# Thermal Triggered Release of Menthol from Different Carriers: A Comparative Study
## 3 Results  
First, the affinity of the menthol with SM and guar was investigated theoretically with DFT investigations. The adsorptive configuration of menthol on SM and guar was optimized and the binding energy was calculated accordingly (Figure 1). Guar is a high molecular weight galactomannan polysaccharide composed by linear D-mannose units joined by 1,4-8-glycosidic linkages. To generate the adsorption model with guar gum, we built a block composed of galactose and mannose to mimic the molecule structure of guar gum (Figure 1B). According to the calculation results, the binding energy of menthol on SM and guar gum was \(-0.915\) eV and \(-0.05\) eV, respectively. These results suggest that menthol favorably binds with the SM and the affinity of menthol with SM was higher than that for guar gum. Such a high affinity between the fragrance and polymeric substrate helps to evenly distribute and stabilize the fragrance in the polymer carrier. Thermal induced fragrance release below 100 \({}^{\circ}\)C is important in food and tobacco applications. Therefore, it is important to identify possible polymer carrier for typical fragrances, which could realize sustainable and controllable fragrance release at a temperature below 100 \({}^{\circ}\)C. As the thermal release performance of the carrier system is closely related to the thermal stability of the carrier, we first evaluated the melting behavior of the polymer carrier. The melting point of SM was measured to be 63 \({}^{\circ}\)C. For guar gum, no distinct melting behavior was observed, indicating the low crystallinity of the material. The fragrance was loaded into the carrier material through mixing of the fragrance and polymer carrier at 80 \({}^{\circ}\)C, under which temperature the mixture exists as fluid. After mixing, the carrier system was naturally cooled down to room temperature. To demonstrate the successful loading of the fragrance into the polymer carrier, an FTIR measurement was performed (Figure 2). Except the typical bands from alkyl chains, characteristic bands associated with the unique functional groups of the polymer carrier were observed. For example, an absorption band at 1735 cm\({}^{-1}\) (C=O, stretching vibration) was present in the FTIR spectra of SM, menthol-SM, and menthol-SM + GG; an absorption band at 1650 cm\({}^{-1}\) (C=C, stretching vibration) was recorded in the spectra of GG, menthol-GG, and menthol-SM + GG. The successful loading of menthol into the polymer carrier was evidenced by the presence of characteristic bands from menthol in the spectra of menthol-SM, menthol-GG, and menthol-SM + GG (e.g., 1345 cm\({}^{-1}\) for the in-plane wagging vibration of CH and 1025 cm\({}^{-1}\) for the stretching vibration of C-O). The incorporation of the fragrance into the polymer carrier influences the melting behavior of the carrier system. As discussed above, the pristine SM had a melting point of 63 \({}^{\circ}\)C. The melting point of the fragrance carrier system with fragrance (menthol): polymer carrier (SM) ratios of 1:10, 2:10, and 4:10 were determined to be 55, 52, and 42 \({}^{\circ}\)C. When further increasing the menthol content in the carrier system (above 4:10), the carrier system tends to behave like a semi-liquid above 30 \({}^{\circ}\)C. The fragrance carrier system at a semi-liquid state may not be suitable for most of the practical applications. Thus, we chose a fragrance: polymer carrier ratio of 4:10 for subsequent studies. Figure 1: (**A**) Optimized configuration showing the adsorption of menthol on (**A**) sorbitan monostearate and on (**B**) a model polymer block mimicking guar. The release of fragrance from the polymer carriers was first characterized with a visualized time-lapse experiment with the melting point meter (Figure 3). At a set temperature of 80 \({}^{\circ}\)C, no obvious phase changes occurred on menthol-GG. Only a small portion of melted substance, which could be menthol (melting point 45 \({}^{\circ}\)C), was observed in some parts of the material.