components are complicated and unstable. It is common to use cyclodextrin inclusion technique to improve the stability of volatile oils and make them easier to be prepared. At present, β-cyclodextrin (β-CD) is the most common inclusion material. The evaluation indicators for inclusion technique usually contain the inclusion rate and the oil content in the inclusion compound. However, the articles about the study on selecting inclusion materials for volatile oils were few. In this paper, menthol, the main active ingredient of mint volatile oil, was used as model drug, while β-CD and hydroxypropyl-β-cyclodextrin (HP-β-CD) were used as the inclusion materials. Inclusion equilibrium constant (K), solubilization ratio were investigated, and the results were combined with IR, DSC and TG to verify the formation of inclusion complexes. It turned out that in the range of 0-15 mmol•L⁻¹, the solubility of menthol was increased linearly with the increase of HP-β-CD concentration, with AL-type phase solubility diagram, K=3 188.62 L•mol⁻¹; in the range of 0-12.5 mmol•L⁻¹, the solubility of menthol was increased linearly with the increase of β-CD concentration, K=818.73 L•mol⁻¹. When the concentration was over 12.5 mmol•L⁻¹, the solubility of menthol appeared to be a negative deviation with the increase of β-CD concentration, with AN-type solubility diagram. The above results showed that the inclusion behavior was different between β-CD and HP-β-CD, laying a foundation for further study on inclusion complexes of volatile oil.