THERMODYNAMICS OF SOLUTION OF HAPTENS AND CYCLODEXTRIN-HAPTEN COMPLEXES IN AQUEOUS AND NON-AQUEOUS MEDIA

Rafic Moussa  N. TRABOULSI

Thermodynamic parameters of solution  ( Δ G°_s, Δ H° _s and Δ S° _s) of some haptens [ortho-, meta-, para-. 5-chloro-2-, 6-chloro-2-, 2-chloro-4- and 4-chloro-3­(parahydroxyphenylazo) sodium benzoate] and three cyclodextrins (α, β and у) were carried out in different reaction media. 'Thermodynamic parameters for the transfer  ( Δ G°_t, Δ H° _t Δ S° _t) of haptens and their anions from water to methanol and from water to N,N'‑Dimethylformamide were derived. In addition, transfer free energy, enthalpy and entropy of cyclodextrins from water toN,N′­Dimenthy1formamide are reported. Thermodynamic parameters of complexation between haptenic anions and cyclodextrins were investigated in water and in N,N'‑Dimethylformamide and their transfer quantities from water to N,N'‑Dimethylformamide are also given.

It was found that the selected haptens (anions) are better solvated in methanol than in water than in N,N'‑Dimethylformamide. The transfer enthalpies of the anions (data based on the Ph₄ AsPh₄B convention) from water to methanol and from water to N,N'‑Dimethylformamide ‑[ΔH°_t (X)] are largely compensated by their transfer entropies [Δ S° _t  (X)].

As far as solution thermodynamic data of cyclodextrins in water and N,N'‑Dimethylformamide are concerned, it was noticed that a compensation effect between the Δ H° _s and Δ S° _s values is taking place in water and in N,N'‑Dimethylformamide.

Only three anions complex with α and у ‑cyclodextrins in water, whereas four haptens form complexes with α, β and у ‑cyclodextrin in N, N'‑Dimethylformamide. Again a compensation effect for cyclodextrin‑anion complexes was observed in water and in N,N'‑Dimethylformamide.

A cavity size effect was shown during the formation of cyclodextrin‑hapten complexes. Anion‑cyclodextrin interaction becomes weaker with an increase in the cavity of cyclodextrin.

Inclusion complexes (axial) are found to take place in water and lid‑type (equatorial) complexes are found between these haptenic anions and cyclodextrins in N,N'‑Dimethylformamide.

The transfer parameters for the cyclodextrin‑anion complexes were calculated using a thermodynamic cycle. This is the first set of data ever reported on the transfer of cyclodextrin adducts among solvents.