trans-Cinnamic acid--alpha-cyclodextrin system as studied by solubility, spectral, and potentiometric techniques

Area of Science:

• Supramolecular Chemistry
• Physical Chemistry

Background:

• Cyclodextrins are cyclic oligosaccharides known for their ability to form inclusion complexes with various guest molecules.
• Cinnamic acid and its ion are common organic compounds with potential applications in pharmaceuticals and materials science.

Purpose of the Study:

• To quantitatively characterize the complex formation between alpha-cyclodextrin and cinnamic acid/cinnamate ion in aqueous solutions.
• To determine the thermodynamic parameters (association constants, enthalpy, and entropy changes) for these complexation reactions.

Main Methods:

• Solubility measurements
• Spectroscopic analysis (UV-Vis, NMR)
• Potentiometric titrations

Main Results:

• Consistent estimates of 1:1 (SL) and 1:2 (SL2) complex association constants were obtained using solubility, spectral, and potentiometric data.
• For cinnamic acid, K11 = 2260 M⁻¹, ΔH°₁₁ = 9.3 kcal/mole, ΔS°₁₁ = -8 e.u.; K12 = 60 M⁻¹, ΔH°₁₂ = -12 kcal/mole, ΔS°₁₂ = -26 e.u.
• For cinnamate ion, K11 = 110 M⁻¹, ΔH°₁₁ = -1.9 kcal/mole, ΔS°₁₁ = +11 e.u.; K12 = 15 M⁻¹, ΔH°₁₂ = 9 kcal/mole, ΔS°₁₂ = -15 e.u.

Conclusions:

• Complex formation in aqueous solutions of cinnamic acid/cinnamate ion and alpha-cyclodextrin can be quantitatively described as 1:1 and 1:2 complexes.
• Thermodynamic analysis indicates that complex formation is thermodynamically spontaneous in the solid phase.
• Complex stability is greater in aqueous solution compared to the solid phase.