*3.2.2. Fourier-transform infrared spectroscopy (FT-IR)*

FT-IR technique can be used to investigate the variation of shape, intensity and position of peaks [138].

FT-IR spectrum of black pepper oil consisted of the prominent absorption bands at 2954, 2923 and 2865 cm−1 for C─H stretching vibration of methylene group, 1638 cm−1 for H─O─H bending, 1447 cm−1 for C─H scissoring vibration, 1369 cm−1 for symmetrical deformation vibration of CH<sup>3</sup> , 886 cm−1 for C─H deformation vibration and 789 cm−1 for S─C absorption. However, FT-IR spectrum of the encapsulated black pepper oil showed that no character similar to the free black pepper oil. All bands of black pepper oil spectrum were totally obscured by HPβCD bands it was possible that black pepper oil entered the cavity of HPβCD and inclusion complex was formed.

In the case of yarrow oil, its FT-IR spectrum of yarrow oil shows prominent absorption bands at 2956 cm−1 for ═CH<sup>2</sup> symmetrical stretching vibration, 2926 cm−1 for C─H stretching vibration of methylene group, 2869 cm−1 for ─CH stretching, 1652 cm−1 for H─O─H bending, 1626 cm−1 for C═C stretching vibration of the allyl group, 1446 cm−1 for C─H scissoring vibration, 1380 cm−1 for symmetrical deformation vibration of ─CH<sup>3</sup> , 1366 cm−1 for symmetrical deformation vibration of ─CH<sup>3</sup> , 1240 cm−1 and 1103 cm−1 for P─O and P═O, 1022 cm−1 for C─O─C stretching vibration, 916 cm−1 for C─S─C stretching vibration, 875 cm−1 and 865 cm−1 for C─H bending of aromatic ring. The spectrum of HPβCD shows prominent absorption bands at 3406 cm−1 for O─H stretching vibration, 2970 cm−1 for ═CH<sup>2</sup> symmetrical stretching vibration, 2930 cm−1 for C─H stretching vibration, 1646 cm−1 for H─O─H bending vibration, 1157 cm−1 for C─O─C asymmetrical stretching vibration, 1083 cm−1 and 1033 cm−1 for symmetric C─O─C stretching vibration [139]. FT-IR spectrum of inclusion complex was identical to HPβCD and no feature similar to yarrow oil. The results indicated that HPβCD covered all the absorption bands of yarrow oil in the spectrum of inclusion complex indicating the entering to the cavity of HPβCD and the formation of inclusion complex.

Finally, FT-IR spectrum of guava leaf oil showed prominent absorption bands at 2921 cm−1 for C─H stretching vibration of methylene group, 1642 cm−1 for H─O─H bending, 1447 cm−1 for C─H scissoring vibration, 1376 cm−1 for symmetrical deformation vibration of CH3, 886 cm−1 for C─H deformation vibration and 789 cm−1 for S─C absorption. FT-IR spectrum of encapsulated guava leaf oil shows no feature similar to the free guava oil. The bands of guava leaf oil spectrum were almost completely concealed by very intense and broad bands of HPβCD. However, the absorption band at 608 cm−1 of HPβCD disappeared in encapsulated guava leaf oil. This change may be related to the interaction between guava leaf oil and HPβCD in the inclusion complex.

The inclusion complex formation of β-CD was also investigated by Liu et al. [140] using FT-IR analysis. The absorption bands of β-caryophyllene were not detected in the spectrum of inclusion complex. The changes were related to the inclusion complex formation of β-CD and the guests which whole of guest could be contained in the CD cavity. Wang et al. [139] have reported similar results. In their study, the inclusion complex formation of soybean lecithin and β-CD was determined by FT-IR. All the absorption bands of soybean lecithin encapsulated in β-CD were obscured by β-CD spectrum showing that inclusion complex of β-CD and soybean lecithin was formed. However, Gomes et al. [141] reported that the absorption band at 1738 cm−1 of the red bell pepper pigments was observed after encapsulation in β-CD indicating that some region of the encapsulated molecules was not contained in the cavity of β-CD.

#### *3.2.3. Ultraviolet-visible spectrophotometry (UV-Vis)*

*3.2.2. Fourier-transform infrared spectroscopy (FT-IR)*

**Figure 7.** SEM micrographs of encapsulated essential oils at 500 times magnification.

peaks [138].

