The thermodynamic study of p-methoxycinnamic acid inclusion complex formation, using β-cyclodextrin and...

Cyclodextrin's ability to form an inclusion complex with a guest molecule is a function of two factors. The first is steric and depends on the relative size of cyclodextrin cavity to the guest molecule, while the second is the thermodynamic interaction between the different system components. This study therefore aims to determine the effect of β-cyclodextrin and hydroxypropyl-β-cyclodextrin as complex formers, on thermodynamic parameter values (ΔH, ΔG, and ΔS) in the formation of inclusion complex with p-methoxycinnamic acid (pMCA).

The pMCA complex formation with β-cyclodextrin or hydroxypropyl-β-cyclodextrin was determined in 0.02 pH 4.0 M acetate buffer and 0.02 M pH 7.0 phosphate buffer, with a 0.2 µ value at 32, 37, and 42 ± 0.5 °C. This experiment was carried out in a waterbath shaker until a saturated solution was obtained. Subsequently, pMCA concentration was determined using UV spectrophotometer at the maximum pMCA wavelength, at each pH.

The result showed pMCA formed inclusion complex with β-cyclodextrin or hydroxypropyl-β-cyclodextrin and exhibited increased solubility with increase in β-cyclodextrin or hydroxypropyl-β-cyclodextrin concentration. This temperature rise led to a decrease in the complex's constant stability (K). Furthermore, the interaction showed a negative enthalpy (∆H<0) and is a spontaneous processes (∆G<0). At pH 4.0, an increase in the system's entropy occurred (∆S>0), however, at pH 7.0, the system's entropy decreased (∆S<0).

The rise in pMCA solubility with increase in cyclodextrin solution concentration indicates an inclusion complex has been formed, and is supported by thermodynamic data.