*3.2. Phase Solubility Studies*

Phase solubility studies were performed according to the method reported by Higuchi and Connors [38] in 0.1 M phosphate buffer of pH 7.4, at 25 ◦C. An excessive amount of RSP (10 mg) was added to 3 mL of solution containing RM-β-CD in concentration of 0–50 mM. The obtained suspensions were vigorously shaken at 25 ◦C for 5 days. After the equilibrium was reached, the samples were filtered using a 0.45 µm nylon disk filter and suitably diluted. The RSP concentration in filtered solutions was evaluated using UV spectroscopic measurements at 277 nm. Spectronic Unicam—UV 320 UV-Visible double beam spectrophotometer (Spectronic Unicam, Cambridge, UK) with 1 cm matched quartz cells was used for all UV spectrophotometric measurements.

The apparent stability constant (*K*1:1) was calculated from the phase solubility diagram, using the following equation:

$$K\_{1:1} = \frac{Slope}{\mathcal{S}\_0 \ (1 - Slope)}\tag{2}$$

where *S*<sup>0</sup> is RSP solubility in phosphate buffer 0.1 M (pH 7.4) in the absence of RM-β-CD.