272 Cyclodextrin - A Versatile Ingredient

of CH<sup>3</sup>

complex.

plex was formed.

bands at 2956 cm−1 for ═CH<sup>2</sup>

rical deformation vibration of ─CH<sup>3</sup>

FT-IR technique can be used to investigate the variation of shape, intensity and position of

FT-IR spectrum of black pepper oil consisted of the prominent absorption bands at 2954, 2923 and 2865 cm−1 for C─H stretching vibration of methylene group, 1638 cm−1 for H─O─H bending, 1447 cm−1 for C─H scissoring vibration, 1369 cm−1 for symmetrical deformation vibration

In the case of yarrow oil, its FT-IR spectrum of yarrow oil shows prominent absorption

vibration of methylene group, 2869 cm−1 for ─CH stretching, 1652 cm−1 for H─O─H bending, 1626 cm−1 for C═C stretching vibration of the allyl group, 1446 cm−1 for C─H scissoring

for C─O─C stretching vibration, 916 cm−1 for C─S─C stretching vibration, 875 cm−1 and 865 cm−1 for C─H bending of aromatic ring. The spectrum of HPβCD shows prominent

rical stretching vibration, 2930 cm−1 for C─H stretching vibration, 1646 cm−1 for H─O─H bending vibration, 1157 cm−1 for C─O─C asymmetrical stretching vibration, 1083 cm−1 and 1033 cm−1 for symmetric C─O─C stretching vibration [139]. FT-IR spectrum of inclusion complex was identical to HPβCD and no feature similar to yarrow oil. The results indicated that HPβCD covered all the absorption bands of yarrow oil in the spectrum of inclusion complex indicating the entering to the cavity of HPβCD and the formation of inclusion

absorption bands at 3406 cm−1 for O─H stretching vibration, 2970 cm−1 for ═CH<sup>2</sup>

vibration, 1380 cm−1 for symmetrical deformation vibration of ─CH<sup>3</sup>

symmetrical stretching vibration, 2926 cm−1 for C─H stretching

, 1240 cm−1 and 1103 cm−1 for P─O and P═O, 1022 cm−1

, 1366 cm−1 for symmet-

symmet-

, 886 cm−1 for C─H deformation vibration and 789 cm−1 for S─C absorption. However, FT-IR spectrum of the encapsulated black pepper oil showed that no character similar to the free black pepper oil. All bands of black pepper oil spectrum were totally obscured by HPβCD bands it was possible that black pepper oil entered the cavity of HPβCD and inclusion comEssential oils contain various bioactive chemicals, which adsorb ultraviolet (UV) or visible light (Vis) at different wavelengths. CD host-guest complex formation would alter UV-Vis absorption spectra [142]. Otherwise, the spectra of the guests appear in line of CD [140]. Therefore, UV-Vis spectrophotometry, evaluated the inclusion complex formation of HPβCD and the three essential oils. The UV absorption spectra of guava leaf oil, limonene and their inclusion complexes were compared. Indeed, maximum absorption value of guava leaf oil was at 214.5 nm, which was mainly attributed to limonene. The absorption peak at 205 nm corresponds to β-caryophyllene and/or pinene. The peak at 275 nm of guava leaf oil was ascribed to 1,8-cineole.

The spectra of the physical mixture of HPβCD with guava leaf oil and with limonene before complexation were consistent with that of guava leaf oil or pure compound. The absorption spectra of the physical mixture of HPβCD with guava leaf oil and with limonene were in accordance to with the spectra of guava leaf oil and pure limonene, respectively. When the active compounds in essential oil or the pure compound were encapsulated into the cavity of HPβCD, the absorption peaks of each compound disappeared in the spectra of the inclusion complexes. To recover active compounds encapsulated in the HPβCD cavity, the active compounds were extracted from HPβCD by dissolving the inclusion complexes in 95% acetonitrile. The encapsulated compounds were released from the cavity of HPβCD and HPβCD was separated from guava leaf essential oil or limonene in solution by centrifugation. The solution was diluted 100 times with acetonitrile and the absorbance was measured by UV spectrophotometer.

After extraction from the inclusion complexes, the absorption peaks of encapsulated compounds in guava leaf oil could be observed. In this line, besides limonene, the absorption peaks at 205 and 275 nm suggested the presence of β-caryophyllene and 1,8-cineole, respectively. The results indicated that the active compounds in guava leaf oil had formed inclusion complex with HPβCD. Therefore, the chemical components of guava leaf oil were successfully encapsulated in the HPβCD.

UV spectrum of yarrow oil shows peaks at 270–275 nm indicated the presence of carvacrol, 1,8-cineole, thymol and camphor. A minor peak at 243 nm attributed to linalool. The spectra of the physical mixture of HPβCD with yarrow oil and with pure compound (carvacrol) conformed to UV spectra of yarrow oil and pure carvacrol, respectively. When the active compounds in yarrow oil or carvacrol were entrapped with HPβCD, the absorption peaks of each compound also disappeared in the spectrum of the inclusion complexes.

