Wirt-Gast-Komplex β-Cyclodextrin und Phenolphthalein

1. Experiment is contained in the WAKER's Experimental Kit for Schools	yes
2. Experimental procedure has been modified	no
3. Video clip available	no
4. Flash animation available	no

4. Binding and Controlled Release of Peppermint Aroma

1 Materials, Chemicals, Time Needed

Snap-cap vials
Filter flask
Büchner funnel
Gucoring
Filter paper
Water jet pump

β-Cyclodextrin
Distilled water
Peppermint oil (Oleum menthae piperitae)

It takes only 10 to 15 minutes to produce the complex, but the samples must be stored in the open for at least three days for a clear difference to become apparent.

2 Procedure and Observations

Prepare a suspension of 0.25 g β-cyclodextrin and 25 ml water, add 2 ml of peppermint oil and shake vigorously for five minutes. Obtain the resultant solid by suction filtration and apply to filter paper. Prepare a control sample by applying 5 drops of pure peppermint oil to filter paper. Leave the two filter papers in the open air for several days. Then compare the odors. Dissolve a spatula tip of the solid in warm water.

Immediately after filtration, the solid smells only of peppermint, whereas the filter paper soaked with oil also has an oily aroma. A clear difference in aroma is discernible after several days. The solid continues to emit a fresh peppermint aroma, while the filter paper soaked with the oil has hardly any odor at all. Dissolving the solid in warm water immediately releases a strong peppermint aroma.

3 Discussion of Results

The results of this experiment demonstrate the properties of cyclodextrin complexes that are desirable in cosmetics, for example. The fresh peppermint aroma persists for longer due to the formation of host-guest complexes with the aroma substances in the peppermint oil, which prevent the guest molecules from evaporating so easily. The aromas encapsulated within the cyclodextrin cavities are also protected from oxidation by atmospheric oxygen. The colorless solid steadily releases aromas for a prolonged period, since the complex dissociates to a minor degree as a result of atmospheric humidity. The formation of a hydration sheath on the exterior of the cyclodextrin molecule can modify the molecular structure of the complex to such an extent that interactions between the host and guest molecules become weaker. Adding the solid to warm water facilitates dissociation of the host-guest complex due to the excess of water. An intense minty aroma is then immediately discernible, as the volatile and extremely poorly water-soluble menthol rapidly disperses in the air.

4 Tips and Comments

For this experiment use Oleum menthae piperitae (peppermint oil), which can be obtained from the drugstore.
This experiment demonstrates the formation and dissociation of the complex. The aroma substance can be released steadily over a prolonged period, or rapidly, depending on the conditions (controlled release).
The following technical terms may be mentioned when evaluating this experiment: intermolecular interactions, hydration sheath, solubility, equilibrium, shift in equilibrium, aroma substances.

5 Supplementary Information

The two stereoisomers (+)-menthol and (-)-menthol are principally responsible for the peppermint aroma (see Fig. 2.25).

Fig. 2.25: The two stereoisomers (+)-menthol and (-)-menthol cause the characteristic peppermint aroma

The menthol-cyclodextrin complex is used in cosmetics, principally because the non-polar menthol is present in the aqueous phase in the complex. The menthol-cyclodextrin complex is also used in the tobacco industry, where it is added to the tobacco in menthol cigarettes. If the menthol was not formulated as a complex, all the menthol would evaporate within a short time.

6 References

Gröger, M.; Kretzer, E.K.; Woyke, A.; Reader mit Hintergrundinformationen zum Thema Cyclodextrine, 2001
www.wacker.com/internet/noc/Products/PT_Bio/P_Bio_Cy/P_CycCom/P_CycCom_SP_Ment/