*3.2.4. Phase solubility*

Phase solubility study is generally performed to evaluate the stability and to classify the inclusion complex when they are in the solution. The phase solubility profiles can be obtained from the interaction between the guests (encapsulated compounds) and the hosts (CDs or derivatives) in the solution. In solution, a fundamental parameter such as stability constant (K<sup>s</sup> ) of inclusion complex formation can be used to evaluate the stability of the inclusion complex [143] – see **Table 3**.


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 success-

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

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

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

Phase solubility study is generally performed to evaluate the stability and to classify the inclusion complex when they are in the solution. The phase solubility profiles can be obtained from the interaction between the guests (encapsulated compounds) and the hosts (CDs or derivatives) in the solution. In solution, a fundamental parameter such as stability constant (K<sup>s</sup>

inclusion complex formation can be used to evaluate the stability of the inclusion complex

) of

compound also disappeared in the spectrum of the inclusion complexes.

in black pepper oil has occurred during inclusion complex formation.

fully encapsulated in the HPβCD.

274 Cyclodextrin - A Versatile Ingredient

during inclusion complex formation.

*3.2.4. Phase solubility*

[143] – see **Table 3**.

**Table 3.** Phase solubility parameters and stability constants (K<sup>s</sup> ) of encapsulated essential oil and their main component.

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> of 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 in Ks values with increasing temperatures were expected for exothermic processes [99].

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 and clove bud extract samples increased with increasing temperatures while the Ks value of the samples decreased with increasing temperature [99]. Kamimura et al. [110] reported that water solubility of the pure carvacrol increased and the Ks value decreased with increasing temperatures.

Regarding to guava leaf essential oil – see **Table 3**, low Ks value were obtained for guava 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 with limonene to form HPβCD complexes. In addition, the decrease in K<sup>s</sup> values with increasing 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 as pharmaceutical applications.
