*3.1. Phase Solubility Studies*

The phase solubility studies evidenced a linear increase of drug solubility with increasing CDs concentration (AL type), as reported in Figure 1. The results indicate, in all cases, the formation of soluble complexes of probable 1:1 mol:mol stoichiometry [18]. The parameters, reported in Table 1, evidenced the difference in the solubilizing and complexing power towards HCT among the different CDs.

**Figure 1.** Phase solubility diagrams of HCT with βCD(-), HPβCD (Δ), HEβCD (-), SBEβCD (X), and RAMEB (). Each value represents the mean of 9 measurements.

**Table 1.** Stability constant (K1:1), complexation efficiency (CE), and solubilizing efficiency (SE) of complexes of the different examined cyclodextrins (CDs) with hydrochlorothiazide in pH 5.5 phosphate buffer at 25 ◦C.


<sup>a</sup> ratio between drug water solubility in presence of the highest CD concentration used (12.5 mM for βCD and 25 mM for other CDs) and alone.

Our results were in full agreement with those of Onnainty et al., 2013 [25], who reported the same type of phase-solubility diagrams for HCT complexation with βCD. The highest values of complex apparent stability constant (K1:1), complexation efficiency (CE), and solubilizing efficiency (SE) were obtained with RAMEB and SBEβCD that gave rise to an approximately five-fold increase in drug solubility as compared with the drug alone (Table 1). The better performance of these derivatives can be attributed to the presence of methyl and sulfobutylether substituents which extended the CD hydrophobic region and improved substrate binding via a hydrophobic effect, as just observed for other lipophilic drugs [16,26].