In the case of black pepper, A linear relationship between the amount of dissolved essential oil or β-caryophyllene and the concentrations of HPβCD in this study with slope ˂1 was classified as a typical AL-type (type A reveals an inclusion complex formation where the amount of encapsulated compounds increase as the HPβCD concentration increases, subscript L indicates a 1:1 molecular ratio formation of soluble complexes) [144]. As the majority of encapsulated compounds are mono- and sesquiterpenoids and phenylpropane derivatives of an average molecular weight of 120–160 g/mol, a 1:1 complex formation is observed [16]. The molar ratio of host to guest molecules is usually 1:1 for inclusion complexes formed in solution, except for complexes with long-chain or bifunctional guest molecules (e.g. guest molecules having two aromatic rings on opposite sides of a small central molecule segment). In aqueous system, black pepper oil and β-caryophyllene show difference in stability of complex form with the K<sup>s</sup>

Black pepper oil-HPβCD 25 104.5 β-caryophyllene-HPβCD 25 132.8 Black pepper oil-HPβCD 35 100.0 β-caryophyllene-HPβCD 35 114.0 Guava leaf oil-HPβCD 25 25.0 Limonene-HPβCD 25 628.0 Guava leaf oil-HPβCD 35 33.8 Limonene-HPβCD 35 605.9 Yarrow oil-HPβCD 25 106.6 Carvacrol-HPβCD 25 360.9 Yarrow oil-HPβCD 35 92.0 Carvacrol-HPβCD 35 309.7

**/M−1 Inclusion complex T/°C Ks**

Encapsulation of Essential Oils by Cyclodextrins: Characterization and Evaluation

) of encapsulated essential oil and their main component.

http://dx.doi.org/10.5772/intechopen.73589

104.5 and 132.8 L/mol at 25°C, respectively. This might be because of the other components in black pepper oil might compete to HPβCD form complex with β-caryophyllene. The decreases

Equivalent results were observed for yarrow oil host-guest complex, as we can observe in **Table 3**. In agreement with the results reported in **Table 3** – for black pepper essential oil and yarrow essential oil – similar Hill et al. [99] and Kamimura et al. [110] have reported observations. The water solubility of trans-cinnamaldehyde, eugenol, cinnamon bark extracts

the samples decreased with increasing temperature [99]. Kamimura et al. [110] reported that

leaf oil than for limonene. They were in the order of those for β-CD complexes according to Connors [145]. This might be due to the competence of the other components in guava leaf oil

ing temperature reflects that complex formation is an exothermic process [99]. However, these results reflect that the aqueous solubility of guava leaf oil can be increased with increasing HPβCD concentration. Considering that very labile complexes (K<sup>s</sup> < 100 L/mol) result in premature release of the guests because of the weak interaction between hosts and guests [92], the very labile encapsulated guava leaf oil could be useful for fast release systems such

values with increasing temperatures were expected for exothermic processes [99].

and clove bud extract samples increased with increasing temperatures while the Ks

water solubility of the pure carvacrol increased and the Ks

**Inclusion complex T/°C Ks**

**Table 3.** Phase solubility parameters and stability constants (K<sup>s</sup>

Regarding to guava leaf essential oil – see **Table 3**, low Ks

with limonene to form HPβCD complexes. In addition, the decrease in K<sup>s</sup>

in Ks

temperatures.

as pharmaceutical applications.

of

**/M−1**

275

value of

value decreased with increasing

value were obtained for guava

values with increas-

After extraction from the inclusion complex, the absorption peaks of entrapped compounds in yarrow oil appeared at 270–275 nm implying carvacrol and also are 1,8-cineole, thymol, camphor and linalool. In this study, the chemical components of yarrow oil were successfully entrapped in the HPβCD, as in the previous case. However, the encapsulation efficiency of yarrow oil was much lower than those of its pure compound. This was likely because the competition of major active compound among other components in essential oil has occurred during inclusion complex formation.

Finally, the absorption spectrum of black pepper oil was recorded with absorption peaks at 200, 205 and 214.5nm for δ-3-carene, β-caryophyllene and limonene, respectively [140]. The maximum absorption peak at 205 nm was ascribed to β-caryophyllene. The spectra of the physical mixture of HPβCD with black pepper oil and with β-caryophyllene accorded with UV spectra of black pepper oil and pure β-caryophyllene, respectively. When the active compounds in black pepper oil or the pure compound (β-caryophyllene) were entrapped into the cavity of HPβCD, the absorption peaks of the compounds also disappeared in the spectrum of the inclusion complex.

After extraction from the complex, the observable peaks of entrapped compounds in black pepper oil could be seen. The spectrum of encapsulated compounds from black pepper oil show absorption peaks at 205 and 214.5 nm indicating β-caryophyllene and limonene, respectively. The UV spectrum indicated that the chemical components of black pepper oil were successfully entrapped in the HPβCD. As in the previous cases, the encapsulation efficiency of active compounds of black pepper oil was much lower than those of its pure compound. This was likely because the competition of major active compound among other components in black pepper oil has occurred during inclusion complex formation.
