Effect of pH on the Encapsulation of the Salicylic Acid / Salicylate System by Hydroxypropyl-β-Cyclodextrin at 25 °c. A Fluorescence Enhancement Study in Aqueous Solutions

The encapsulation of the acid/base conjugated system salicylic acid(HSA)/salicylate (SA-) by hydroxypropyl-β-cyclodextrin (HPBCD) has beenstudied through fluorescence emission enhancement measurements in aqueoussolutions at 25 °C. With the aim of analyzing the crucial importance ofa proper and cautious choice of the pH of the medium (i.e. choice of thebuffer), the study has been carried out at pH = 1, 2, 4 and 7. Since the pKa of the HSA/SA- system is 2.95 at 25 °C, the presence of the protonated(HSA) and non-protonated (SA-) forms suitable for inclusion by cyclodextrinvary appreciably within the different pH conditions: 99% HSA,1% SA- at pH = 1, 90% HSA, 10% SA- at pH = 2,10% HSA, 90% SA- at pH = 4, and ≈0% HSA,≈100% SA- at pH =7. The association constants K CD : HSA and K CD: SA- have been determined in all cases by using a nonlinear regressionanalysis of the experimental data at three different λ em. The effectof the pH of the medium on all the equilibria involved as well as in the Kvalues is fully discussed. The 8-anilino-1-naphthalene sulfonate (ANS) +β-cyclodextrin (β-CD) system, widely reported in literature, hasinitially been studied to check the experimental protocol and the numericalmethod.