Trans- Cinnamic Acid–α-Cyclodextrin System as Studied by Solubility, Spectral, and Potentiometric Techniques

Abstract

Complex formation in aqueous solutions of trans-cinnamic acid or trans-cinnamate ion (the substrate, S) and α-cyclodextrin (the ligand, L) can be described quantitatively as the 1:1 and 1:2 complexes, SL and SL2. The solubility, spectral, and potentiometric data over a wide range of ligand conccntrations yielded consistent estimates of the complex association constants. For cinnamic acid at 25°, K11 = 2260 M−1, ΔH°11 = −9.3 kcal/mole, and ΔS°11 = −8 e.u.; and K12 = 60 M−1, ΔH°12 = −12 kcal/mole, and ΔS°12 = −26 e.u. For cinnamate ion at 25°, K11 = 110 M−1, ΔH°11 = −9 kcal/mole, and ΔS°11 = +11 e.u.; and K12 = 15 M−1, ΔH°12 kcal/mole, and ΔS°12 = −15 e.u. (all entropy changes are unitary quantities). Thermodynamic cycles for the complexes, using solubility data, reveal that complex formation in the solid phase is thermodynamically spontaneous but that complex stability is greater in aqueous solution than in the solid phase.